Lec 5 d Heat treatment.pptuyjghcgggghhdhdhd

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1. Engr. Dr. Aqib Mashood
Khan
[email protected]
1
Lec 5 Heat treatment

2. Heat
Treatment of
Metals

3. 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.

15. Types of Heat-Treatment (Steel)
 Annealing
 Tempering, andQuenching
 Precipitation hardening
 Case hardening

16. 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:
•Relieve stresses of cold working
•Increase softness, ductility and toughness
•Produce specific microstructure

18. 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

19. Iron-C Phase Diagram
A
B

20. Temp Ranges in Fe-C Phase Diagram
-
A1. Lower
critical Temp
A3. Upper
critical Temp for
Hypo- eutectoid
steels
Acm. Upper
critical Temp for
Hyper- eutectoid
steels
Eutectoid
α+Fe3C

 

 
Fe
3C
T

21. Temp Ranges for Annealing Processes

22. 1. Stress-Relief Annealing
 It is an annealing process
below the transformation
temperature A
1, 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

23.  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

24. 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

25.  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

26. 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:
•Cooling rate
•Cross-sectional size of the
work- piece, and
(3)Composition of
the steel

27. 2. Normalizing
 A heat treatment process consisting of
austenitizing at temperatures of 50–80˚C
above upper critical temperature (A
1 ,
A
cm) 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 A
Cm transformation
temperature

28. Normalizing – Heating and
Cooling
A3
A1
Purpose of soaking:
1.To allow metal to
attain uniform temp
1.All the austenite
transform
intopearlite, especially
for hyper-
eutectoidcompositions

29. 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

30. 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

31. 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

32. 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

33. 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).

34. 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

35. 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 A
1 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.
 
Fe
3C
 
Fe
3C

36. 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.

37. 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.

38. 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

39. 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:
•High proportion of ferrite
•Uniformly distributed pearlite grains
•Fine lamellar pearlite grains

40. 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
 
Fe
3C

?