Heat Treatment of Steel

Prepared By
Md. Mostafa Kamal
Lecturer
Department of Mechanical Engineering
RUET, Rajshahi-6204
 Heat Treatment

▪ Heat Treatment is the controlled heating and cooling of metals to alter their
physical and mechanical properties without changing the product shape.

▪ Steels are particularly suitable for heat treatment, since they respond well to heat
treatment and the commercial use of steels exceeds that of any other material.

▪ Generally, heat treatment uses phase transformation during heating and

cooling to change a microstructure in a solid state.
Heat Treatment Vs Thermomechanical Treatments

▪ In heat treatment, the processing is most often entirely thermal and modifies
only structure.

▪ Thermomechanical treatments, which modify component shape and structure,

and thermochemical treatments which modify surface chemistry and
structure, are also important processing approaches which fall into the domain
of heat treatment.
Classification of Heat treatment
Effect of different Heat Treatment Process
▪ Heat treating is a generic term for many different thermal processes used on different
materials. The processes are used to enhance or modify the properties of the material to
meet the requirements of the intended application. Heat treatment is used to give parts
their initial, intermediate and their final properties.

➢ soften metal or plastic (annealing)

➢ harden metal (through hardening, case hardening, carburizing, nitriding)
➢ soften or harden just one area on a part (induction, flame)
➢ homogenize plastic injection molded parts (annealing)
➢ remove stress from material before or after machining (stress relief)
➢ remove stress from formed, extruded, cut or bent material (stress relief)
➢ add resilience (spring) to a metal (aus-tempering, quenching)
➢ change magnetic permeability (magnetic anneal, metal anneal)
 Application of Heat Treatment Process
Improving Workability and Machinability:
Heat treating improves manufacturability by removing internal stresses from previous
fabrication processes like cold work, hot work, machining, stamping and welding. For instance, if
a metal is too hard to machine or bend, it can be annealed or stress relieved to reduce the
hardness. If a plastic deforms when it’s machined, it can be stress relieved or annealed to keep
it from deforming. Heat treating, by flame or induction, can also soften just one area of a part,
leaving the rest of the part unchanged.

Improving Wear Resistance and Durability:

Heat treating can improve wear resistance by hardening the material. Metals (including steel,
titanium, inconel, and some copper alloys) can be hardened either on the surface (case
hardening) or all the way through (through hardening), to make the material stronger, tougher,
more durable and more resistant to wear. This is a great way to increase the durability of an
inexpensive steel such as A-36 or 1018.
Application of Heat Treatment Process
Improving Strength and Toughness:
Strength and toughness are a trade-off, as increasing strength (as measured by hardness) may
reduce toughness and introduce brittleness. Specifically, heat treating can affect yield strength,
tensile strength and fracture toughness. Case hardening or through hardening will increase
strength, however the parts will need to be tempered or drawn back to decrease brittleness.
The amount of tempering is determined by the ultimate strength desired in the material. Often,
if material is too brittle as received, it can be heat treated (annealed or re-tempered) to make it
more usable (ductile).

Improving Magnetic Properties:

Several materials, such as 1008 or 316, tend to gain magnetism (measured as “magnetic
permeability”) when they are work-hardened (machined, formed, stamped, bent, etc). A
specific annealing process will reduce the magnetic permeability, which is especially important
if the part will be used in an electronic environment.
Purpose of Heat Treatment
➢ Improve the toughness

➢ Increase the hardness

➢ Increase the ductility
➢ Improve the machinability
➢ Refine the grain structure
➢ Remove the residual stresses
➢ Improve the wear resistance
 Important Terminology

▪ Eutectoid Reaction
▪ Hypereutectoid Steel
▪ Hypoeutectoid Steel
▪ Upper Critical Temperature
▪ Lower Critical Temperature
▪ Pearlite Structure
 Steps in Heat Treatment
▪ The 1st step in the heat treatment of steel is to heat the material to some
temperature in or above the critical range to form austenite.

▪ Rate of heating to the desired temperature is less important than the other factors in
the heat treating cycle.

▪ Highly stressed materials produced by cold work should be heated more slowly than
stress free materials to avoid distortion.

