ASNT NDT Handbook Volume 3 Infrared and Thermal Testing

procedure for discontinuity
characterization.

A series of abacuses are provided for
this purpose. Despite some simplifications
(heat losses were disregarded and the heat
pulse duration was infinitely short), the
results give a synthetic overview of the
performances that can be achieved with
pulsed thermography in nondestructive
testing.

436 Infrared and Thermal Testing

References

1. Vavilov, V. "Thermal Nondestructive 7. Rantala,]. and]. Hartikainen.
Testing: Short History and "Numerical Estimation of the Spatial
State-of-Art." Quantitative Infrared
Themwsraphy (QIRT '92) Resolution of Thermal NDT
rch<ltenay-Malabry, France, july
1992]. Eurotherm Seminar 27. Paris, Techniques Based on Flash Heating."
France: Editions Europeennes Research in Nondestructive Evaluation.
Techniques et Industries {1992): Vol. 3, No. 3. Columbus, OH:
American Society for Nondestructive
p 179-189. Testing (1991): p 125-139.
8. Sante}', ~vf.B. and D.P. Almond.
2. Krapez J.-C. and P. Cielo "Defect Sizing by Transient
"Thermographic Nondestructive Thermography II: A Numerical
Evaluation: Data Inversion Treatment." foumal of Physics D:
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Research i11 Nmzdestructil•e Eva/uaUm1. United Kingdom: Institute of Physics
Vol. 3, No. 2. Columbus, OH: (1995): p 2539-2546.
American Society for Nondestructive 9. Houlbert, A.S., A.S. Lamine and
A. Degiovanni. "Modelisation d'un
Testing (1991 ): p 81-100.
3. Krapez ].-C., X. Maldague and P. Cielo Defaut Limite en Vue du ContrOJe
Nondestructif des Matfriaux
"Thermographic NDE: data inversion Anisotropes. 11 lntemationalfoumal of
procedures. Part II: 2-D Analysis and Heat ami Mass Transfer. Vol. 34,
Experimental Results." Research in No. 4/5. New York, NY: Pergamon
Nondestructiw Evaluation. VoL 3,
No. 2. Columbus, OH: American Press (1991): p 1125-1138.
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(1991): p 101-124.
S. Didierjean, A.S. Lamine and
4. "NDT Abstracts: Thermography on A. Degiovanni. "Non-Destructive
Composites." Nondestructive 'Ii'sting
and Evaluation ltlternatioual. Vol. 26, Thermal Evaluation of Delaminations
No. 2. Oxford, United Kingdom:
in a Laminate: Part 1 -Identification
Elsevier Science Limited (1993): by Measurement of Thermal Contrast;
p99-107. Part 2- The Experimental Laplace
Transform Method." Composites
5. Krapez, j.-C., D. Boscher1 P. Delpech, Science and Technology. Vol. 47.
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Elsevier Applied Science Publishers
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Stimulated Infrared Thermography (1993): p 137-172.
11. Degiovanni, A., A. Bendada,
Applied to Carbon-Epoxy Non
Destructive Evaluation." Quantitath•e ].C. Batsale and D. Maillet.
11 Analytical Simulation of a
Infrared "/Jrermosraplly (QIRT '92) :~vfulti-Dimensional Temperature Held

{Chfitenay-Malabry, France, july Produced by Planar Defects of Any
1992]. Eurotherm Seminar 27. Paris,
Shape: Application to Nondestructive
France: Editions Europfennes Testing. 11 Quantitative Infrared
Techniques et Industries (1992): TlrermoJmphy (QJRT '94) [Sorrento,
p 195-200. Italy, August 1994]. Eurotherm
Seminar 42. Paris, France: Editions
6. Cesini, G., M. Paroncini and R. Ricci.
Europeennes Techniques et Industries
Thermal Distribution in Circular (1995): p 253-259.
Slabs: A Thermographic Method."
QlmJrtitatil•e f11{rared 71zennograp!Jy 12. Almond, D.P. and S.K. Lau. "Defect
(QIRT '92) IChatenay-Malabry, France,
Sizing by Transient Thermography I:
July 1992J. Eurotherm Seminar 27. An Analytical Treatment." joumal of
l'aris, France: Editions Europfennes Physics D: Applied Physics. Vol. 27.
Techniques et Industries (1992): London, United Kingdom: Institute of
Physics (1994): p 1063-1069.
p 272-277.

Thermal Contrasts in Pulsed Infrared Thermography 437

13. Vavilov, V. and X. Maldague. 21. Legrandjacques, L., J.·C. Krapez,
"Optimization of Heating Protocol in
F. Lcpoutre and D. Balageas.
Thermal NOT, Short and Long Pulses: 11 Detection of Open Cracks by a
A Discussion." Research in Photothermal Camera." Review of
Nvnclestructiw Evaluation. VoL 6, Progres$ in Quantitatiw Nondestmctil'e
No. 1. Columbus, OH: American El'llluationj1997j. Vol. 17B. New York,
NY: Plenum Press (1998):
Society for Nondestructive Testing p 1959-1964.
(1994): p 1-17.
22. Krapez, J.·C. "Spatial Resolution of
14. Vavilov, V., E. Grinzato, P. Bison and
S. Marinetti. "Peculiarities of Thermal the Flying Spot Camera with Respect
to Cracks and Optical Variations.''
Inspection of Materials with Short lOth Intemational Conference 011
Plwtoacoustic mul Plwtothermal
Observation Times." QIWiltitatiw
Phenomena. AlP Conference
Infrared Thermography (QIRT '94) Proceedings 463 {Rome, Italy,
[Sorrento, Italy, August 1994].
August 1998]. \Noodbury, NY:
Eurotherm Seminar 42. Paris, France:
Editions Europeennes Techniques et American Institute of Physics (1999):
p 377-379.
Industries (1995): p 44-49. 23. Degiovanni, A. 11 Correction de
15. Grinzato E., P. Bison, S. Marinetti and Longueur d'Impulsion pour la Mesure
de Ia DiffusivitC Thermique par
V. Vavilov; "Nondestructive Methode Flash." International Journal
Evaluation of Delaminations in Fresco of Heat and Mass Transfer. Vol. 30,
No. 10. New York, NY: Pergamon
Plaster Using Transient Infrared Press (1987): p 2199-2200.

Thermography." Research in 24. Boscher, D., D. Balageas, A. DCom and
Nondestructive Emluation. Vol. 5, G. Gardette. "Nondestructive
No. 4. Columbus, OH: American
Society for Nondestructive Testing Evaluation of Carbon Epoxy
(1994): p 257-274. Laminates Using Transient Infrared
Thermography.'' 16th Sympositlm on
16. Vavilov, V., E. Grinzato, P. Bison, Nondestructiw Evaluation [San
S. Marinetti and M. Bales. '1Surface
Antonio, Texas]. San Antonio, TX:
Transient Temperature Inversion for Nondestructive Testing and Analysis

Hidden Corrosion Characterization: Center, Southwest Research Institute
Theory and Applications." (1987): p 324-327.
lntemational Journal of Heat and A·!ass
Tramfer. Vol. 39, No. 2. New York, NY: 25. Krapez, J.-C. and D. Balageas. 11Early
Pergamon Press (1996): p 355-372.
Detection of Thermal Contrast in
17. Vavilov, V., X. Maldague, B. Dufort,
Pulsed Stimulated Infrared
F. Robitaille and J. Picard. 1'Thermal Thermography." Qua11titatil'e I11{mred
Thermograph)' (QIRT '94) !Sorrento,
NDT of Carbon Epoxy Composites:
Italy, August 1994]. Eurotherm
Detailed Analysis and Data Seminar 42. Paris, France: Editions
Processing." Nondestructive Testing aml
Evaluation Jntemational. Vol. 26, EuropCennes Techniques et Industries
No. 2. Oxford, United Kingdom: (1995): p 260-266.

Elsevier Science Limited (1993): 26. Vavilov ,v., D.G. Kourtenkov,
p 85-95.
E. Grinzato, P. Bison, S. Marinetti and
18. Balageas, D.L., J,·C. Krapez and
C. Bressan. "Inversion of
P. Cielo. "Pulsed Photothermal
Experimental Data and Thermal
Jvlodelling of Layered Materials." Tomography Using Thermo. Heat and
Thermidge Software." Qttatltitative
joumal ofApplied Ph)'sics. Vol. 59.
London, United Kingdom: Institute of lnfrmed Thermography (QIRT '94)

Physics (1986): p 348-357. rsorrento, Italy, August 1994].
19. Krapez, ].·C. 11Inversion Method for Eurotherm Seminar 42. Paris, France:

Effusivity Depth Profile Retrieval: Editions EuropCennes Techniques et
Industries (1995): p 44-49.
Application to the Characterisation of
Case Hardened Steel." Presented at 27. Vavilov, V. "Transient Thermal NDT:
5th lntemational H'orkslwp on Conception in Formulae" Qwmtitatiw
AdvanC<'d ll'l{mred Teclmolvgy and
Applimtiom [Venice, Italy, September lnfimed Thermography (QIRT '92)
1999].
[Chatenay-Malabry, France,
20. Kaufman, 1., P.T. Chang, H.S. Hsu, july 1992]. Eurotherm Seminar 27.
W.Y. Huang and D.Y. Shyong.
11 Pl10tothermal Radiometric Detection Paris, France: Editions Europeennes
Techniques et Industries (1992):
and Imaging of Surface Cracks." p 229-234.
foumal of Nondestmctive Evaluation.
Vol. 3, No. 2. New York, NY: Plenum
Press (1987): p 87-100.

438 Infrared and Thermal Testing

28. Krapez, j.-C., D. Balageas, A. Deom,
F. Lepoutre. "Early Detection by
Stimulated Infrared Thermography,
Comparison with Ultrasonics and
Holo/Shearography." Adwmces in
Sig11al Processing for Non Destructi1•e
Emlflation of Materials {Quebec,
August 1993]. NATO AS! Series E:
Applied Sciences. Vol. 262. Dordrecht,
Netherlands: Kluwer Academic
Publishers: p 303-321.

29. Vavilov, V.P. and V.V. Shiryaev.
"Method of Determining Transverse
Dimensions of Internal Flaws by
Thermal Control." Defektoskopiya ~
The Soviet Joumal of Nondestmctivc
Tes.ting. No. 11. New York, NY:
Consultants Bureau (1979):
p 1005-1007.

Thermal Contrasts in Pulsed Infrared Thermography 439



CHAPTER

Infrared and Thermal
Testing of Metals

Thomas Benziger, Otto-von-Guericke Universitiit,
Magdeburg, Germany (Part 4)
Mario Bertolotti, National Institute for the Physics of
Matter (INFM) and the University of Rome, Rome, Italy
(Part 1)
Bryan A. Chin, Auburn University, Auburn, Alabama
(Part 3)
Maria C. Larciprete, National Institute for the Physics
of Matter (INFM) and the University of Rome, Rome,
Italy (Part 1)
Grigore L. Liakhou, National Institute for the Physics of
Matter (INFM), Rome, and the Technical University of
Moldavia, Kishinau, Moldavia (Part 1)
Roberto Li Voti, National Institute for the Physics of
Matter (INFM) and the University of Rome, Rome, Italy
(Part 1)
Chee-Ang Loong, National Research Council Canada,
Boucherville, Quebec, Canada (Part 2)
Sundaram Nagarajan, ITW Hobart Brothers, Troy, Ohio
(Part 3)
Ky T. Nguyen, National Research Council Canada,
Boucherville, Quebec, Canada (Part 2)
Yoshizo Okamoto, East Asia University, Shemonoseki,
Japan (Part 5)
Stefano Paoloni, National Institute for the Physics of
Matter (INFM) and the University of Rome, Rome, Italy
(Part 1)
Marc Prystay, Oerlikon Aerospace Incorporated, St.
)eansurRichelieu, Quebec, Canada (Part 2)
Andres E. Rozlosnik, Sl Termografia lntrarroja, Buenos
Aires, Argentina (Part 6)
Concita Sibilia, National Institute for the Physics of
Matter (INFM) and the University of Rome, Rome, Italy
(Part 1)

PART 1. Crystallography of Metals

The following discussion briefly describes temperature between 1183 K (910 oc =
the crystallography and the phase 1670 °!') and 2205 K (1932 oc ~ 3510 °F)
diagram of steels, recalls the standard the structure turns into a (i1ce centered
hardening processes to improve the cubic (I'CC) gamma ferrite (see Fig. !b)
mechanical properties cmd discusses the whereas for higher temperature the
correlation ·with the thermophysical structure is again body centered cubic
properties. Some examples of with a different reticular parameter called
nondestructive testing by photothermal
techniques are shown. the delta ferrite phase.
The primitive cell is the smallest
Crystallographic Structures
of Steels structural element of the lattice in a
crystalline material. ln this simple scheme
The allotropy of iron is at the basis of the each atom, represented as a rigid sphere,
different mechanical and thermal occupies a site called reticular point: the
properties of steels. Pure iron, in fact, at crystalHne lattice adses from the geometry
different temperatures, shows different of the reticular points. The empty spaces
reticular structures (allotropic states): 1,2 are called interstices and may be classified
under 1183 K (910 oc = 1670 oF) the iron -for example, octahedral (Fig. 2a) or
is stable in body centered cubic (BCC) phase tetrahedral (Fig. 2b) depending on the
(see l:ig. la) known as alpha ferrite. For
position of the surrounding atoms.
FIGURE 1. Cubic centered structures: The body centered cubic cell is formed
(a) body structure of alpha ferrite and delta
by an atom just in the middle of the cell
ferrite; (b) face structure of gamma ferrite. and by eight other atoms on the eight
(a)
corners; considering that each of these
(b) atoms is shared with seven neighboring
cells, the examined ce1l contains only
12.5 percent of it (see Fig. l a). This means

that a body centered cubic cell is made by
1+8(1/8) = 2 atoms. In the body centered

cubic cell are also present both octahedral
and tetrahedral irregular interstices. In

this cell an interstitial atom can enter
with a maximum radius of r = 0.291 R in
the tetrahedral interstice and only r =
0.154 R in the octahedral interstice (where
R is the iron atom radius 0.124 nm).

On the other side the face centered
cubic cell is made of eight atoms on the

corners and six atoms on the faces of the
cube (see Fig. lb). Each atom on the face

is shared with another cell. This means

that there are 8(1/8) + 6(1/2) = 4 atoms
per cell corresponding to a higher density

with respect to body centered cubic cell.
In the face centered cubic the

octahedral interstices (r = 0.414 R) are
bigger than the tetrahedral interstices

(r = 0.225 R). Because of this different size,
in the body centered cubic cell the

possible guest atoms must be smaller than
in the celJ.Z To study the lattice density it

is necessary to look at the ratio between
atomic volume and the total volume in
the elementary cell. The cell has a ratio of
0.74 whereas the body centered cubic cell
has a value of 0.68. From crystallography
comes out a different solubility of carbon

into iron for body centered cubic and
therefore at different temperatures. A

442 Infrared and Thermal Testing

simple example is given by the hardening Phase Diagram
process of steels: the thermal treatment
increases the temperature, turns body The iron-to-iron carbide metastable phase
centered cubic into face centered cubic diasmm helps to illustrate the different
and makes possible the superficial phases that may be found in a steeL In
diffusion of carbon at a percentage higher the diagram (Fig. 3) the steels may have
than in body centered cubic. up to 2 percent carbon; cast iron up to
6.67 percent, the maximum solubility of
On a macroscopic point of view, the carbon into iron.3 In this diagram the
described crystalline structures are areas represent the fields of existence as a
repeated with the same orientation many function of the temperature and carbon
times in space, depending on the grain content.J
size, 10 pm to 1 mm (4 x 10-~ to
4 x I0-2 in.). Grains are separated by Iron carbide (Fe3C), or cementite, is an
boundaries that are important for iron carbon intermetallic compound, hard
mechanical properties. '!'he mechanical and brittle. lron carbide has negligible
resistance in fact increases with the solubility limits and contains 6.67 percent
capability of boundaries to obstruct the carbon by weight and 93.3 percent iron
plastic deformation. The study of the by weight.
crystallographic structure of a metal is
useful to understand if and how a guest Fi'rrite is a solution of carbon in body
element can find a place in the lattice centered cubic iron. This phase comes in
giving rise to a solid interstitial solution. two forms. The high temperature form is
This is the case of carbon into iron. called delta ferrite and the low temperature
Carbon has an atomic radius of 0.077 nm. form is called alplla ferrite.

fiGURE 2. Interstices: (a) octhahedral; Delta ferrite is a solid solution of
(b) tetrahedral. carbon in delta iron. It has a body
centered cubic structure and the
(a)

fiGURE 3. Metastable phase diagram of iron to iron carbide.
(L+y~l321 K[1148oC;2098°F).)

