What is quenched and tempered steel?
Hardening of steel and iron involves heating the material to a temperature above its austenitizing temperature, thereby transforming the material structure to austenite. After the material is completely transformed into austenite, rapid quenching is carried out to transform the material structure into martensite, thereby increasing the hardness and strength of the material to the highest possible level. After quenching, the material is tempered to the desired final hardness and strength level by reheating to a lower subcritical temperature, which significantly increases ductility and toughness as hardness and strength decrease.
Neutral hardening occurs in a neutral atmosphere, with no net gain or loss of carbon, nitrogen, or other elements from the surface of the material. This is typically used for finished or near-finished parts that do not allow surface decarburization.
Open flame hardening is carried out in an air atmosphere furnace, where carbon can leave the surface of the material by reacting with air, causing surface decarburization to occur. This is usually done on raw or rough parts where all surfaces will be ground or machined after hardening, which will remove the decarburized surface layer.
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