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Heat treatment is a method used to alter the
physical, and sometimes chemical properties of a
material. The most common application is
It involves the use of heating or chilling, normally to
extreme temperatures, to achieve a desired result
such as hardening or softening of a material
It applies only to processes where the heating and
cooling are done for the specific purpose of
altering properties intentionally
How do we fabricate metals?
Blacksmith - hammer (forged)
Molding - cast
Rough stock formed to final shape
Hot working vs. Cold working
• T high enough for • well below Tm
recrystallization • work hardening
• Larger deformations • smaller deformations
• Rolling (Hot or Cold Rolling)
(I-beams, rails, sheet & plate)
• Forging (Hammering; Stamping)
Metal Fabrication Methods - I
ductile metals, e.g. Cu, Al (hot)
(rods, wire, tubing)
die must be well lubricated & clean
• Sand Casting
(large parts, e.g.,
auto engine blocks)
• Investment Casting
(low volume, complex shapes
e.g., jewelry, turbine blades)
Metal Fabrication Methods - II
• pattern is made from paraffin.
• mold made by encasing in
plaster of paris
• melt the wax & the hollow mold
• pour in metal
Metal Fabrication Methods - III
• Powder Metallurgy
(materials w/low ductility)
at low T
by diffusion at
(when one large part is
• Heat affected zone:
(region in which the
microstructure has been
Adapted from Fig.
11.9, Callister 7e.
(Fig. 11.9 from Iron
Walton and T.J.
Opar (Ed.), 1981.)
piece 1 piece 2
fused base metal
filler metal (melted)
base metal (melted)
heat affected zone
Steel Crystal Structures:
•Ferrite – BCC iron w/
carbon in solid solution
(soft, ductile, magnetic)
•Austenite – FCC iron
with carbon in solid
solution (soft, moderate
Compound of carbon
and iron FE3C (Hard and
•Pearlite – alternate
layers of ferrite and
•Martensite – iron –
carbon w/ body
centered tetragonal –
result of heat treat and
HT: ferrite then austentite then martensite
TTT diagram is a plot of temperature versus the
logarithm of time for a steel alloy of definite
It is used to determine when transformations begin
and end for an isothermal heat treatment of a
previously austenitized alloy
TTT diagram indicates when a specific
transformation starts and ends and it also shows
what percentage of transformation of austenite at a
particular temperature is achieved.
The TTT diagram for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised at 900°C
Types of Heat-Treatment (Steel)
Annealing / Normalizing,
Tempering, and Quenching
Utilize heat treatments to design optimum microstructures and
mechanical properties (strength, ductility, ardness….)
Strength in steels correlates with how much martensite
remains in the final structure
Hardenability: The ability of a structure to transform to
Has the Strongest microstructure.
Can be made more ductile by tempering.
Therefore, the optimum properties of quenched And
tempered steel are realized if a high content of
martensite is produced.
It is difficult to maintain the same conditions
throughout the entire volume of steel during cooling:
The surface cools more quickly than interior, producing
a range of microstructures throughout.
The martensitic content, and the hardness, will drop
from a high value at the surface to a lower value in the
interior of the specimen.
Heat treatment of Steels
Controlled heating and cooling of metals to alter their
physical and mechanical properties without changing
the product shape,
associated with increasing the strength of material,
alter certain manufacturability;
improve formability, and
restore ductility after a cold working operation.
Annealing: Heat to Tanneal, then cool slowly.
Based on discussion in Section 11.7, Callister 7e.
Thermal Processing of Metals
• Process Anneal:
Negate effect of
cold working by
• Stress Relief: Reduce
stress caused by:
• Normalize (steels):
Deform steel with large
grains, then normalize
to make grains small.
• Full Anneal (steels):
Make soft steels for
good forming by heating
to get g, then cool in
furnace to get coarse P.
• Spheroidize (steels):
Make very soft steels for
good machining. Heat just
below TE & hold for
Decarburization during Heat
Decrease in content of carbon in metals is called
It is based on the oxidation at the surface of
carbon that is dissolved in the metal lattice
In heat treatment processes iron and carbon
usually oxidize simultaneously
During the oxidation of carbon, gaseous products
(CO and CO2) develop
In the case of a scale layer, substantial
decarburization is possible only when the gaseous
products can escape
The strength of a steel depends on the
presence of carbides in its structure
In such a case the wear resistance is obviously
In many circumstances, there can be a serious
drop in fatigue resistance
To avoid the real risk of failure of engineering
components, it is essential to minimize
decarburization at all stages in the processing
Annealing: a heat treatment in which a
material is exposed to an elevated
temperature for an extended time period and
then slowly/controlled cooled.
