5. The four types of
heat treatment processes:
1. Annealing
2. Normalising
3. Hardening
4. Tempering
6. Purposes of Annealing Processes
1. To soften steels
3. To improve micro-structure of steels
2. To relieve internal stresses induced by
cold-working.
<Click here to view wire drawing>
< Click here to view grain refining>
18. Spherodising Annealing
Apply to high Carbon Steel ( 0.5%)
to improve machinability and ductility
Cementite plate changes to
cementite spheroids
Heat up the steel to 650 to 700 oC,
hold it for 24 hour, cool it in furnace
23. (c) quench in water or oil
or other media.
(b) Hold at that temperature for sufficient
time to form austenite;
Heating
Soaking
(a) heat the steel to austenitising temperature
i.e. 30-50 0C above UCT for hypoeutectoid and
30-50 0C above LCT for hypereutectoid;
24. PRINCIPLE OF HARDENING
At temperature above UCT,
microstructure is Austenite
slow cooling
Ferrite/cementite
form first when temp.
below UCT
Rapid quenching
Martensite
Hard brittle
Pearlite form at LCT
36. To improve Toughness Ductility
To relieve internal stresses
Aims of Tempering process
37. How to carry out tempering process
Heat the quenched steel to a
temperature below the lower critical
temperature
Hold the steel at the temperature
Cool slowly to room temperature
Hardening
Temp.
Time
Tempering
Austenitising
Below LCT
Hardening
38. Effect of tempering temperature
Properties
Tempering Temperature
200° 400° 600°
Strength
Hardness
Toughness
Ductility
/
39. Ductility
Effect of tempering temperature
Toughness
Tensile strength
Hardness
200° 400° 600°
Martensite
Tempering Temperature
Properties Low temp. e.g. 200°
C:
high hardness and
wear resistance are the
prime consideration
Use for applications
workshop file,
cutting tools
40. Effect of tempering temperature
200° 400° 600°
Martensite Tempered martensite
Tempering Temperature
Properties At higher
tempering temp.
e.g. 480 0C
good strength
toughness
Use for
applications
e.g. crankshaft
Strength
Hardness
Toughness
Ductility
41. Effect of tempering temperature
Properties
Tempering Temperature
200° 400° 600°
Strength
Hardness
Toughness
Ductility
! 0 ! %
700 0C
1 2 3
43. To refresh what has been covered
1. Which of the following structures of
steel is the hardest ?
a. Ferrite
b. Austensite
c. Martensite
d. Pearlite
44. 2. Which of the following heat treating
processes is employed to improve
the toughness of a steel which has
just been hardened ?
a. Tempering
b. Process annealing
c. Spheroidising annealing
d. Full annealing
45. 3. Which of the following heat treating
processes is employed to improve
the grain size before hardening
process ?
a. Normalising
b. Tempering
c. Spheroidising annealing
d. Full annealing
46. 4. Which of the following structures is
transformed into martensite after the
hardening process ?
a. Ferrite
b. Pearlite
c. Cementite
d. Austenite
47. To refresh what has been covered
1.Which of the following heat treating
processes is used to soften high carbon
steel to improve its machinability ?
a. Process annealing
b. Spheroidising annealing
c. Full annealing
d. Normalising
48. 2. Which of the following heat treating
processes employs air to cool the
parts /components to be heat treated ?
a. Process annealing
b. Spheroidising annealing
c. Full annealing
d. Normalising
49. 3. Which of the following heat treating
processes is employed to soften steel
so that it can be cold worked further ?
a. Process annealing
b. Spheroidising annealing
c. Full annealing
d. Normalising
50. 4
+
5 $
+
Temp. 0C
A:
B:
Soaking temp. A: 763 0C; B: 2000C
Microstructure: Martensite
Properties:
After Hardening: Hard Brittle
After Tempering: High hardness
wear resistance
