The 2nd term to discuss is the hardening process and what transformation the steel goes through during the hardening process.
Hardening (A specific heating and cooling process based on the chemistry of the material) is performed on all ferrous steels to develop the proper microstructure and optimal properties of the material.
The 1st stage of hardening is the preheat cycle. Preheating the steel helps to prevent thermal shock & helps the material adjust to the higher hardening temperatures.
During the heat treating cycle, the material goes through many transformation stages, usually 10-12 stages. During this process the material will shrink & grow. It will form austenite (material will shrink and become non-magnetic), then martensite (Growth larger than its starting size at room temperature), then tempered martensite (returns to dimensions very close to starting datums at the start point).
Note: Proper hardening processes will produce as close to a homogeneous (tight) microstructure as possible, which produces a better tool or part.
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