Annealing temperature and the control cooling
rate depend on the alloy composition and the type
of the annealing treatment.
Three stages of annealing
1. Heating to the desired temperature (austenite or
2. Holding or “soaking” at that temperature
3. Cooling, usually to room temperature 50 - 20 ºC/hr
1. Stress-Relief Annealing
It is an annealing process
below the transformation
temperature Ac1, with
subsequent slow cooling, the
aim of which is to reduce the
internal residual stresses in
a workpiece without
intentionally changing its
structure and mechanical
Causes of Residual Stresses
1. Thermal factors (e.g., thermal stresses
caused by temperature gradients within the
workpiece during heating or cooling)
2. Mechanical factors (e.g., cold-working)
3. Metallurgical factors (e.g., transformation
of the microstructure)
How to Remove Residual Stresses?
R.S. can be reduced only by a plastic deformation in
This requires that the yield strength of the material
be lowered below the value of the residual stresses.
The more the yield strength is lowered, the greater
the plastic deformation and correspondingly the
greater the possibility or reducing the residual
The yield strength and the ultimate tensile strength
of the steel both decrease with increasing
Stress-Relief Annealing Process
For plain carbon and low-alloy steels the
temperature to which the specimen is heated is
usually between 450 and 650˚C, whereas for hot-
working tool steels and high-speed steels it is
between 600 and 750˚C
This treatment will not cause any phase changes,
but recrystallization may take place.
Machining allowance sufficient to compensate
for any warping resulting from stress relieving
should be provided
Stress-Relief Annealing – R.S.
In the heat treatment of metals, quenching or rapid
cooling is the cause of the greatest residual stresses
To activate plastic deformations, the local residual
stresses must be above the yield strength of the
Because of this fact, steels that have a high yield
strength at elevated temperatures can withstand
higher levels of residual stress than those that have a
low yield strength at elevated temperatures
Soaking time also has an influence on the effect of
Relation between heating temperature
and Reduction in Residual Stresses
Higher temperatures and
longer times of annealing
may reduce residual
stresses to lower levels
Stress Relief Annealing - Cooling
The residual stress level after stress-relief annealing will be
maintained only if the cool down from the annealing
temperature is controlled and slow enough that no new
internal stresses arise.
New stresses that may be induced during cooling depend
on the (1) cooling rate, (2) on the cross-sectional size of
the workpiece, and (3)on the composition of the steel
A heat treatment process consisting of
austenitizing at temperatures of 30–80˚C
above the AC3 transformation temperature
followed by slow cooling (usually in air)
The aim of which is to obtain a fine-grained,
uniformly distributed, ferrite–pearlite
Normalizing is applied mainly to unalloyed
and low-alloy hypoeutectoid steels
For hypereutectoid steels the austenitizing
temperature is 30–80˚C above the AC1 or ACm
Normalizing – Austenitizing
Effect of Normalizing on Grain Size
Normalizing refines the grain of a steel that has
become coarse-grained as a result of heating to a high
temperature, e.g., for forging or welding
Carbon steel of 0.5% C. (a) As-rolled or forged; (b)
normalized. Magnification 500
Need for Normalizing
Grain refinement or homogenization of the
structure by normalizing is usually performed
either to improve the mechanical properties of
the workpiece or (previous to hardening) to
obtain better and more uniform results after
Normalizing is also applied for better
machinability of low-carbon steels
Normalizing after Rolling
After hot rolling, the
structure of steel is usually
oriented in the rolling
To remove the oriented
structure and obtain the
properties in all
directions, a normalizing
annealing has to be
Normalizing after Forging
After forging at high temperatures,
especially with workpieces that vary
widely in crosssectional size, because of
the different rates of cooling from the
forging temperature, a heterogeneous
structure is obtained that can be made
uniform by normalizing
Normalizing – Holding Time
Holding time at austenitizing temperature may be
calculated using the empirical formula:
t = 60 + D
where t is the holding time (min) and D is the
maximum diameter of the workpiece (mm).
