Common heat treatments on steel
Softening – is a process where steel is subjected to controlled heat in a Heat treating equipment to reduce strength and hardness, eliminate residual stresses, improve toughness, restore ductility, refine grain size and change its electromagnetic properties. The restoration of ductility and elimination of residual stress is an important operation prior to cold working. Annealing, spheroidizing, normalizing and tempering are the ways with which steel is softened.
- Annealing is a process where steel is heated to a specific temperature in a Heat treating equipment and then allowed to cool slowly so that it is soft enough to be cut and shaped.
- Spheroidizing – is a process that is limited only to steels that are in excess of 0.5% carbon. In the softening process, steel is heated at 727oC so that cold worked ferrite will recrystallize and the iron carbides present in pearlite will form spheroids.
- Normalizing – is a process that requires a soaking temperature of 30oC to 50oC before it is cooled in still air. The applied temperature and cooling rate will produce smaller grain sizes that will improve toughness and yield strength.
- Tempering – is a process carried out on hardened steels to remove internal stresses and brittleness resulting from a severe rate of cooling. Steel is treated inside the Heat treating equipment with a temperature range of 200oC to 600oC according to the final properties desired. The process of tempering will result in a reduction in hardness and slight increase in ductility.
Material modification – is a heat treatment process that modifies the properties of steel in addition to softening and hardening. If the behavior of steel is modified in a beneficial manner, its life can be maximized. For example, in stress relieving steel is subjected to heat in the Heat treating equipment so that the tensions in the material will be minimized without any changes in its microstructure or strength. This will make machining easier and more efficient. Cryogenic heat treatment on steel is carried out as a supplemental process following the hardening treatment. Cryogenic treatment will result into the transformation of retained austenite into martensite with the precipitation of the fine carbide particles that are uniformly distributed in the martensite grains. This will result into an increase in wear resistance, mechanical strength and toughness with low coefficient of friction due to the presence of hard and fine carbide particles.