▪ Slowing the heating rate to minimize the thermal stresses and distortion.
Different Heat Treatment Process in Fe-Fe3C Diagram
 Heat Treatment Process
Heat Treatment Process Heating Cycle Cooling Conditions Purposes

Full Annealing Heating from 30 – 50°C Slowly Cool inside the ▪ To soften steel and to
above the upper critical furnace. improve its
temperature. machinability.
▪ To refine grain size and
remove gases.
▪ It removes the internal
stresses developed
during the previous
process.
▪ To obtain desired
ductility, malleability,
and toughness.
▪ It modifies the
electrical and magnetic
properties.
 Heat Treatment Process
Heat Treatment Process Heating Cycle Cooling Conditions Purposes

Normalizing ▪ Heating the steel ▪ cooling the hot steel ▪ Promote uniformity of
component above the component to room structure.
A3 temperature for temperature in still air. ▪ To secure grain
hypo eutectoid steels refinement.
and above Acm (upper ▪ To bring about
critical temperature for desirable changes in
cementite)temperatue the properties of steel.
for hypereutectoid ▪ To improve the
steels by 30C to 50C or machineability
100F.
▪ The second step
involves holding the
steel component long
enough at this
temperature for
homogeneous
austenization.
 Heat Treatment Process
Heat Treatment Process Heating Cycle Cooling Conditions Purposes

Full Annealing Heating from 30 – 50°C Slowly Cool inside the ▪ To soften steel and to
above the upper critical furnace. improve its
temperature. machinability.
▪ To refine grain size and
remove gases.
▪ It removes the internal
stresses developed
during the previous
process.
▪ To obtain desired
ductility, malleability,
and toughness.
▪ It modifies the
electrical and magnetic
properties.
 Annealing

▪ Annealing is one of the most important processes of heat treatment. It is one of the most
widely used operations in the heat treatment of iron and steel and is defined as the
softening process.

▪ Heating from 30 – 50°C above the upper critical temperature and cooling it at a very slow
rate by seeking it the furnace. The main aim of annealing is to make steel more ductile and
malleable and to remove internal stresses. This process makes the steel soft so that it can
be easily machined.
 Process of Full Annealing
▪ Depending on the carbon content, the steel is heated to a temperature of about 50°
to 55°C above its critical temperature range. It is held at this temperature for a
definite period of time depending on the type of furnace and nature of work. The
steel is then allowed to cool inside the furnace constantly.
 Process of Full Annealing
▪ When this steel is heated, no change will occur until
the A1 lower critical line is crossed. At that
temperature the pearlite areas will transform to
small grains of austenite by means of the eutectoid
reaction, but the original large ferrite grains will
remain unchanged. Cooling from this temperature
will not refine the grain.

▪ Continued heating between the A1 and A3 lines will allow

the large ferrite grains to transform to small grains of
austenite, so that above the A3 (upper-critical) line the
entire microstructure will show only small grains of
austenite, Subsequent furnace cooling will result in small
grains of proeutectoid ferrite and small areas of coarse
lamellar pearlite.
 Process of Full Annealing
▪ Refinement of the grain size of hypereutectoid
steel will occur about 500F above the lower
critical temperature A(3,1) line.

▪ Heating above this temperature will coarsen the

austenite grains, which, on cooling, will transform
to large pearlitic areas.

▪ The micro-structure of annealed hyper-eutectoid

steel will consist of coarse lamellar pearlite areas
surrounded by a network of pro-eutectoid
cementite. Because this excess cementite
network is brittle and tends to be a plane of
weakness, annealing should never be a final heat
treatment for hypereutectoid steels. The
purpose of a thick, hard, grain boundary will also
result in poor machinability.
 Normalizing
▪ The main purpose of normalizing is to produce a harder and stronger steel than full annealing, so
that for some applications normalizing may be a final heat treatment. Therefore, for hyper-eutectoid
steels, it is necessary to heat above the Acm line in order to dissolve the cementite network.

▪ Normalizing may also be used to improve machinability, modify and refine cast dendritic structures
and refine the grain and homogenize the micro-structure in order to improve the response in
hardening operations.

▪ The increase in cooling rate due to air cooling as compared with furnace cooling affects the
transformation of austenite and the resultant micro-structure in several ways.

▪ There is less time for the formation of the proeutectoid constituent, consequently, there will be
less pro-eutectoid ferrite in normalized hypo-eutectoid steels and less pro-eutectoid cementite in
hypereutectoid steels as compared with annealed ones.
 Normalizing
▪ The faster cooling rate in normalizing will also affect the temperature of austenite transformation
and the fineness of pearlite. In general, the faster the cooling rate, the lower the temperature of
austenite transformation and the finer the pearlite. The difference in spacing of the cementite
plates in the pearlite between annealing and normalizing is shown in the following fig.

▪ Ferrite is very soft, while cementite is very hard. With the cementite plates closer together in the
case of normalized medium pearlite, they tend to stiffen the ferrite so it will not yield as easily, thus
increasing hardness.

