PE theory

PRODUCTION

1 MATERIAL SCIENCE

PROPERTIES OFMATERIALS : Manufacturing a component is normally influenced by the mechani-
cal and thermal properties of the work material.Also the mechanical properties are affected by the manufac-
turing process.
1. Strength : The resistance offered by a material on application of external force is called strength
based on the type of load it is called tensile, compressive, shear, bending etc. when the load is applied the
material may elastically deforms, which is called strain. The deformation may be elastic or plastic.

The tensile strength is measured by UTM. The standard test specimen has gauge length equal to
5.65 (d)0.5, where d = dia of specimen
From UTM we can measure load and elongation. By using the load Vs elongation data we can calculate
stress and strain correspondingly. From the data we can calulate engg. strain.

True stain = loge (1+e), where e = engg strain.
2. Hardness :- Resistance offerd by material to indentation. It can generally be measured by the
indentation made by a harder material. The indentation. It depends upon the applied load, the sharpness of
the indenter and the time for which the applied load is maintained.

The most commonly used tests are
BH test - here a sphere (of dia 10  0.01 mm ) made of steel or tungsten carbide is indented with
a gradually applied load at right angles to the specimen surface and the indentation diameter made on the
specimen measured.

P
BHN = ----------------------------

--- D [ D -(D2 - d2) ]

where P = applied load in kg, D = dia of ball (mm), d = dia of indentation (mm)
For steel Tensile strength = K x BHN MPa
Where K = 3.296 for alloy steel

= 3.342 for plain carbon steel

Rockwell test - it utillizes the principle that the depth of penetration of the indenter is proportinal to
the meterial hardness. So hardness measurement is faster. It uses a sphero - concal diamond cone1200 angle
and a spherical apex of radius 0.2 mm is used to make indentation and the depth of the indentation, t. the
Rockwell hardness number = R = 100 - 150t. whrer t = depth of indentation. Depending upon the load for
indentation there are a number of scales A, B, C etc available in Rockwell test. These are used for materials
different hardness.

- In Rockwell B test a steel ball of 0.0625 inch dia is used with load of 100 Kg. This is
used for LC and MC steels. It should not be used for materials whose hardness is

above RB 100.

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- For materials with hardness above this value, RC test ( 30 RB corresponds 0 RC ) is more
generally used.

Vickers hardness test- a square base pyramid diamond indenter having 1360 between the opposite
faces is used. VHN = 1.85 P/D2 , where P applied load in Kg = 30 120 kg available, D = measured avg.
diagonal of the indentation in mm.

The schelriscope uses a different principle to measure the hardness which is based on the rebound
height of a ball from the specimen. It is mainly used for measuring hardness of rubber.
3. Ductillity: It is the measure of the amount of plastic deformation a material can undergo under tensile
forces without fracture. It is the ratio of elongation of material at fracture during the tensile test to the original
length expressed as percentage. The final value of elongation obtained during the tensile test immediately
after the fracture could be taken as ductility.
But it is of length dependent. So it can also be taken as the ratio of reduction in CS area in fractured
specimen to the original CS area and it is the more convenient way of measuring ductility because it is
independent of gauge length.

It is also termed as ability of a material to be drawn into wires since only ductile materials can be
drawn into continuous wires.

4. Toughness : This is the property which signifies the amount of energy absorbed by a material at the
time of fracture under impact loading or it is the capacity to take inpact loads. It is equal to total area under
the stress-strain curve (because it indicates work done). It the parameter, which is considering both strength
and ductility.
Toughness of a material can be measured by inpact tests. These are two types Charpy

and lzod impact tests
The Energy used = m g ho - m g h

= m g ( ho - h)
In both tests loss of height of weight is measured, so that energy absorbed = weight x

loss of height
The annealed materials have better toughness than the corresponding normalized or Quenched specimens.
Coarse-grained structures would tend to have higher ductility than fine grain structures

and consequently better toughness.

5. Stiffness :Ability of a material to resist deformation under stress. The modulus of elasticity is the
measure of stiffness.
6. Elasticity : It is the property of material ot regain its original shape after deformation when the
external force are removed. This property is required for materials used in tools and m/c. so steel is more
elastic than rubeer.

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7. Plasticity : It is the property of a material which retains the deformation produced under load
permanently.
8. Brittleness : It is oppsite to ductility. It is the property of breaking of material with little permanent
deformation. CI is a brittle material.
9. Malleability : It is a special case of ductility which permits material to rolled or hammered into thin
sheets. Malleable metal should be plastic but it is not strong. The malleable materials commonly used are
lead, soft steel, wrought iron, copper and aluminum.
10. Resilience : It is the properties of material to absorb energy and to resist shock and impact loads.
It is the amount of energy absorbed per unit volume within elastic limit. This is realised for spring materials.
11. Creep : when a part is subjected to a constant stress at high lamp for a long period of time, is will
undergo a slow and permanent deformation called creep. This property is required when designing IC
engines, boilers, turbines etc.
12. Fatigue : Propertyof the material which fails below its yield point of stress when we go for repeated
cycles (variable or cyclic loads).

Deformation of metals : Deformation is the change in dimensions or forms of material under the action of

applied forces. The deformation of metals is necessary to form various types of shapes without rupture.

Deformation is based on the type of strain produced due to loading of metal. There are mainly two types of

deformation such as

1) elastic deformation 2) placstic deformation

Elastic deformation : Elasticity of a material is defined as the ability of material to return to its original
position after the removal of stress. When a metal crystal is subjected to external stress a change of shape
takes place because of the fact that aloms are displaced from their normal equilibrium position. Removal of
stress will allow the atoms to return to their normal equilibrium positions, so long as material is elastic.

In elastic deformation the strain produced is linear strain and lateral strain. The ratio between the
stress and linear strain under elastic deformation is called modules of elasticity.

Plastic deformation : It is a permanent deformation. It may occur in the following ways

a) by slipb) by formation of twins c) deviation form regular positions of stoma.

Plasticity is the properly of a meterial which enables the formation of permanent deformation without frac-

ture.At this condition the material appears to glide or slip along planes called slip planes which are planes of

max inter planner distance and are planes of weakness within the grain.

Slip : It is a common deformation mechanism, where plastic shear moves many inter atomic distance with
respect to their initial position. It is called of shear deformation, it represents a large displacement of one part
of crystal with realtion to another, which may glide more readily along others. It

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occurs in certain planes in the most dense atomic plane and in certain directions called slip directons. Gen-
erallyslip plane is the most dense atomic plane and the close packed direction with in the slip plane is the slip
direction. In order to produce slip, the critical resolving stress must occur. So shear stress which produces
slip on a crystal plane is called the critical shear stress.

All metals of similar crystal structure slip on the same crystallographic planes and the same crystal-
lographic directions. Slip occure the shear stress resolved along these planes reach a certain value known as
the critical resolved shear srtess. It is a fuction of temp and constant but not of the crystal sttructure.

Material Cu Al Mg Cobalt Iron Molybdenum
730
Critical 5 8 4.5 67.5 280

Resolved shear

Stress kg/cm2

Critical shear stress can be affected by

- pure metals reduces critical stress

- Temp increases the thermal mobility and hence decreases critical stress.

- Surface effects like surface films greatly increases critical shear stress.

- Rate of deformation and initial deformation will also raise critical stress.

Twinning : It is a method bywhich some crystals deform plastically. In twinning the atoms in a part of crystal
structure on each side of twinning plane form mirror image. It accounts, for a much smaller part of the total
deformation by slip.

Twinning occure frequently in BCPand HCPstructures. It is generallycaused by impact and thermal
treatment

- Generallyslip is limited to certain planes only, but twinning is uniform throughout the volume
- In polycrystalline metals slip takes place in preferred direction. (In wire drawing and rolling
the crystals in a metal try to align themselves in a common axial direction called preferred
direction)

Crystal Structuer : During solidification of liquid metal from its state of fusion, . the atoms of liquid metal
arrange themselves in a systematic patter called crystal structure. The repeating 3-D pattern in crystals is due
to atomic co-ordination within the material. In addition, the cavity sometimes controls the external shape of
the crystal.

The space lattice bydefinition is an infinite arrayof points in three dimension in which everypoint has
surrounding indentical to those of every other point in the array.
If the centres of the atoms are considered to be connected together by staight lines, then a system will be
obtained comprising a great number of equal parallel pipes. This system in known as space (or) crystal
lattice.

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The unit cell is the smallest component of the space lattice. The distance from one atom to another

atom measured along one of the axes is called space constant (or) lattice constant. In the cubical cell, the

lattice constant has the same value in all the three dimensions.

There are only fourteen independent ways of arranging points in 3-D space, such that each arrange-

ment conforms to the definition of a space lattice. these the space lattices are called Brave lattices. They are

1) Cubic - BCC, FCC are Corner C - ex : CaF2 , NaCl, Al
2) Monoclinic - 2 (Corner and top bootom)

3) Triclinc - 1 (Corner)

4) Tetragonal -2

5) Ortholombic - 4

6) Rhombohedral - 1

7) Hexagonal - 1 / 14 c

Actually these are primitives and 3 angles   ) >
 >

a = b = c,  ) b
a

The most common types of space lattice unit cells, with which metallic element crystallize are

a) BCC structure b) FCC structure c) HCP

structuers

BCC :Aunit cell of BCC consists of an atom at each corner and another at the body centre of the cube, For

example iron at room temp.Geoneric evironment of each atom is same

Lattice Constant =9 Atomic packing factor = 0.68

Total number of atoms = ( 1/8 x 8 ) + 1 = 2 atoms C
Ex. metals possessing BCC structure are Mo, V, Mn Ta etc

AC2 = AB2 + BC2

4r a

A B
< >

a

For atomic packing factor

Atoms per unit cell = 2
Volume = 2 x 4/3  r3

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Volume of unit cell = a3
Atomic Packing factor = 0.68

Simple cued : In this structuer atoms touch each other along the lattice as shown

a=2r; r=a/2

Atomic packing factor = Volume of atoms in the unit cell

Volume of unit cell

Atoms per unit cell = 1/8 x 8 = 1

Volume of unit cell = a3
Atomic packing factor = 0.52

FCC : In this atoms are located at the corners of the cube and at the centre of each face. Metals having this
structures are Cu , Al , Pb, Ag and Ca etc.

A metal with FCC structure has four times as many atoms as it has in simple cubes. This shows that
it is more densely packed with a packing factor of 0.74.No. other structure possesses such a large number
of close packed planes and directions. For this reason, metals with FCC can be deformed critically

Coordinate number = Lattice constant = 12
Total number of atoms = 1/8 x 8 + ½ x 6

= 1 + 3 = 4 atoms
Atomic packing factor = 0.74

HCP : It has an atom at each corner of the haxagon, one atom each at the centres of the two hexagonal

faces and one atom at the centre of the line connecting the perpendicular in the three Rhombuses. Ex. Be,

Mg, Ca, Zn

Lattice constant = 12 number of atoms = 1/6 + 6 + ½ x 2 + 2 = 4

Atomic packing factor = 0.74

Miller indices : These indices which identify a family of planes in crystal structure. Let the intercepts m, n,
& p for any plane with in a crystal give the reciprocals 1/m, 1/n, & 1/p, which may be changed to a common
denominator resulting in the numerator h, k & 1 respectively. These numerators to a common denominator
resulting in the numerator h, k & 1 respectively. These numerators when written as (h k l) identifythe family
of planes to which the specific plane belongs.
For example m, n & p are a, b/2 & 3c. i.e. numerical parameters are 1, ½ , 3.1/m, 1/n & 1/p are reciprocals
1/1, 2/1, 1/3 can be changed with common denominator as 3/3, 6/3, 1/3 so miller indices (h k l) are (3 6 1).

