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The term martensitic hardening refers to a heat treatment process consisting of austenitization and cooling under conditions that produce an increase in hardness through the more or less complete transformation of austenite into martensite.
Austenitization is the treatment step in which the workpiece is brought to the austenitizing temperature and full phase transformation and carbide dissolution occurs changing the structure of the steel to austenitic.
Austenitization is followed by the cooling step. To ensure that the entire workpiece assumes a martensitic structure, the speed of the temperature drop must be greater than the critical cooling rate of the particular steel.
Cooling can be carried out in a range of different media characterized by their cooling effects in the different temperature ranges.
Case hardening is one of the thermochemical processes. In this process, the surface layer of components and tools is carburized with a carbon-emitting medium and then quenched, improving the mechanical properties (e.g. wear) of the component's surface layer. Carburization is usually at temperatures of 880 to 950° C. After the hardening of the carburized parts, annealing is generally required to reduce the stresses resulting from hardening and to achieve the required strength.
A range of different systems such as chamber furnaces and continuous furnaces are available for case hardening. Oils are generally the media used for quenching.
The conventional way to increase hardness or strength is by hardening and tempering. A second, somewhat more specific method is austempering, or bainitic hardening.
In this form of heat treatment, the component is austenitized in the same way as for other forms of hardening, i.e. it is heat-treated at temperatures of 800 to 900° C, depending on the material.
Quenching then takes place in a hot salt bath. The temperature of the salt bath depends on the material, and can range from 220 to 450° C. The part remains in the salt bath at a constant temperature (isothermal) until the structural transformation of austenite to bainite is complete (intermediate stage). No martensite is formed. Depending on the material, the transformation can be completed in a few minutes; but can sometimes also take several hours.
Bainite structures have very specific properties, characterized by high strength (hardnesses), maximum toughness and, generally speaking, relatively little distortion. Very low residual austenite contents that can otherwise only be achieved by deep-freezing are also possible.
For special components that require partial hardening by laser due to high wear requirements, we have developed laser hardening processes. Because the heat input is low, the hardening is produced by "self-quenching" – and ´there is no distortion.
In the process of inductive heating, medium or high frequency alternating electrical current is used to generate an induction current in the workpiece through an inductor adapted to the contour to be hardened, producing heat.
The increase in hardness is caused by a transformation of the heated layer into martensite (during quenching), and the hardness that can be achieved depends on the carbon content and composition of the alloy.
Quenching is controlled, with the time window depending on the material, and is usually done as a supplementary quench with a synthetic polymer solution.