2 percent Cast iron--+-
carbon L+ Feic

j-steels

1873 (1600) [2912] L+o

0

1673 (1400) [2552]

1473 (1200) [2192]

E

...-----·......-- -------. G 1273 (1000) [1832] A, "m 4.3
'-' 'f + Fe3C
(b) A,
"~- 1073 996 K(723 QC = 1333 ~F)
0.02 0.8
~"
Q. 873 (600) [1112]

E
>"
U+ h•3C

673 (400) [752] Fe3C--+-

473 (200) [392]

2 3 4 5 6 6.67

Carbon content in steel (percent)

legend

A= hardening temperature
E"' eutectoid point
Fe3C"' iron carbide
L "' liquid ph;ne
ft = alpha ferrite
0 = delta ferrite
y = ga1nma iron

Infrared and Thermal Testing of Metals 443

maxilmlm carbon solubility in it is 0.09 time-temperature transformation diagram

percent at 1768 K (1495 oc = 2723 °F). for a low silicon gray iron. Each cooling

Alpha ferrite is a solid solution of carbon path in Fig. 4 defines the ,
in alpha iron. It has a body centered cubic
time-temperature t·ooling relationship
crystal structure but with a maximum
required to produce a specific
carbon solubility Of 0.02 percent at 996 K
microstructure. The position of the
(723 oc = 1333 °F). As a result alpha ferrite
transformation zone on the
has high plasticity.
Austmite is a solid solution of carbon in time~temperature transformation diagram,

gamma iron (denoted gamma on the defined by start and finish curves,
diagram). Its structure is and it has a
determines the rate and extent of cooling
greater carbon solubility than for both
required to avoid certain transformations
alpha ferrite or delta ferrite: 2.1 percent by
and promote others.
weight at 1421 K (1148 oc = 2098 °F).
If the cooling rate is increased up to
Hm\'ever, carbon solubility in austenite
the so called inferior critical rate ~ that is,
decreases to 0.8 percent at 996 K (723 oc
for eutectoid steel ZOO K·s-1 (200 oc.s-1 =
= 1333 °F). As can be seen in the diagram
this phase is stable only above 996 K 360 oJ:.s-1) ~in the structure a percent of
(723 °C = 1333 °F) but can be stabilized by
martensite appears. If a speed is reached
adding elements to increase the field of
existence of gamma iron. ~,!anganese is called superior critical rail' then the result is
one such element.
only martensite. Under these conditions
The most important point of this
the material has reached the maximum of
diagram for hardening treatments is the
point E in Fig. 3, the so called eutectoid hardness and at the same time of fragility.
point, at 996 K (723 °C = 1333 °F) and
So there are two important parameters:
0.80 percent carbon. A slow cooling of a
steel of this composition (eutcctoid (1) the superior critical rate over which
composition) creates a fine dispersion of
austenite turn into martensite only and
alpha ferrite and cementite that is called
pearlite. This phase has low hardness and (2) the inferior critical rate under which
low mechanical resistance. The structure
is lamellar, with an alternation of pearlite there is no martensite at all. To ensure

and cementite lamellae. The mechanical that a treated component is entirely
properties change a Jot depending on the
distance between two lamellae. A martensitic, the slowest cooling rate must
distinction for steels may be done
depending on the t.arbon content if lower be sufficiently fast to avoid the nost' of the

than 0.8 percent or higher than 0.8 transformation zone. Martensite is a

percent. In the first case, the products of metastable phase: it is a saturated solid
the cooling are pearlite and ferrite; in the
solution of carbon in the alpha ferrite.
second case, pearlite and cementite.
If the cooling rate is too fast to reach The carbon content higher than the

the equilibrium conditions, the time of solubility gives place to a distortion of the

the process is not enough to allow the lattice. So the martensite has a body
nucleation and growing of the lamellae of
centered tetragonal lattice, instead of
pearlite. During the cooling the carbon
first has to pass from a condition of cubic. The new reticular parameters of

solubility of 0.8 percent to 0.02 percent this lattice depend on carbon percentage.
only in the alpha ferrite. Therefore it is
During the transformation of austenite
necessary some time for the exceeding
carbon to go out of the austenitic to martensite there is also an increasing of

structure and to nucleate into lamellae of volume (4 percent) higher than in the
cementite. The force of this process is the
transformation of alpha iron to gamma
carbon diffusion. The faster the t·ooling is,
the thinner are the final products. If the iron. That is why high tensions arise.

rate is further increased a new product These tensions make this compound hard
appears instead of pearlite. This
and brittle. ln Fig. 5 is represented the
intermediate compound, bainite, has a
martensite lattice in the bain model: the
resistance higher than the pearlite but still
keeps some plasticity. starting structure is the face centered

The standard practice to display these FIGURE 4. Time-temperature transformation diagram.

transformations is the time-temperature 1073 (800) [1472] Ferrite
transformation diagram (see Fig. 4) also
t 873 (600) (1112) Bainite
known as isothermal transformation or C
cmve because of its shape. This diagram is 673 (400) [7521 Orig'rnal
473 (200) 13921 temperature
useful in selecting heat treatment
practices for steeL Figure 4 shows <t typical Final

10'
Time (s)

444 Infrared and Thermal Testing

cubic (white spheres) that is distorted and C < 0.8 percent or between A1 and Auu
can be seen as a tetragonal cel1 if the (for steels with C > 0.8 percent).
atoms in the center of the faces are taken Hardening temperature Ac3 varies with
as corners (gray spheres). During fast
cooling the carbon does not have time to composition over 996 K (723 <>c ==
diffuse outside the austenite; meanwhile
the energy stored is suddenly released as 1333 'F). The sample is kept at this
the undercooling increases. The atoms in temperature for the time specified for the
the austenite, metastable at low complete austenization. This time ensures
temperature, suddenly change positions a uniform austenitic structure all over the
in the lattice at the sound speed in the specimen. This step should last long
material. This transformation, out of enough to guarantee both the carbon
equilibrium, is characterized by the initial diffusion outside the existing phase and
and final temperature (J:ig. 4). They the austenite nucleation. Moreover the
depend on many factors: for example the carbon content is higher in cementite
carbon content. than in ferrite. This gradient of
concentration makes the austenite grow
Hardening faster from the side of the cementite. The
presence of other elements normally
The thermal treatments in general are <I dissolved in the steel to form carbides
succession of thermal procedures applied slows down this process. Even when the
to metals to obtain particular structures steel will finally be completely austenitic,
and final macroscopic properties different a nonuniform carbon composition higher
from the initial ones. Normally these in the positions previously occupied by
processes are solid state treatments in cementite and carbides may be observed.
controlled ambient conditions. The Considering all these factors the time and
hardening processes may be divided into the temperature of this process must be
three steps: heating, holding at a specific chosen carefully.4
temperature and cooling according to a
special law. In most cases also some The second step of a hardening process
chemical reactions are joined to the is the fast coo1ing under the ten1perature
thermal treatment in the second step. A 1 to avoid totally or in part the
formation of equilibrium phases. The steel
The first part of a hardening process is is quickly brought to the desired
always the austenization. The sample is temperature and held there in a salt bath.
heated at the hardening temperature A3 After the time requested to have a partial
(see the diagram on Fig. 3) for steels with transformation, in the third step the steel
is cooled until room temperature in air (or
FIGURE 5. Bain's model for martensitic transformation. Face water or oil): all the residual austenite is
centered cubic structure (wllite atoms) can be considered so turned into martensite. Finally, to relax
like body centered tetragonal (dark atoms). the tensions and to decrease the high
fragility of the ne·w structure, the
00 recovering may be performed by heating
the piece under A1•
{v7\<_ ·o---------j--0·-·- :D. 0 0
0 0 Surface Treatment
''
Sometimes the residual tensions due to
10 0 the hardening process are too high and
undesired. Because of the strong
0:.::6.-L---------6-.-. i 0 temperature difference between the
:D 0 surface and the bulk of the piece the
transformation from austenite to
00 martensite starts at different times in
different depths of the piece. This causes
great tensions that may give place to
distortions or even cracks, in the worst
case. Sometimes the treatment is
performed to reach only a strong surface
holding a plastic core. '10 avoid these
problems the hardening process is often
limited to the surface. These treatments
are of industrial relevance and call for
various heating techniques.

Induction Hardening. The temperature
increasing is reached by means of electric
induction at high frequencies (1 kHz to
1 MHz). In the piece there is induced a
current that produces a temperature rise.
The frequency drives the penetration of
the current and hence the size of the

Infrared and Thermal Testing of Metals 445

heating region. To obtain a small 3. liquid State. This kind of carburizing
is performed in cyanide salt balhs.
hardening depth of 0.3 to 1 mm (0.01 to Carbon together with nitrogen
penetrates into thC steel. For this
0.04 in.) high frequency current between reason it is usually called
10kHz and 2 MHz must be used. carbonitridiug. The reactions involved
are Z(NoCN) + 0 2 => Z(NaCNO);
Flame Hardening. To heat the piece a 4(NaCNO) => Na2C03 + 2(01oCN) +
flame is made by oxygen and CO+ ZN, 2(CO) = C02 + C; and
oxyacetylene. Just behind the flame the NaCN+C02 => NaCNO + CO. The
instrument's water jet cools the piece carbon or nitrogen percentage is
immediately. A different gas like propane driven by chemical composition of the
or oxygen and air can be used. bain. This process is called
nitrorarburizing when the percent of
laser Transformation Hardening. In this nitrogen is higher than for carbon.
case the bulk of the sample is not
changed but a power laser directs a great The typical hardness profile created by
amount of energy. The advantage of such these thermochemical processes is due to
a technique over alternative processes are the diffusion of the guest element; for this
chemical cleanliness, controlled reason the profile cannot be steplike but
penetration and noncontact processing..'> weakly decreases from the surface to the
bulk value. The steels for thermochemical
In these three cases {induction hardening must be soft (carbon< 0.15
hardening, flame hardening and laser percent) and with a low percent of other
transformation hardening), the treatment elements. These elements are used to
is only thermal. The structure and the improve the hardness: a low percent of
properties of the steel are changed manganese and chromium, for instance,
without varying the chemical allmvs the growth of iron carbides and
composition. The depth profile of the nitrides. Both have extremely small size
reached hardness is near to a step profile and will precipitate at the metal grain
because there is a good control of the boundaries. Moreover their presence at
time of the treatments and there is no the grain boundary may prevent the grain
problem with the diffusion of elements motion as well as the martensitic small
inside the lattice. Another possibility is to grains in precipitation hardening.
use thermochemical processes to modify
the superficial composition by diffusing Relation between
carbon or nitrogen into the piece. These Hardness and Thermal
two elements contribute to the hardness Properties
of the steel because they can occupy the
interstitial positions of the structure. It is already wen known that for each
material there is a strong correlation
Carburizing between the local thermal properties and
the microstructure. The propagation of
From the industrial point of vic"w this is heat in solids is strongly affected by the
one of the most important diffusion microscopic properties of the material and
processes. It can be divided in two parts. overall by the internal distribution of
During the first one the diffusing element interfaces (Jike grain boundaries in steel).
is put on the surface of the piece by These internal surfaces may limit the free
chemical reactions. In the second part the path length of energy carriers like
element diffuses into the piece and the electrons and phonons; therefore, the
chemical composition of the surface is thermal conductivity also decreases. Now
completely changed. From the that it is clear what happens to a piece
technological point of view, this process when hardened, is possible to understand
can be performed in three different states. the correlation between the grade of
hardness and the thermal propertie~.
1. Gas State. The process is realized using
an atmosphere of methane or carbon ln a hardened piece a lot of structural
monoxide: Z(CO) = C (y Fe) + C02 or changes occur. tvlacroscopically there is a
CH 4= C (y Fe)+ 2H2. When the region with a different structure from the
equilibrium is reached at the working bulk. The structure of the martensite
conditions the carbon is deposited on introduces distortions into the lattice. The
the surface. grain size of this new phase is smaller
than in the phase at the equilibrium and
2. Solid State. The reaction involves solid the number of grain boundaries is higher
state elements. This is the case in the hardened surface than in the bulk
Jwrdenilzg: a mixture of coal and of the piece. This is because during the
carbides i5 put into cases with the fast cooling there is no time for the grain
pieces. At the temperatures of work to grow but the nucleation of new grains
there is the boudard equilibrium:

BoC03 = Bao + C02 and c + C02 =

Z(CO). After the reactions the carbon
may diffuse into the workpiece.

446 Infrared and Thermal Testing

is favored. This represents another All techniques have wide applications.
obstacle for the heat conduction. The common principle is that the
Moreover if the steel was chemical hardn.:ss number is the rate H = P-S-1
treated, then the concentration is where Pis the load and S the surface of
changed from the starting 0.2 percent to the print. Particular attention must be
around 1.1 percent of carbon that work paid when measuring this print because
like a fine dispersion of inclusions. If the edges are never on the original plane but
steel was also alloyed there are carbides always under or over it.w
that are Jess conductive than steel and so
are obstacles for heat conduction. In the case of vickers hardness tests,
commonly used for steels, a square based
Generally what works very well to pyramidal indenter is applied that
increase the hardness of a piece on the penetrates the sample under a load of 10
other hand makes heat conduction to 1200 N (1 to 120 kg,) for a 10 to 20 s
diffiCult. Therefore there is a correlation loading time. The indenter has an angle
between the profile of hardness and the of 136 degrees on its pinch. In this case
thermal conductivity (or diffusivity). As the hardness number is given by H = (2P
an example, in Table 1 the thermal sin 68 degrees)·d-2 where dis the average
properties of some steels are reported length of the diagonals of the print. In
together with the kind of treatment. It is practice the result is an averaged value
possible to see that the thermal over five measurements.
conductivity in hardened steel is lower
than in annealed steel.0 Another important point is the
preparation of samples. The surface
Conventional Hardness preparation for hardness tests must be
Measurements according to common standard
procedures.7 Hardness testers are
Hardness tests are commonly applied to calibrated on plates of known thickness.
control the hardening process and to Special care must be taken about testing
measure hardness depth. Mechanical equipment verification. Particularly, the
testing is most often used because it is specimen thickness should be such that
easy, fast and cheap but on the other no effects of the applied load appear on
hand it is destructive. In a hardness test
an indenter is loaded onto the sample at the back.
specific loads and loading time (tens of Moreover the specimen surface should
second). The harder the material is, the
smaller is the indentation. The hardness is be so finished that the ends of the
evaluated from the dimension of the print diagonals can be dearly defined and can
left on the surface. Many standard tests be read with a precision of ±5 pm
use different loads and indentation size (2 x 10-4 in.). The procedure is applied on
and shape (spherical for brinell/ conic for specimen cross section and is repeated at
rockwell8 and pyramidal for vickers.9 The different positions from the surface to the
choice of test depends on the test material bulk to reconstruct the hardness depth
and on the dimension of the test surface. profile. 11

An example of hardness profile
measurements (I~ig. 6) has been obtained
on two samples of 201vfnCrS steel (\'\1orld
number 17147, EN 10085} with hardness

depth of 0.4 and 0.8 mm (0.016 and

TABLE1. Thermal properties of low alloy steel.

AISI" Chemical Composition (percent) Conductivity Specific Heat at
SAEh Carbon Manganese Chromium Nickel (W·m-1·K-1) 323d to 373 Ke

Type at 273 K< at 373 Ke (J·kg-1·K-1) Treatment

1008 0.08 0.031 0.045 0.07 59.9 57.8 481 annealed
1010 65.2 60.2 450 unknown
1025 0.10 0.40 51.9 51.1 486 annealed
1042 51.9 50.7 486 annealed
1078 0.23 0.64 trace 0.074 47.8 48.2 490 annealed
1524 46.0 45.8 477 annealed
4130 0.42 0,64 trace 0.063 477 hardened
4140 42.7
5140 0.80 0.32 0.11 0.13 42.7 hardened
44.8 452 hardened
0.23 1.51 0.06 0.04

0.30 0.50 0.95

0.41 0.67 1.01

0.39 0.79 1.03

a. American Iron and Steel fnstilule, Washington, DC.

b. Society of Automotive Engineers, Warrendale, PA.

c 273 K (0 "C"' 32 <f).

d. 323 K(50 oc = 122 "F).

e. 373K(100°C=212°F).

