21) What is meant by lower and higher critical points ?
While heating a steel part, the temperature raises and it changes the structure of steel part .The temperature point at which the change starts is called lower critical point and the temperature at which the change ends is called higher critical point.
22) What are the common heat treatment processes used in industries ?
The following heat treatment processes are generally used in manufacturing :
- Case hardening
23) What do you mean by Carburising ?
Carburising is a case hardening process in which carunaceous materials such as leather scraps, charcoal dust, nut shells, bone powder etc. are packed with low carbon steel parts and heated together, thus dissolved carbon combines with the outer surfaces of the parts. After reheating the parts to above critical temperature it is suddenly cooled for making outer surface harder.
25) What is Case hardening ?
It is a process of making outer surfaces harder of the steel part. Generally it is done for the following purposes,
- To make outer surface harder.
- To bring the cost cheaper
- To utilize low carbon steel.
26) Describe the Annealing process ?
The process for softening the steel part is called Annealing. Generally it is done for the following purposes,
- To soften the steel for machining.
- To refine the grain size in the steel.
- To increase ductility in the steel.
- To relieve internal stresses.
- To change electrical and mechanical properties.
27) Define Normalising ?
Normalising is a process to make the steel part to its normal stage. The main purposes are
- To reduce grain size of steel.
- To achieve desired mechanical properties.
- To remove internal stresses caused by working.
28) What is Nitriding ?
It is the case hardening process in which work piece is heated in a steam of ammonia at 500 to 550 degree C.
29) Why is it not possible to harden unalloyed tool steel part right through its full thickness ?
Because it requires very high cooling rate which is only possible for its outer surface.
30) What are the advantages of flame hardening ?
- The work piece can be hardened on the required spot.
- Heat can be given to required portion of the work piece.
- Time can be saved.