Normalizing - Cooling
Care should be taken to ensure that the cooling rate
within the workpiece is in a range corresponding to
the transformation behavior of the steel-in-question
that results in a pure ferrite–pearlite structure
If, for round bars of different diameters cooled in
air, the cooling curves in the core have been
experimentally measured and recorded, then by
using the appropriate CCT diagram for the steel
grade in question, it is possible to predict the
structure and hardness after normalizing
3. Isothermal Annealing
Hypoeutectoid low-carbon steels as well as
medium-carbon structural steels are often
isothermally annealed, for best machinability
An isothermally annealed structure should have the
1. High proportion of ferrite
2. Uniformly distributed pearlite grains
3. Fine lamellar pearlite grains
Principle of Isothermal Annealing
Bainite formation can
be avoided only by very
cooling, but with such
a slow cooling a
results (hatched area)
Process - Isothermal Annealing
Austenitizing followed by a fast cooling to the
temperature range of pearlite formation (usually about
Holding at this temperature until the complete
transformation of pearlite
and cooling to room temperature at an arbitrary
4. Spheroidizing Annealing
It is also called as Soft
Any process of heating and
cooling steel that produces a
rounded or globular form of
It is an annealing process at
temperatures close below or
close above the AC1
temperature, with subsequent
Spheroidizing - Purpose
The aim is to produce a soft structure by changing all hard
constituents like pearlite, bainite, and martensite (especially in
steels with carbon contents above 0.5% and in tool steels) into a
structure of spheroidized carbides in a ferritic matrix
(a) a medium-carbon low-alloy steel after soft annealing at 720C;
(b) a high-speed steel annealed at 820C.
Spheroidizing - Process
Heat the part to a temperature just below the
Ferrite-Austenite line, line A1 727 ºC.
Hold the temperature for a prolonged time,
Fairly slow cooling. Or
Cycle multiple times between temperatures
slightly above and slightly below the 727 ºC
line, say for example between 700 and 750 ºC,
Slow cooling, or
For tool and alloy steels heat to 750 to 800 ºC,
Hold for several hours,
Slow cooling. 42
Spheroidizing - Uses
Such a soft structure is required for good
machinability of steels having more than
0.6%C and for all cold-working processes
that include plastic deformation.
Spheroidite steel is the softest and most
ductile form of steel
Spheroidizing - Mechanism
The physical mechanism of soft annealing is based on
the coagulation of cementite particles within the
ferrite matrix, for which the diffusion of carbon is
Globular cementite within the ferritic matrix is the
structure having the lowest energy content of all
structures in the iron–carbon system
The carbon diffusion depends on temperature and
Annealing - summary
•Most heat treating operations begin with heating the alloy into the
austenitic phase field to dissolve the carbide in the iron
Recommend temperatures for the process annealing,
annealing, normalizing, and spheroidizing of 1020,
1077, and 10120 steels.
Example: Design Heat Treatment Temp.
Material is quenched above the temperature when
Martensite forms MS, around 215 ºC ( Eutectoid steel)
Hold longer at this temperature, the Austenite
transforms into Bainite
Tendency to crack is severely reduced.
Martempering is similar to
Austempering except that the part is
slowly cooled through the martensite
The structure is martensite, which
needs to tempered just as much
as martensite that is formed
through rapid quenching.
The biggest advantage of
Martempering over rapid
quenching is that there is less
distortion and tendency to crack.
Process done subsequent to quench hardening
Quench-hardened parts are often too brittle.
Brittleness is caused by a predominance of
This brittleness is removed by tempering.
Tempering results in a desired combination of
hardness, ductility, toughness, strength, and
The mechanism of tempering depends on the steel and
the tempering temperature.
Martensite is a somewhat unstable structure.
When heated, the Carbon atoms diffuse from
Martensite to form a carbide precipitate and the
concurrent formation of Ferrite and Cementite,
which is the stable form.
Tool steels for example, lose about 2 to 4 points of
hardness on the Rockwell C scale. Even though a
little strength is sacrificed, toughness (as measured
by impact strength) is increased substantially.
Springs and such parts need to be much tougher —
these are tempered to a much lower hardness.
Tempering at temperatures 300°C - 400°C.
– Soaking time varies (2 to 8 hr)s depending on the parts size.
– At these temperatures martensite transforms to trostite (very
fine mixture of ferrite and cementite).
– Trostite is softer than martensite and more ductile.
Tempering at temperatures higher than 400°C but lower
than lower critical point (A1).
Soaking time varies (2 to 8 hrs) depending on the parts size.
At these temperatures martensite transforms to sorbite (fine
mixture of ferrite and cementite).
Sorbite and trostite are principally similar structures
differing only in the particles size.
Sorbite is more more ductility and toughness, and less strong
Example: Design of a Quench and Temper Treatment
A rotating shaft that delivers power from an electric motor is made
from a 1050 steel. Its yield strength should be at least 150,000 psi,
yet it should also have at least 15% elongation in order to provide
toughness. Design a heat treatment to produce this part.