Infrared and Thermal Testing of Metals 447

0.032 in.). These samples are made from u etchant comprising S percent nitric acid
typicul steel for carburizing and have been and 95 percent alcohol.
treated by case hardening. As expected,
the profile presents a slowly decrease of On the macroscopic (20x) scale it h
hardness going into the bulk. The possible to see the microstructure
hardening depth is represented by the variations due to the treatments (see
distance where the hardness goes to half Hg. 7). The phase near the surfuce is
of the surface value. expected to be tempered m<lftensite. The
phases are often difficult to distinguish at
Microscopy this magnification.

Optical examinations can be also The transition from surface to bulk
performed on steels to examine the microstructure can easily-be seen at low
hardening structure. The structure magnification. The phase transition is
changes from specimen to specimen and gradual: the grain size increases going into
depends on the hardening treatment. The the bulk. To be more accurate it is
visual technique can be performed on two necessary to have a microscopic image.
different scales: macroscopic and On the same sample with higher
microscopic. magnification (100x) it is possible to

On the macroscopic scale using low FIGURE 7. Micrograph of 20MnCr5 sample
magnification (20x) a cross section of the
sample can show the different structures hardened by case hardening. Hardening
of surface and bulk (in this case it can be
possible to distinguish the total hardening depth 0.4 mm (0.1 0 in.).
depth). This technique offers the
advantages of being easy and fast. _...._ 1 mm (0.04 in.) __.._

The microscopic technique takes more
time but is more precise and more
accurate. These optical techniques are
very simple and fast but the sample must
be etched with an acid solution to reveal
grain boundaries. As an example the
photomicrographs have been shown in
Figs. 7 to 9 for a sample of S percent
manganese and 20 percent chromium
steel. They represent the cross section
after chemical etching with a nita!

fiGURE 6. Vickers hardness test for two samples of 20MnCr5 steel treated by case hardening.

700

650

600

" 550

.l!

~

~ 500

c

1I•' 450

400

350

300 _______,_
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
{0.008) (0.016) (0.024) (0.032) (0.04) (0.048) (0.056) (0.064) (0.072) (0.080)

Depth, mm (in.)

Legend

---+----""' h<1rdening depth: 0.4 mm (0.016 in.)
-------=hardening Depth: 0.8 mm (0.032 in.)

448 Infrared and Thermal Testing

recognize the different structures of the (see Hg. 9). Some sphemidized cementite
surface (see Fig. 8) and of the bulk (see is visible in the pearlitic regions, probably
Fig. 9). From the t·.~·n figures it can be seen bee<tuse of preliminary heating.
that the grain size in the hardened part is
smaller than in the untreated material. It Thermal Diffusivity
is also possible to see that the color is Measurements
different. This difference occurs because
the metallographic etching proceeds with Photothermal Deflection
different speeds: it is faster in the higher Technique
energy zone (martensite} and slower in
the bulk. The microstructure of the bulk is An example of the application of this
about 50 percent ferritic (white areas) and technique is shown in Hg. 10, which
50 percent pearlitic (dark speckled areas) refers to five samples of a 20MnCrS steel
(World number 17147, EN 10084). The
FIGURE 8. Micrograph of 20MnCr5 sample samples have the same hardening depth
hardened by case hardening. Hardening of 0.8 mm (0.032 in.) but a different
depth 0.4 mm (0.016 in.). Magnification surface hardness ranging from 450 to
100x. Distance from surface: 0.1 mm 880 vickers. The effective thermal
(0.004 in.). diffusivity for these samples is measured
by using a standard photothermal
1-+--100 fJm (0.004 in.)__..., deflection scheme (see Fig. 11). 12 In
ng. 11 the phase of the lateral deflection
FIGURE 9. Micrograph of 20MnCr5 sample signal is plotted as a function of the
hardened by case hardening. Hardening lateral offset at different working
depth 0.4 mm (0.016 in.). Magnification frequencies for sample 2 (20 percent
1OOx. Distance from surface = 1 mm manganese, S percent chromium) in
(0.04 in.). Table 2. All curves show an asymptotically
linear slope as expected.12 The initial
distortion from linearity is due to the
probe beam height with respect to the
surface, 100 pm (0.004 in.) and the pump
beam size (typically between 0.01 and
0.09 mm H. x I0-·4 and 4 x 1o-3 in.).

By using the least squares technique on
the asymptotic linear piece,- it is possible

FIGURE 10. Schematic representation of standard
photothermal deflection setup.

Ve1tical
deflection Z

Probe fI Horizontal o,.L. Jsft '
beam

tVertical~--~/

offset Z

Lateral
delleftiOn Y

1-+-- 100 !Jm (0.004 in.) z

L
X

Infrared and Thermal Testing of Metals 449

to work out the lhermal diffusivity FIGURE 11. Phase of lateral photothermal deflection signal
reported in Table 2 for all five samples of versus lateral offset for hardened steel sample 2. Different
20 percent matlgaJii.:~C1 5 percent curves refer to different modulation frequencies.
chromium steel. The experimental results
in Table 2 on lhe average thermal 100
diffusivity are plotted versus the expected
hardness in Fig. 12 where the expected " 50
anticorrelation between thermal
diffusivity and hardness is proven. ~

Photothermal Radiometric :"ws'
Technique
~0
This technique may be used for the .rco
nondestructive evaluation of the
diffusivity profiles of hardened steel ~
samples. As an example the frequency
domain depth profiling for a steel sample -50
is reported/ in which the hardening
process modifies the surface hardness 0 0.2 0.4 0.6 0.8 1.0 1.2
from 200 to 1000 (vickers 0.05), the (0.008) (0.016) (0.024) (0.032) (0.040) (0.048)
thermal conductivity from 0.65 to 0.3
\V-cnr1-K-1 and the thermal diffusivity lateral offset, mm (in.)
from 0.2 to a 0.08 cm2·s-1•13 By comparing
the thermal diffusivity depth profile FIGURE 12. Diagram of thermal diffusivity to hardness for
reconstructed by thermal wave 20MnCrS steels hardened to depth of 0.8 mm (0.032 in.).
backscattering theory and singular value
decomposition procedure with the vickers ' 0.15 (1.40) • ••
microhardness depth profile measured ••
from the samples cross section/ the .Ec 0.14 (1.30) 100
anticorrelation may be found between 300 500 700 900
diffusivity and hardness. A similar '"'c 0.13 (1.21)
anticorrelation between conductivity and
hardness may be found by using other 0
photothermal measurements.t4
-~ 0.12 {1.12)
TABlE 2. Thermal diffusivity and hardness
for 20MnCr5 hardened steels. 1

Sample Surface Average Thermal u 0.11 (1.02)
Hardness
(vickers) Diffusivity !5-
(cm2·s-l)a
:~ 0.10 (0.93)

0

"' 0.09 (0.84)

'6

"E 0.08 (0.74)

.>wc-- 0.07 (0.65)

880 0.099 ± 0.005 Hardness (vkkers 100)

2 670 0.103 ± 0.005

3 580 0.132 ± 0.005

4 450 0.135 ± 0.005
5 unhardened 0.140 ± 0.005

450 Infrared and Thermal Testing

PART 2. Heat Transfer in Molds and Dies for
Aluminum and Plastic15

Infrared and thermal testing finds various in complex systems is less evident. At
applications in the primary metals present, molds are instrumented with
industry. Mold and die cooling systems subsurface thermocouples and
are evaluated using infrared thermal occasionally infrared pyrometers are used
imaging techniques. Single thermal to monitor the melt temperature. In
images provide surface temperature addition, hydraulic and cavity pressure
distributions; transient techniques are probes are used to measure pressure drops
used to obtain information ahout how and detect when the gate or sprue freezes.
thermal energy is dissipated. Some manufacturers have found these
sensors useful; others have found the
Comparing thermal signatures of results difficult to interpret and resort to
molds and parts provides information experience in running their machines.
regarding cooling deficiencies, part
shrinkage and thermal stresses. Thermal Infrared thermal imaging may be used
analysis of dies in aluminum casting can to help evaluate and optimize cooling
be used to study the effectiveness of die systems in molds and dies. Infrared
spraying and evaluate the design of images are used to obtain surface
subsurface cooling channels. temperature distributions. Transient
thermal imaging provides information on
Thermal techniques have been applied how thermal energy is dissipated.
to measurement of waH thickness and
detection of incomplete core removal in General Considerations in
investment castings.l6 Thermography has Plastic Injection Molding
also been used to monitor the condition and Aluminum Die Casting
of a torpedo car, a molten metal holding
vessel on a railroad track in a foundry. 17 Examination of the relative thermal
properties of common plastics and tool
Introduction steels is helpful in interpreting the
temperature variations seen on a mold
Causes of warpage in injection molding of surface. It is important to realize that a
plastics can be broadly classified into mold is a large thermal mass with
problems with flow or solidification continuous cooling whereas the plastic is
{heat). Each of these dassifications is a relatively small thermal mass injected
interrelated. Operating conditions that intermittently. Thermal conductivities of
provide minimal \\'arpage while mold steels are 100 to 500 times higher
maintaining the economics of processing than those of plastics and thermal
are not always obvious. diffusivities are about 100 times higher.1R·2
Subsequently, thermal energy which
Similar problems exist in aluminum die slowly diffuses through plastic is quickly
casting. Although the effects of stress are dissipated in the mold. Heat capacities of
not as pronounced in terms of warpage most plastics are 3 to S times higher than
and shrinkage, the problems exist just the steel; but their densities are of the order of
same. Directional solidification is a 7 to 10 times lower. Thus thermal energy
function of heat flow. Sometimes from 100 to 1000 g (0.22 to 2.20 Ibm) of
problems with heat flow are evident only plastic is readily absorbed by the
after completion of metallographic surrounding steel. The net result is that
analysis of the part. Incorrect changes in molding parameters tend to
microstructure, porosity and microcracks result in only minor differences in the
are discontinuities that degrade the mold surface temperature distribution.
quality of the part. The net result is
inferior mechanical properties and poor Analysis of dies in aluminum casting is
fatigue strength. quite different. The temperature of
molten aluminum is over 923 K (650 "C ==
Some of these problems can be 1202 °F) and single shots requiring 20 kg
minimized using the correct mold and die (44lbm) of material are not uncommon.
design. Proper heat transfer often In addition, aluminum releases about 400
improves the flow of the material, allows }g-1 of latent heat during solidification.27
control over solidification and helps
minimize warping or stress in the
manufactured part. However, a robust
methodolot,•y for evaluating heat transfer

Infrared and Thermal Testing of Metals 451

As a result there is significant thermal dissipated when cold water is flowing
energy being dissipated by the die. through the channels during production.
These results are best viewed at early
The thermal conductivity and times after the onset of heating.
diffusivity of aluminum are higher than
those of steel (opposite to plastics Allowing hot oil flow to heat a mold or
molding). Accordingly, heat energy tends die to steady state pro\' Ides no additional
to diffuse quickly from the part and build information regarding <:ooling dynamics.
up in the surface layer of the die. The results of a study are shown in
Diffusion of this energy then becomes Fig. 14. A graphite coated die used in
critical for controlled solidification of the aluminum casting was heated by placing
parl. Increased cooling channel flow and an insulating fireproof blanket hetwern
spraying of the die surfaces with a water the die halves, closing the die and letting
soluble die lubricant are required to hot oil circulate through the channels.
remove the large amount of heat. After five hours thermocouples inside the
die reached steady state. The die was
Once the filling stage is complete, opened, the blankets were removed and a
cooling and solidification of either plastic thermal image of the surface was
or aluminum is influenced by the rate of recorded. The result (Fig. 14a) shows a die
heat transfer in the steel and the design uf hot at the center and cooler at the
the cooling system. Cooling channels extremes. This may suggest that the
near the surface of a mold or die provide center region has the greatest heat
steep thermal gradients for cooling but transfer capacity and should be cooled lhe
the surface temperature may be most during production. However, after
nonuniform (Fig. 13). In contrast, cooling casting aluminum parts for several hours a
channels placed farther into the mold clear hot spot was evident on the center
generate smaller temperature gradients surface (Fig. 14h).
but provide a more uniform surface
temperature. Evidence of these gradients FIGURE 14. Thermal images of die surface:
may be obtained by flowing hot oil (a) painted die surface heated by internal
through a cold mold. ~vfeasuring the mold cooling channels (c.enter region of die is
surface temperature as a function of time hottest even at edges of die);
provides evidence of how the mold heats (b) temperature profile along line ll01 in
and, by deduction, hmv thermal energy is fig.l4b is plotted below image; (c) hot
region is detected during production run.
FIGURE 13. Illustration of cooling effect provided by mold or (a)
die cooling channels: (a) channels machined dose to surface
provide steeper thermal gradients from surface but uneven Cool
cooling may result; (b) channels machined farther from
surface provide shallower thermal gradient but more uniform (b)
surface temperature.

(a)

Mold surface

Channel
depth

Channels (c)

(b)

Mold surface

ICh.onel
depth

Hot region on
die surface

Channels

452 Infrared and Thermal Testing

This hot spot is due to thick bosses in unnecessary reengineering and
the die that contain no cooling. Thus, subsequent unbalancing of a well
although transient warming of a die may designed cooling system.
provide information on the location of
cooling channels, temperature A better way to evaluate cooling lines
distributions seen on a die or mold might be to compare thermal images of
surface after warmup are not a reflection molds and parts. Hot spots or read tlmmgil
of the heat transfer capacity during on plastics often is due to insufficient
operation. (The images presented in this local cooling in the mold. Generally, this
work have been filtered to enhance visual is recognized as a correlation between
contrast. Analysis was carried out on thermal signatures from the mold and
unfiltered data.) part. Insufficient mold cooling generates a
hot mold, which in turn creates a local
Considerations Specific to temperature maximum on the part.
Plastic Injection Molds
The part produced in the mold of
Low emissivities of injection molds Fig. 15 is shown in J:ig. 16. There are three
(typically c ~ 0.08 to 0.30) make direct spots that are hotter than the rest of the
thermal imaging a difficult way to surface. One hat the top of the part1 one
measure temperature. An image of a mold in the center and one at the bottom. The
surface presented in I;ig. 15 shows how only hot spot that can be matched to the
reflections from one half of a mold can thermal signature of the mold is the
lead to apparent hot spots on the mold center spot. This is due to trademarks
face being imaged. The image in Fig. 15a being molded into the surface. The melt
was taken without shielding. A large hot surrounds protruding ridges of the mold,
spot is seen on the bottom half of the heating it on three sides. The hot spots at
mold. A temperature profile across the the top and bottom of the part may be
surface indicates a maximum of 366 K (93 due to the other half of the mold,
geometry effects (such as local ribs) or
oc ~ 200 °F). The image in Fig. lSb was
FIGURE 16. Part made in mold of Fig. 15:
taken using a fireproof blanket placed (a) bottom surface; (b) top surface in
over the opposite mold. The hot spot is contact with mold surface above.
no longer seen. The temperature at the
location of the hot spot now measures (a)
about 339 K (66 °C ~ 150 °1'). Initially, it
may appear that the differences between
the images is minor. However, plastics are
sensitive to temperature gradients and
errors in measurement can lead to

FIGURE 15. Thermal images of mold surface:

(a) without shielding; (b) with shielding.
Apparent temperature difference Ll Tis

about 28 K(28 oc ~ 50 °F).

(a)

:~::~ ~,__ --

24B.1 (b)
l'N.6

:·;~~_..·, ..:....._/,· ··x

'"'·" J'IJ•I,~

1>.'7.•-• - - - - - - -

Hot mo!d surface

(b)

'"·'l~;~-272,1) .

2•18.4
224.8
2•.li.J

----x171.7

15<1.1 __ .---· ---
13u,& -

1(1].(1-- --- - ' -

Cool mold surface

Infrared and Thermal Testing of Metals 453

shrinkage away from the mold surface FIGURE 18. Thermal image of automotive fan:
during the cooling phase of the cycle. (a) differences in .<.urface temperature
correspond to variations in wall thickness;
The effects of plastic shrinkage in the (b) same image with wider span and
mold are most easily distinguished by temperature profile along line ll01;
measuring the thermal transient of the (c) profile.
part after ejection. An example taken (a)
from injection molding of polyethylene
terephthalate preforms is presented in (b)
Fig. 17. Transient thermal analysis of the
preforms was performed after ejection. line ll01
Regions that shrink away from the mold
during cooling have an elevated surface (c)
temperature that does not rise
significantly after ejection. This lack of
temperature change is due to the thermal
contact resistance caused by the air gap.

