The salt bath method of hardening is predominant with high-speed steel tools. With correct operating conditions, tools can be heat treated without carburization, decarburization, and scaling. The surface will be fully hard with a minimum of distortion. The four types of salt baths generally used are preheating baths, high-temperature baths, quenching baths, and tempering baths.
Molten salts of various compositions are well adapted to all operations in the heat treatment of tool steels. For tools that cannot be ground after hardening or for tools that require an excellent surface condition and the maintenance of sharp edges, salt bath heating provides excellent results. These salt baths contain different compositions of BaCl2, NaCl2, and KCl as well as additional additives to avoid decarburization. The higher the working temperature, the higher the content of BaCl2 usually is. Higher additions of CaCl2 are typically used in salt baths for the quenching step.
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R Schneider; R. Mesquita; W Schützenhöfer, Distortion in Tool Steels, ASM International, 2014, https://doi.org/10.31399/asm.hb.v04d.a0005958
