We ended by mentioning that there were other heat treatments including hardening, cold treating, or cryogenic treating. Let’s get right into learning about a few of the treatments.
Annealing
Annealing is the process of heating a metal to a specified temperature than cooling it at a rate that will refine the microstructure. It may soften a metal, make it easier to machine, or enhance its electrical conductivity. It may be done to improve cold-working properties. Ferrous alloys are full annealed or process annealed. Full annealing is a slow process that results in coarse pearlite. Process annealing aims to produce a uniform microstructure. Large ovens are used for annealing. Gas-fired conveyor furnaces are used for high-volume production. Pieces are often left in the oven to control the cooling. Others are removed, some are quench hardened.
Normalizing
Normalizing is a sub-process of the annealing treatment. This technique results in uniform grain size and composition. Normalizing eliminates columnar grains and dendritic segregation, byproducts of casting. Normalizing results in harder, stronger steel. The process involves heating to relieve stress and then air-cooling. A ferrous alloy will be heated above the transformation temperature and cooled in air to well below the transformation range. Normalizing may be done to improve stability for further heat treatment.
Resistive Heating
Resistive heating, also a sub-process of annealing, is done to anneal copper wire. The heating system employs a controlled electrical short circuit. Resistive heating does not require a temperature-regulated furnace. Wire passes across two conductive pulleys after it is drawn. The electrical potential of the pulleys causes the short circuit. The Joule effect raises wire temperature. The speed the wire can be drawn depends on the voltage applied.
Stress Relieving
To remove or reduce internal stresses in a metal caused by cold working or non-uniform cooling stress relieving is done by heating a metal below the lower critical temperature and then cooling uniformly.
Cold and Cryogenic Treatment
Cold treatments often follow quenching and precede tempering. Cold treating will increase hardness, wear resistance, and reduce internal stresses. There is a risk of cracking, however. Cold or cryogenic treatment is often done on tools or bearings requiring good wear resistance. High-carbon or high-alloy steels are good candidates for cold treatment. Quenching will transform austenite to martensite, but not all. Some will remain. Further precipitation of austenite into martensite occurs with slow cooling and cryogenic treatment. The metal may reach -115 or even -315 degrees Fahrenheit.
Please comment below if you’d like a more in-depth article on any particular process.