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The features of miller indices are
- it not only defines a prticular plane but a set of parallel planes.
- Plane which is parallel to any one of the principle exes has intercept of infinity and whose miller
index is zero.
- Only the ratio of indices is important
- Negative indices are represented by putting a bar over the digit (0 1 0)

Work hardening : Ductile materials show increase in strength and hardness when plastically
deformed at temp lower than the recrystallisation temp. This is called work hardening or strain
hardening. The rate of strain hardening decreases repidly beyond the elastic and becames a decreased and
the rate of chemical reaction increased.

Work hardening might cause development of internal stresses, increase in corrosion, elastic distrtion
etc.

Recovery, recrystallisation and grain growth : In cold working plastic deformation breaks up the struc-
ture of the crystal lattice and causes distortion and which makes the metal unstable. To make the metal the
metal is to be heated sufficiently, then the metal tries to approach equilibrium through 3 stages called recov-
ery, recrystallistion and grain growth.

Recovery is function of temp and is the first stage of annealing. Recovery eliminates void or vacan-
cies which were created by the interaction of dislocations. The recovery process is used to relieve stresses
trapped during welding, casting, forging and extrusion operations without affecting strength obtained.Also
electrical conductivity increases and distortions produced by residual stresses are reduced.

Recrystallisation temp is the approximate minimum temp at which complete recrystallisation of a
cold worked metal occurs with in a specified time. It is generally 1/3 to ½ of melting point of most of the
metals. Recrystallisation temp also depends on amount of cold work received by the metal. Higher the cold
work lower will be the recrystallisation temp.

Heating beyond the recrystallisation temp range causes the size of the recrystallised grains to in-
crease. Grain growth lowers the energy of solid. The only way to decrease grain size is to cold work and
plastically deform the existing graine and start new grains.

Theory of alloys :
Solid solution are two types substitutional and interstitial solid solutions.

Substitutional solid solutions : In this type of solid solutions the atoms of the solute substitute for the atoms of
the solvent in the lattice structure of the solvent. For example silver atoms may substitute for gold atoms
without loosing the FCC structure of gold. Other examples are brass (Cu and zine), Monel (Cu and nickel).

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The factor that will affect the range of solubility are

- crystal structure - For complete solubility both elements will have same crystal structure.
- Relative size factor - It must be less than 15% Ex. lead and silver - both FCC but due to

RSF20%, the solubility is limited to 0.1% in solid state, antimony and bismuth- complete soluble,
RSF is 7%.
- Chemical affinity factor- Greater the chemical affinity of two metals the more restricted is
their solid solubility
- Relative valence factor -A metal of lower valance tent to dissolve more of a metal of higher
valance than vice versa.

Interstitial solid solutions: These are formed when atoms af small atomic redii fit into the spaces or
interstices of the lattice structure of the larger solvent stoms. Atoms with size less than 1 (0.77), nitrogen
(0.71) and oxygen (0.6).

Gibbs phase rule:
p+F=C+n

As we know that F 0, so F = C + 1 - P,
C + 1 - P  0, P  C + 1
Where P = no. of phases which may coexist under rquilibrium condition.

F = no. of degrees of freedom
C = no. of components
N = no. of external factors

= 2 (temp and pressure) for chemicals
= 1 for metals (temp only)

Phase diagrams :
Different reaction in phase diagrams are
Eutectic reaction (ex. Cu-gold, Cu-nickel, Cu-platintum, iron-nicked & bismuth-tin)

Colling
Liquid1  solid1 + solid2

Heating

Eutectoid reaction (lead-tin, iron-corbon, lead-silver) maximum solubilityof lead in tin is 2.5% and that of tin
in lead is 19.5%. cooling

Solid 1  Solid2+Solid3
Heating

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peritectic reaction

Cooling
Liqud1 + Solid  Solid2

Heating

Peritectoid reaction

Cooling
Solid1 + Solid 2  Solid3

Monotectic reaction

Cooling
Liquid 1  Liquid 2 + Solid1

Heating

Allotropy of iron : The reversible phenomenon by which certain metals may exist in more than one crystal

structure is called allotropy. If not reversible the phenomenon is termed as polymorphism.
Pure iron can freeze at 1539C, on further cooling forms as -iron with BCC structure.At 1404C it

can changes into FCC -iron. At 910C it again transform into BCC -iron which is non magnetic. On
further cooling at 768C the BCC -iron becomes magnetic. The same can be possible in heating also. So it

is called allotropy.

1539 A4 BCC Delta iron
1404 A3 FCC Gamma iron
BCCAlpha iron magnetic
910 AA12
768 BCCAlpha iron magnetic

723

IRON-CARBON EQUILIBRIUM DIAGRAM :
Mirco-constituents of I-C diagram:
Austenite: It is solid solution of ferrite and iron carbide in gamma iron which is formed when steel contains
carbon upto 1.8% at 1130C. on cooling below 723C it starts transforming into pearlite and rerrite.It is
unstable at all temps.

Ferrite : It is a BCC iron phase with very limited solubility of carbon. The solubility of carbon in ferrite is
0.025% at 723C. ferrite does not harden when cooled rapidly. It is very soft and highly magnetic.

Cementite: These are extremely hard in nature. Cemenitite inreases gradually with increase in carbon per-
centage. It is found in steel containing more than 0.8% carbon. It is magnetic below 200C.

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Pearlite : It is combination of about 78% of ferrite and 13% of Cementite. Steel with 0.8% carbon is wholly
pearlite, less than 0.8% is hypo eutectoid contains ferrite and pearlite and is soft. More than 0.8% is hyper
eutectoid steel which contains pearlite and Cementite which is hard and brittle.
Bainite: It is the product of isothermal decomposition of austenite. These are two types feathery Bainite and
acicular Bainite.Also it is more tough.

Martensite: This is obtained by rapid cooling of austenite. It is magnetic and has a carbon content upto
2%. It is extremely hard and brittle. The decomposition of austenite below 320C starts the formation of
martensite.

Troosite and sorbite: Both are produced by transformation of temperd martensite. Troosite is weaker than
martensite.

1539  +L Liquid
Temp
1130

-iron

Ledeburate

Cementite

-iron Pearlite 723

Fe3 C

0.8 2.0 4.3 6.67

HEAT TREATMENT

Heat treatment is na operation involving the heating of solid metals to definite temp, followed by
cooling at suitable rates in order to obtain certain physical properties. The purpose of heat treatment is

- to relieve internal stressess
- to improve machinability
- to change grain size
- to soften metals for further working as wire drawing etc.
- to improve mechanical properties
- to improve trapped gasses.

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Different heat treatment processes are
i) Annealing
ii) Normalizing
iii) Hardening
iv) Tempering
v) Case hardening
vi) Surface hardening
The temp range of different heat treatment processes are shown below

Annealing: Various types of annealing processes are
a) Full annealing : temp range of heating is shown above.After heating it for sufficient time depending on the
thickness of work and then slowlycooling it in the furnace. The purpose is to soften the metal, relive internal
stresses and refine the grain structure. Holding time is about 3 to 4 min for each mm thickness. The cooling
rate will be about 30 to 200C per hour.
b) Process annealing : It is usually carried out to remove the effects of cold working and to soften the
steel to make it suitable for further plastic deformation as in the sheet and wire drawing industries.

Temp Critical temp
Holding time

Time

c) Spheroidise annealing : In this type of annealing Cementite in the granular form is produced in the<
structure of steel. This process is usually applied to high carbon steels which are difficult to machine.<

Holding

Temp Lower critical temp
Transformation
Time

d) Diffusion annealing : In order to remove the heterogeneity in composition of heavy casting diffu-
sion annealing is used. This process homogenizes the austenite garin when heated to above the upper critical
temp. this proess is always followed by full annealing.
Normalizing : The process consists of heating steel to a point 40 to 50C abvoe the upeer critical temp
holding at that temp for a shor duration and subsequently cooling in still air at room temp. this

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process is suggested for manufacturing operation like hot rolling and forging which are carried out on steels
in austenite range. Normalized steels have better machanical properties than annealing. But if the mechanical
properties is not the main aim of heat treatment, then better machinability and greater removal of internal
stresses is possible by annealing than normalizing.
Hardening: The process is carried out in 3 stages,
i) heating the work to a temp above critical point
ii) holding the work at that temp for a definite period
iii) quenching in a suitable medium called water, oil or molten salt bath
Cooling at a rate hagher than the critical cooling rate enables the austenite to super cool to the martensite
point.After hardening steel must be tempered to
- reduce the brittleness
- relieve internal stresses caused by hardening
- obtain predetermined mechanical properties.
The purpose of hardening followed by tempering are
- to increase hardness and wear resistance
- to improve ductility, strength and toughaness.
Hardening process depends upon
- carbon and alloy content
- heating rate and time
- quenchingmedium
- quenching rate
- size of the part
- surface conditions
Hardenability : It defines how much depth the required hardness has been achieved during hardening
process. It is a measure of ease with which steel can be hardened by heat treatment and which determines
the depth and distribution of hardness induced by quenching. Fully hardened articles will have the same
properties through out their cross section.

Depth of hardening is usually taken as the distance from the surface to the semi-Martensitic zone i.e.
50% martensite and 50% pearlite. Fully hardening of carbon steels is obserrved in articles of diameter or
thickness upto 20mm.

Factors affecting Hardenability are
- composition of steel process of manufacture
- the quenching media and method of quenching
- the size and shape of piece
- presence o non-metallic inclusions

Hardenability in steel increase as the percentage of carbon increases and it is further improved by the
addition of such alloying element as Mg, Cr and vanadium. But cobalt is the only one known to decrease
hardenability are

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- by the appearance of the fracture
- by the distribution of hardness along the cross section
- by jominy end test
Jominy end quench test can be done on a standard specimen of 25 mm dia and 100 mm long. Rockwell
hardness reading are taken every 1.5 mm from quenched end.
Hardening methods are
- quenching in a single medium
- quenching in two media’s
- stepped quenching or Martempering
- isothemal quenching orAustempering

ISOTHERMAL TRANSFORMATION OR TTT DIAGRAM :
The I-C diagram will have some disadvantages such as
- it does not show time as a variable so the effects of different cooling rates on the structure of

steels are not revaled
- Equilibrium condition are not maintained in heat treatment.

The I-T diagram for 1080 eutectoid ateel is shown below. Above Ael austenite is stable.
The area to the left of the beginning of transformation consists of unstable austenite. The area to the right of
the transformation line is the product to which austenite will transform at constant temp.

