Introduction to Basic Manufacturing Processes and ... - always yours

136 Introduction to Basic Manufacturing Processes and Workshop Technology
Line PSK
This line occurs near 723°C and is a horizontal line and is known as lower critical
temperature line because transformation of steels starts at, this line. Carbon % has not effect
on it that means steel having different % of carbon will transforms at the same temperature.
The range above the line up to GSE is known as transformation range. This line tells us the
steel having carbon up to 0.8% up to 0.8% will starts transforming from ferrite and pearlite
to austenite during heating.
Line ECF
It is a line at temperature 1130°C which tells that for cast iron having % of C from 2%
to 4.3%. Below this line and above line SK, Cast iron will have austenite + ledeburite and
cementite + ledeburite.
8.6.3 Critical Temperatures
The temperatures at which changes in structure takes place is known as critical temperatures,
these are as follows:
1. The temperature along GSE is known as upper critical temperature. The
temperature along GS during heating as (upper critical temperature) where austenite
+ alpha iron changes into austenite and vice versa.
2. The temperature along GS during cooling as A 3
where austenite changes into
austenite + alpha iron and vice versa during heating.
3. The temperature along line SE during heating as Acm changes into austenite from
austenite + cementite and vice versa.
4. The temperature along PSK is known as lower critical temperature when
pearlite changes into austenite on heating as denoted, by A 1
.
8.6.2 Objectives of Heat Treatment
The major objectives of heat treatment are given as under
1. It relieves internal stresses induced during hot or cold working.
2. It changes or refines grain size.
3. It increases resistance to heat and corrosion.
4. It improves mechanical properties such as ductility, strength, hardness, toughness,
etc.
5. It helps to improve machinability.
6. It increases wear resistance
7. It removes gases.
8. It improves electrical and magnetic properties.
9. It changes the chemical composition.
10. It helps to improve shock resistance.
11. It improves weldability.
The above objectives of heat treatment may be served by one or more of the following
heat treatment processes: