Week 01 Introduction to Heat Treatment

10/11/2011

MME444 Heat Treatment Sessional

Week 01

Introduction to Heat Treatment

Prof. A.K.M.B. Rashid
Department of MME
BUET, Dhaka

Introduction

 Can you control the microstructure that formed during
cooling of an alloy of fixed composition?
 Yes, by controlling the transformation mechanism
by varying temperature and time of transformation.

 Manipulating phase transformations is a major tool
for controlling the properties of metals and alloys.

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Introduction

 Phase diagrams represents microstructure that SHOULD
develop, assuming that temperature is changed slowly enough
to maintain equilibrium at all time.

 In actual practice, materials processing is rushed, and
time becomes an important factor.

 The practical aspect of this is HEAT TREATMENT,
the temperature versus time history necessary to generate
a desired microstructure.

 The fundamental basis for heat treatment is KINETICS, which is
defined as the science of time-dependent phase transformation.

Time – The Third Dimension

 Time did not appear in any quantitative way in the discussion
of phase diagrams.

 Phase diagrams summarised equilibrium states and these
equilibrium structures take time to develop, and the approach
to equilibrium can be mapped on a time scale.

T  Time for solidification to go to completion
is a strong function of temperature.

completion  The reaction proceeds slowly near the
of reaction melting point .

IA  The reaction is fastest at some
intermediate temperature.

 The reaction becomes slow again as the
temperature is decreased further.

B

x Schematic illustration of the approach to equilibrium

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Time – The Third Dimension

Example: Solid-State Transformation

 All diffusion controlled solid-state transformation processes
commonly proceed by the nucleation and thermally activated
growth mechanism

 Since these transformations involved the formation of at least
one new phase that has different structure and/or composition
from that of the parent one, some atomic rearrangements via
diffusion are required, which is a time-dependent phenomenon.

Time – The Third Dimension

 For solid-state transformations, the fraction of material
transformed y over a time period t is given by the Avrami
equation:

y = 1 – exp (-ktn)

where k and n are time-independent
constants.

By convention, the rate of
transformation r is taken as the
reciprocal of time required for 50%
completion of the transformation:

r= 1
t 0.5

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Time – The Third Dimension

 Temperatures have profound influence on kinetics and,
thus, on the rate of transformation.

 For most reactions, rate of transformation increases with
temperature according to the following Arrhenius equation:

y (%) r = A exp (-Q/RT)

A = temperature independent constant
Q = activation energy for the reaction

log t (min)
Recrystallisation of Copper

Time – The Third Dimension

 Now, the overall transformation rate depends on both the
nucleation rate and the growth rate.

Temperature growth / The nucleation rate increases
diffusion with decreasing temperature.

overall The growth rate is slower the
transformation lower the temperature
(a diffusion-controlled process).
nucleation /
driving force

Rate (s -1)
Temperature dependency of
overall transformation rate

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Time – The Third Dimension

 In practice, a phase transformation does not always take place
exactly at the transition temperature.

For example, liquids may undercool before solidification commences.

Near Tt, the overall transformation rate

Transformation temperature, Tt is slow.
(faster growth rate, slower nucleation rate)
few nucleus formed, grow rapidly to form coarser
Temperature T1 growth / grains
diffusion

At much lower temperature than Tt,

overall the overall transformation rate is slow

transformation again.
(faster nucleation rate, slower growth rate)
T2 many nuclei formed, finer grains resulted due to
nucleation / slower growth rate
driving force

Rate (s -1) The maximum transformation rate occurs
Temperature dependency of at a temperature range where the driving
overall transformation rate forces for solidification and diffusion rates
are both significant.

The TTT Diagram

 If, instead plotting the rate against temperature, we plot the start and
finish times for a transformation at various temperature, we obtain a
graph which is a mirror image of the overall transformation curve.

Temperature Untransformed End of
Temperature structure Transformation

increasing % transformation

Re-plot Start of
Transformation
Transformed
structure

Rate (s -1) log (time)

 This plot is known as Time – Temperature – Transformation (TTT) diagram.
 Commonly used to represent the temperature dependence of a transformation.

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Types of Phase Transformations

1. Diffusion-dependent with no change in phase composition
or number of phases present

(e.g. melting, solidification of pure metal, allotropic transformations,
recrystallization, etc.)