The area between the two lines is labeled as A+ F + C consists of three phases, austenite, ferrite &
Cementite. Ms is a borizonntal line which represents the start of tranformation of austenite into martensite
into, similarlyMf is the finish of transformation into martensite.
Position of cooling curves : There are only two types factors which will change the position of I-T diagarm,

- chenical composition
- austente grain size
An increase in carobon content or in grain of the austenite always retards transformation i.e. curves move
to the right. This in turn slows up critical cooling rate making it easier to form martensite.

Martempering :Also known as hardening by interrupted quenching. Steps to be followed in martempering
are

- steel is heated to hardening temp
- quenched in a medium salt bath slightly above Ms line (about 150 to 300C)

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- aricle is held for some time to get uniform temp but not longer than time
- then cool it to room temp in air or oil.
This treatment provides a strucuture of martensite and retained austenite in the hardened steel.Advantgages
of martempering over hardening are
- less volum changes occurs due to presence of large amounts of retained austenite.
- less wrapping due to because transformation is uniform \
- less danger of quenching cracks.

Austempering :
Also called isothermal quenching. It is similar to martempering, but the deference’s are
- Quenching time the salt bath is longer.
- Salt bath temp forAustempering is abvoe the martensite point to ensure a sufficiently com-

plete austenite decomposition into brainite. Bainite structure produced in this way are free from cracks,
softer than martensite and process good impact resistance.

Tempering : This process can be done after hardening to induce ductility in the steel. Generally tempering
is carried out to

- increase toughness
- decrease hardness and increase ductility
- Stabilize structure
- relieve internal stresses
There are three types tempering methods,
i) Low temp tempering: done at the temp range of 150 to 250C and its purpose is to reduce internal
stresses and increase toughness without loss in hardness. Mainly used for carbon tool steels, alloy tool steels
and surface hardened or case carburised parts. Observing the colours appered on the surface does temp
control in this process.
ii) Medium temp tempring: done at temp range of 350 to 400C and its purpose is to toughen the steel
at the expensse of hardness. Used for coil and laminated springs and provides the highest attainable elastic
limit with sufficient toghness.
iii) High temp tempering: done at the temp range of 500 to 650C. almost completelyeliminates internal
stresses and provides the most favorable ratio of strength to toughness for structural steels.

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Sub-zero treatment: A certain amount of retained austenite may always be foud in hardened steel. It
reduces hardness, wear resistance and thermal conductivityand make the dimensions of the article unstable.
So subzero treatment can be reduces it. It consists of metal to cool it to subzero temperature. This is
possible only when Mf is below zero.
With thish treatment the retained austenite transformed into martensite and increases the hardness of the part
and its dimensions become more stable. Subzero treatment is usually carried out in the temp range of -30 to
-120C and holding time at this temp is 1 to 1.5 hours. It is mainly used for HSS tools, measuring tools,
carburised gears etc.

Case hardening : In general some of the machine components will requir case to be harder and core to be
softer which are tough, shock resistant and capable of carrying high stresses, for example gears, can shafts,
bearing surfaces etc and also which are made by LC steels and it cannot be respond to heat treatment so the
carbon or nitrogen or both can be introduced into the surface by diffusion from CO gas which is brought into
contact with the surface at an appropriate temp (870 to 950C) by some controlled means.

There are three methods of case hardening processes
i) Carburizing

- pack carburizing
- liquidcarburizing
- gas carburising
ii) and Nitriding
iii) cyaniding carbonitriding
Pack carburising: the reactions involved are
- component is heated in box which releases CO upto 900C
- CO can be dislocated as 4CO  2Co2 + 2C
- Case enrichment with carbon as 2CO + 3 Fe  Fe3 C + CO2
In this packed boxes are properly sealed and place in a furnace to heat.At 900C iron will have affinity to
take carbon and forms it on the surface as HC steel. The time for heating is about 5 hours.

Liquid carburising : In this the steel is heated in presence of liquid medium which contains 75% sodium
carbonate, 15% sodium chloride, 10% Sic. Main advatages are

- uniform heating
- case depth is more
- heating time is less
- wide range of parts can be carburised
- suitable for mass production
But disadvantage is poisonous salts which require careful operation in presence of fumes and risk of explo-
sions.

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MECHANICAL ENGINEERING

Gas carburizing : Here steel is heated in presence of CO gas. The advantages are
- surface quality is good
- accurate case depth are obtained
- process is rapid

Nitriding : when a portion of the part can be case hardened then we will use Nitriding. For example cylinder
liners of IC engines.

Nitriding is the process of saturating the surface of steel with nitrogen by holding it for increases
hardness at a temp between 480C and 650C in an atmosphere of ammonia. The process increases hard-
ness of the surface to a very high degree and also increases wear resistance, fatigue limit and resistance to
corrosion etc. it produces hard case without quenching or any further heat treatment. It is generally applied
for MC steels. Advantages of this process are

- good surface finish
- less distortion and cracks
- good wear resistance
- used for mass production
- better than carburising

Disadvntages are
- long operational times 100 hours for 0.038mm depth
- all alloys steels can be used
- special equipment are needed
- oxidation due to prolonged heating

Cyaniding : In this carbon and nitrogen are added to the layer of the steel to increase hardness wear
resistance and fatigue limit. The steel is heated in a molten cyanide salt bath maintaining at 950C followed by
water or oil quenching case thickness from 0.075 to 1.5 mm can be botained molten salts like NaCl, sodium
carbonate, sodium cyanide, soda and barium chloride are commonly used in this process.
Advantages are

- can be applied to LC and MC steels
- bright finish of arts can be obtained
- cracks and distortions are minimized
- most suitable for parts subjected to high loads.

Disadvantages are
- risk of splitting of poisonous salts
- unhealthyfumes are formed

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Carbonitriding : Here the parts are heated in presence of gases cyanide i.e. matural gas and ammonia in a
3:1 ratio at the critical temp followed by quenching and tempering. It is slower process and used for LC and
HSS steels.
Surface hardening : It is also similar to case hardening but here no change in chemical composition and
heating and quenching of the metal is rapid and thus the core of the metal remains unaffected. There are two
methods here

i) Induction hardening : In this process the surface to be hardened is surrounded by a copper
inductor through which a high frequercy current of about 2000cycles per second is passed. The inductor
acts as a primary coil of a transformer. The work to be hardened is placed in such away that it does not
touch the inductor. Steels containing 0.35 to 0.55 % carbon are most frequently induction hardened.

The hardening temp is about 768C (curie point) in order to increase depth of current penetration.
This process is used for camshafts, gears, axels and many other automobile parts, tractor parts and similar
wearing surfaces. Here heating times are short (1 to 5 sec), automation is possible. The difficulties are not
economical for mass production, cost of equipment is high, restricted to MC and LC steels only.

ii) Flame hardening : Here an oxy-acetylene flame is used to heat the work above its critical temp
and quenching is done by means of a spray of water directed on the surface.

This is used for cast gears, mill rolls, worms etc. the main advantage is low cost of equip-
ment and simplicity of operation. But problem is danger of overheating, and uncontrollable hardness pro-
duced at two different location, due to uncontrollable temp.

DEFECTS IN HEAT TREATMENT OF STEELS :

Decarburisation : Heating of article for long periods at high temp in oxidizing atmosphere cause the loss of
carbon from the surface. Heating in protective atmosphere can decrease this effect.

Oxidation: It will result in a thick layer of scale formed on the suface of the article. This also be avoided by
using right atmosphere.

Quenching cracks: Occurs when cooling rate is more than critical rate.Avoided by tempering immediately
and avoiding sharp corners.

Warping : produced by non-uniform heating

Overheating : Heating long periods at high temp, which produces coarse grained micro structure and
fractuers, resulting in the loss of ducilityand impact strength. Prevented byannealing and normalizing.

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MECHANICAL ENGINEERING

Soft spots: This are appear due to localized decarburisation, bubble formation and inhomogenity of initial
structure.Avoided by effective quenching.

Corrosion and erosion : Produced when heating in excessive oxidizing and reducing conditions, Can be
prevented bycarefully controlling the flame heating and salt compositions in salt baths.

Excessive or insufficient hardness after tempering : Insufficinet or excessive holding time while tem-
pering produces this defect.Aproper tempring temp and holding time or subsequent annealing can prevent
this defect.

EFFECT OFALLOYING ELEMENTS
Carbon : Carbon content in steel affects

- Hardness
- Tensile strength
- Machinability
-Melting point
Nickel : Nickel
- Increase toughness and resistance to impact
- Lessens distortion in quenching
- Lowers the critical temperatures of steel and widens the range of successful heat treatment
-Strengthens steels
- Renders high-chromium iron alloys austenitic
- Does not unite with carbon
Chromium: Chromium
-Joins with carbon to form chromium carbide, thus adds to depth hardnenability with improved
resistance to abrasion and wear.
Silicon : Silicon
-Improves oxidation resistance
- Strengthnes low alloy steels
- Acts as a deoxidizer
Titanium : Titanium
-Prevents localized depletion of chromium in stainless steels during long heating
-Prevents formation of austenite in high chromium steels
-Reduces martensitic harness and hardenability in medium chromium steels
Molybdenum : Molybdemun
-Promotes hardenability of steel
-Makes steel fine grained
-Makes steel unusually tough at various hardness levels
-Counteracts tendency towards brittleness
-Raises tensile and creep strenght at high temperatues
-Forms abrasion resisting particles.

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Vanadium : Vanadium
-Promotes fine grains in steel
-Increases hardenability (when dissolved)
-Imparts strength and toughness to heat-treated steel
-Causes marked secondary hardening

Tungesten :Tungsten
-Increase hardness (and also re-hardness)
-Promotes fine grain
-Resists heaet
-Promotes strength at elevated temperatures

Manganese : Manganeses
-Contributes markedly to strength and hardness (but to a lesser degree than carbon)
-Counteracts brittleness from sulphur
-Lowers both ductility and weldability if it is present in high percentage with high carbon content in
steel

Copper : Copper (0.2 to 0.5%) added to steel
-Increase resistance to atmospheric corrosion
-Acts as strengthening agent

Boron : Boron
-Increases harenability or depth to which steel will harden when quenched.

Aluminium :Alumiuium
- Acts as a deoxidizer
-Produces fine austenitic grain size
-If present in an amount of about 1%, it helps promosting nitriding

Coblat : Cobalt
-Contributes to red-hardness by hardening ferrite
-Improves medchanical proterties such as tensile strengths, fatigue strength and hardness
-Refines the graphite and pearlite
-Is a mild stabilizer of carbides
-Improves heat resistance
-Retards the transformation of austenite and thus increases hardenabilityand freedom from cracking
and distortion

Vanadium : Vanadium (0.15 to 0.5%)
-Is a powerful carbide former
-Stabilizes cementite and improves the strucutre of the chill.

**********************

“Sucess doesn’t make you and failure doesn’t break you”

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MECHANICAL ENGINEERING

2 CASTING

“PATTERN MAKING & MOLDING”

Casting is a process of forming metallic products by melting the metal, pouring it into a cavity known
as the mould and allowing it to solidify. When is removed from the mold it will be the shape as the mold.