What happens during rapid cooling?
Phase diagrams only show stable phases that are
formed during slow cooling
If cooling is rapid, the phase diagram becomes invalid
and metastable phases may form
In the case of steel, the formation of ferrite and cementite
requires the diffusion of carbon out of the ferrite phase.
What happens if cooling is too rapid to allow this?
The crystal lattice tries to switch from fcc (austenite)
to bcc (ferrite).
Excess carbon distorted body centred lattice (BCT)
Hardening and Tempering
Steels can be heat treated to high hardness and
strength (wear properties) levels. Structural
components subjected to high operating stress need the
high strength of a hardened structure. Similarly, tools such
as dies, knives, cutting devices, and forming devices
need a hardened structure to resist wear and deformation
As-quenched hardened steels are so brittle that even slight
impacts may cause fracture.
Tempering is a heat treatment that reduces the brittleness
of a steel without significantly lowering its hardness and
strength. All hardened steels must be tempered before use.
Hardenability: is the ability of the Fe-C alloy to
be hardened by forming martensite. Hardenability
is not “hardness”.
It is a qualitative measure of the rate at which
hardness decreases with distance from the surface
because of decreased martensite content.
Hardenability depends on
Hardening and Tempering
1. Quenched end cools most rapidly and contains most
2. Cooling rate decreases with distance from quenched end:
greater C diffusion, more pearlite/bainite, lower hardness
3. High hardenability means that the hardness curve is relatively
0.40 wt% C, + different additional alloying elements
pearlite, bainite :
•Can also define
terms of cooling
Direct Hardening – Austenitizing and quench:
Austenitizing – again taking a steel with .6% carbon or
greater and heating to the austenite region.
Rapid quench to trap the carbon in the crystal
structure – called martensite (BCT)
Quench requirements determined from isothermal
transformation diagram (IT diagram).
Get “Through” Hardness!!!
Heat to austenite
range. Want to be
temperature to get
fine grain structure.
For this particular steel want to cool from about 1400 F to <400 F in
about 1 second!
Depending on how fast steel must be quenched
(from IT diagram), the heat treater will determine
type of quenching required:
Water (most severe)
Gas/ Air (least severe)
Many phases in between!!! Ex: add water/polymer to
water reduces quench time! Adding 10% sodium
hydroxide or salt will have twice the cooling rate!
Same requirements as austenitizing:
Must have sufficient carbon levels (>0.4%)
Heat to austenite region and quench
When only desire a select region to be hardened:
Knives, gears, etc.
Object to big to heat in furnace! Large casting w/ wear
Flame hardening, induction hardening, laser beam
Direct Hardening - Selective Hardening :
Diffusion Hardening (aka Case Hardening):
Carbon content to low to through harden with previous
Desire hardness only in select area
More controlled versus flame hardening and induction
Can get VERY hard local areas (i.e. HRC of 60 or greater)
Interstitial diffusion when tiny solute atoms diffuce into
spaces of host atoms
Substitiutional diffusion when diffusion atoms to big to
occupy interstitial sites – then must occupy vacancies
High temp (> 900 F)
Host metal must have low concentration of the diffusing
Must be atomic suitability between diffusing species and
Case hardening or surface hardening is
the process of hardening the surface of a
metal, often a low carbon steel, by infusing
elements into the material's surface, forming
a thin layer of a harder alloy.
Case hardening is usually done after the part
in question has been formed into its final
Flame and induction hardening
Flame or induction hardening are processes in
which the surface of the steel is heated to high
temperatures (by direct application of a flame, or
by induction heating) then cooled rapidly,
generally using water
This creates a case of martensite on the surface.
A carbon content of 0.4–0.6 wt% C is needed for
this type of hardening
Application Examples -> Lock shackle and
Carburizing is a process used to case harden steel
with a carbon content between 0.1 and 0.3 wt%
Steel is introduced to a carbon rich environment
and elevated temperatures for a certain amount
of time, and then quenched so that the carbon is
locked in the structure
Example -> Heat a part with an acetylene torch
set with a fuel-rich flame and quench it in a
carbon-rich fluid such as oil
Carburization is a diffusion-controlled
process, so the longer the steel is held in the
carbon-rich environment the greater the
carbon penetration will be and the higher
the carbon content.