2. Diffusion-dependent with changes in phase compositions
and/or number of phases

(e.g. eutectoid transformations, where one solid transforms into a
mixture of two solids)

3. Diffusion-less phase transformation produces a metastable
phase by cooperative small displacements of all atoms in
structure in no time !!

(e.g. martensitic transformation)

Types of Phase Transformations

Diffusional vs. Martensitic Transformation

Untransformed 50 % Diffusional transformations
structure  occur relatively slowly
 usually result in the formation of
Temperature Transformed
structure equilibrium phases through diffusion
MARTENSITIC of atoms
DIFFUSIONAL  transformation occurs only when
cooling time is increased
curve
Martensitic transformations
log (time)  very rapid, no diffusion involved
 systematic coordinated shearing of

the lattice
 produce a metastable phase
 transformation occurs only when

temperature is decreased

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Phase Transformations in Steels

800 Eutectoid temperature Coarse
TTT Diagram
for Eutectoid 723 Austenite Pearlite
Steel (0.8% C)
Fine
600

T (C) → 500 Pearlite + Bainite

400 Bainite

Isothermal 300 Austenite
cooling 200 MS
100
Non-isothermal M90
cooling

0.1 Martensite 105
1 10 102 103 104

t (s) →

Phase Transformations in Steels

800 Eutectoid temperature
TTT Diagram
for Eutectoid 723 Pearlite
Steel (0.8% C)

600

T (C) → 500

400 Bainite
Curve 1
Continuous 300 Austenite

cooling curve 200 MS
(constant rate)

M 90

100 Curve 2
Martensite

0.1 1 10 102 103 104 105
t (s) →

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Phase Transformations in Steels

800 Eutectoid temperature

Eutectoid 723
Steel (0.8% C)

600

T (C) → 500

400

Different 300 MS Coarse P
Cooling M90
Treatments 200 M+P Fine P
M
100 1 10 102 103 104 105
t (s) →
0.1

Phase Transformations in Steels

Formation of Pearlite

Pearlite microstructure
containing coarse alternate
layers of ferrite (white) and

cementite (black)

 Nucleation and growth process
 Heterogeneous nucleation at grain boundaries
 Interlamellar spacing is a function of the temperature of transformation
 Lower temperature → finer spacing → higher hardness

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Phase Transformations in Steels

Formation of Bainite

Bainite formed at 348 C Bainite formed at 278 C

 Nucleation and growth process

 Acicular, accompanied by surface distortions

 Bainite plates have irrational habit planes

 Ferrite in Bainite plates possess different orientation relationship
relative to the parent Austenite than does the Ferrite in Pearlite

Phase Transformations in Steels

Formation of Martensite

FCC Possible positions of
Austenite Carbon atoms

Only a fraction of
the sites occupied

C along the c-axis
obstructs the contraction

FCC Austenite
Alternate choice

of Cell

20% contraction of c-axis Tetragonal In Pure Fe after
12% expansion of a-axis Martensite the Matensitic transformation

c=a

Austenite to Martensite → 4.3 % volume increase

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Phase Transformations in Steels

Formation of Martensite

 The martensitic transformation occurs without
composition change.

 The transformation occurs by shear without need
for diffusion.

 The atomic movements required are only a fraction of
the interatomic spacing.

 The shear changes the shape of transforming region
→ results in considerable amount of shear energy
→ plate-like shape of Martensite

 The amount of martensite formed is a function of the temperature to which the sample
is quenched and not of time.

 Hardness of martensite is a function of the carbon content
→ but high hardness steel is very brittle as martensite is brittle

 Steel is reheated to increase its ductility
→ this process is called TEMPERING

Phase Transformations in Steels

Tempering of Martensite

' (BCT ) Temper (BCC) Fe3C (OR)

Martensite Ferrite Cementite

 Heat below Eutectoid temperature
→ wait → slow cooling

 The microstructural changes which take place during
tempering are very complex

 Time – temperature cycle chosen to optimize strength
and toughness

 Example: For tool steel,
As quenched (Rc 65) → Tempered (Rc 45-55)

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Phase Transformations in Steels

Harness of Martensite Hardness (Rc) 60
as a function of 40
Carbon content

20 % Carbon
0.2 0.4 0.6

Properties of 0.8% C steel

Constituent Hardness (Rc) Tensile strength (MN / m2)
Coarse pearlite 16 710

Fine pearlite 30 990

Bainite 45 1470

Martensite 65 -

Martensite tempered at 250 C 55 1990

Heat Treatment of Steels

 It is an operation or combination Temperature holding
of operations involving
cooling
 heating a metal or alloy in its solid state
to a certain temperature, heating

 holding it there for some times, and Time

 cooling it to the room temperature at a
predetermined rate to obtain desired
properties.