The sequence of steps involved in casting are * Mould and core making
*Pattern making * Fettling
* Melting and pouring
* Inspection

Pattern Making : pattern is the replica of casting to be made. It gives its shape to the mold cavity where the
molten metal solidifies to desired form and size. The size of pattern is slighty greater than the casting by an
amount called allowances, which are

* Shrinkage allowance : Shrinkage of metal during casting will takes in three stages
1. Shrinkage of molten metal when reducing from pouring temp to freezing temp.
2. Shrinkage of molten metal during ferrzing.
3. Shrinkage solid metal when reducing from freezing temp to room temp.

The first two will taken care byproviding riser during casting. But third will be provided as a shrinkage
allowance in the pattern.Also shrinkage is different for different metals and is more for cast steel.

* Machining allowance or finishing allowance - the excess in the dimensions of the casting (the dimen-
sion of the pattern) over the finished casting is called machining allowance or finishingallowance. Machining
allowance in addition to shrinkage allowance is given to the pattern. In general ferrous metals require more
machining allowance than non-ferrous metals.

* Draft allowance: It is the allowance provided on vertical surfaces to avoid tearing of edges during
removal of pattern from mould. The amount of draft for external surfaces is varies from 10 to 20mm per
meter and for internal surfaces is about 60mm per meter. Draft may be expressed in mm per meter on a side
or in degrees.

* Distortion allowance : distortion allowances are applied to the casting of irrgular shapes that are dis-
torted in cooling because of metal shrinkage. Ex. ‘U’ shaped component will certainly spread out when
cooling shown in figure.

* Shake or rapping allowance - all the above are positive but it is negative allowacne and is to be applied
only to those dimensions which are parallel to the parting plane.

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Types of patterns : the type of pattern used depends upon the design of casting, complexity of shape,
number of castings required, molding process, surface I finish and accuracy. Different types of patterns are

* Solid or single piece pattern- best suited for limited production.
* Split pattern- used for intricate casting or castings of unusual shapes. in three-piece pattern middle one is
called cheek.
* Match plate patterns - used in machine molding and also for producing large number of small castings by
hand moulding. Production efficiency and dimensional accuracy is improved bythis method.
* Gated pattern - eliminates the time required to cut gating by hand. Suitable for small quantity production.
* Sweep pettern - suitable for simple symmetrical casting.
* Skeleton pattern - used for making large castings in small number.
* Loose piece patterns - used to produce the catings having projection in the sides.
* Follow bord pattern - used for those castings where there are some portions which are structurally weak
and if not supported properly are likely to break under the force of ramming.
* Shell pattern - it is hollow costruction and its outsinde shape is used as pattern while inside is used as core
box for making core.

Pattern materials :- the selection of pattern material is based on the following factors.

* production quantity * dimensional accuracy required

* molding process used * size and shape of the casting.

The most commonly used pattern materials are
* Wood - is easily available, low weight, easily shaped and relatively cheap but will absorb moisture. For
very large castings wood may be only practical pattern material. Common are pine wood, teak wood and
mahogany.

* Metal matters - most common are aluminum and white metal. these are light, easily workable and are
corrosion resistant.Also white metal has very small shrinkage.

* Plastics - these are cheaper than metals, high strength, lightweigth, dimensionally stable, resistance to
corrosion and easy to repair. Common are plaster of Paris, gypsum cement & wax. Wax patterns are used
in investment castings and they provide good surface finish and diensional accuracy.

Color codes for patterns : The following color - codes are used in foundry.

* Surface to be left un-machined black

* Surface to be machined red

* Core prints and seats for loose core prints yellow

* Parting surfaces on a split pattern clear

* Seats and loose piece red strips on yellow background

* Stop-off or supports black strips on yellow background.

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MECHANICAL ENGINEERING

Molding sand : Main ingredients of moulding sand are

* Silica grains - forms major portion
* Clay as binder - most popular clay used are kaolinite or fire clay (has MP 1750 to 1787C), bentonite (has
MP of 1250 to 1300C and can absorb more moisture which increase its bonding power)
* Water - develops platicity any strength. Normal 1% is 62to 8%.

Beside the above 3 cereal is used to upto 2% to increase strength, saw dust upto 2% to improve
collapsibilitybyslowlyburning and permeablilty.

Different molding sands are
- Based on its origin

* Natural sand - it contains 5 to 20% of clay, less refractory but it can maintain moisture content for long
time &

Permitting easy patching and finishing of mold. Used for non-ferrous and gray iron castings.

* Synthetic sand - contains high silica grains and no clay, they are mixed with clay and water to develop
mold
properties. Used for steel castings.

*Special sands - made by high refractory materials and with this sand high surface finish of the casting is
obtained.

Some of the materials are zirconimum (used for of brass and bronze casting), olivinate (high refracto-
riness and surface fiinish, used for steel casting), chromite (used as facing mixture in molds, it is stable
volumetrically) magnesite and chamotte.

- Based on initial condition

* Green sand - foundry sand containing moisture is called green sand and green refers to moisture content
but not color. It has silica sand, clay ( 18-30%) and water ( 6-8%). Used for small and medium size castings.
The adv. is it can be reused many times reconditioning with water and clay. But non suitable for intricate and
accurate castings.

* Dry sand - no moisture, high strength and used for making large castings.

* Loam sand - it is a green sand with clay 50%, used for making large castings.

* Facing sand - applied to surface of the pattern to obtain smooth surface finish and withstand for pressure
and heat of molten metal during pouring.

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Proerties of molding sand :
Porosity : It is the structural property of sand, the sand is to be porous to provide passage to steam and
gases. The ability of sand to allow the gas to pass through it is called permeability. It depends on size and
shape of grain, moistrue content and degree of compaction. The rate of flow of air passing through a
standard specimen under a standard pressure is termed as permeability number.

The permeabilitynumber is given by P = (VH) / (pAT)

Where, V= volume of air in cm H = height of the sand spcimen in cm

p= air pressure, g/sq.cm A = C.S. area of sand specimen sq. cm

T= time in minutes, standard test condition are 2000, 5.08, 10 (980pa), 5.08 dia respectively

Substation of std valume, P - 201.28 / pT

Strength : It can be done on universal sand strength testing machine. The strength measured can be com-
pression, shear and tension.

Green strength : It is also called green compression strength which refers to stress required to rupture the
sand specimen under compression loading. The range is 0.03 to 0.16 Mpa.

Green shear strength : The stress required to shear the spcimen along the axis is called green shear
strength The range is 0.01 to 0.05 Mpa.

Dry strength : In refers to compression strength on dry sand. Range is 0.04 to 1.8.

Mold hardness : It is the hardness of mold which is measured similar to Brinell hardness test.

Adhesiveness : Ability to molding sand to adhere to the surface of molding boxes.

Cohesiveness : Ability to molding sand particles to stick to each other. It also refers to strength. Increases
with increase in clay and decrees with increase in grain size.

Grain size : Sieve analysis test used to determine grain size, and is denoted by grain fineness number
(GFN). Normal value of GFN = 50.

Additives : Different materials are added to molding sand to improve molding properties. They are

Coal dust : Sruface finish and resistance to metal penetration

Saw dust / wool flour : Provides collapsibility and resistance to expansion defects.Also helps in maintain-
ing uniform mold density.

Starch and dextrin : Increases resistance to deformation skin hardness and expansion defects.

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MECHANICAL ENGINEERING

Fluidity : is the ability to fill the mold cavity. It depends on casting material (viscosity of melt, heat content
of the melt, surface tension, freezing range and sp. weigth of the liquid metal) and mold properties (thermal
characteristics and permeadibility).

Fluidity viscosity
Surface tension
Freezing range

Heat conent
Permeability

Fluidity (cm) = 37.846 X CF + 0.228 X T - 389.6

Where T = pouring temp in deg C

CF = composition factor = %C + 0.25 X %Si + 0.5 X%P

NOTE : Gray CI is most fluid of all ferrous alloys.

Machine molding : For producing large batches of the same type of casting, machine molding is used. they
are

Jolting : Fill the sand into the flask, rise the flask to certain height and allow to fall freely on a solid bed plate,
repeat this till required hardness of the mold has got. It gives uneven packing with height force at bottom and
low at top.

Squeezing : Squeezing plate ( which is slightlyless inside dimensions of flask) s placed on the top of the sand
filled flask and uniform pressure is applied on the plate and resulting mold compact uniformly. For applying
differential pressure for the contour of the pattern, diaphragm is used. Hardness is high at top and low at
bottom.

Jolting and Sqneezing : It is more common for uniform ramming

Sand slinging : Here sand is trown into flask rapidly with great force and it develops uniformly high mould
hardness and uniform ramming. Initial cost is high.

Cores : These are the materials used for making cavities and hallow projecitons which cannot normally be
produced by the pattern alone. Cores are normally made by CO2 molding. In CO2 molding sodium silicate
is used as a binder and the sand is treated with CO2 for 2 to 3 min so that dry compressive strength 1.4 Mpa
is reached. Here no reinforcement is required and self life of the sand mixture is less. Develop non-uniform
strength due to the difficultyin achieving uniform gassing.

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PRODUCTION

- Aslo core should be free clay, moisture and any other impurities.

- The GFN for sand grain is 55 to 85 and sodium silicate from 3 to 4.5% by weight.

- CO2 gas pressure varies from 0.14 to 0.28 Mpa depending on thickness of section to be gasses.

- Cores are two types, green sand cores and dry sand cores.

- Green sand cores are low strength and requires large amount of draft. Cannot be for deep holes.

Dry sand cores are made by special core sands in a separate core box, baked and then placed in mould
befor pouring.

CORE PRINTS : These are used to position cores in cavity. These are designed such that it can take care
of weight of core before pouring and upward metallostatic force of molten metal after pouring.

* The main force acting on the core during pouring of molten metal is buoyancy force which is given by
P = V (-d)

Where P = buoyant forec, N, V= volume of core in the mold cavity, cc,
= Density of liquid mererial, N//cc, d = density of core meterial = 1.65 X 10-2 N/cc

Volume of core = (T1 / 4)D2 H

P = 11 / 4 (Dt2 - D2) H
V = total volume of Core
Also P < = 350 A
A = core point area mm2

According to Russia methodology

P Vd / 

Where V = total volume of core including prints in cc,
= compression strength of the molding sand N/cm2 .

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MECHANICAL ENGINEERING

Chaplets are metallic support often kept inside the mold cavity to support the cores. these are of same

composition as that of the pouring metal so that it becomes part of the casting. Normally a weak joint is

formed between casting and chaplet, to avoid this chaplet is to be cleaned thoroughy. Chaplet area is

calculated based on unsupported load which is equal to

P - 350A

Where A = core print area mm2 P= buoyancy force N

If unsupported load is , no chaplet is required.
Unsupported load is > 0, for every one N, 29 mm2 area chaplet is required.

Forces on moulding flasks : There are two types of forces will acts on flasks.
\- Buoyancy force exerted by core is transmitted to cope and would tend to lift the cope from drag.

- Metallostatic force exerted by molten metal in directions but our interest here is upward force.

Fm = Ap  (h - c)

Where Fm = metallostatic force Ap = projected area
= density of molten metal h - c = head

“ GATING DESING”

Gating System : It includes the elements connected with flow of molten metal from ladle to mould cavity.

They are

* pouring basin * sprue * runner * ingate * riser

*Agood gating system requiers

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PRODUCTION

* filling time of mould cavityshould be low * smooth flow of molten
metal without any turbulence

* unwanted material like slag, dross etc should not be allowed

* aspiration effect should be minimum (air entrapment)

* no erosion takes place during flow of molten metal

* Disign of gating system should be such that enough metal reaches mould cavity.

Pouring Basin : It acts as a reservoir of molten metal from which in moves smoothly into sprue skin core in

pouring basin stops slag to enter into mold cavity. Skim core

•<

Pouring Basin

Sprue : It connects the flow between pouring basin and runner. Normally it is tapered to avoid aspiration

effect. .
. .. .
air at low ... . .. .................... . .. . .
pressure .. . .. . .. . . . . .. .
.. ... . .. ... ..
.. . .. . . .. .
..

Runner : It is on parting plane and connects sprue to its in gates. Runners are trapezoidal C.S. for ferrous

metals runners are cut in the cope and in gates in grag, because to trap the slag and dross which are lighter

and thus trapped in the upper portion of the runners.Also for effective tapping of slag, runners should flow

full.

Sprue > Cope Ingate

Runner > Cavity
drag

Ingates : These are openings through which molten metal enters the mold cavity. The shape and CS of the
ingate should be such that it can readily be broken off efter casting solidification. These are top, bottom and

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MECHANICAL ENGINEERING

middle (parting) gates.

Top Gating :As the metal falls directly into mould cavitythrough a height it is likely to cause mold erosion.
It is good for ferrous alloys and not suggested for non-ferrous alloys. Suitable for simple casting shapes
which are essentially shallow in nature and to reduce mould pencil gates are provided.

Bottom Gate : No mold erosion, and used for very deep molds, but takes more time for filling. These cause
unfavourable temp gradients compared to top gating, so additional padding is required. Bottom gate in
conjunction with side gating gives improved performance.

Parting Gate : More popularly. used one. It has derived the advantages of top and bottom gating. It is on
the parting line.

Step Gate : Used for heavy and large castings. Molten metal to cavity enters through a no. of ingate
arranged in vertical steps. Size is progressively increases form top to bottom.
Chills : Used to provide progressive solidification or to avoid the shrinkage cavities. Chills are head sinks. It
provides directional solidification.

Fig.

Shrinkage cavity Metal chill

Casting yield = Mass of actual casting W
---------------------------- = ---------- X 100
Mass of metal poured into mold w

Casting yield depend of casting material and complexity of shape, Highly shrinking material have low yield.
Also massive and simple shapes have high yield, but small and complex parts have low yield. Ex.Al-025 to
045, simple shapes and massive - 0.85 to 0.95.

NOTE : Gray cast iron has negative shrikage that is during solidification graphatisation takes place and due
to that volume of metal increases.

Bearing bronze - 7.3 % Al - 6.6% Gray CI - 1.9 (-ve) % MS - 2.5 %

Design of Gating System : The flow through different channls in the mold obeys Bernolli’s equation

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PRODUCTION

There fore velocity, V = 2gh At various places

Q = A1 V1 = A2V2
- The time required for filling cavity with molten metal in called pouring time. It should be as low as

possible.

- Tool long a pouring time requires higher pouring temp and too less means turbulent flow, which make

casting defect prone.

- Pouring time is chosen based on experimentation and experience only. For example gray CI needs

less pouring time because of high K and so it loses heat very fast.

- Non-ferrous metals need longer pouring time because it loses heat slowly and tends to from dross if

metal is poured quikly. pouring time = K (1.41 + T/14.59) M secs,
Gray Ci with mass less than 450 Kg

K= fluidity of iron in inches/40, T =average section thickness, mm, M-mass of casting, Kg

- Gray CI, M> 450 Kg pouring time = K

(1.236 + T/16.65) (M)1/3 secs,
- Steels - pouring time = P.T. K1 M sec, Kl = 2 to 2 (thinner to thicker sections)

- Copper alloys - P.T. = K2 (M)1/3 sec, K2 = 1.3 to 1.8

Choke Area : It is main control area which meters the metal flow into the mold cavity so that the mold is
completely filled within the calculated pouring time . in general the choke area happens to be the bottom of
the sprue.

ing, kg M Where M = mass of cast-

Choke area (A) = ------------------- c = efficiency factor
tc 2gH t = pouring time, sec

d = mass density of molten metal, kg/cu.mm

H = effective metal head, mm = h for gating = h - s/2

For bottom gating = h - a/2s for parting gate

h = height of sprue a = height of cavity in cope s = height of mold cavity

In sprue flow at top and bottom are AtVt = AbVb
At = Ab Vb / Vt = Ab ht/ht -----------------------eq.1

hb = ht + H ht = height of molten metal above top of sprue H = height of sprue

Fig 1 fig2

ht

>

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MECHANICAL ENGINEERING

By seeing eq.1 the shape of sprue should be parabolic but it is difficult to make parabolic sprue so we prefer
tapered straight sprue.
- To avoid vertex - shedding flow, it is necessary to run the pouring basin full. For achieving this provi-
sion is made by using delay screeen or straight core.
- The delay screen is thin tin sheet which get melts by taking heat from molten metal.

Gating Ratio is the proportion of the CS areas between sprue, runner and ingate.
Gating ratio = sprue area : runner area : ingate area.
Gating systems are two types.

- Non-pressurized : here sprue bottom will have minimum area, reduces turbulence, used for casting
drossy alloys like Al and Mg alloys. The air aspiration may occur due to partial flow and to reduce this
tapered sprues must be used.Also for maintaining runner full, runners are kept in drag and gates are kept in
cope. Also casting yield reduces due to large metal lying in runners and gates. Best is a 1 : 4 : 4

- Pressurized, here ingated have minimum area so that back pressure will be maintained through the
gating system. Due to this back pressure the metal flows more turbulent and flows full and so air aspiration
cab be minized even when a straight sprue is used (after initial stages of pouring ).
Here yield is more. Due to turbulence and dross formation it is not suitable for light alloys best is 1:2:1

NOTE - With respect to heat loss factor circular runners are preferred
- To reduce turbulence trapezoidal CS runners are preferred. the ratio of width to depth is 2
- For trapping slag runners are kept in cope and ingates are in drag
- For casting of light alloys runners in drag and ingates in cope because dross is heavier thanAl.

Ingate design :

Free height of gate = h= 1.6 (Q2/gb2)1/3 + V2/2g mm

Q = metal flow rate cu. mm/sec

b = width of gate mm V = velocity in runner, mm/sec g = acceleration due to

gravity

gate height = h1 = h - 5 mm gates higher than this will not fill completety and those lower than this will

increase velocities of stream entering to it.

- In ingates ratio of width to depth is 4.

- Ingates should preferably be placed along the logitudinal axis of mould wall

- Ingates should not be placed near a core print or chill.

- CS area of ingate should be smaller than smallest thickness of the casting.

- In multiple ingate system to make flow uniform, the ingate area should be reduced progressively from

first to last.

- In general the first metal moves into gating system contains slag and dross and to avoid this runner is

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PRODUCTION

extended beyond the ingate.
- Whirl gate is used tosuccessfully trap slag from entering into mold cavity. Thus utilizes centrifugal
action to throw the dense metal to the periphery and retain slag at center.Also ratio of inlet to exit area is 1.5
to build up metal at center quickly. Also metal has to revolve 270deg before reaching exit gate to gain
enough time for separation.
Fig

..........................................................................................< ..........................................................................................
ht< ...................................................................
.................
....................-..-.-.....-..-..-...-..-............ ht

Top Gating : ...........................

hm

V = 2ght

Bottom average
t = Am/Ag/2/g (ht -ht-hm)

or

make average

Am= CS area of mold Ag = CS area of gate hm=height of mold
ht= total
height

Solidification Time : TS (V/As)2
Solidification time TS = K (V.As)2

Where V=volume of casting As = surface area of cast-
ing K = mould constant

Aspiration Effect : If the mold is made by permeable material (sand), we should see that pressure in the
liquid metal stream the pressure does not fall below atmosperic pressure. Other wise gases may enter from
sand to moltan metal stream producing porous castings called aspiration effect.Applying Bernoulli’s equa-
tion

So we make straight sprue with constant cross section, aspiration effect will be ther. To avoid this sprue is
made always tapered.
For determining size of sprue, let P2 =0 abd P3=0

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MECHANICAL ENGINEERING

Pouring Basin: Reducing eroding force

Strainer : Made by ceramic, reduce dross

Splash Core : Made by ceramic, reduces eroding force

Skim Bob : It is a trap, to prevent lighter and heavier impurities

Effect of Frition and Velocity Distribution in all the above we assumed that there is no fricton and

velocity is uniform, But in actual practice

i) Velocity of molten metal just after pipe wall is zero and max at the center. But we are taking average

velocity.

ii) There is always frictional exists in sprue because of roughness and so loss of head due to friction will

comes into

picture. Ef1= 4 f1 v2 / 2gd
iii) Loss of energy due to 90 deg dending also exists

Ef2 = ef v2 /2
ef = friction loss factor
f = 16/Rs for laminar flow (Re less than 2000)

f = 0.0791 (Re) -1/4 for turbulent flow (Re greater than 2000)

d, 1 are dia and length of sprue

v = average velocity

after combiningall

Where v = Cd 2 ght  = constant = 1 for turbulent = 0.5 for laminar
Cd = (1/ + e + 4fl / d)-1/2

f

“RISER DESIGNING & DEFECTS ”

Risering Design : There are three types of shrinkage occur during costing of any metal.
1. Liquid shrinkage during cooling of molten metal from pouring temp to freezing temp.
2. Liquid shrinkage during freezing of molten metal.
3. Solid shrinkage during cooling of solidified metal from freezing temp to room temp.

In the above the first two will be taken care by providing riser and third one will be taken care by
providing shrinkage allowance on pattern.

All metal swill have positive shrinkage where as gray CI sometimes mayhave negative shrinkage,
this happens because with higher carbon and silicon contents, graphatisation occrs which inreases the
volume and so risering may not be critical. But forAl and steel more volume contraction takes place and
so elaborate risering is required.

By seeing the sequrncing of the cooling in a cube a void will be formed at the center called hot
spot. To avoid this we have to design the system such that the solidification starts at farthest point so that

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liquid metal can be fed to casting during solidification.

- The phenomena of starting of solidification at remotest place from feeder is called directional
solidification.

- Disign of riser is such that the time taken fro solidification of liquid metal in the riser should be

more than the time taken for solidification of liquid metal in the cavity.
(Ts) riser (Ts) casting

- Riser can be designed by using four methods. d) shrinkage volume
a) canines method b) modulus method c) navel research method
consideration method.

Caines Method :
(V/As) casting

Freezing ratio (X) = --------------------
(V/As) riser

also
a

X = ------------ - c
Y-b

Where Y = (V) riser / (V)casting a, b, c are constants whose value depends on type of metal

The line shows the locus of points Y
that separate the sound castings
and casting with shrinkages >

Modulus Method : This was proposed by wlodawer and said if the modulus of the riser exceeds the
modulus of the casting by a factor of 1.2, the feeding during solidification is satisfactory.As we know
modulus = Volume / surface area = D/6, D = 7.2 Mc for side risering
D = 7.2 Mc for top risering
Where D = dia of riser and Mc = modulus of riser
Even though it is considering cooling effect of riser but it does not exactly considering the amount of
feeding molten metal required to compernsate shrinkage of cating. To consider this the quotation is

D3 - 5.46 Mc D2 - 0.05093 VC = 0 --------- (valid for h = D type risering only)
Where ‘VC’ is the volume of casting
In the above equation if the shrinkage is neglected.

