Tools that are heat treated in molten salt baths are heated up much more rapidly than by convection or radiation, thereby heat treating times are significantly shortened. The molten salt provides a ready source of heat and maintains the rate of heating necessary to reach equilibrium with the rate of heat absorption.
Although steels come in contact with heat only through the tool surfaces, the heat is quickly drawn to the core by thermal conductivity. The high rate of heat transfer is also essential in the quenching step from austenitizing temperature to the temperature of the quenching bath, thereby avoiding the formation of pre-eutectoid carbides at grain boundaries, which can cause embrittlement.The figure below shows a possible arrangement of a salt bath line for high-speed steels. Depending on the loading, full but also partial hardening of the tools is possible. A softer shaft is often desired, especially for twist drills and millers.
For more information, click on the link below (subscription required). Then scroll to Figure 2.
R. Schneider ; R. Mesquita ; H. Altena ; T. Müller ; P. Seemann,Processes and Furnace Equipment for Heat Treating of Tool Steels, ASM International, 2014, https://doi.org/10.31399/asm.hb.v04d.a0005958
