TEMPERING

Tempering is a process of reheating of hardened steel below critical range and cooled at the decreased rate (approximately 4–5 min for each mm of the section). There is partial transformation of martensite to secondary constituent troostite and sorbite. The purposes of tempering are: (a) to reduce some amount of hardness produced during hardening and increase the ductility, and (b) to remove strain produced during heating.

Low Temperature Tempering: Steel is heated to 150–250°C and cooled down. This is used to remove internal stress, reduce hardness, and increase ductility without changing the steel structure.

Medium Temperature Tempering: Steel is heated to 350–450°C and cooled down. Martensite is changed to secondary troostite. It results in reduction in strength and hardness, and increase in ductility. It is used for the part which is to be used in impact loading such as chisel, hammer, spring, and spring plates.

High Temperature Tempering: Steel is heated to 500–600°C and cooled down. Martensite is changed to sorbite. Internal stress is relived completely. This is used for the part subjected to high impact and stress such as gear wheels, shafts, connecting rod, etc.