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MECHANICAL ENGINEERING

D3 - 5.46 Mc D2 = 0 D3 = 5.46 Mc D2 D = 5.46 Mc = 6 Mc

Chunky Casting : Ex :Cubes, volume component is negligible.

Rangy Casting : Ex : Plate, volume component has more influence.

NOTE : The ring will be treated as solid body if the ratio of outer to core dia is greater than 3.75
Novel Research mathod : It is a form of simplification of Caines method and defines shape factor in
place of freezing ratio

Length + width

Shape factor = ---------------------

Thickness

Using this shape factor, from graph the volume of “Y” can be obtained and

Volume of riser

Y = ----------------------

Volume of casting

For top risering D = 2H Side risering D =H

NOTE: For circular plates length = width = diameter
For cylinders witdth = thickness = diameter
In general risering should be located at heaviest sections as they themselves act as feeders for thin sec-
tions.

Shrinkage volume Consideration Method : If volume shrinkage on % of casting is given then we have

to calculate shrinkage volume and then

Volume of riser = 3 x shrinkage volume

And then check again so that the
(Ts) riser  (Ts) casting

If the above equation is not satisfied then

Assume that (Ts) riser = (Ts) casting , and determine the size of riser

From the above it is seen that it should satisfy two conditions
1. volume riser should  be 3 X shrinkage volme of
2. Ts riser  Ts casting

Optimum relation between dimensions of riser

For side riser :

D=H ( because of two side surface )

For top risering :

So, D = 2H or R = H ............................. Here one side (bottom side) no cooling effect and it is

and it is integral to casting

Skin Thickness :
t = K T + C

Where t = thickness of molten metal solidified T = time t = K1
K and c are constants depends on casting meterial

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Defects in Casting : The defects in casting may arise due to the defects in one more of the following.

* Design of casting and pattern * Moulding sand and design of mold and core

* Metal composition * Melting and pouring * Gating and risering

Blow : It is a wall-rounded cavity produced by the gasses and occurs on a convex casting surface and
can be avoided by proper venting and an adequate permeability.

Scar : It is shallow blow found on a flat casing surface.

Blister : Scar covered by a thin layer of metal

Porosity : Very small holes uniformly dispersed throughout a casting and is due to decrease in gas solubil-
ity during solidification.

Inclusion : It is the non-metallic particle in the metal matrix.

Dross : Lighter impurities present on the top surface of a casting. It can be avoided by keeping strainer
and skim bob in the pouring stage.

Misrun : Freezing of molten metal before reaching farthest point of the mould cavity is called misrun and
due to insufficient superheat.

Hot Tear : Crack developed in the casting due to residual stresses.
Shrinkgae cavity : Due to improper riser.
Shift : It is due to misalignment between two halves of a core.

Miscellaneous Casting Processes .

Dry Sand Mold Casting :
* It uses expandable mold ( mold is used only once )
* It uses green sand mold baked at 100-250C and sand mix contains 1-2% of pitch.
* Blows and porosity are less in dry sand mould casting.
* Hot strength of mold can be increased by oxidation and polymerization of pitch.

Shell Mold Casting :

* It is a semi precise method for producing small casting repetitively in large numbers.

* Mold material contains Phenolic resin with fine dry silica.

* No water is used in mixing of sand.

* Dwell time in making mold is 20-30 seconds.

* Thickness of shell formed in making mold is 6mm and it can be varied with dwell time.

* More accurate dimensions will be obtained. * Mechanization is possible.

* Permeability of sand is high and therefore on gas inclusions occur.

* Is economical only when more number castings were made say above 15000.

* The size of casting limited to 200 kg only.

* Highly complicated shapes cannot be made.

*Applicatons are cylinders and cylinder heads for IC engines, automobile transmission parts, cast tooth

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MECHANICAL ENGINEERING

bevel gears, gear blanks, refrigarator value plates, small crank shafts etc.
Investment Casting :
* Suitable for wide range of shapes and contours in small size parts which are made by hard to machine
materials.
* No parting line and gives excellent surface finish.
* Suitable for cating turbine and jet engine parts made of high temp and high strength alloys.
* Wax is used as pattern material and determining the dimensions of pattern is the only tedious taks and
requires considerable experimentation.
*An expandable wax/plastic pattern is given a precut with silica flour, water and some bonding agent.
after removing pattern from die, it is rinsed in alcohol to remove grease and dirt, then precut is given and
is now the pattern is placed inthe steel can which is filled with self hardening refractory concrete, then can
is kept in a oven for 24 hours so that most of the wax or plastic melts and flows out lf mold leaving a
*The process is limited to small castings say less than 5kg.
*Applications are jewellery, surgical instruments, vanes and blades for gas turbine, shuttle eyes for
weaving, wave guides for radars, stainless steel valve bodies etc.

Gravity Die Casting :
*Apermanent non-expandable mold is used and liquid metal is poured under the force of gravity.
* It is used for casting cast iron and the mold is made of heat resistant cast iron, fins are provided on the
outer surface for efficient air-cooling. Inner surface of the mold is sprayed with oil-silica mixture before
each pouring.

Die Casting :
* The process is used for casting low melting temp material likeAl and zinc alloys.
* The most common die material is medium carbon, low alloy tool steel.
* The die is cooled by water for an efficient cooling and to increase life of the die.
* Depending upon the melting furnace which is integral part of the or not we can divide it as hot chamber
or cold chamber die-casting.
* The liquid metal is forced into die with high external pressure, so much thinner sections can be cast by
this process.
* The process when applied to plastic then it is called injecton molding.

Centrifugal Casting :
* It uses permanent mold which is rotated during the solidification of a costing.
* The speed of rotation is maintained high so as to produce centripetal acceleration of the order of 60g to
75g.
* The centrifuging action segregates near the center of rotion the less dense nonmetallic inclusions
* No core is required for producing hallow section in this process.
* Solid parts can be cast by placing entire mold cavity on one side of the axis of rotation
* Castings produced by this process are very dense and more than one casting also can be made simulta-
neously.

Slush Casting :
* In this liquid material is poured into an permanent mold and inverting the mold after a thin layer of the
liquid has solidified on its surface. The process results in shell like castings which are widely used for
ornamental objects like lamp shades and toys.

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* In general low temp materials such as tin, lead and zine are used in this process.
Co2 Process :
* It is sand molding but does not contain oil, resin or clay as bonding agent which eliminates dryers and
heading cycle.
* But the sand mix consists of 2-6% of sodium silicate solution and it has very high flow ability to fill
corners and intricate shapes.
* The sand is hardened by passing CO2 for one minute.
* The CO2 forms a week acid that hydrolyses the sodium silicate solution to from amorphous silica which
acts as a bond.

***********************

“You may have to fight a battle more than once to win it”

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MECHANICAL ENGINEERING

3 WELDING

“INTRODUCTION AND GAS WELDING”

Welding is a process of joining two or more pieces of metal together such that a metallugical bonding
achieved between the parts joined. This unity is obtained by heat and or pressure with or without filler metal

Welding 




Fusion Non-Fusion (Pressure)
 
 
Heat created by Hot Cold

   
Arc Gas Chemical (Thermit) Black smith Resistance

   

Spot Projection Seem Butt

   
Carbon Metal TIG Submersed Atomic
Arc Arc MIG
H2

  
 oxy oxy
oxy

Acetylene

Gas Welding :

Oxygen : R.H. Threads, Block, 120 kg/cm2, 2pressure gauges.

C2H2 : L.H. Theads, maroon, 15kg/cm2, 2pressure gauges.

C2H2 + W2 > CO + H2 + 1 heat
3

2CO + O2 > 2CO2 + 1 heat
3

H2 + 1 O2 > H2O + 1 heat
3 3

Types of Flames :
1. Neutural flame : O2 / C2H2 = 1.04 to 1.14 produces hissing noise highest flame temperature 3260oC,
Applicationa : Welding and cutting fo ferrous metals and non-ferrous metals.
2. Oxidizing flame : O2 / C2H2 = 1.15 to 1.17, 3380oC, Loud roarApplication:- Welding of Zinc

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PRODUCTION

Alloys, brasses, ferrous metals like Monel steel.

3. Carburizing (Reducing) Flame : O2 / C2H2 0.85 to 0.95, 30540o, H.C steeling
Applications : for welding of high carbon steels some non-ferrous metals and for hard surfacing.

The carburizing flame is also called reducing flame with this carburizing flame free carbon is deposited

in a very fine form. Use of this free carbon in welding has been taken advantage in welding steels because of

thin layer of carburizing iron is formed which will melt at temperature lower than the iron itself. The presence

of this mixture is conducive to fusion and shortens the period of heating so that the rate of welding is faster

than neutral flame.

Torch angle : This is the angle between joint and torch tip taken in the longitudinal plane, it increases with

thickness of metal, M.P., and thermal conductivity.

Ex : For a particular thickness

For Cu - = 80oC

Pb - < 10oC

With refernce to thickenss for L.C steels “” is as follows

Upto 1 mm __________ 10o
20o
1 to 3mm __________ 30o
40o
3 to 5mm __________ 50o
60o
5 to 7mm __________ 70o

7 to 10mm __________

10 to 12mm __________

12 to 15mm __________

Also the torch angle can veryduring the process of weling, in the beginning more heat is received to rise the
temperature of work piece for obtaining pool of molten metal quickly is greatest i.e., 80o to 90o. But at the
end of joint in order to avoid burning through and to fill up the crater  is to be reduced to minimum say 10o

so that flame almost touches the joint, in between the torch angle is maintained according to 3 factors which

are mentioned earlier.