After ejection1 the profiles of
temperature versus time are constant or
drop. A part that remains in good thermal
contact with the mold has a cold surface
temperature at ejection. It then heats up
as the front surface is warmed from
within. The differences in U1e curves are
quite evident. It is noted that these curves
came from the same spot on different
parts molded using different operating
parameters.

Another source of surface temperature
variations is differences in the part
thickness. The temperatures are a
reflection of the thermal transit times
required to cool the part to the core.
Assuming transient heat conduction out
of the part1 cooling times go up as a
square of half the thickness. As a result
thickness differences as low as a few
micrometers can be distinguished. A fan
presented in Fig. 18 clearly shows a wide

FIGURE 17. Transient thermal analysis of polyethylene terephthalate preforms after ejection.
Solid curve suggests poor thermal contact between part and mold in cavity. Dashed curve
suggests better thermal contact between part and mold.

398 (125) ]257] -----

393 (120) ]248]

388 (115) ]239] .... . .. .-......,.~... ~.....~ ......, ,, ...... , .
''
"':?.... -. .. - ' ~ ... ~

E 383 (110) ]230]

"~- 378 (lOS) ]221]

il

~

a~. 373 (100) ]212]
E
~ 368 (95) ]203]

363 (90) 1194] PO<t eje<!ioo 20 30 L _ __ _ so
/ 10 Time (s) 40

358 (85) 1185]

353 (80) ]176)
0

454 Infrared and Thermal Testing

temperature variation that correlates to strategies used in injt:'ction molding.
part thickness. Optical temperature measurement of
aluminum is technically difficult because
In summary, thermal techniques may the emissivity drifts while lhe part cools.
be able to identify causes of thermal Variations in apparent temperatures are
gradients in the parts by comparing difficult to validate. In contrast, thermal
thermal signatures of the part to the mold oxidation of dies creates a surface layer
and studying hmv the part cools after with unique infrared absorption
ejection. characteristics. \,Vith filters the surface
temperature can be measured reliably. 21>,2Y
Applications of mold coatings such as As a result, it is more reliable to image the
release agent, graphite and paints have die than the part.
been investigated as methods to increase
the emissivity of mold surfaces. This Spraying
approach was not successful for two
important reasons. Hrst, about 10 shub Die surfaces are sprayed with water
are required to reach steady state soluble lubricants between shots to
operating conditions after application of minimize soldering and to augment heat
the coating. Typically coatings begin to transfer. Typically, operators aim to cool
wear off before useful information can be the surface to a predetermined
obtained. Second, measured temperature temperature range where production is
distributions of the mold are possibly a optimal and the lubricant is designed to
reflection of paint thickness variations operate. However, spraying large dies with
rather than a true reflection of heat water can have surprisingly little effect on
transfer in the mold. Thick paint coatings the die temperature.
result in decreased heat transfer and
hotter mold surfaces. The following case will serve as an
example. After the die opens and the part
This effect has been noticed in plants is ejected the surface temperature at two
where problems with cooling are partially points read 616 K (343 'C ~ 650 °F) and
alleviated by cleaning and polishing the 500 K (227 'C ~ 440 "F). The die
mold surface. Residual films may serve to immediately begins to cool due to
increase emissivities of molds but also act conduction to internal channels, radiation
as thermal resistance, which reduces the losses and convective cooling at the
diffusion of heat from the part. surface. Spray is applied 35 s after the die
opens. However, after 20 s of spraying the
Considerations for Dies in surface temperature drop is negligible.
Aluminum Casting There are tvw important factors that
contribute to this effect. First is the skillet
Strategies in thermal analysis of c((ect. \.Yater sprayed onto the hot die
aluminum die casting is different from

FIGURE 19. Cooling curve of die surface sprayed manually three times. Surface does not
significantly cool until third spray. (See Table 19.)

Infrared and Thermal Testing of Metals 455

rapidly heats and changes to a gas. In the Cooling Channels
phase change there is a large volume
expansion that temporarily prevents Liquid aluminum is usually poured
subsequent spray from reaching the die between 923 to 97i !< (650 and 700 °C;
surface. Second, water that does reach the 1202 and 1292 "F). Cooling water flows
surface does not have the heat capacity to into the die at about 283 to 288 K (1 0 to
remove a significant amount of energy.
Thus 5 s after spraying stopped the die 15 °C; so to 59 °F). Channels placed too
temperature is comparable to that which
would have occurred without spraying the close to the surface generate steep thermal
surface. (Predicted die temperature ·was gradients that cause thermal fatigue of the
obtained by extrapolating the slope of the
curve before ·water spray.) FIGURE 20. Analysis of cooling lines (positions of interest are
marked as spots 1 to 4): (a) after spraying is complete die
The curve in Fig. 19 shows the effect of temperature is monitored; (b) if cooling lines are not
cooling a die using multiple spray passes. optimally designed then surface temperature rises because of
The first pass spr<)yed the surface for 3 to energy stored in die; (c) if die's cooling system is adequate
S s without a decrease in die temperature. then die temperature will stay level or continue to drop after
After about SO s the die was sprayed a spraying.
second time for about S s. The
temperature response shows the die (a)
surface temperature returns to about
(b)
522 K (249 oc ~ 480 °F) before a third pass
573 (300) (572] c.,- - - - - - - - - - - - - - .
is completed. Following the third spray
the temperature profile requires a longer 553 (280) {536] -..........,.....,
period to recover suggesting that the
depth of cooling is greater and the final 533 (260) [500] ~-, ....... .__,,_ Spot
surface temperature drops over 311 K
E 513 (240) {464] ____.../..------:.~~-...~. ::-._number
(38 oc ~ 100 "F) to about 466 K (193 oc ~ ;·-'/·-~-:::_~·~...~..~_~_,--.-_ -.__
r ..._____-.:.:::::G~
380 °F). However it is not clear if the drop 493 (220) (428] -_ 2
in temperature can be attributed to the 473 (200) (392]
spray alone. Calculations show that after 3
about 100 s the internal cooling lines i 453 (lao> (356J
remove a significant amount of thermal e3
energy from the die, which would have 433 (160) [320] 4
contributed to the temperature drop.
1i
The time required for the surface
temperature to recover following spraying E 413 (140) [284}
might be useful in estimating the depth of
cooling. Results of calculations that use !" 393 (120) !248]
the thermal properties of the tool steel are
presented in Table 3. These data show 373 (100) [212] .
that the spraying of this die is truly a
surface cooling operation. 3S3 (80) {176} 50 100 150 200
0
For the calculations presented here a
constant thermal diffusivity a::::: 9 x 10--6 Time (s)
m2·s-1 was assumed. The cooling depth 8
was calculated from the recovery time t as (c)
o ~ v(mo:). Finite element models (FEM)
can be used to improve precision if 573 (300) 1572] ~-------------,
required but this calculation should
provide an order of magnitude estimation. 553 (280) [S36]

533 (260) 1500]
,,,
TABLE 3. Cooling depths calculated from .......' - ,
cooling curve in Fig. 19. E 513 (240) [464]

r ---,- 493 (220) [428]

C\£' 473 (200) [392] ·-.............
.~3 .~, ~-~ ....... ......, ~M
Time at End Recovery Approximate Depth 453 (180) [356]
,, -......___ number
of Spray Time t 8 of Cooling
~ 433 (160) [320] "~~ ........................,__
(s) (s) mm .(in.) ............._......_ 2
~E 413 (140) [284] '"';:-::::.::.:::.-..-..::.:-.-:-:-~-:~...:..~::~-----.~~
26.5 1.7 7 (0.3) 393 (120) [248]
56.8 9.3 16 (0.6)
90.5 25.2 27 (1.1) 373 (100) [212] -.....---...... ....... 3
4
353 (80) (176] L__ _L__
0 50 _ c_ __ l_ _-'.L

100 ISO 200

Time (s)

456 Infrared and Thermal Testing

die material and lead to cracking. As a regarding interface physics between the
result, cooling channels are often placed part and the mold. For example,
deeper beneath surface in aluminum dies aluminum has a relatively small linear
than in plastic molds. coefficient of thermal expansion (at least
compared to plastics). As a result the die
Analysis of internal cooling channels is and part remain in fairly good thermal
best implemented by breaking the contact during most of the cooling stage.
production cycle after spraying and Eventually, however, heat transfer is
recording the surface temperature as the diminished due to shrinkage and the
die cools for 60 to 120 s. Dies with formation of air gaps. The effect of this air
adequate internal and external cooling gap is unclear because the heat capaCity of
should cool with time. Die surface air is relatively small compared to the
temperatures that rise after the energy released by the cooling aluminum.
completion of the cooling stage may have Consequently, the decrease in required
a hot spot beneath the surface. cooling time may not be significant.

An example is presented in Fig. 20. The Conclusions
four points used to evaluate the channels
are marked as Fig. 20a shows. The Single infrared images of dies, molds and
two-dimensional cluster formation curves parts can l1e used to identify local hot
in Fig. 20b all exhibit evidence of spots and temperature gradients after part
inadequate cooling. !;or example, curve 1 ejection. Images of the surface can be
rises in te1':nperature for 3 min. This recorded and compared in time to follow
suggests a large thermal reservoir exists at changes in temperature due to changes in
the top of the sliding core. Curve 2 is too the operating conditions.
hot after cooling, which suggests that
spray is not reaching that region of the ~vlore importantly, transient thermal
die. Temperatures plotted by curves 3 and imaging may provide insight into not
4 on the face of the core rise for about only surface temperatures but how
30 s and then begin to fall. thermal energy is dissipated. Through
cooling curves, recovery of subsurface
Using the finite element model temperatures and surface heat flux values
calculations the depth of this thermal may soon be available. App1ying these
source was estimated. As a whole, techniques to the analysis of cooling
information obtained suggests that channel design may help quantify heat
thermal energy was not being efficiently dissipation in tool steels.
dissipated by the cooling channels.
Modifications were made to internal water
flow and the analysis was repeated.
Curves plotted in Fig. 20c show that after
modifications the temperatures fell with
time after cooling was completed. The net
effect on production was a decrease in the
cycle time and a decrease in rejected parts.

Discussion

Rather than optimizing the surface
temperature of dies and molds it may be
better to optimize a combination of
surface temperature and subsurface
temperature gradients. This optimization
would provide better control of heat
transfer and it may be possible to use the
technology to control both solidification
rates and the direction of solidification.

A similar analysis could also be useful
from a design point of view. For example,
the ideal location of cooling channels
may be compromised by the requirement
of an ejector pin. Identification of internal
hot spots would help establish whether
the pin or channel receives priority.
Complex part geometry often leaves
design engineers balancing expensive
machining against cooling requirements.
Thermal imaging is a tool to understand
the dynamics of local heat flow.

Analysis of thermal transients often
requires an interpretation or assumption

Infrared and Thermal Testing of Metals 457

PART 3. Online Monitoring of Arc Misalignment
in Gas Tungsten Arc Welding3°

Infrared thermography has been used to monitors and updates the position of the
monitor quality control of welds since the arc with respect to the joint.

1970s.31,32 Several sensing techniques - such as
through-the-arc sensing, J:J laser
The reliability and consistency of welds stripping/" magnetic wave controt,J~
produced may be enhanced through real direct coaxial viewing~6 and infrared
time monitoring to assess and control the thermography37,:nc~ - have been developed
performance of the welding system. to monitor and control the position of
Infrared thermography has been used as the arc with respect to the joint. ~·1ost of
an online sensor to monitor arc these sensing techniques can monitor
misalignment in gas tungsten arc ·welding only a single parameter and have had
processes. Obtained plate temperature success only in limited weld joint
distributions are displayed as isotherms, configurations. \·York on infrared
regions of equivalent temperattiteS. A thermography indicates that, in addition
qualitative analysis of the temperature to arc misalignment, several other
distributions reveals that arc displacement welding perturbations can be detected by
with respect to the joint produces a analyzing the surface temperature
definite asymmetry in the isotherms. To distrlbu tion.3 7,31l
demonstrate computer control of the arc
position, image analysis techniques have Because the arc joint position error
been developed to quantify the observed signal is derived from the temperature
asymmetry into an arc joint position error distributions, this technique can be used
signai.30 The thermal image analysis to control the welding process regardless
technique involves comparison of radii of of the joint configuration.
an isotherm about the calibrated torch
position. Averaging techniques have been Experimental Procedure
implemented to reduce the noise level in
the derived error signals. The schematic shown in Fig. 21 gives an
overview of the welding and infrared
Background camera equipment. The infrared camera,
shown mounted on the torch assembly,
Success of robotic welding systems has monitors the temperature distribution
been limited to repetitive and large scale around the weld pool. A custom built
fabrication jobs. There are many reasons interface transfers the information from
for this but basically robotic welding the camera to the computer to analyze
systems do not incorporate the adaptive
skiH of a human welder. In a manual FIGURE 21. Infrared and welding equipment setup.
welding situation, the welder alters the
welding parameters according to what the
welder observes. Absence of such adaptive
features in robotic welding systems
necessitates high tolerance control in part
preparation procedures and fixturing to
maintain the arc position over the center
of the weld joint. Another cost effective
solution is to make the robotic welding
system adaptive to adjust to the changing
environment. The most challenging task
in conceiving such a machine control
system to emulate the functions of a
welder is the development of a
comprehensive online sensing system.
The primary function of an online sensor
is to provide process status information of
the parameter being controlled. For
example, in arc position control
application, the sensor continuously

458 Infrared and Thermal Testing

and update the position of the arc with offset of 12.7 n11n (0.5 in.) to the left of
respect to the joint. the seam and to linearly cross the seam
and end ·with an offset 12.7 null (0.5 in.)
All expC'rinwnts were conducted using to the right of the seam (Fig. 23). The
6.35 mm (0.25 in.) thick American Iron welding conditions used in the
and Steel Institute (AISI) type 1008 steel experiments are listed in Table 4.
plates measuring 305 x 152 mm
(12.0 x 6.0 in.) in size. The edges of the FiGURE 23. Experimental weld path
plates were milled to produce a precisely configuration: (a) parallel seam; (b) cross
fitted butt joint. The surfaces of the plates seam.
were prepared using standard preparation
techniques. \-\'elding was performed with (a)
a water cooled torch. The torch was
manipulated by an X, Y positioning table Ll
(Fig. 22). This positioning table was
controlled by computer through a data ~2
acquisition and control unit. rt 3

A camera detected the infrared ~
radiation used to characterize the thermal
distribution of the plates being welded. t .~
The infrared image from the camera was N
transferred to a computer in a digital
format and was also recorded on video tE
tape for future analysis. The infrared
camera determines the temperature ~ E
distribution by sampling a portion of the
emitted energy within a 'Wavelength band g
of 8 to 12 pm. Each scan of the camera tM
was transferred as a frame consisting of 1~ X
192 x 250 discrete temperature
measurements. The temperature I
distribution of the plates being welded I II
was represented as isotherms, regions of
equivalent temperature. 150 mm (6 in.) 150 mm (6 in.)

Two types of experiments were P.1rallel se3m
performed to derive the relationship
between the arc misalignment and the (b)
asymmetry of the thermal profiles. In
both the experiments, the arc r' I
misalignment was introduced by .~
offsetting the torch from the center of the 1-2 N
joint. In the first type of experiment, 1-, :::.
welding was done along a straight line E
with a constant offset from the seam. In If E
the second one, the experimental weld 0
path was designed to start ·with a torch r- 0
M
fiGURE 22. X-Ypositioning table. !
1
f-

tf--x

150 mm (6 in.) 150 mm (6 in.)
Cross 5e3m

TABLE 4. Welding conditions.

