Material Engineering, Heat treating (or heat treatment) is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching

1. HEAT TREATMENT

2. Heat Treatment
Heat Treatment is the controlled
heating and cooling of metals to alter
their physical and mechanical
properties without changing the
product shape.
Heat treatment is sometimes done
inadvertently due to manufacturing
processes that either heat or cool the
metal such as welding or forming.

3. • HEAT TREATMENT
• Heat treatment may be defined as an operation or
combination of operations involving the heating of metals or
alloys in the solid state to definite temperatures, followed by
cooling at a suitable rates to obtain desired physical and
mechanical properties. The physical and mechanical
properties depend on the size, shape and form of the micro
constituent present.
• The cooling of material after it is being undergone heat
treatment is called as QUENCHING. Quenching may be
done at different rates depending on the desired properties of
material.
• The medium in which material is cooled is known as
Quenching media. Quenching media may be water, brine or
still air.

4. Why to Heat Treat?
• Heat Treatment is often associated with increasing
the strength of material, but it can also be used to
alter certain manufacturability objectives such as
improve machining, improve formability, restore
ductility after a cold working operation.
• Thus it is a very enabling manufacturing process that
can not only help other manufacturing process, but
can also improve product performance by increasing
strength or other desirable characteristics.

5. Factors to consider before heat
treatment :-
There are certain factors which are to be
considered before heat treating a material.
1. Chemical composition of a material.
2. Mode of manufacture of a material i.e cast,
forged etc.
3. Whether any previous heat treatment has
been carried out and what is its structure.
4. Heat treatment operations to be performed
and properties and structural requirements
desired.

6. Objects of Heat Treatment :-
When steel is subjected to the heat
treatment operations, it undergoes many
structural changes due to which the
properties of materials change. The
following points need consideration.
1. Structural considerations of the object
before heat treatment.
2. Structural changes occurring during the
heat treatment
3. Structural conditions permanently retained.

7. The following are the Main Objectives of Heat
Treatment.
• To soften the material that has been
hardened by previous heat treatment or
mechanical working.
• To harden the steel and increase its strength.
• To adjust its other mechanical and physical
properties like ductility, malleability,
permeability corrosion resistance etc.
• To stabilize the dimensions of the steel
instruments so that they do not expand or
contract with time.

8. • To refine the grain size of the steel.
• To reduce and eliminate the internal stresses.
• To produce special micro structures e.g. to
spheroidize in a tool steel for improved
machinability or to completely retain austenite
structure in stainless steel.
• To eliminate gases.
• To alter the composition of surface layers on
a ductile interior.
• To improve electrical and mechanical
properties.

9. The Principles of Heat Treatment
• Heat treatment consists of Heating-Up and Cooling-Down
process.
• Heating up the steel will change the microstructure to Austenite.
• Cooling down the steel at different cooling rates will change the
microstructure from Austenite to different structures
correspondingly.
• Change in Microstructures result in change in mechanical
properties.
• By heat treatment, we can change the mechanical properties of
moulds and machine components to our desired state.
• For example, to harden the mould will increase strength and wear
resistance resulting in longer mould life.
• To anneal a hard steel bar will soften it to a state good for
machining.
• To normalize a steel bar will toughen it to a state good for impact.

10. Change in Micro-structure
• If a steel bar is heated, it is found that at a specific
temperature, which differs with each class of steel,
important structural alterations begin to take place in it.
This specific temperature marking the beginning of the
structural change is known as the 'lower critical
temperature'. This change concern the composition of
the steel, that is soft ferrite iron and a hard, brittle
substance called Cementite (Fe3C).
• The lower critical temperature is the point at which ferrite
begins to transform to another structure called 'austenite'
and iron carbide starts to dissolve in the 'austenite'.