80o to 90o  10o

Beginning of the joint During the course of welding At the end of joint

Leftward (Forhand) Technique :

filler rod ------- Dir<ctiowneoldf ing 80o to 90o

30 to 40o < <
<
<

1 to 1.5 mm

Advantages : Pre heating of edges takes place
Disadvantages : No addition heat to weld, no protection from atmospheric oxidation and used for thin

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MECHANICAL ENGINEERING

sections only less than 4.5 mm
Rightward (Backward) Technique :

Advantages : Additional heat to weld, protection from atmospheric oxidation, more speed of welding
Disadvantages : no preheating.
Note : In leftward above 4 - 5 and upto 6mm thickness the weld moment is

Note : In Rightward welding above 6mm thick the weld moment is

Welding positions Vs technique
1. Flat or down hand position : - Two plates are in Horizontal plane and joint is horizontal and metal

deposited down.
2. Vertical : - Two plates are vertical, joint is vertical and weld is done from botton to top.
3. Horizontal - Vertical : - Two plates are vertical plane, joint is horizontal, both leftward and rightward
techniques are used
4. Over head : - The two plates are overhead and welding is done from beneath
The formation of gas weld depends on
1. Pressure and direction of weld
2. The motion of filler rod
3. The weight of drop of molten metal
4. The surface tension of molten metal
Preparation of work piece for welding
1. Edge preparation
2. Cleaning of edges (free from scale, rust, oil grease etc) done by flame blasting Tack welding - used to
keep the plates in correct alignment,
Fluxes for Gas Welding
a) Function : -

• To deoxidize the melt
• To form a blanket over the molten weld pool these by preventing metal from fur char oxidation

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PRODUCTION

• To release entrapped gases, slag and non-metallic inclusions.
b) Characteristics : -

• Should readily melt i.e. M.P. less than parent and filler metal
• Should readily react with oxides
• Its specific gravity should be less than that of metal
• Should readily apread over the molten metal of form as a blanket for protecting metal from atmo-
spheric oxidation and come off easilyupon solidification
• Should not have injurious effect on the metal
c) Fluxes for different metals : -
• For nickel and carbon steel there is no need for a flux, if any oxides are formed the CO and H2 in the
intermediate envelop of the flame can easily reduce them
• For Cu and copper base alloy-boric acid, borax, sodium phosphate, potassium carbonate, and
NaCI
• For C I - Borax, Naco3, potassium carbonate, sodium nitrate, sodium bicarbonate etc.
d) Filler rod : - The composition of the filler rod should readily match with base metal. Therefore the
chemical composition of the filler rod is so selected as to compensate for the loses during welding. In
addition molten metal should flow smoothly and truly to readily fuse with base metal producing sound and
clean welds.
As per IS 122 size of filler rods are 1.6,2.5, 3.15,5,6.13,8,10mm
The composition of filler rods for gas welding of L.C steels is

C- 0.25 to 0.3% De oxidizers
Mn -
Si - }1.2 to 1.5%

0.3 to 0.5%

Note - Some times, filler rods are not recovered upto certain thickness of sheets (upto 3mm).

When the filler rod is not used the gas welding is called pudding.

Gas Welding Torches :

1. Low pressure torch or injector type torch

2. High pressure torch

The mixing chamber can be located either in the torch body or in the detachable nozzle head, later is

commonly used one.

Nozzles or Torch Tips:

1. Solid or single piece type 2. two piece nozzle

The selection of type of torch tip is determined by whether or not the operator prefers the single piece

or two piece welding tips. The advantage of single piece tip is that it is lower in intial cost then two piece tip,

however if the tip is exposed to more than the normal wear the price of replacing the tip will offset the

cheaper original price because the entire tip must be replaced. But with the two piece tip it is required to

replace only a short part at the end. The overall functioning of both the tubes are identical. The number

stamped on the tip indicates the working gas pressure needed for efficient welding. If the torch is injector

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MECHANICAL ENGINEERING

type the number indicates the oxygen pressure and the torch is high pressure type the number indicates

working pressure of O2 and C2H2 in kg/cm2. Usually H.P torches are provided with piece tips and L.P
torches are provided with single piece tips.

Acetylene : -

a) Properties :

• Inflammable

• Comparativelysmell

• Possesses distintive smell

• Gives highest flame temperature around 3200oC

• It is unstable and liable to decompose at elevated temperature or at pressure greater than A atmo-

sphere.

• When mixed with air it forms on explosive mixture in a proportion greater than 2% C2 H2
• When in contact with copper and copper alloy having 70% and above Cu it forms a violently explo-

sive compound copper acetylene.

b) Production of acetylene :
Cao + 3C  CaC2 + CO


Quick lime Calcium Carbide
CaC2 + 2H2 O  C2 H2 + (Ca(OH)2
Acetylene can be produced in two ways.

1. Water to carbide - used in production of L.P acetylene and where small gas yields

and portability are required

2. Carbide to water type - used in huge plants producing C2 H2 on large scale at L.P
Oxygen : It is colourless, odourless and active gas. The oxygen production consists of

(i) Removing mechanical impurities, moisture, CO2 from air
(ii) Washed air is compressed to 6 to 200 at a depending on the refrigeration plant used

(iii) Reduce the temperature of compressed air in inter cooler

(iv) Reducing the pressure of air byexpanding through a nozzle known as throttling, to create cool liquid

air.

(v) Rectifying the liquid air and separating N2 from O2 by proper heating up as follows.

Compound B.P

N2 - -198.8oC
Argon - -185.7oC

O2 - - 182.96oC

So from N2 and argon will get boiled before O2 and will be removed so that only liquid O2 will
present. This will be gasified in the factory or can be transported as liquid O2 to the user industry where it is
gasified.

Advantage of liquid oxygen :

• Weight of containing for liquid O2 is several times smaller than gas O2 container

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PRODUCTION

• A smaller no. of O2 cylinders as to be kept circulation
• It is safe because pressure of liquid O2 in the container is not greater than data.
• When it is produced at the siter where it is to be used gaseous O2 contains less moisture.
Disadvantages: Loses are inevitable due to evaporation during storage, transportation, and gasification say
15%
Oxy Hydrogen Welding :
The Oxy-hydrogen weling is a gas welding process in which the required heat is supplied by a gas
flame obtained from the combustion of H2 with O2 . filler metal may or may not be employed to effect the
weld as in the case of oxyacetylene welding. The equation of combustion is
2H2 + O2  2H2 O + heat

• Caracteristic of the oxy hydrogen flame is that no carbon is present. This is an advantage where

carbon pick up must be avoided.

• Oxygen flame can not be distinguished with a naked eye in the forms of neutral flame, oxidizing

flame, and reducing flame as in case of oxy acetylene flame.

• Generally used ratio of H2 to O2 is 2.5 to 6 by volume.
• Increased proportion of H2 results in a lowering of flame temperature, but at the same time it
assumes a reducing atmosphere in the flame

•Areducing atmosphere with oxy hydrogen flame does not cause any it effects because it leaves no

carbon deposits.

• The temperature of oxy hydrogen flame is 2880oC.

• Used mainly for low M.P. metals such as AI and its alloys, Mg, Lead, Bronze welding alloys etc.

• Due to its lower temperature of flame distortion and residual stresses will therefore be less.

Air acetylene welding :

• The temperature of flame is much lower than any other flame obtained with any fuel gas plus O .
2

• The torch used for air acetylene welding operates no Bunsen burner principle that is acetylene

flowing under pressure through a Bunsen jet aspirates the appropriate amount of air to provide combustion.

• The C2H2 is usually supplied from a small cylinder with a pressure reducing value.
• Because of the lower heat available the air acetylene flame is suitable for welding light sections of

lead, brazing operations (refrigerators andA/C) and soldering.

• Quality of weld is satisfactory.

Pressure gas welding : These are two types

a) Closed joint method

For LC steels - Initial pressure < 100 kg/cm2
Final pressure  250 kg/cm2

For HC steels and many alloys - constant pressure of about

b) Open joint method

After the faces get heated, burner is removed and pressure of 250 kg/cm2 is applied.

Oxy acetylene cutting :

Width of cut is 0.8mm for t < 10mm 6mm for t=300mm

Fe + 0.5 O2  FeO + 64.3 kcal/gram mole

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MECHANICAL ENGINEERING

3Fe + 2O2  Fe3O4 + 266.9 kcal/gram mole
2Fe + 1.5 O2  Fe2O4 + 198.5 kcal/gram mole

All the metals oxidized when exposed to oxygen in the atmosphere. In this metallic oxidation par-

ticles of base metal and oxygen combined into some times give protective coating for the metal. In most of

the ferrous metals, the oxide is a very large porous coating. This looseness allows more of the base metal to

be exposed to the atmosphere, more and oxide is formed, the process continuing in the ferrous material until

eventually the whole structure turns into ferrous oxide.An increase in the amount of pure O2 directed against
the metal increases the rapidity of the reaction. Similarly higher temperature also increases the reaction,

which is provided by oxy acetylene fuel gas.

Factors affecting flame cut ability of metals :
1. The M.P should lie above its kindling temperature with oxygen (kindling temperature is the point at
which the metal is readily oxidized by the oxygen jet striking its surface. For LC steels it is about 1350oC
where as M.P is around 1500oC).

As carbon content increases the M.P of steel goes down and its kindling temperature with oxygen
increases thereby reducing its flame cut ability.

2. Oxides of metal should melts at temperature lower than the metal itself and below the temperature

developed by cutting otherwise surface oxides will prevent any combustion and separation of metal for

example chrome nickel steels - oxide is Cr2 O3 melts at 2000oC, Al2 O3 melts at 2050oC. So both chrome
nickel steels andAl cannot flame cut.

3. The heat produced by combustion of metal with jet O2 must be sufficient to make the cutting pro-
cess self sustained. However it is difficult to satisfy this condition because some of the heat produced is lost

to atmosphere, therefore preheating flame should continue to supply heat. The usual the temperature is got

from the exothermic reactions and remaining 30% of the heat is got from the preheating flame.

4. The metal to be flame cut must not have excessive thermal conductivity, for example in case of Cu

andAl and its alloys it is very difficult to bring the metal to the kindling temperature.

5. The oxides formed in cutting should flow readilyfor example in case of Cl, Sio formed is not highly
2

refractive but also highly so CI can not be flame cut.

Oxygen LANCE Cutting :
• The lance cut is started by raising the tip of the pip (lance) and the point of cutting to kindly

temperature by

a) Oxyacetylene torch (or)
b) Carbon arc (or)

c) Resistance heating

•Allow oxygen at 1 or 2 atmosphere now the tip of lance and job get ignited (get oxidized) heat is

liberated.

• The increase the pressure of oxygen to 5 to 6 atm. and advance the lance into the block.

The cutting action is sustained by the heat liberated, the slag found flows out through the clearance

between the holes and the lance. The lance should be moved to and fro or else the hole may be filled with

slag. As the lance advances it should given half turn in each direction periodically or else the hole will be
distorted in its out line.

The lance is made of LC steel, the cross section of the lance depends on the hole to be prepared.

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For example thick walled lance

}Do = 17 to 19 mm Along with a rod of 5 mm dia of LC steel

di = 6 to 8 mm

The result of both the lance and the rod supply heat by oxidation.Also the rod reduces the area of

effective opening giving greater velocity to oxygen.

•Applications are

a) Removal of risers and spruces from huge steel castings

b) Sintering holes in large shafts

c) Opening of top holes in blast furnace.

d) Metals upto 1.5 to 2 m can be drilled through cut by this process.

• The main disadvantage is for one cm of hole burnt requires 2.5 to 5 cm of thick walled lance or 12

cm of a thin walled lance.

• To over come the heavy consumption of lance, non consumable lance is being used of late along

with the non-consumable lance metallic powder is utilized. This powder gives a lot of heat for the purpose of

lance cutting. This process specially advantages in burning holes in stainless steels which are verydifficult to

cut.

Atomic hydrogen welding:

It is a welding process where in the coalescence is produced by heating a job with an electric arc is

maintained between two tungsten electrodes in an stmosphere of H2 which also acts as a shielding gas filler
rod may be used wherever necessary.

