Preheating normally is employed as a safeguard against the cracking and extreme distortion resulting from the thermal shock undergone by a cold workpiece when it is exposed to the high temperature of the austenitizing furnace. It also is important to avoid large differences in temperature from surface and core regions in large tools with a thickness over 100 mm.
Preheating tool steels before austenitizing is sound practice, but it is not always required. For small pieces of simple shape, preheating may be eliminated.Preheating is especially beneficial for the highly alloyed hot work and high-speed steels, because it gives them a greater length of time to reach thermal equilibrium, and it eliminates most of the risk of prolonged exposure to austenitizing temperatures, such as decarburization and the tendency for surfaces of large sections to experience longer exposure times.
For more information, click on the link below (subscription required). Then scroll to Figure 4.
Rafael A. Mesquita; Reinhold E. Schneider, Introduction to Heat Treating of Tool Steels, Vol 4D, ASM Handbook, Edited By Jon L. Dossett, George E. Totten, ASM International, 2014, https://doi.org/10.31399/asm.hb.v04d.a0005946