11. General types of Heat Treatment:-
• It accounts for more
than 80% of all
metals. The various
processes may be
broadly classified
as:

12. Heat Treatment
Hardening
Softening
Thermo Chemical
Process

13. Hardening
Hardening
Tempering

14. Softening
Annealing
Normalizing

15. Thermo chemical
process
Or may be called as
Case Hardening

16. Hardening :-
• This process is intended to produce through
hardened structure by quench-hardening.
This is the most common heat treatment
practice for most of mould making industries.
• Hardening increases wear resistance and
strength of material and provides toughness
after Tempering, so it is widely used for
increasing the life of moulds as well as
mechanical parts of machinery.

17. • Hardening:-
• The work piece is heated slowly, to obtain the finest grain-sizes,
to its hardening temperature - much higher than annealing
temperatures. The metal is kept at this temperature only until
uniform heat distribution and completion of the thermal
transformation. Once the metal has been uniformly heated to
temperature, it is removed from the furnace and placed directly
into a quenching tank. This rapidly cools the metal and the
metal retains its new qualities.
• There is a range of quenching media of varying severity, water
or brine being the most severe, through oil and synthetic
products to air which is the least severe.

18. • Tempering:-
• After quenching the steel is hard, brittle and internally
stressed. Before use, it is usually necessary to
reduce these stresses and increase toughness by
'tempering'. There will also be a reduction in
hardness and the selection of tempering temperature
dictates the final properties
• Tempering is always done below lower critical range.
The operation consists of heating the hardened steel
to a range of temperature which extends from room
temperature to below lower critical range
temperatures.

19. Tempering temperatures
Tool or
Article
Temperature
ºC
Temper
Colour
Turning tools 230 Pale straw
Drills and
milling
cutters
240 Dark straw
Taps and
shear blades
250 Brown
Punches,
reamers
260 Brownish
purple
Cold chisels 280 Dark purple

20. The Softening Processes
• Softening is done to reduce strength or hardness,
remove residual stresses, improve toughness,
restore ductility, refine grain size or change the
electromagnetic properties of the steel. As softening
process increases the toughness of the material so it
may also be called as Toughening Process.
• This process is intended to produce a structure
possesses good strength and ductility in steels by
means of Normalizing. Improved machinability, grain
structure refinement, homogenization and
modification of residual stresses are among the
reasons for which normalizing is done.

21. • Annealing :-
• Used variously to soften, relieve internal stresses,
improve machinability and to develop particular
mechanical and physical properties.In special silicon
steels used for transformer laminations annealing
develops the particular microstructure that confers
the unique electrical properties
• The operation consists of heating the steel to certain
predetermined temperature, soaking at a constant
temperature for a sufficient time to allow the
necessary changes to occur and then cooling at a
very slow rate.

22. • Annealing is the heating and then cooling of metal to
make the metal less brittle, or more malleable and
ductile.
• This will soften the steel that was previously
hardened and reduce internal stresses. Annealing is
done by heating the metal to a temperature beyond
the critical temperature and holding it there for a
period of time. The metal is then cooled with the
furnace and not removed until the furnace is cold.

23. • Normalising:-
• Heating the forged part just beyond the critical
temperature and then allowing it to air-cool completes
normalizing.
• This allows the grain-size to be refined and, if not
held at that temperature too long, will result in a
newly formed crystalline structure. The internal
stresses, if any, will be relieved, hardened steels will
be softened, overheated steels will have a more
favorable, normal fine-grained structure, and
structural distortion will be removed.

24. • This requires heating to above the
critical range temperature, holding for
sufficient time to allow temperature
equalization followed by air cooling. It is
therefore similar to annealing but with a
faster cooling rate.

27. It should be noted that not all steels will respond to all heat treatment processes, Table
1 summaries the response, or otherwise, to the different processes.

28. Thermo-chemical Processes (Case
Hardening):-
• These involve the diffusion, to pre-determined
depths into the steel surface, of carbon,
nitrogen and, less commonly, boron. These
elements may be added individually or in
combination and the result is a surface with
desirable properties and of radically different
composition to the bulk.
• This process is employed to produce a 'case'
or surface layer substantially harder than the
interior or core of the work piece. They
include carburizing, nitriding and induction
hardening

29. Following tables will make the point clear about thermo chemical
processes

30. Table 2

31. Questions:-
• ?