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Hardening describes the process of making a workpiece harder as a result of increased martensite formation by undergoing thermal treatment (austenitisation and quenching). The austenitisation temperature (T >800°C) varies depending on steel type. After austenitisation the workpiece is quenched in a medium that is suitable for the steel and its construction (e.g. oil, water, polymer solution). Hardening and subsequent cleaning needs to be followed by tempering in order to transform the very stiff and brittle material into a usable spring. This process reduces hardness while at the same time increasing toughness. The tempering temperature leads to the required properties (e.g. tensile strength, yield strength, elongation at brake) depending on the material of the product. Precise adherence to all tempering parameters is essential in order to accomplish the desired spring function.
Unlike conventional tempering (hardening and tempering), austempering employs a bainitic microstructure instead of a martensite microstructure. In this thermal treatment the workpiece is also cooled down after austenitisation. However, in general a salt bath with a temperature of approx. 300°C - 400°C (and therefore well above the martensite formation temperature) is selected as the quenching medium in order to achieve the required mechanical properties, such as hardness, tensile strength and yield strength. The material to be hardened remains in the salt bath until the end of the bainitic transformation. Subsequent cleaning finishes off the hardening process. A bainitic microstructure is very beneficial to complex-shaped components and those liable to cracks, as the likelihood of cracks and warping decreases due to fewer stresses related to hardness. At the same time the absence of tempering brittleness increases the dynamic load-bearing capacity.