T (mm) Electrode dia (mm) Current (amp) H2 flow rate ltrs/min

0.8 1.5 18 19
2.4 2.5 33 19
4.8 4 43 21
9.5 5 60 26

• H2 gas deforms the arc in the shape of a fan
• The heat of the will dissociate the molecular H2 , it is an endothermic reaction
• The atomic H2 after coming out of the arc fan being unstable will reunite forming molecular H2 ,
given out a lot of heat (exothermic reaction)

• The molecular H2 also acts as a shielding gas and reducing agent. Thus there is no need for any

additional flux for atomic H2 welding.
• In this process one eletrode is stationary, the moment of other electrode is controlled by trigger

machanism which adjusts the distance between the electrodes (arc gap). In general flow of H2 is controlled
automatically.

• Parameters are current - 15 to 150 amp, arc voltage 50 to 75V, electrode dia - 1 to 5mm.

• Applications - It was extensively used in earlier days and finde limited use now. Some of the

applications are, welding of tool steels containing tungsten, nickel and Molybdenum, for hard surfacing and

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rapair of moulds, dies tools etc and manufacture of ally steel chains.
• Advantages
a) Uniform, strong and ductile welds are formed
b) No flux or separate shielding gas is required
c) Job does not form part of electrical circuit
d) Even thin sheets can be welded
e) The process can be used for welding of metals of specific composition for which sepa-

rate coated electrodes are not availble
• Disadvantages
a) Speed is low
b) Un economical for certain applications due to high cast
c) Can not be used for depositions large quantities of metal

“ARC WELDING”

AnArc is generated between two conductors of electricity, cathode and anode, when they are touched
to establish the flow of current and then separated by small distance.

Because of very high velocity of electrons the KE possessed by electrons is very high. Upon impinge-
ment on the surface of anode this KE is converted into heat energy.

The heat distribution between cathode and anode will be roughly about 33% and 67% respectively and
a temperature of about 6000oC is generated at anode.
DCSP and DCRP :

• DCSP (DCEN) gives greater depth of penetration, more heat at job, used for welding of thicker
plates

• DCRP (DCEP) gives less heat at job, less depth of penetration and used for welding thin gauge sheets
also used for welding ofAl and Mg, irrespective of the thickness because of oxide cleaning action possessed
by DCRP
Arc length and power calculation in welding :

Let V0 = open circuif voltage
Is = short circuit current
L = arc length

By approximating both the characteristics as straight lines
V = a + bl - arc length characteristics
Where a, b are constants characteristic
V = V0 - V0 I/Is - power source characteristic
Where I current flowing in circuit
At equilibrium condition V in arc is equal to V in power source
By equating (1) and (2) we will get I in terms of “I”

Then power = P = VI
= (a+b1) (I interms of 1)

for max power output

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d(P)/dl = 0  1 = ----------------------- optimum arc length.
By substituting “l” in power equation we will get max power
Duty cycle : It is the % of time during which the m/c can be operated without overheating the vital connec-
tions in the m/c

Duty cycle % = Arc on time X 100

Total time

Total time =Arc on time + Rest time

For a welding transformer

Id2 Dd= Ir2 Dr

Where Id = Desired output current in Amp Ir Rated output current in Amp

Dd = Desired duty cycle % Dr = Rated duty cycle %

Note : - If arc length decreases current increases and vice versa. Arc length means gap between tip of the

electrode and work piece.

Electrodes : Electrodes used inArc welding are two types, consumable and non-consumable.
When the arc is obtained with a consumable electrode, weld metal under the arc melts and also tip of

the eletrode. The molten metal from eletrode and that obtained from the base metal gets intimately mixed
under the arc and provides necessary joint after solidification. So in this process electrode get consumed
continuously and so electrode is to be moved towards work to maintain constant arc length. So the elec-
trode acts as a filler rod and heat source. Consumable electrodes are made of various meterials depending
on the purpose and chemical composition of metals to be welded. Thus they maybe made of steel, cast iron,
copper, brass, bronze or Al.

It is also possible to use non-consumable electrodes made of carbon, graphite or tungsten. Carbon and
graphite are used only in DC welding, where as tungsten electrodes are used both in AC and DC weling.
The filler metal required has to be deposited through a separate filler rod. So it is possible to control heat
input and filler metal because of separate control.

When the consumable electrodes are used the weling is called metal arc welding, where as non-consumble
electrodes are used they are called by meterial or tungsten arc weling.

The consumable electrode used in welding can be either bare or coated. The problems of associated
with bare wire electrodes are.

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• Oxides are formed, which cause loss of fusion and hence strength. Oxides also reduce ductility
• Nitrodes pickup which result in formation of nitrides causing embitterment of the weld
• Water vapour pick-up. This complicates metallurgical reaction and also leads to porositic.
• Thus the use of bare wire leads to lack of strength, lack of ductility and lack of soundness
• Low burn off rates of electrodes, low penetrating power of arc and the arc is unstable that is difficult
to start and difficult to control.
•Applications are galvanized surfaces, surfacing, tack welding etc where less quality is tolerable and
where cast should be less.
Electrode Coatings :
The functions of coating are
• Formation of a protective gas shield
• Formation of a protective slag
• De oxidation of the melt
• Stabilization of arc
• Provide alloying elements to weld
• Reduce spatter of weld metal
• Increase the deposition efficiency
• Influence the depth of penetration
• Slow down the coating rate of weld
• Coatings also contain materias, which can control the slag to be viscous or fluid. Viscous slag will be
useful for making welds in vertical position
• The only problem with coated electrodes is the moisture pick up by the coating. This moisture when
entered into molten puddle, dissociates into O2 and H2 with H2 being absorbed by liquid metal and subse-
quently released during solidification, causing porosity. So some additive are added to reduce the moisture
pick up, but they do not ensure complete safety from moisture pick up. So normally electrodes are kept in
oven and taken out whenever required and they should not be kept outside more than 4 hours.
Intredients of coating
1. For shieldingpurposes - shielding gas is most commonlyprovided byvaporization of orgnic compoundes
in the electrode coating. Cellulose is the most important ingredient provides H2 , CO and CO2 when vapor-
ized in the arc. In some situations H2 causes cracking of welds and difficulties in welding high Sulphur steels
and hence should be eliminated. Lime stone (CaCo3) which decomposes to CO and CO2 gases and a
calcium oxide slag is then substituted for the cellulose materials.
2. Slag forming oxides - Tio2 , Fo, lime stone, Sio2 Mno2 are common materials are added to coating to
form a slag. Under the heat of arc they liquefy and coat the weld metal with non-metallic slag cover, which
is later removed from the solid metal.
3. De-oxidized constituents - Ferro silicon and ferromanganese are used for steel welding
4. Arc stabilizing constituents - easily ionized constituents such as potassium compounds
5. Alloying constituents - To improve properties of weld ex:- for steels either in the pure powder form or
ferrous alloys the following alloying elements may be added like V, Co, Mo,Al, Zn, Cr, Ni, Mn etc.
6. Binding constituents - For binding the ingredients to electrode, sodium silicate and potassium silicate is
commonly used to bind the various intgredients together and attach them to the electrode core wire.

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7. Rutile - Tio2 based mineral can be readily used as slag forming constituent
Heat flow characteristics n arc welding : In general heat source is point source or line source but heat

distributing is 3-dimensional. In most cases, the heat is liberated in a small zone which is idealized alonga line

and the heat surce is idealized as a line source and in such situations heat flow is 2-dimensional. After

studying both cases, on estimate of the minimum heat input rate required to form a weld of a given width for

3-D with point heat source is

(Q = 5 5 uW
4 IIWKm 4 + 4

Where Q = Rate of heat input (W) W = Width of the weld (m)

K = Thermal conductivity of the work material (w/m k)

m = MP of work material above the intial temperature

u = welding speed (m/sec)  = Thermal diffusivity of work material (m2/sec)

K (= density, c = specific heat, K = Thermal Conductivity)


But for 2-D heat source, the corresponding equation is given by
Q = 8K .m.t (1/5 + uw/4)  (2)

Where t = plate thickness

In arc welding the heat input rate is given by
Q = CVI  (3)

Where V = arc voltage, I = arc current

C = fraction of total time during which arc is on

If heat input rate given equation (3) is less than heat required for welding given in equation (1) and (2),

a lack of side fusion occurs.

Magnetic arc blow (arc blow) : The magnetic forces created by the electic current tend to draw the arc

from its shortest line of action and make the arc flame flutter, this is known as magnetic arc blow and mainly

occurs in DC arc. InAC arc each magnetic force is immediately follow by one in the opposite direction, the

arc deflecting influence thus being automaticallycancelled.

The factors which may lead to arc blow are

• Magnetic fields produced in the work piece adjacent to the welding arc because of the current flow

• Presence of but bar carrying large D-C’s in the neighborhood of the plates where the welding is being

carried out.

• Multiple welding heads - arc at one electrode may bne affected by the magnetic field of arc at the

other electrode.

• The magnetic field produced in the work piece around the earth connection may deflect the arc from

the point where the connections is made. This magnetic field is produced because of the current between

earth connection and work piece.

Types of arc blow

1. Forward arc blow at the starting end of the weld and the backword arc blow at the finishing of the

weld.

2. Sideword deflection arc blow, This is reduced by changing the earth clamp.

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3. Arc rotation - Indicates that under certain conditions of arc blow perhaps arc experiences magnetic

field lines parallel to the arc axis.

Effects of arc blow

• increased arc blow results in an unstable arc and presents considerable difficulty to a welder to

carryout welding properly.

• Poor weld bled appears

• Irregular and erratic weld deposition

• Under cutting and lack of fusion

• Weld spatter

• Un even and weak welded joint

• Entrapment of slag

• Porosity

Arc blow is more when thick plates are welded and arc blow is more when thickly coated electrodes

are used, because slag formation runs under the arc causing incomplete fusion and excessive weld spatter.

Remedies for arc blow : The arc blow can be minimized in the following ways

1. set up a magnetic field of sufficient strength to neutralize the force caused by the magnetic flux

2. welding away from the earth ground connection (or) change the position of earth connection on the

wok piece.

3. wrapping the welding electrode cable a few turns on the work

4. using a run out tab and starting plate

5. reducing welding current or electrode dia

6. welding towards a heavy tack or welding towards a portion already welded

7. reducing the rate of travel of the weld

8. decreasing are length

9. change the power supply from DC to AC

Arc welding electrodes

Core wire dia - 3.15, 4.0, 5.0, 6.3, 8.0, 10 and 12.5mm

Core wire length - 350 and 450mm

According to amount of coating electrodes are divided into light coated and heavy coated electrodes.

Light coating is applied in thin layer to the core wire. It serve only the purpose of stabilizing the arc.

Therefore theyare known as stabilizing electrodes. There are no ingredients in the coating to prevent oxida-

tion or to cause de oxidation of the melt and no slag is formed and the mechanical properties of the weld

metal are not improved. Hence a shielded arc electrodes. These are used to obtain a Coding of heavy

coated electrodes is as per IS 815 - 1994 consists of

1. It consists one prefix letter

2. Code no. of 6 digits

3. In some cases one or more suffix letters indicating specific property.

Prefix letter : It indicates the method of manufacturing the electrode

E - Solid extrusion R - Extruded with Reinforcement

Code number : The first 3 digits decribes the performance coating and last 3 digits give the mechanical

properties of all metal deposits

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