2. Heat-Treatment
Heat treatment is a method used to alter the
physical, and sometimes chemical properties of a
material. The most common application is
metallurgical
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
Generally, heat treatment uses phase
transformation during heating and cooling to
change a microstructure in a solid state.
3.
4. Types of Heat-Treatment (Steel)
Annealing
Tempering, and Quenching
Precipitation hardening
Case hardening
5. Annealing
A heat treatment process in which a metal is exposed to an
elevated temperature for an extended time period and
then slowly cooled.
Purpose:
1.Relieve stresses of cold working
2.Increase softness, ductility and toughness
3.Produce specific microstructure
6. Annealing
α+Fe3C
T
Three Stages of Annealing
1. Heating to a desired temperature
2. Holding or soaking at that temperature
3. Cooling usually to room temperature
Note: Time in above procedures is important
- During heating and cooling temp gradients exit b/w inside and
outside portions of part. If rate of temp change is tool high,
temp gradients will induce internal stress in part and hence
cracking
2
Time
Time
α+Fe3C
T
1 3
11. 1. Stress-Relief
Annealing
It is an annealing process
below the transformation
temperature A1, 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
properties
12. 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 recovery & recrystallization may take place.
Machining allowance sufficient to compensate for
any warping/distrotion resulting from stress
relieving should be provided
1. Stress-Relief
Annealing
13. Causes of Residual Stresses
1.Mechanical factors (e.g., cold-working during
metal forming/machining)
2.Thermal factors (e.g., thermal stresses caused by
temperature gradients within the work-piece during
heating or cooling)
3.Metallurgical factors (e.g., phase transformation
upon cooling wherein parent and product phases have
different densities
- In the heat treatment of metals, quenching or rapid
cooling is the cause of the greatest residual stresses
14. Higher temperatures and
longer times of annealing
bring residual stresses to
lower levels
All kinds of times (heating
time, soaking time, cooling
time)
Stress Relief Annealing –
Temperature & Time Vs
Stresses
15. Stress Relief Annealing –
Cooling Rate Vs
Stresses
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:
(1)Cooling rate
(2)Cross-sectional size of the
work- piece, and
(3)Composition of
the steel
16. 2. Normalizing
A heat treatment process consisting of
austenitizing at temperatures of 50–80˚C
above upper critical temperature (A1 , Acm)
followed by slow cooling (usually in air)
The aim of which is to obtain a fine- grained,
uniformly distributed, ferrite– pearlite
structure
Normalizing is applied mainly to unalloyed
and low-alloy hypo-eutectoid steels
For hypereutectoid steels the austenitizing
temperature is 50–80˚C above the ACm
transformation temperature
17. Normalizing – Heating and
Cooling
A3
A1
Purpose of soaking:
1. To allow metal to
attain uniform temp
2. All the austenite
transform into
pearlite, especially
for hyper-eutectoid
compositions
18. Normalizing – Austenitizing
Temperature Range
1. Depend on
composition
2. Increase in C %
reduces temp for
hypo-eutectoid steels
3. Increase in C %
increases temp for
hypo-eutectoid steels
19. Effect of Normalizing on Grain Size
Normalizing refines (reduces) the grains of a steel that
have become coarse (long and irregular) as a result of
heavy deformations as in forging or in rolling
The fine grains have higher toughness than coarse
grains,
Steel
with
0.5% C
20. Normalizing after Rolling
After hot rolling, the
structure of steel is usually
oriented in the rolling
direction
To remove the oriented
structure and obtain the
uniform mechanical
properties in all
directions, a normalizing
annealing has to be
performed
21. Normalizing after Forging
• After forging at high temperatures,
especially with work-pieces that vary
widely in cross sectional 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 is also done to improve
• machinability of low-c steels
22. 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).
23. 3. Full
Annealing
- For compositions less than eutectoid, the metal is heated above
A3 line to form austenite
- For compositions larger than eutectoid, the metal is heated
above A1 line to form austenite and Fe3C
- Cooled slowly in a furnace instead in air as in Normalizing.
Furnace is switched off, both metal and furnace cool at the same
rate
-Microstructure outcome: Coarse
Pearlite. In Normalizing,
structure?
-Structure is relatively softer than
that in Normalizing
-Full annealing is normally used
when material needs to be
deformed further.
Usually applied for low
and medium C steel
24. 4. Spheroidizing Annealing
It is also called as Soft Annealing
Any process of heating and cooling steel that produces a
rounded or globular form of carbide (Fe3C)
It is an annealing process at temperatures close below
or close above the A1 temperature, with subsequent slow
cooling
Used for Medium & High C-Steels
- Spheroidite can form
at lower temperatures but the
time needed drastically
increases, as this is a diffusion-
controlled process.
Fe3C
Fe3C
25. Spheroidizing: How to Perform
By heating alloy at a temp just
below A1 (700C). If pre-cursor
structure is pearlite, process time
will range b/w 15 & 25Hrs
Heating alloy just above A1 line
and then either cooling very slowly
in the furnace or holding at a Temp
just below A1
Heating & cooling alternatively
within ±50C of the A1 line.
26. Spheroidizing - Purpose
The aim is to produce a soft structure by changing all hard
micro-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 soft annealed at 820C.
27. 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
28. 5. Isothermal Annealing
Spheroidizing is more useful for improving machinability of
high C steel than that of low and medium C steels.
In fact, spherodized low and medium C steels become over
soft for machining and give long shavings which accumulate
on tool cutting edge and produce poor surface.
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 following
characteristics:
1. High proportion of ferrite
2. Uniformly distributed pearlite grains
3. Fine lamellar pearlite grains
29. Process – Isothermal Annealing
Austenitizing
followed by a fast
cooling to the
temperature range of
pearlite formation
(usually about 650˚C.)
Holding at this
temperature until the
complete
transformation of
pearlite
and cooling to room
temperature at an
arbitrary cooling rate
Fe3C
?