▪ The net effect is that normalizing produces a finer and more abundant pearlite structure than is
obtained by annealing which results in a harder and stronger steel.
Hardening
 Hardening
▪ Hardening is a heat treatment process in which steel is rapidly cooled from
austenitising temperature. As a result of hardening, the hardness and wear
resistance of steel are improved.

▪ Hardening treatment generally consists of heating to hardening

temperature, holding at that temperature, followed by rapid cooling such as
quenching in oil or water or salt baths.

▪ The hardening temperature depends on chemical composition. For plain

carbon steels, it depends on the carbon content alone. Hypoeutectoid steels
are heated to about 30 – 50˚C above the upper critical temperature,
whereas eutectoid and hyper eutectoid steels are heated to about 30 – 50˚C
above lower critical temperature.
 Hardening
▪ Quenching is a process of rapid
cooling of materials from high
temperature to room temperature
or even lower. In steels quenching
results in transformation of
austenite to martensite (a non-
equilibrium constituent).
 Martensitic Structure
▪ Martensite is formed in carbon steels by the rapid cooling (quenching) of the
austenite form of iron at such a high rate that carbon atoms do not have time to
diffuse out of the crystal structure in large enough quantities to form cementite
(Fe3C). Austenite is gamma-phase iron (γ-Fe), a solid solution of iron and alloying
elements. As a result of the quenching, the face-centered cubic austenite
transforms to a highly strained body-centered tetragonal form called martensite
that is supersaturated with carbon.

▪ Martensite is a very hard form of steel crystalline structure.

 I-T Diagram and Cooling Curves
▪ Phase Diagrams are limited in their usefulness because they can only predict the
microstructure that will result for equilibrium conditions, i.e. very very slow cooling.

▪ Non-equilibrium cooling will result in different microstructures resulting in altered

properties.

▪ Time and temperature of austenite transformation has profound influence on the

transformation products and subsequent properties of steel.

▪ For study of non-equilibrium cooling, information about isothermal transformation

diagram is needed
 I-T or TTT Diagram
▪ Since austenite is unstable below the lower critical
temperature Ae1, it is necessary to know at a
particular subcritical temperature how long it will
take for the austenite to start to transform, how long
it will take to be completely transformed, and what
will be the nature of the transformation product.

▪ T (Time) T (Temperature) T (Transformation) diagram

is a plot of temperature versus the logarithm of time
for a steel alloy of definite composition. It is used to
determine when transformations begin and end for an
isothermal (constant temperature) heat treatment of
a previously austenitized alloy.
 Construction of TTT Diagram
The steps usually followed to determine an isothermal-diagram are as under.
Step 1: Prepare a large number of samples cut from the same bar. Their cross section has to be small in
order to react quickly to change in temperature.
Step 2: Place the samples in a furnace or molten salt bath at the proper austenitizing temperature. For
1080 (eutectoid) steel, this temperature is approximately 1425°F. They should be left at the given
temperature long enough to become completely austenite.
Step 3: Place the samples in a molten salt bath which is held at a constant subcritical temperature (a
temperature below the Ae1 line), for example, 675°C.
Step 4: After varying time intervals in the salt bath, each sample is quenched in cold water or iced brine.
Step 5: After cooling, each sample is checked for hardness and studied microscopically.
Step 6: The above steps are repeated at different subcritical temperatures until sufficient points are
determined to plot the curves on the diagram.
 Construction of TTT Diagram
▪ We are really interested in knowing what is happening to austenite at 675°C, but the samples cannot
be studied at that temperature. Therefore, we must somehow be able to relate the room-
temperature microscopic examination to what is occurring at the elevated temperature. Following
two facts will help in correlating microstructure at room temperature to the microstructure at the
elevated temperature.
 Construction of TTT Diagram
Fact 1: Martensite is formed only from austenite almost
instantaneously at low temperatures.
Fact 2: If austenite transforms at a higher temperature to
a structure which is stable at room temperature, rapid
cooling will not change the transformation product. In
other words, if pearlite is formed at 675°C, the pearlite
will be exactly the same at room temperature no matter
how drastically it is quenched, since there is no reason for
the pearlite to change.
Based on above facts and study of microstructures of
various samples at room temperature, two points may be
plotted at 675°C, namely, the time for the beginning
(point A corresponding to time T1) and the time for the
end of transformation (point B corresponding to time T2)
as shown in the figure given below. It is also common
practice to plot the time for 50 percent transformation
(point C).
Thank You!