Parameter Specification

Current 200 A direct current
Voltage
Polarity 32 v
Electrode composition
Electrode diameter straight
Shielding gas
Shielding gas flow tungsten with 2 percent thmia
Torch speed 3.175 mm {0.125 in.)
argon
0.314 dm 3·s- 1 (0.67 ft 3·min- 1)
2.267 mnH·l (6.3 in.·min' 1)

Infrared and Thermal Testing of Metals 459

Results and Discussion either side of the joint. This balanced
thermal distribution yields isotherms that
The first task in the procedure adopted to are symmetric about the center of the
develop an arc joint position error signal torch. However, in off~seam welding
was to identify the asymmetry of the conditions, the arc is offset from the seam
thermal profile caused by arc and therefore the heat input is unequally
misalignment. To identify the asymmetry, distributed on either side of the joint.
thermal profiles of on~seam and off-seam This inequality in heat distribution is
weld conditions were displayed as caused by the thermal contact resistance
isotherms and visually compared. Ou-seam of the seam. The unequal thermal
welding, in this discussion, refers to an distribution is manifested in the
arrangernent in which the arc is aligned asymmetry of thermal profiles.

with respect to the joint position; offseam Before discussion of comparison
condition refers to an arrangement in techniques used to quantify the observed
asymmetry, it is necessary to understand
which the arc is misaligned. the important features of the isotherm
Figure 24 contains two isotherm patterns. Each one of these isoth('rm
displays consists of several continuous
patterns depicting the effect of arc bands of different colors, each
misalignment on the thermal distribution representing one rarige of temperatures.
in the plate. The thermal profiles obtained
during on-seam welding is _illustrated by The color coding of the isotherms is
the pattern on the right; the pattern on user defined. In the upper half of the
the left depicts the thermal profiles isotherm pattern, the outline of the
obtained during off~seam ·welding. ceramic nozzle of the welding torch can
be seen. The cross hair at the center of
In on-seam welding conditions, the arc the isotherm display is the calibrated
is aligned with the center of the seam; torch position determined before the
hence, the heat input to the plates by the plate was welded. It is essential to note
welding torch is equally distributed on that the isotherms displayed on the
computer screen are mirror images of the
FIGURE 24. Radii comparison technique: thermal distributions on the surfaces of
(a) right radius greater than left; (b) radii the plates. The mirror reflecting radiation
dimensions change with different angle. into the camera inverts the thermal
(a) images. Therefore, the plate temperature
distributions on the right side of the
(b) torch are illustrated by the features of the
isotherms on the left of the torch.

After identification of the asymmetry,
the next step was to quantify the
asymmetry into an arc joint position error
relationship. The asymmetry was
quantified by comparing the features of
thermal profiles on either side of the
calibrated torch position. The asymmetry
in the thermal distribution reduces the
size of isotherm on one side of the torch
in comparison to the other. Therefore, lhe
asymmetry of the isotherms was
quantified through comparison of half
isotherm size.

In off-seam welding conditions, the
contact resistance of the seam restricts the
heat flow to the plate farther from the arc,
reducing the maximum distance a single
isotherm can reach in that plate. Because
the heat flow is unrestricted in the plate
toward which the torch is offset, the
maximum distance reached by any single
isotherm is greater than for the other
plate. Thus the asymmetry of isotherms is
manifested in their size. The radius of the
isotherm from the torch position is a
good measure of the size of the isotherm.
The radius of any isotherm is the distance
between the calibrated torch position and
the outer edge of the isotherm.

Appropriately, the radius of an
isotherm on the side toward which the

460 Infrared and Thermal Testing

arc is offset was found to be greater than Percent Difference
the radius on the side farther from the (1) difference in radii
torch. For example, when the arc is offset
to the left of the seam, it is expected that in radii Width of
the left side radius should be greater than isotherm
the right side radius. This would be true
only if the welding process were viewed The cross scam data were analyzed by
directly by the camera. Because the calculating the percent difference in radii.
process was vie\ved through a mirror, the The magnitude of the error signal for the
camera perceived only the mirror 0 degree radii comparison and 45 degree
reflection of the images. Hence the right radii comparisons was improved
radius is greater than the left (see Fig. 24). tremendously. The variations in the error
In the figure, radii measurements along 45 signal ·were stabilized by using a running
degrees in the direction opposite to the average technique, in which the average
torch motion are superimposed on the of percent differences in radii of five
isotherms. Because the shape of the prl'ceding frames is computed. The
isotherms is different from that of a circle, computed average is plotted in the Y axis,
the magnitude of the radii varies with the ·with the frame number along the X axis.
angle at which it is measured. This reduces the noise level in the error
Comparison of radii to identify an error signal.
signal was attempted along different
angles. A comprehensive analysis revealed The results obtained from the analysis
that the radii measurements along the of the data through radii comparisons
torch row (0 degrees direction) and 45 along different angles are unique. It has
degrees in the direction opposite to the been proven that the arc misalignment
motion of torch yielded useful error can be detected by analyzing the thermal
signals. distributions. Hu·i\'ever, the time frame in
which the sensor detects the arc position
This technique was used to analyze depends on the regions of the isotherms
parallel seam experimental data and cross being analyzed to obtain the error signal.
seam experimental data. The frame For instance, the radii comparison of the
number is inconsequential because the arc isotherm along the torch row (0 degrees)
offset remains constant but the frame provides information about the current
number is very useful in the cross seam status of the arc position with respect to
experiment because the arc offset is a the seam. Instantaneous arc position
function of time. The frame number in detection in this case is possible because
such an experiment is an indirect measure the thermal distribution to the side of the
of the arc offset. In the parallel seam torch is affected more drastically by the
experiment, because the torch offset is current position of the arc.
constant1 the difference in radii remains a
constant in the entire data file. However, Analysis of the regions behind the arc
the magnitude of the difference in radii is would reveal the torch position with
found to vary with the direction of radii respect to the seam for past torch
comparison. The magnitude of the error positions. Therefore, the error signal
signal for 0 degrees radii comparison is obtained by comparing the radii along 45
smaller than for the error signal obtained degrees in the direction opposite to the
through radii comparison along torch motion has a time delay in
45 degrees against the direction of the detecting the torch offset. '"'hen the
torch. The radii comparison analysis of effects of arc blow are neglected, the
cross seam data yielded an error signal degree of arc misalignment is obtained by
that starts with a negative difference in measuring the torch offset. In the
radii and linearly crosses the X axis, zero experiments conducted, the torch offset of
error, to terminate with a positive error any frame of data is determined by using
magnitude. the time information recorded during the
data acquisition run and the ·welding
The radii comparison error speed.
identification technique has its
limitations. The magnitude of the radii In the cross scam data, it \Vas found
difference is on the order of tens of pixels. that the torch offset was zero in the
This small magnitude of the error signal is twenty~ fifth frame of data. As explained
affected by perturbations occurring during earlier1 the instantaneous detection of
welding. To improve the magnitude of the torch position is possible with radii
error signal, percentage difference instead comparison along 0 degrees direction, so
of absolute difference in radii is used as the error signal obtained through radii
the error signal. Percentage difference in comparison along 0 degrees has a zero
radii is computed as follows: error magnitude at the 25th frame of data.
However, in the error signal obtained
through radii comparison along 45
degrees in the direction opposite to the
motion of the torch, zero error magnitude
occurs at a later frame. Enhanced data

Infrared and Thermal Testing of Metals 461

acquisition and feedback response
minimize the disadvantage of deriving the
error signal from the regions behind the
torch.

Conclusions

The follm\•ing conclusions resulted from
this investigation.

1. The radii comparison technique was
found to be sensitive to the angle of
radii measurement.

2. The radii comparison along 0 degrees
direction and 45 degrees in the
direction opposite to the motion of
the torch yields a usable error signal.

3. Comparison of the percent difference
in radii instead of the absolute
difference ·was found to effectively
quantify the asymmetry.

4. A running average scheme for five
data points reduces the noise level in
the error signal.

462 Infrared and Thermal Testing

PART 4. Thermal Imaging of Laser Welding

Basics means of information acquisition, the
algorithms of image processing have to be
laser beam welding is a flexible and adapted. It is either possible to use binary
productive process for joining parts. A images for geometrical assessment or gray
large variety of materials, metal as well as level images to calculate temperature
nonmetal, are suited for welding by laser. gradients.

The process stability of laser beam Image Acquisition, Sensor
welding is influenced by the technological Calibration, Filtration
parameters and characteristics of the work
piece, of the laser and of the positioning Usually the thermal condition monitoring
system. The process stability itself is performed with infrared thermal
influences the quality of the product. imagers. In this case a special solution
Process deviations can lead to adapted to the application is preferred for
imperfections, distortions, reduced seam various reasons.
quality and degraded mechanical
properties. This results in unnecessary The sensor has to he mounted directly
manufacturing steps. at the laser head. This avoids problems
arising from relative motions between
Laser beam welding can be automated laser and camera. As a rule the laser head
easily due to the integration of robots and is positioned by a robot. The mass of the
computers for handling and control of the laser head must not exceed a certain value
technological process. Information about due to problems arising from inertia
the process, obtained from the interaction (acceleration, deceleration and
of laser beam \Vith material, are important positioning). And the robot's freedom of
for full automation and for quality action must not be restricted by
assurance. attachments.

A lot of experimental work has been The requirements are fulfilled best by a
done for testing the suitability of different special adapted charge coupled device
techniques of process control. Known for camera with a silicon sensor.
their application in the field of process
monitoring during laser welding are Silicon can be used as sensor up to a
acoustic emission, plasma monitoring, wavelength of about 2 pm. Beyond this
micromagnetic technique and also wavelength, silicon becomes transparent
thermographic observation. The aim of and cannot be used for sensory purposes.
the work is generally to acquire signals
from the interaction of beam with The radiation of the laser plasma
material and to use them for process extends from the ultraviolet to the near
control. infrared spectral range. This ·wide band is
caused by the large energy range of the
Thermal monitoring of the laser particles. The steel materials emit
welding process is especially suited for radiation in the visible and near infrared.
process control because beam material Additionally heated gases used for the
interaction affects heat transfer, 'Which in technological process cause radiation and
turn affects seam properties. Information contribute to the disturbing radiation.
about the location of the heat source, the
geometry of the weld pool and the l;igure 25 shows schematically the
surrounding temperature field can be spectral distribution of the radiation
obtained from the thermal images. So it is portions. It is to be seen that the useful
possible to control the process by feeding signal (heat radiation from the work
back signals obtained from the processed piece) is interfered by intense disturbing
information to the laser welding process. radiation. Therefore the filtration of the
incident radiation is inevitable.
There are two different ·ways to extract
information from the acquired thermal The most favorable conditions for the
images. They are related to the aims of process monitoring were found in the
condition monitoring at laser welding: interval between about 780 nm and
(1) detection of geometrically describable 1200 nm. The term tllermooptical imaging
process deviations, such as edge applies to the spectral range between
misalignment; (2) determination of the visible and near infrared radiation.
mechanical properties of the weld and the
heat affected zone. Depending on the Because of the high level of disturbing
radiation (plasma, evaporated metal,

Infrared and Thermal Testing of Metals 463

shielding gas molecules and other sources) The locally different filtration causes
it has been found that narrow band another problem immediately related to
filtration in the range between 780 and temperature measurement by radiation.
1200 nm guarantees the best conditions The level of calibration temperature Vl'l\ll.',
for monitoring the surface temperature gray for the filtered region of the image i\
distribution. for this purpose metal different from its level for the unfiltered
interference filters with various regions. If the whole image is analyzed,
wavelengths have been tested. The metal the different calibrations for the two
interference filter has to be tilted for some rt-gions have to be considered. This
degrees against the optical axis to suppress discrepancy can be accommodated by
reflections that lead to shadows in the fitting the gray level lines.
image.
The calibration of the sensor system
The cooling cycle during laser beam (consisting of camera and filter system)
welding of most steels usually covers the was done· applying the usual setup,
temperature range from about 1773 to consisting of furnace, thermocouple and
773 K (1500 to 500 oc; 2732 to 932 °F). blackbody source. A calibration curve for a
The usual range of 256 gray levels is not low alloyed steel is shown in Fig. 27. At
suitable for the visualization of this large this point it has to be emphasized that
temperature difference. the calibration process has to be repeated
for every particular material.
Three solutions are conceivable:
(1) application of sensors with lateral Image Processing
logarithmic detectivity; (2) application of
gray filters ·with lateral different density; The first step in image processing is the
(3) filtration of only the bright region filtration of thermal noise and other
around the weld pool. Solution by disturbances. Typical origins for these
logarithmic sensors requires the greatest unwelcome effects are quantization,
technical effort; by filtration, the least. thermal reflections, johnson noise and
others.
For these reasons, a special filter setup
that allmvs partial filtration of the scene is The filtration process must not change
advantageous. This requires the filter to he the information content of the image.
mounted directly on the camera chip to The result of the filtration is an improved
avoid diffraction effects. Because of the image that shows more clearly the desired
higher ·wavelength of the radiation, information.
compared with visible light, the
diffraction effects become more serious. A Some digital filters are usual in the
region with a height of about 10 to 20 preprocessing of thermal images. The
pixels remains unsharp and can not be effects of average, mean and median filter
used for image processing. applied to the acquired raw images have
been compared.:w A three~by~three average
Figure 26 shows the layout of the filter was found to be sufficient for the
optical system for wavelength and elimination of noise. Jvfedian filtering
intensity filtration of radiation from the however leads to disappearance of small
region around the weld pool. objects.

FIGURE 25, Spectral distribution of emissions during laser Afterwards either image binarization or
welding. densitometric algorithms must be applied

1.0 FIGURE 26. Filter setup for charge coupled
device camera.
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Metal interference filter~
Wavelength (pm)
legend Telephoto tens~ ~

- - = laser plasma ~
········· = heated gas
~Ch"g' '""pled d"ke
- · = heat from workpiece (752 x 582 pixels)

I/f

Ch"9'/
coupled

device
chip Gray
filter

464 Infrared and Thermal Testing

to acquire information from the images applicabi1ity for image measurement
and to reduce the amount of data. The algorithms. The transformation of gray
latter is especially important if it is taken level images to binary images divides each
into account that- assuming 25 gray image into two regions - object region
level images per second with dimensions and background region. In this special
of 512 x 512 pixels- a data stream of case the object is the region of the weld
about six megabytes per second would pool and the heat affected zone, the
have to be processed. background is the colder region in a
greater distance around the interaction
In contrast, data reduction with the zone. Hence the process of binarization is
gray level line technique leads to data sets a special kind of segmentation, because
of 25 x 512 bytes per second. These some the image is split into segments.
12 kilobytes can be handled better by far.
The pixels of binary images have either
The situation of processing binary the value 0 or the value 1. Therefore it is
images is similar. Because binary images necessary to establish a criterion for the
contain only the values zero and one, it is transformation of the gray level image
possible to reduce the volume of data by into a binary image. In other ·words, the
using appropriate image formats and by image binarization requires a suitable
indexing of colors to the same degree. threshold. Attribute for the suitability of a
certain threshold is a resulting binary
Binary Image Formation image that sati5fies the follmving
demands.39
Binary images are widely used in image
processing because of their good 1. Every pixel of the image belongs to
exactly one region.
FIGURE 27. Calibration gray level as function of temperature:
(a) scale; (b) plot. 2. The predicate of unity results in a true
value for every pixel of the region.
(a)
3. The size of the region is maximal.

The effect of a ·wrong tlueshold is
characterized as follows (Hg. 28). If the
threshold is too )ow, the object region is
enlarged and does not only cover the
weld pool and the heat affected zone. In
the case of a high threshold, parts of the
heat affected zone are considered to
belong to the background. In both cases,
it is not desired to change the shape of
the objects (weld pool, heat affected
zone). For the purposes of thennooptical
condition monitoring, a threshold of
about 180 (for a maximum of 256 gray

(b) fiGURE 28. Influence of wrong threshold on result of
binarization: (a) gray level image; (b) threshold too low;
:n~ 2SO / (c) correct threshold; (d) threshold too high.
200 (a) (c)
r"o "·c' 150
100 (d)
>"
-"'0

~£

u ->rco.:rc0o

50

873 973 1073 1173 1273 1373 1473 1573
(600) {700) (800) (900) (1000) (1100) (1200)(1300)
[1112] [1292] [1472] [1652] [1832] [2012] [2192] [2372}

eqTemperature, K, {°F]

Infrared and Thermal Testing of Metals 465

levels in the original image) was found to prediction of mechanical properties. For
be optimal. the purposes of experiments a wavelength
independent emissivity value of 0.8 was
Densitometry of Gray level assumed.
Images
A more exact calculation of the
Gray level images, acquired from the temperature field, however, should take a
welding process, are suited for the variation of emissivity E as a function of
determination of temperature gradients. A the temperature into account. So the
prerequisite for this procedure is a dependency of gray levels to the
calibrated image as described above. That temperature can be approximated \Vith an
means that every gray level in the image equation of the fourth order:
can be assigned to a certain temperature
value. (2) G - G0 a·T(i.t + b·T(1,)3

It is not desirable or necessary to + C·T(1,)2 + d·T(i,) + e
calculate the temperature gradients for
every pixel in the image and every Coefficients can be estimated for a duplex
direction. This procedure would lead to steel as follows: a::::: 6.79 x JQ-11;
enormous calculation times, which are
not available. The better and faster b ~ s.21 x w-'; c ~ -o.ooB; d ~ o.97;
solution is the determination of
temperature gradients at certain positions e ~ -226.79.
and in particular directions. Although the
data processing is done \Vith gray level FIGURE 29. Region of heat affected zone is
images, data are reduced in two images white: (a) in binary image; (b) in gray level
(Fig. 29) that show the heat affected zone image.
as a white region in the scene. The
amount of reduction can be estimated by (a)
the ratio of white to black pixels in the
left image. (b)

The determination of temperature
gradients in the region of the heat
affected zone is of special interest because
the thermal processes that determine the
properties of the material take place in the
solidified region around the melt pool.
Hence information about the temperature
field in the heat affected zone permits the
specific control of the seam properties by
feedback to the laser system.