The carburized section will have a carbon
content high enough that it can be hardened
again through flame or induction
The carbon can come from a solid, liquid or gaseous
Solid source -> pack carburizing. Packing low
carbon steel parts with a carbonaceous material and
heating for some time diffuses carbon into the outer
A heating period of a few hours might form a high-
carbon layer about one millimeter thick
Liquid Source -> involves placing parts in a bath of a
molten carbon-containing material, often a metal
Gaseous Source -> involves placing the parts in a
furnace maintained with a methane-rich interior 78
Nitriding heats the steel part to 482–621°C in an
atmosphere of NH3 gas and broken NH3.
The time the part spends in this environment
dictates the depth of the case.
The hardness is achieved by the formation of
Nitride forming elements must be present in the
workpiece for this method to work.
Advantage -> it causes little distortion, so the part
can be case hardened after being quenched,
tempered and machined
Cyaniding is mainly used on low carbon steels.
The part is heated to 870-950°C in a bath of
sodium cyanide (NaCN)and then is quenched
and rinsed, in water or oil, to remove any
The process produces a thin, hard shell (0.5-
0.75mm) that is harder than the one produced
by carburizing, and can be completed in 20 to 30
minutes compared to several hours.
It is typically used on small parts.
The major drawback of cyaniding is that cyanide
salts are poisonous
Carbonitriding is similar to cyaniding except a
gaseous atmosphere of ammonia and
hydrocarbons (e.g. CH4)is used instead of
If the part is to be quenched then the part is
heated to 775–885°C; if not then the part is
heated to 649–788°C
Design of Surface-Hardening Treatments for a Drive Train
Design the materials and heat treatments for an
automobile axle and drive gear.
Precipitation hardening (or age hardening),
is a heat treatment technique used to increase
the yield strength of malleable materials
Malleable materials are those, which are capable
of deforming under compressive stress
It relies on changes in solid solubility with
temperature to produce fine particles of an
impurity phase, which blocks the movement of
dislocations in a crystal's lattice
Since dislocations are often the dominant
carriers of plasticity, this serves to harden
The impurities play the same role as the
particle substances in particle-reinforced
Alloys must be kept at elevated temperature
for hours to allow precipitation to take
place. This time delay is called aging
Two different heat treatments involving
precipitates can change the strength of a material:
1. solution heat treating
2. precipitation heat treating
Solution treatment involves formation of a
single-phase solid solution via quenching and
leaves a material softer
Precipitation treating involves the addition of
impurity particles to increase a material's strength
QUENCHING and TEMPERING
In quench hardening, fast cooling
rates, depending on the chemical
composition of the steel and its
section size, are applied to prevent
formations in the pearlite range and
to obtain a structure consisting
mainly of martensite and bainite
However, the reduction of
undesirable thermal and
transformational stresses usually
requires slower cooling rates
To harden by quenching, a
metal must be heated into the
austenitic crystal phase and
then quickly cooled
Cooling may be done with
forced air, oil, polymer
dissolved in water, or brine
Upon being rapidly cooled, a
portion of austenite
(dependent on alloy
composition) will transform to
Cooling speeds, from fastest to slowest, go from
polymer, brine, fresh water, oil, and forced air
However, quenching a certain steel too fast can
result in cracking, which is why high-tensile
steels such as AISI 4140 should be quenched in
oil, tool steels such as H13 should be quenched in
forced air, and low alloy such as AISI 1040 should
be quenched in brine
Metals such as austenitic stainless steel (304,
316), and copper, produce an opposite effect
when these are quenched: they anneal
Untempered martensite, while very hard, is too
brittle to be useful for most applications.
In tempering, it is required that quenched parts
be tempered (heat treated at a low
temperature, often 150˚C) to impart some
Higher tempering temperatures (may be up to
700˚C, depending on alloy and application) are
sometimes used to impart further ductility,
although some yield strength is lost
Tempering is done to toughen the metal by
transforming brittle martensite or bainite into a
combination of ferrite and cementite or
sometimes Tempered martensite
Tempered martensite is much finer-grained
than just-quenched martensite
The brittle martensite becomes tough and
ductile after it is tempered.
Carbon atoms were trapped in the austenite
when it was rapidly cooled, typically by oil or
water quenching, forming the martensite
The martensite becomes tough after being
tempered because when reheated, the
microstructure can rearrange and the carbon
atoms can diffuse out of the distorted body-
centred-tetragonal (BCT) structure.
After the carbon diffuses out, the result is
nearly pure ferrite with body-centred
Design of a Quench and Temper Treatment
A rotating shaft that delivers power from an electric motor
is made from a 1050 steel. Its yield strength should be at
least 145,000 psi, yet it should also have at least 15%
elongation in order to provide toughness. Design a heat
treatment to produce this part.