The effect of tempering temperature on deformation is mainly caused by the transformation of the structure during tempering. If the phenomenon of "secondary quenching" occurs during the tempering process, the retained austenite is transformed into martensite, and the specific volume of the martensite generated is larger than that of the retained austenite, which will cause the mold cavity to expand; For some high-alloy tool steels, such as Cr12MoV, high temperature quenching is used for the main requirement of red hardness. When multiple tempering, the volume will expand once for each temper. If tempered in other temperature regions, the specific volume decreases due to the transformation of quenched martensite to tempered martensite (or tempered sorbite, tempered troostite, etc.), and therefore, the cavity tends to shrink.
In addition, the relaxation of the residual stress in the mold during tempering also affects the deformation. After the mold is quenched, if the surface is under tensile stress, the size will increase after tempering; on the contrary, if the surface is under compressive stress, it will shrink. But of the two effects of organizational transformation and stress relaxation, the former is the main cause.