Two directions of gray level lines are
distinguished for the calculation of
temperature gradients: (1) in the direction
of workpiece feed and weld seam;
(2) perpendicular to the weld seam
direction.

The temperature gradients in these two
directions are influenced by the thermal
properties of the material and the relative
velocity beam material. The velocities of
heat diffusion and workpiece feed are
superimpoSed during the welding process.
This is reflected also by the elliptical
shape of the temperature field in the
thermal image. Therefore the analysis and
calculation of temperature gradients is
more simply to be done perpendicular to
the feed direction. Additionally the step
in the attenuation function caused by the
gray filter would have to be taken into
account, if the temperature gradients in
the direction of the work piece feed are
calculated.

The algorithm for the determination of
temperature gradients was additionally
simplified. The assumption of a constant
emissivity valueW leads to the fact that
the gray level images are sufficient for the

466 Infrared and Thermal Testing

Thermooptical Images deviations. In other cases, for example,
edge misalignment and lateral placement
Assessment of Characteristics deviation of the laser spot, the histograms
showed only slight alterations.. These. are
If the gray level image is used for process not sufficient for the process dJagnosts.
control (to determine the influence of the
thermal cycle on materials properties), The selection of a suitable region of the
there are two procedures to evaluate the image has a substantial influence on the
degree of process deviation: (1) gray level appearance of the histograms. T!1e gray
lines in certain positions and directions level distributions of the whole tmage are
(not mentioned up to now) and very different from those of the upper
(2) histogmms of gray levels either in th~ region (melt pool, heat affected zone) and
whole image or in \veil defined parts of 1t. those of the lower region (the cooling
weld seam) of the thermooptical image.
Gray level histograms of image parts
differ from those of the whole The other posslble procedure for the
thermooptical image. This means that the identification of the process status is the
restriction to particular regions can be assessment of gray level lines. Using
considered as a preclassification. This is special placed and orlet_Jted gwy level
desired because only information carrying lines, temperature gradients can be .
parts of the image are analyzed. These calculated and compared with predefmed
image parts are regions of interest. optimal values. This proc~~ute seen~s t~
be more suitable for condition momtormg
In Fig. 30 an example for the variation
of a gray level histogram from the. than is the histogram evaluation.
thermooptical images due to the k111d of A criterion for an optimal temperature
process deviation is shown. This
histogram takes only the region of lower gradient is the materials property, which
temperatures (heat affected zone) into should be influenced by the thermal cycle
account. in a defined way. To apply gray level lines
for the estimation of the thermal cycle it
The detectability of process deviations is necessary to determine the function
using gray level histograms is, hmvever,
limited to a subset of the examined gray level = f (temperature), as mentioned

above.
The thermal treatment during the laser

welding has an influence on the
mechanical properties, for example the

FiGURE 30. Histograms of different process deviations. ~-­ -----

7000
6000
5000
'N
~

i'? 4000

c

~

o~- 3000

~

~

2000
1000

0

Gray levels 50
(arbitrary units)

