Steel hardenability

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Hardenability of steel - the ability to form a martensitic structure during hardening.

The main factor affecting the hardenability of steel is its chemical composition, ie the content of carbon and alloy and technological elements. In addition to the chemical composition, grain size and homogeneity of austenite and the presence of other, undissolved particles also have an impact on hardenability. Alloy additives (except cobalt) increase steel hardenability by reducing the critical cooling rate. The critical cooling rate, i.e. the lowest cooling rate allowing to obtain a martensitic structure, is related to the durability of the supercooled austenite until its transformation begins, for carbon steels it can be as much as 400-500 ° C / s. The temperature of the beginning and end of the martensitic transformation depends to a large extent on the carbon content of the austenite (see charts illustrating phase transitions as a function of time and temperature, so-called CTP diagrams). Alloy additions affect the hardenability of steel, and hence the ability to harden items with a larger cross-section. The carbon content in steel is the main factor affecting the hardness of steel after hardening. Higher hardenability allows for lower cooling rates (quenching in oil), thus avoiding hardening stresses. The depth of hardening that can be achieved in carbon steels is within 3 to 10 mm, depending on its composition. Additions in alloy steels allow to increase the hardening depth.

Hardenability can be determined by the methods:

With steel hardenability, the concepts are:

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