 All basic heat-treating processes for steels involve the
transformation of austenite.

 the nature and appearance of these transformation products determine the
physical and mechanical properties of heat treated steels.

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Heat Treatment of Steels

Heating Period

 Heating steel to above upper critical temperature (A3 or Acm) to form single-
phase austenite.

 Rate of heating is usually less important, except for
[1] highly stressed materials, or [2] thick-sectioned materials.

Holding / Soaking Period

 Holding at the austenitising temperature for complete homogenisation of structure.
 Usually 1 hour per 1 inch section is enough for holding.

Cooling Period

 Cooling rate that determines the nature of transformation products of austenite.
 Depending on cooling rate, heat treatment of steels are classified as:

[1] annealing, [2] normalising, and [2] hardening.

Heat Treatment of Steels

Annealing of Steels

 Annealing is a generic term denoting a treatment that consists
of heating to and holding at a suitable temperature followed
by cooling slowly through the transformation range, primarily
for the softening of metallic materials.

 Generally, in plain carbon steels, full annealing (commonly
known as annealing) produces coarse ferrite-pearlite structures.

 Purposes of annealing

 Refining grains
 Inducing ductility, toughness, softness
 Improving electrical and magnetic properties
 Improving machinability
 Relieve residual stresses

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Heat Treatment of Steels

Normalising of Steels

 Normalising is done by heating the steel to above the upper
critical, followed by slow cooling to room temperature in still air.

 Cooling rate is no longer under equilibrium conditions

 Phase diagram cannot predict the proportions of phases.

 There will be less pro-eutectic constituent and more pearlite.

 Finer and stronger pearlite produced. Cementite
Ferrite

 Purposes of normalising:

 Modifying and refining cast dendritic structure

 Refining grains and homogenising the structure ANNEALED NORMALISED
 Inducing toughness coarse lamellar medium lamellar
 Improving machinability
pearlite pearlite

Heat Treatment of Steels

Temperature for Annealing and Normalising

Schematic on selection of heat treatment temperatures

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Heat Treatment of Steels

Hardening of Steels

 Hardening is done by heating the steel approximately to

 50 C above the upper critical temperatures (A3 line)
(for hypeutectoid steels).

 50 C above the lower critical temperatures (A3,1 line)
(for hypereutectoid steels)

followed by drastic cooling to room temperature.

 Purposes of hardening:

 to improve hardness
 to improve wear resistance

Heat Treatment of Steels

Hardening of Steels

 Martensite appears microscopically as needle or acicular
structure, sometimes described as a pile of straw.

Martensite needles (black) in
retained austenite (white background)

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Heat Treatment of Steels

Hardening of Steels

Quenching medium

 Severity of cooling medium influences the cooling rate.
Air  slow cooling rate  low hardness
Oil  moderate cooling rate  moderate hardness
Water  fast cooling rate  high hardness

Part geometry

 Thicker the sample, more variation in the cooling rate between the
centre and surface of the sample.
Centre  slow cooling rate  low hardness
Surface  faster cooling rate  high hardness

Alloy Content

 Addition of alloying elements slows down the diffusion process, thereby
making it easier for the steel to form martensite.

Heat Treatment of Steels

Tempering of Steels

 In the as-quenched condition, the steel is too brittle for most
applications. The formation of martensite also leaves high
residual stresses in the steel.

 Tempering is done almost immediately after hardening to relieve
residual stresses and to improve ductility and toughness. The
increase in ductility is attained at the sacrifice of some hardness
or strength.

 In tempering, the hardened steel is heated and held to a
temperature (which is below the lower critical), and then cooled
to room temperature.

The selection of heating temperature depends upon desired properties.

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Heat Treatment of Steels

Tempering of Steels

 During tempering, the excess carbon atoms, trapped in
martensite, gradually come out as extremely fine cementite
particles and the metastable BCT martensite transforms into
stable BCC ferrite.

 The resulting microstructure (fine cementite dispersed in
ferrite matrix) is called tempered martensite.

Heat Treatment of Steels

Tempering of Steels

Effect of tempering temperature on mechanical properties of a 1050 steel

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