60 62

legend

~=1600W

~~~=large gap
~ = double layer weld
~ =high alloy steel

Infrared and Thermal Testing of Metals 467

hardness of the heat affected zone. To The following equation is the basis for
estimate the suitability of the technique the calculation of vickers hardness (1--!V):w
for the prediction of hardness values,
experiments as well as comparative (3) HV (310 + 494C
calculations have been carried out. The
hardness was calculated using the + 620C2 + 18Mn)M
characteristics of the thermal cycle and
the composition of the material and was + (234 + 122C) Zw
compared with the values obtained from + (98 + 275C + lSMn)FP
hardness testing.
where M is content of martcnsite1 Zw is
The base for the calculations is a content of bainite, and FP is content of
specially adapted ·weld time conversion ferrite perlite.
diagram that takes the cooling times and
line energies during the laser welding into In the region of the melt pool no
account. Regression equations also have hardness values can be calculated because
been adapted to the conditions of the no temperature gradients can he obtained
laser welding process:10 from the gray level image.

Figure 31 compares measured and Not only temperature gradients but
predicted values of vickers hardness for a also the shape of the heat affected zone
steel St52-3N (5355, according to changes in comparison vl'ith the
European standard EN 1002741). undisturbed process. The assessment of
this change can be realized with binary
The hardness testing was done images.
perpendicular to the seam. The dynamical
alteration of the hardness as a result of a A human operator with the appropriate
variation of the workpiece feed is shown knowledge can identify geometrical
comparing the values from experiment process deviations without problems,
(hardness testing) and simulation based on the operator's experiences. The
(calculation on the base of regression automation of the laser welding process
equation and weld time-temperature control requires the reproduction and
conversion diagram). adaptation of the algorithm that identifies
the shape alteration.
FIGURE 31. Comparison of vickers 0.3 hardness values:
(a) measured; (b) calculated. To adapt the algorithm, characteristics
must be found that make it possible to
(a) distinguish different process deviati011s.
These characteristics must have certain
;:;; properties:42 (1) independence and
distinguishability, (2) small number,
0 400 (3) reliability and (4) reproducibility.

w~ 350 The extraction of characteristics was
~ carried out in a combination of heuristkal
u and analytical techniques. These
300 techniques mean that a set of
2- characteristics is compiled larger than
250 needed for classification. By fitness
ili 200 evaluation of particular characteristics a
reduction of the characteristics set is
c carried out. Finally a suitable combination
"E of characteristics has to he determined.
:mr: 150 80 This combination should guarantee that a
(3.2) 60 definite classification of t'Vt>ry process
situation can be performed.
(2.4) 10
40 The following characteristics havt' bet>n
tested amongst others for their efficiency
(1.6) 0 -5 0 5 (0.4) concerning the recognition of proce.~s
20 (0.2) deviations: (1) area of object;
(2) characteristics of the surrounding
Object length (0.8) rectangle; (3) width of object; {4) length
of object. These characteristics in differ<:'nl
rnrn (in.) -10 (--0.2) Object length combinations are suited for the
(-0.4) recognition of various process deviatiom.
mm (in.)

(b)

M 400
0
~ 350
J.i
300
u
~ 250

200

150 Classification
3
\'Vith the characteristics of binary image~,
(0.12) the dassifiahility of various proress
2 conditions has been tested. Accordingly,
the paraJlelepiped classifier and the tree
(0.08) classifier exemplarily were tested.

1

Object length (0.0<1)

mm (in.) o_Jo

(-.().4) Object length
mm (in.)

468 Infrared and Thermal Testing

Following some results of the energy) has been successfully tested.
classification are described. Using these diagrams, it is possible to
predict mechanical properties resulting
The parallelepiped classifier belongs to from the thermal treatment. The
the exact classifiers. For its application it mechanical properties of the heat affected
is necessary to define lower and upper zone can be calculated using the cooling
limits. These limits are used for the time (from the weld time-temperature
decision whether an object belongs to a conversion diagram) and the romposition
certain class or not. of the material using regression equations.

Among others the characteristics object For 1ow alloyed steels, the cooling time
width and object area were used as criteria between the temperatures of 1123 and
for the recognition of the process status. 773 K (850 and 500 "C; 1562 to 932 of)
has an important influence on the
The motion of the melt pool causes mechanical properties of the welded
some thermograms to show combinations material. The characteristics of duplex
of characteristics outside the clusters. This steels arc mainly influenced in the
makes the classification more difficult and temperature range from 14 73 to 107:{ K
suggests the desirablity of (1) expanding (1200 lo 800 oc; 2192 lo 1472 °F).
the cluster limits, (2) enlarging the
number of characteristics and (3) using By prediction of materials properties
fuzzy algorithms for the classification. using information from the thermal cycle,
it is possible to provide signals for the
The application of a next neighbor control of the technological process.
classifier is difficult because of the partial
louching or overlapping cluster Condition monitoring by classification
boundaries. Therefore this classifier is not of binary images allows the assessment of
well suited for the application in process geometrically describable process
diagnostics during laser welding. deviations. The efficiency of several
classifiers has been tested. It could be
A tree classifier is a hierarchical shown that those classifiers ctlll identify
classification scheme. It uses a tree the process status under certain
structure with nodes to decide about the circumstances.
class affiliation of an object. In contrast to
the other classifiers mentioned above, the The behavior of laser welding greatly
decision is made in several steps. One influences the suitabilitv of a classifier. For
advantage of this classifier is the fact that instance the applicatiori of the next
at every node the value of only one neighbor classifier is difficult because of
rharacteristic has to be determined for a the overlap of object classes.
decision. A disadvantage is the possible
high number of decision steps. For the introduction into industrial
applications further experiments have to
The tree structure depends on the be carried out. These experiments improve
sequence of decisions in the hierarchy. the knowledge about a large variety ot
Systematical experiments concerning the steels a~ well as nonferrous metals and
optimization of classifier trees are known alloys.
from the literature.43

Results and Conclusions

The monitoring of the thermal cycle
during laser welding allows the prediction
of mechanical properties especially for the
material in the heat affected zone.

Low line energies are characteristic of
laser welding. This causes high cooling
rates and possibly degradation of seam
quality. The properties of weld seam and
heat t~ffected zon<;> can change in an
unwelcome manner. Therefore it is
desirable to have knowledge about the
materials transformation and their
dependency on the thermal cycle.

The prediction of mechanical
properties requires knowledge about the
structural transformation in the material.
A prerequisite for the prediction of the
transformation behavior is a known
chemical composition.

The adaptation of weld
time-temperature conversion diagrams,
well known from arc ·welding,40 to laser
welding (higher feed rates, lower line

Infrared and Thermal Testing of Metals. 469

,ART 5. Infrared Tribology

Introduction and contact electric re~istance near the I
friction interface to analyze the interface
Tribological interfaces under relative temperature.47 It is very difficult to I
motion arc widely used in industrial measure and analyze the two·dimensional
components such as sliders, fasteners, and transient temperature distribution of
dampers, brake disks1 tires, bearings and the friction interface itself.
seals. Sliding and rotating interfaces such
as sliders, bearings and seals are used to The two-dimensional transient
eliminate a friction force of the interfaces temperature distribution of the friction
using appropriate tribological materials interface is continuously visualized and
and lubricants. In contrast, friction analyzed infrared thermography to dl'tect
interfaces like fasteners, damper, brake flashing. spots and streaks and to evaluate
disk and tires arc also used to increase and friction behavior under the wearing
control the friction force of the interfaces condition.
to keep an appropriate value.44··t<>
Applications of infrared thermography
A generated dynamic energy in those have included detection of flashing spots
interfaces by the relative motion is and abnormal temperatures of the
transferred to the friction energy. The following components: bearings,
friction force F dissipates the dynamic couplings, supports, cutting tools, skis and
energy and transfers to the heat enerb')': snowboards.

(4) F ~PA The present discussion discusses several
techniques for detecting and evaluating
where p is the friction coefficient, P is the friction interface and its related
contact pressure and A is contact area of mechanisms: (1) friction and wear
the friction interface. The generated heat behavior of dry reciprocation interface,
flux qf in the interface of tribology (2) friction and wear behavior of dry
clements increases with increase in rotating interface, (3) thermal evaluation
friction coefficient p and P·V value: of friction and wearing behavior,
(4) thermal analysis of cutting tools and
(5) 'if (5) temperature measurement of car tires.

where vis mean relative velocity between Test Apparatus and Its
static and moving interfaces. Friction Interface

It is quite important to regulate the The transient temperature of th(;'
friction heat release in the friction reciprocating interface het\\'t'Cn
interface and limit the temperature rise polyoxymethylene (!'OM) and
!J.T. In cases such as softening, melting polyphenylene (PPS) resins is visualized
and '.vearing, where the interface and observed with infrared thermography
temperature becomes larger than the to evaluate friction and wear behavior of
limiting value, the friction interface the dry friction interface for use in dry
generates a higher friction coefficient and hl'aring materials.
heat that influences stable operation and
the life of mechanical components. The A schematic illustration of the testing
temperature of the dry friction interface apparatus b shown in Fig. 32. In the
under heavy ·wear conditions, variously reciprocation interface, the upper
referred to as llot spots and flasllins stationary pad measures 30 x 30 x 20 m m
temperatures, increases by several hundrt'd (1.2 x 1.2 x 0.8 in.) and the lower
degrees through shearing disruption of reciprocating pad measures SO x 200 x
both interfaces. Those phenomena play 10 mm (2.0 x 8.0 x 0.4 in.). The load ce-11
an important role on engineering that measures the V(;'rtical force F
damages, such as rupture, seizing and vertically loads the stationary upper pad.
heavy wear of friction components. The lower pad that measures the friction
force is horizontally and reciprocally
Bowden and others have measured sliding under the stationary pad. A
point temperature by the thermocouple chrome! alumel thermocouple is inst'rted
in the stationary upper pad to measure
the temperature distribution of the
interface materiaL An infrared radiometer,

470 Infrared and Thermal Testing

with mercury cadmium tellurium sensor temperature T; becomes smaller than the
real temperature T;, as shown in Fig. :B.
and detection wavelength of 8 to 13 pm,
is installed on the right side. It displays \,Vhen the ambient temperature T.. is

the radiation temperature distribution T; 262 K (-11 oc ~ 12 °F), the radiation
temperature T; is expressed by F.q. 6 for
of pad surfaces. Thermography of the -
polyoxymethylene and hy Eq. 7 for
interface is continuously recorded by polyphenylcne:

personal computer. (6) T; ~ 63.0 + 0.707 'J;

Figure 32 shows the apparatus of the (7) T; ~ 58.4 + 0.804 1;
rotating interface for testing the thrust
The emissivities of polyoxymethylene
rotation bearing. The cylindrical hollow and polyphenylene materials become 0.6
test pieces are composed of the upper and 0. 74 respectively, using measured

rotating and lower stationary parts, values of 7~, T; and '/~. Uncertainties of
25 x 20 mm (1.0 x 0.8 in.) in normal cross
the temperature and emissivity in the
section and 25 mm (1.0 in.) in height. experiment are 3 percent and 5 percent
respectively.
The upper cylindrical rotating part is
made of the polyoxymethylene resin and Friction and Wear Behavior
of Dry Reciprocating
the stationary lower part is made of Interface
polyoxymethylene and polyphenylene
materials. A throb motor controls the Figure 34 shows thermography of the
thrust load F and revolution velocity v friction interface using the infrared
(m-s-1). Th~ friction force F is measured by radiometer at P = 0.33 lviPa (481bcin;·2) in
pressure and I'= 0.3 m·s-1 (59 ft·min-1) in
the torque meter. The radiation mean velocity after 1, 5, 20 and 60 min.
Figure 35 represents the transient
temperature distribution of the interface is radiation temperature distribution of the
measured by the infrared thermography. reciprocation interface at P = 0.33 MPa
(48 lbrin.-2) in pressure and v = 0.3 m-s·-1
Calibration of Radiation (59 ft·min- 1). It is observed that a flashing
Temperature and Its
Uncertainty

The two-dimensional radiation

temperature of the interface materials T;

is not equal to the real temperature 7~. It
is necessary to calibrate the radiation

temperature T; using ambient temperature

'/~. Polyoxymethylene and polyphenylene
resins are used as a dry friction interface
in the experiment. The radiation

FIGURE 32. Schematic illustration of testing apparatus. FIGURE 33. Relation between temperature and radiation
temperature of polyoxymethylene and polyphenylene
materials.

fRotating part~

pMt"'~-,~StoHonMy E 420 (149) (296}
""'~~~-:
E
Thermocouple ' I
Friction force ; I ,"_, 380 (107) (224}
J
l__.-- ~

1 e3 340 (67) {152}

Stationary : ~
pad
1 "E
I
I !'! 300 (27) {80]

: Infrared camera "~

~

/ __ l __~ 260 (-13) IBJ _j ___

260 300 340 380
(-13) (27) (67) (107)
{80] [152]
181 1224]

Radiation temperature T,' K ('C) {~FJ

legend

- - =black body
_._ -• = po!yoxymethylene
___._ ~ · · ~ po!ypheny!('ne

Infrared and Thermal Testing of fvleta!s 471

FIGURE 34. Thermography of reciprocation temperature streak ·with one peak at s min
interface at P= 0.33 N·mm-2 and and two pe(lks nonnal to the reciprocating
v ~ 0.3 m-s-1: (a) t ~ 1.0 min; (b) t ~ direction is pulsating at intervals of 20 to
5.0 min; (c) t ~ 20 min; (d) t ~ 60 min. 30 min. In qualitative terms, the heat
generated by the deposition of the wear
(a) powder increases in the one-peak
temperature with the flashing streak at
the temperature of 360 K (87 oc ~ 188 °F).

The deposited powder at the friction
interface and the heat exudes, or is
expelled, to the outside of the friction
interface. Therefore, the temperatures of
the two peaks decrease each in their turn.

fiGURE 35. Radiation temperature distribution at
P ~ 0.33 N-mm-' and v ~ 0.3 m·s-1. (a) chart;

(b) photograph of worn suface.

(a)

(b) 390 (117) [242] ~----,------ ,-------,-

G:' l

'-" I'

2I

:. 360 (87) {188] rc

~

1"'

a~.
E
~ 330 (57) [134]

.,c

.0

0

'6

0
~

(c) 300 (27)

--15 0 ;15

(-0.6) (+0.6)

X direction, mm (in.)

(b)

(d)

legend

A,--- 1 min
B=21 min
C=41 min
D"" 51 min
E: "'56 min
F '--'58 min

472 Infrared and Thermal Testing

FIGURE 36. Thermography of reciprocation The temperature transition from a
interface at P == 1.08 N·mm-2 and
v ~ 0.3 m·s-1: (a) t ~ 1.0 min; (b) t ~ single peak for the flashing streak to the
5.0 min; (c) t ~ 20 min; (d) t ~ 60 min. two peaks is caused by deposition and

(a) expulsion of the powder generated by
heavy wear at the interface.

The thermography of the friction
interface using the infrared radiometer in
case of pressure P == 1.08 N·mm-2 and
mean velocity v = 0.3 m·s- 1 is shown in
Fig. 36 at 1, S, 20 and 60 min after
starting. Figure 37 represents radiation
temperature distribution with time as a
parameter. As already shown in rig. 34, it

FtGURE 37. Radiation temperature distribution at
P = 1.08 N·mm-2 and v = 0.3 m·s-1: (a) chart;
(b) photograph of worn suface.

(a)

- -----,------;--- 1 ~,~[

(b) 420(147) [296]

~

"-..

G
"--
" 380{107) {224] F

>-"
~
3
~
ID

<>
E
340 (67) 11 52]
~
c
0

'."~'
•'0

~

(c) -15 0 +15

(-0.6) (-t0.6)

X direction, mm (in.)
(b)

(d)

legend

A= 1 min
B =-2 min
C = 3 min
D = 4 min
E = 40 min
F =61 min

Infrared and Thermal Testing of Metals 473

is observed that the two peaks m·min-1 and is kept in steady conditiOn.
temperature across the width become
At the same time, the temperature rise /1'1'
uniform after 5 min. The temperature of at v = 28.1 m·min-1 is pulsatin_g from 30
the local flashing spot becomes about to 40 K (30 to 40 'C; 54 to 72 'F) and
friction coefficient p is pulsating from
390 K (117 oc ~ 242 °F) higher than that
FIGURE 38. Temperature difference of material interfaces at
of Fig. 34 and causes the severe wear. The
P~ 98 Nand v ~ 0.3 m-s-·1: (a) interface of
photograph in Fig. 34b shows the worn polyoxymethylene to polyoxymethylene: (b) interface of
polyoxymethylene to polyphenylene.
surface. The white scratched streak
eventually becomes larger than shown in (a) ---1--·· ---------r----

Fig. 34 because the upper limit of Pv G:- 30 (54)
becomes larger than shown in Fig. 34. "-'

The flashing temperature T3 increases i!
with increase in the friction coefficient p 0

and decreases with decrease in p, because ,'"_'. 20 (36)

the temperature rise D.T is proportional to <l
the friction heat flux pFv.
u~c
Friction and Wearing Behavior of
Dry Rotating Interface ~

Thermal behavior of the transient ~
temperature T(x,1:) was represented in the
!uE 10 (18)
dimensionless temperature e:
~
(8) e T(x,t) - T(O,t)
T(x,O) - T(O,t) 0

Figure 38 shows the temperature ~ -20 0 20 40
(-0.8) (0.8) (1.6)
difference of the rotating interfaces of ~
polyoxymcthylene to polyoxyrnethylene
~
and of polyoxymethylene to
polyphenylene at P ~ 98 Nand 0.
v = 0.3 m·s-1• One-peak temperature
distribution is observed and is increasing E
~
with increase in time. The temperature
distribution becomes symmetrical in a 0

radial direction, because the -40
polyoxymethylene material is used in (-1.6)

stationary and rotating pads and the location, mm (in.)
generated heat is transferred from the (b)

interface at same conduction heat rate in ---,----- - -·
the axial direction.
~ 20 (36)
Thermal behavior of transient
0
temperature distribution is analyzed by
solving a one-dimensional heat ",_'.

conduction equation. <l
The revolution interface generates the
~
friction heat flux qr = pFv·A-1 and transfers
the conduction heat to both cylinders. ~
~ 10 (18)
Analytical dimensionless temperature is
u
expressed in a straight line. Experimental
data are scattering around the analytical ~
line because of heat conduction rate to
3
both interfaces and end heat loss of
~g_
rotation cylinders.
The transient interface temperature E
~
difference D.T and the friction coefficient
p of the polyoxymethylene and 0
polyoxymethylene interface materials
·-'---··---- _.L__ 20
have been measured at force F = 98 N and (0.8)
v = 0.268 m·s-1 (16.1 nHnin-1 = -20 0
52.8 ft·min-1) and 0.4683 m·s-1 (-0.8)
(28.1 ITI·min-1 = 92.2 fl·min-1) in mean
location, mm (in.)
velocity under the heavy wear condition.
The temperature rise and the friction legend
coefficient p for v = 16.1 m·min-I
becomes smaller than that for 28.1 30'
~ = 60s

---- =120s
--£)- =- 180 s
---+-- =- 240 s
-<>--- == 300s
-~· = 600 s

474 Infrared and Thermal Testing

0.04 to 0.08 at the time interval of 15 to (9)
20 min. That recuperation is mainly
caused by the wear powder production where L is the stroke of reciprocating
and its expulsion from the interface to
outside. motion, p 1 is thermal conductivity of
polyoxymethylene and P2 is that of
The thermogram test of the friction
interface using the infrared radiometer polyphenylene. Friction lleat flux q1 is
makes it possible to detect the location of related to dimensionless numberS.
the flashing temperature spot by the high
friction coefficient p and generation of (1 0) 1; )q ~tFVL
heavy wear.
Az t."I;A1, 1
Thermal Evaluation of Friction and
Wearing Behavior The root mean square value of Sis

Mechanical bearing element is mainly 0.672. Good correlation among
composed of journal rotation, thrust
rotation and reciprocation. A lot of mechanical bearing elements like journal
self~lubricated bearing materials are tested rotation, thrust rotation and reciprocation
to obtain experimental data with friction,
wear and temperature as parameters to arc obtained to evaluate the temperature
evaluate thermaHy the friction and wear
performances of the dry lubricated rise !lT from the friction heat flux f/!·
interface. To obtain the correlation equation, the

Hgure 39 shows the relation bet\veen weight loss of lV per time Tis measured
the heat flux qf = pFt•.Jl-1 (W·m-2) and
temperature rise !lT (kelvin) for eight dry for the reciprocating friction interface per
friction materials. Data scattering depends timeT. The friction heat flux €Jr = Fl'·Jl~l is
mainly on the wear behavior of interface
materials. The dimensionless number of related to the relative wear rate:
friction heat flux to thermal conduction
flux is derived, considering the transferred (11) K' ),1 VI'
ratio to both interfaces: ), 2 P1•t t."I;

FIGURE 39. Temperature difference !l T and friction coefficient The relative wear rate K' decreases with
increase in the friction heat flux CJt under
~Fv·A-1 • normal friction conditions and is
gradually increasing under heavy wear
conditions of lJt > 10.

Thermographic testing is useful to
evaluate the friction heat nux Cfr and
relative wear rate K' measuring the
temperature rise of the friction interface.

G:' 200(360) • -
3
"-' Temperature Measurement of Car

\-' • Tire

0 • •• The car tire is composed of tread, circus
and bead and makes contact with the
"h' road surface to transfer a driving torque
and load to the road surface. Composite
<l materials of tile tire wheel are made of
composite materials of laminated natural
~uc 100(180) h • and synthetic rubber and fiber net of
A nylon and polyester plastics and frictional
" ........~ a ~t!'=~ A A 0 addition agent.
A 0
~ 0 AA In general, dynamic impact load and
shearing friction force of the tire are
0 absorbed by the viscoelastic stress,
"~ 00 deformation and bending under driving
grlo •-:- •eil • condition. The dynamic enerm' is
transformed to generate internal heat in
~':tl· $0 •I '6a. a~ J.. .~. 0 the tire. The surface temperature rise of
.";;! the tread surface generated by friction has
0 a great influence on friction and wear
performances of the tire itself.
E ~.1.0 co
The loaded tire rotates on a driving
'"~ 0 2 dmm so that the surface of the tread can
he viewed and measured with a
"' 0 synchronized charge coupled device
camera and dynamic infrared imager.
Friction coefficent (1 os w·m-2) Several dynamic performance test~ of the

legend

• = polyoxymethylene sample
o = polyoxymethylene sample
• = po!yoxymethylene sample
a= high strength brass casting plus graphite

+ = fluorocarbon resin compound

0 = high strength brass casting plus fluorocarbon resin plus lead
A = high strength brass casting plus graphite
A = copper tin nickel alloy plus graphite

Infrared and Thermal Testing of Metals 475

car tire are carried out: (1) detection of tire tread shows that the overheating
structural discontinuitic~s; (2) quantitative temperature of the invisible internal
evaluation on temperature distribution of discontinuity becomes higher than that of
the tire tread and wear behavior; the improved tire and its temperature rise
(3) evaluation of frictional force and
quantity of wear; (4) evaluation of the is about +IS K (+IS oc = +27 oF).
coefficient of rotational resistance;
(5) input data for use in temperature Thermographic data from the side of
evaluation and stress analysis inside the the car can be available to measure the
tire; (6) correlation of tire profile with its overheating temperature, excess heat
durability; and (7) relation between generation, brake slip distance and wear
temperature distribution and quantity of rate of treadi to detect abnormal local
·wear on the tire tread with slip and tuw spots; and to reveal local high
angle. temperature zones caused by excess
deformation, consequent deterioriation
The heat generated in the tire is and eventual rupture.
transferred to the tire surface by thermal
conduction and is released to the ground Thermal Analysis of
interface and wheel by conduction, Cutting Tool
convection ancl radiation. ln cases where
the excess and partial deformation of the The cutting tool is used to manufacttue
tire, local contact of the ground interface products from the materiaL The cutting
and excess friction of the boundary area process presents a key problem in process
bet\veen the tire and wheel arc generated, control because of high flux heating at
the thermography can detect hot spots by the loading interface. As the cutting
abnormal and nonuniform heat process separates chips from the
generation. machined surface, the interface between
cutter and chip converts a large quantity
A driving endurance test of the tire of dynamic friction energy into heat
with artificial internal discontinuities is energy.
carried out on the road at the speed of
72.2 m·s-1 (260 km·h- 1 ~ 162 mi·h-1). The

thermography test of the surface on the

FIGURE 40. lsothermic map of cutter's temperature distribution.

1 mm (0.04 in.)

0 ~ Cutter

legend

A = 683K (410 oq [770 "F}
B = 653K (380 oq [716 <F]
C = 623K (350 "C) {662 "FJ
0 = 593K (320 oq [608 <f-]

E == 563K (290 ~q [554 "f-)
F == 533K (260 "C) [500 °F]

G = 503K (230 "C) [446 CFJ

H = 473K (200 "C) [392 <f-]

476 Infrared and Thermal Testing

The relative positions of the infrared
sensor and cutter, as we11 as the
mechanism for chip removal, must be
considered before placement of the visible
and infrared camera and before
interpreting their images. The cutting
material is settled on the stationary bed.
The visible and infrared imagers are
installed on the movable sliding frame,
which holds the cutter in place during the
cutting. The chip breaker separates the
chip produced during cutting. The side
walls of the cutter and cutting material
are coated with acrylic paint to avoid
reflection from the environment. The
surface temperature of the cutter is
calibrated in a preliminary step.

Figure 40 shows an isothermic chart of
the interface of cutter and chip obtained
with thermography. The figure makes it
clear that the maximum temperature of
the interface is generated at the position
of 1.0 mm (0.04 in.) apart from the corner
of the cutter and becomes 683 K
(410 "C ~ 770 "F). The conduction heat
flux of the interface is 210 MVV·m-2
(1 kW·cnr2 ~ 3.17 x 106 BTUrclrl.ft-').
The ratio of heat rate of the cutter and
chip becomes one to four. It is clear that
the heat generated by the shearing
deformation of the chip during cutting is
the cause of the local heat elevation.

Infrared and Thermal Testing of Metals 477

PART 6. Infrared Thermography of Steel Wire
Drawing

Infrared thermographic techniques lend furniture, compressor springs, electronic
themselves to quality control applications hardware products and wir(• for weaving,
in the metal industries, especially process braiding and knitting.
control of forming operations such as
forging and stamping50.SJ of sheet metal, The industrial drawing process reduces
rods, billets and wire. The forming process the section of the wire rod by pulling the
frequently heats the metal so that thermal material through a hard die (Fig. 41 ). This
profiles can reveal variations in material process generates heat by deformation
integrity. and friction. The heat generated must he
immediately removed after every diameter
Steel Wire Drawing reduction; otherwise the final quality of
wire and the drawing performance ·will be
The wire industry seeks to improve the poor. For this reason the improvement of
metallurgical properties of steel wire, to the cooling efficiency in the drawing
provide better and more uniform product machine is very important. The more
quality, to optimize processing speed and efficient the cooling is, t11e greater the
to reduce rejection rates. wire quality and the more productive the
process will be. Infrared thermography
Hnal products obtained from a wire makes it possible to analyze how the
drawing include the following: dry drawing process affects cooling efficiency
cleaning hangers, armoring ·wire, broom and how the drawing process may be
and brush ·wire, core wire, wire rope, high improved.
carbon wire (many reinforcement
purposes), hose reinforcement wire, music \·Vire drawing machines arc basically
wire, wire for fasteners and belts, flexible simple units having capstans driven by
shaft wire, mechanical spring wire, electrical motors and die holders, with
prestressed concrete strands, automotive lubricating and cooling systems (Fig. 42).
·wire, bag tie wire, surgical wire, welding Through years of experience, the
wire, nail wire, belt reinforcement wire, technology as in other fields has
galvanized wire, low carbon wire, binding developed machine and system designs to
wire, building mesh, stainless steel wire increase the efficiency of wire production.
mesh, residential and agricultural fencing, Manufacturers have expended great effort
barbed wire, barbless fence wire, to improve cooling capabilities and to
reinforcement mesh for construction, increase speeds in wire drawing machines.
welding wire, speedometer cables, bicycle, Increase in speed entails more heat in the
drawing process. Heat is a natural enemy
of quality in the drawing of carbon steel

FIGURE 41. Die drawing process. Heat develops from plastic deformation. For each die (section
reduction) there is one capstan for cooling.
Die
(,"Coo,og >Y'''"'
_ _ :- - -Wire already cooled
~- -: Wlre to next capstan to coot down
in previous capstan

1or inlet process
I :; before further section reduction or

/,: ., ,,/~ output to w1re earners
-',C>;. I

_____... - •• - •• - •.. - I . • - . •••
•
• T ..

•••

....

\.

.

lubricant

478 Infrared and Thermal Testing

wire. \.Vhile the process is checked, it is Change in Drawing Parameters. During
possible to look for maintenance normal industrial operation it is often
problems, in some cases with a view to necessary to produce new products or to
redesigning the cooling system. improve the quality of products already
manufactured. To make changes the
Applications of Infrared operators modify drafting, die geometry,
Thermography to Steel Wire drawing speed and lubricants. Changes
Drawing may affect the heat extraction pattern in
different drawing machine steps.
Infrared thermography has been applied Sometimes if production supervisors do
to several aspects of the industrial not want to reduce speed or quality for a
drawing process (Fig. 43). specific product, thermography will help
to decide \Vhether to use additional direct
Predictive Maintenance. For regular cooling or to recommend the purchase of
checking of every capstan cooling a new drawing machine.
efficiency to control clogging of capstan
nozzles, abnormal wear or rust of capstan, Modification or Redesign. On an existing
discontinuity in cap~ tan walls (Fig. 43b) drawing machine it is desirable to check
or carbonate deposit inside walls the effect on heat transmission of the
(Figs. 43c and 43d). Also to check over following conditions: (1) changes in
heating in upper bearing (Fig. 43e) of nozzle type, (2) distribution changes in
capstan (blocks) of the drawing machine. water flow and pressure on gap and

fiGURE 42. Capstan test: (a) front view; (b) overhead view; (c) detail.

(a) (b)

-1 degree jField of -1 degree Field of Indirect water
(capstan conical view (ca~stan view cooling system
con1~al shape
shape to slip t~ slip the \ Cool wire
w1reup) to nexl die
the wire up) ~

-f: r\.·-·-·-·\-_------__u_jt_----·----~-,----
flll\ ::accumu!~Wti,ocne: L\·--------··--·--·--·--···/J
he<ght.
(capstan
: • I. , .• • :

r--120 degrees-----,. ,

:: I
W.~celcomlo.tj -~ill:··capstan gu1ded
: Wire to

and tensed by ' next die
d~ 1 ~~~::;n~;di-----------T-----------------
\ pulleys

Place to take sample for Maximum wire
emissivity calculation temperature
(bottom of capstan)

(c) -1 degrees

Indirect water Camera
cooling system
9Radiation Camera Legend
Possible rust ____-r
A = 60 degrees
deposit can drop B = 90 degrees
heat transfer 50 C = 120 degrees
percent

L__ _ _ _ _ _ oe·~·:•een wire and capstan
and lower taps make
tighter fit on capstan.

Infrared and Thermal Testing of Metal~ 479

plating, (4) coating of the capstan's manufacturers) high speed, low
interior surface, (5) changing the taper of maintenance cost, low energy
the capstan or (6) coating the capstan's consumption, long capstan life and finally
outside surface to increase wear resistance. the best ·wire cooling system. The goal is
to obtain excellent quality wire at the
Purchase of New Drawing Machines. highest qitality standard with high
production efficiency. Therefore, the
\¥hen it is time to choose a new drawing control of cooling efficiency on the ne\V
machine offered will be decisive factor for
machine there are many alternatives. final decision.
Many installations offered in the market
will offer (according to their

fiGURE 43. Thermograms of wire drawing capstan: (a) well filled capstan and good thermal
profile; (b) capstan with severe flaw; (c) capstan with cooling problems; (d) capstan with
anomalous temperature drop; (e) capstan with upper bearing problem; (f) capstan with lack
of wire fill.

(a) (d)

(e)

480 Infrared and Thermal Testing

Wire Drawing Process salt bath at 673 K (400 oc ~ 752 °F) to
773 K (500 oc ~ 932 °F) or in air. The
Die Geometry and Materials
purpose of this procedure is to develop in
During the wire drawing process the the wire a microstructme favorable for
drawn material is reduced in cross section subsequent drmving to make it more
to that of the outlet aperture of the die by strong and ductile and to improve tensile
means of pulling forces working along the strength. Patenting is mainly used to
axis of the wire and through forces acting produce wires for springs, wire ropes (steel
perpendicular to the \Valls of the drawing cables) and piano wire.
cone, thus stretching the wire
proportionally to the reduction of the Deformation in Wire Drawing
cross sectional area. Generally die surfaces
are made of cemented carbide or of An explanation of the mechanism of cold
industrial diamonds (Fig. 41) ~natural deformation is facilitated by reference to
diamonds, polycrystalline diamonds or the crystal lattice. Crystallography makes
artificial single crystal diamonds - in it clear that, in this symmetrical crystal
sizes ranging from 30 mm (1.2 in.) to structure, plastic deformation proceeds in
3 pm (1.2 x 10-4 in.). such a way that, when exceeding the limit
of the crystal stress, a mutual
Wire Surface and Heat Treatment displacement of the· single crystal parts
before Drawing in Multiple Step occurs along the slipping planes. Slippage
Machine on several surfaces inclined against each
other can cause deformation. The
Cleaning Wire Surface, The steel wire rod resulting tensile strengthening through
formed in the final shapes by hot ·working cold deformation can be atttributed to an
is unavoidably covered ·with scale that increased crystal flow obstruction
causes the surfaces to be hard and produced by a blocking up of the crystal
abrasive. Furthermore, the contraction of slipping planes due to local disturbances
the steel as it cools from the finished in the crystal lattice.
temperature is not constant and exact
dimensions cannot be obtained. To In cold deformation the plasticity of
produce a smooth surface and accurate the material is successively reduced with
dimensions the \vire rod needs a cold increasing deformation when working
drawing process. The removal of scale can with large cold deformations, as is usual
be done partly mechanically and partly during ·wire drawing. Plastic deformation
chemically (by pickling). of steel and other metals is sensitive to
the temperature and the rate at which the
Coating Wire Surface. After the ·wire rod is deformation takes places.
descaled or pickled its surface is coated.
The lime coat on the surface of the wire Elastic deformation, a function of yield
saponifies the organic drawing grease and strength and Young's modulus, takes place
so influences favorably the further when a temporary stretching or bending
drawing of the wire. The borax acts in a of bonds is involved between atoms. The
similar way as lime coating but borax also deformation is reversible (the material
forms a crystal coating that carries recovers its former shape) after the load is
drawing soap to die. removed. In cases where plastic
deformation is not reversible, high forces
Thermal Treatment of Wire are applied and permanent deformations
occurs. Plastic deformation involves
Annealing. Through drawing, the wire is breaking of bonds, often by the motion of
strengthened, or cold hardened, and with dislocations.
further deformation puts up a steadily
increasing resistance to drawing. Through Alteration of Mechanical
annealing, a far reaching change of wire Properties through Wire Drawing
properties occurs. The strength gained
through drawing is lost and the flexibility To test the evolution of the mechanical
and malleabiHty are regained. Annealing properties of drawn ·wires, the tensile test,
is genera11y used for low carbon steel the bending test and the torsion test are
wires. often used. During the drawing process
the yield limit is increased and surface
Patenting. In the wire industry, this area is reduced; at the same time
process is defined as a process of heat elongation and necking values are
treatment of medium to high carbon steel reduced. How much these changes affe<:t
wire, consisting of cooling the ·wire material properties depends on the
quickly from temperatures generally chemical composition of the wire material
- its carbon content and heat treatment.
between 1123 K (850 oc ~ 1562 or) and

1373 K (1100 oc ~ 2012 °F), in a lead or

Infrared and Thermal Testing of Metals 481

Heating of Wire and Die during wire on the capstan will not produce the
wanted cooling effect because wire
Drawing capstan contact area is reduced (Fig. 43[).
On the other hand the increase of
The wire temperature increases within drawing speed leaves less time for
every pass. This temperature sometimes extracting the heat frori1 the wire than
changes every 1 ms. The measurements does drawing at low speed. Consequently,
have shuwn that temperature normally the "\Vire temperature in a multi hole
rises from 333 K (60 oc ~ 140 °F) to 353 K drawing machine will be increased if all
(80 oc ~ 176 °F) for mild steel and 373 K the generated heat in the previous die has
not been properly extracted before
(100 oc ~ 212 °F) to 433 K (160 oc ~ entering the next reduction step.

320 oF) for high carbon steel. Researchers have investigated the
If this progressive rise in temperature various systems to capture the heat of
wire hetween drawing steps. Basically
were not controlled on a multihole there are two ways to extract heat from
drawing machine, the properties of high the wire: (1) indirect water cooling and
carbon steel wire would be adversely (2) direct cooling.
affected by a phenomenon described as
static straiu aging emiJrilllement. Strain agi11g 1. Indirect water cooling through the
is aging induced by cold working. Aging is capstan has been commonly
considered to be a spontaneous change in implemented to cool wire in a wire
the physical properties of some metals drawing machine.
and occurs on standing at atmospheric
temperatures after final cold working. 2. Direct cooling of the ·wire has been
developed as a way to extract
The heating of wire and die during additional heat while the wire is on
drawing is due to the plastic deformation in the capstan.
the wire and the friction between wire and
drawing cone, or die. Some investigations Infrared Method
have shown that, with steel, the
deformation work is transformed into 90 Infrared thermography of a drmving
to 100 per cent heat whereas only a machine provides a clear example of the
maximum of 10 per cent of the work used advantages of noncontact temperature
for deformation remains as latent energy measurement over contact temperature
in the drawn material. Lubricants are used measurement. It is impossible to have a
to diminish the friction between the die temperature sensor in contact with the
and the wire. Temperature rises in wire~to-capstan rotating surface, a moving
drawing process with parameters such as target. In particular, this test object has
{1) main deformation strength, (2) friction the following characteristics.
coefficient, (3) drawing speed, (4) length
of drawing cone die and (5) percentage of 1. The surface is nonplanar.
area reduction. 2. The target is moving - that is, the

The rise in temperature could also capstan rotates.
cause a breakdown in lubrication, 3. The emissivity varies as the wire
resulting in scoring of wire and eventual
wire breakage. Because of the effects moves from one capstan to another.
mentioned above it is important to
minimize and quickly dissipate the heat The test properly could be defined as a
generated during the drawing process. passive infrared test. Quantitative analysis
in most cases takes place at ambient
Cooling Systems temperatures inside a building \Vhere wire
is manufactured.
Any cooling system must incorporate a
good lubrication system (hydrodynamic As in other infrared applications, the
lubrication, boundary lubrication or both) following must be taken in account:
to keep temperature within desired radiosity (radiant exitance), surface
bounds. It is important to maximize and features, emissivity variations and target
maintain good lubrication during wire motion (rotation).
drawing. The lubricant divides the surface
of the wire from the surface of the Radiosity (Radiant Exitance)
drawing die. The purpose of the lubricant
is not only to reduce the friction but also The mechanism of capstan-to~wire
to absorb the heat. The higher the radiosity (radiance exilance) follows the
viscosity the greater the separation power. Stcfan-noltzmann law:
And the higher the temperature of the
lubricant, the lower its viscosity. where Q is heat flux density, or radiosily
(exitance) (\'\'·m-2). In the heat exchange
The heat generated by operation of a
multihole drawing machine must be
removed between drafts mainly by
capstan cooling because heat dissipation
from dies is negligible. Also the lack of

482 Infrared and Thermal Testing

bE'tween the wire surface and its the mesh of the wire on the capstan has a
surroundings, e is wire emissivity; g is geometry that seems to improve
geometric factor wire accumulation; and emissivity values and has uniform
Stefan-Boltzmann constant a== 5.670 x reflection properties.
10~ \-\'·m- 1-K-4•
A wire turned on;t capstan presents a
In a wire capstan, at which wavelength wavy compact surface. '.\fire emissivHy
does the peak radiosity (\.Y-m-2-sr-1) occur? commonly varies from one capstan to
Detailed thermal images of a rotating wire another. Sometimes the wire emissivity
capstan (carbon steel) could have stays relatively constc:mt during process
temperature values between Tmin = 303 K but this does not happen in most cases.
As the drawing process advances, wire
(30 oc ~ 86 °F) and Tm., = 493 K surface emissivity changes. The wire
(220 oc = 428 °F). These are threshold almost always suffers superficial changes
through friction and other mechanisms as
values. An average value or the it goes by the die.
temperature of interest could be around
T,v =' 403 K (130 °C = 266 °1'). Therefore 1. Plastic deformation occurs.
according to Wien's displacement law 2. The wire could be impregnated with
(),"'" = 2897.8 1"'), the peak scenic
radiation from wire (assumed here to be a lubricant, which changes emissivity
blackbody) to capstan could oscillate values.
between about 5.87 pm and 9.56 pm with 3. The friction in each die makes the
an average peak wavelength of surface of the wire more bright as the
),= 7.19pm. process moves forward.

Nonplanar Surface In the same spool are small differences
in visual patterns because the wire does
From theoretical point of view radiation is not have an absolutely uniform surface
nonuniform because the capstan surface is from patenting or coating. Also, across the
curved. For blocks of bigger diameters <Jt die wire surface are small variations in
the same focal distances the angle of friction and lubricant deposit. The steel
emitted radiation to the line of sight of emissivity varies as wavelength varies. The
the camera will be less and less radiant 3 to 5 pm region (atmospheric window)
energy will be missed. A larger diameter has different values for steel emissivity
capstan can be assumed to have a planar than does the 8 to 14 pm region. Steel
surface. On the other hand a spool of wire emissivity also varies with temperature.
as a whole target (rather than an
individual wire) has good emittance. The Rotation
wavy uniform surface exhibited by wires
together one over another can generate The capstan's rotational speed varies as
multiple reflections with increasing the process moves forward because, as the
radiance values (\·V-m-2-sr-I). wire gets longer and as its cross section is
reduced in each die in turn, so the next
Emissivity Variations cylinder gets faster than the previous one
in a process called the kinematics elwin.
According to the physics of infrared The linear velocity of the process always
radiation, energy on a surface may be refers to the velocity of the last capstan.
absorbed energy a, reflected energy p or For infrared monitoring of the rotating
transmitted energy 1. According to capstan two techniques could be used.
conservation of energy their sum must be
equal to one: 1. Record several images of the capstan
with a portable camera and observe
(13) (J. + p + ' 1 each image thoroughly. The
measurements above are based on this
For metals such as steel wire the technique.
transmittance is zero, or opaque:
2. Synchronize (with phase locking) or
(14) (J. + p ~ interlace the rotating capstan with the
sweeping camera to develop only one
Kirchhoff pointed out that under image for the 360 degree capstan spool
equilibrium conditions the absorptivity of circumference. The data can be viewed
a sample is exactly equal to its emissivity. in a flat data array image. An operator
Emissivity values are important in looking in a capstan Gill synchronize
temperature measurements because a wire the acquisition of lines and see exactly
spool is far from being a blackbody. when each trigger point is passed. In
Emissivity of a given object is a result of that way every line can be referred to
two effects: inherent surface properties a certain place on the capstan.
and gross three-dimensional surface
geometry. The surface that conforms to Before undertaking all the fine tuning
(emissivity, nonplanar, motion and
others) it is efficient to consider first
whether the camera has enough
resolution to detect variations of concern.

Infrared and Thermal Testing of Metals 483

Infrared Measurements

A drawing machine infrared report must
have the useful information of the process
at the moment that the thermographic
test is carried out. Is neither more nor less
than the general configuration and the
thermal situation of a drawing machine at
the moment of the test. It is important
that process control, condition
monitoring and inspection personnel
keep records of the following dra\ving
machine information: (1) drawing
machine designation, (2) test date,
(3) wire velocity in last cylinder or
capstan (m·s-1), (4) material characteristics
(including alloy type) of the wire that is
being drawn, (S) number of capstan,
(6) atmospheric and ambient temperature,
(7) capstan/die cooling ·water temperature,
(8) wire diameter variation (mm)1 (9) wire
section reduction (percent), (1 0) wire fill
(accumulation heights) in each capstan,
(II) lubricant used in the die, (12) die
angle (degree), (13) average wire
emissivity (0 to 1) and (14) maximum and
minimum temperature (kelvin) in each
capstan.

In the ways suggested above, infrared
thermography is a useful tool for the
process control of drawn steel wire.

484 Infrared and Thermal Testing

References

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Infrared and Thermal Testing of Metals 485