Microstructure evolution and kinetic analysis of DM hot-work die steels during tempering

Utilizing the hardness measurement, an attempt is made to study the tempering kinetics of DM hot-work die steels, combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that the increasing of tempering temperature will result in precipitation and coarsening of the intra-lath spherical carbides. Meanwhile, it is significantly observed the dissolution of needle-like Mo2C carbides with increasing of tempering temperatures. The growth mechanism of carbides is coarsening of large spheroids after complete and impingement precipitation on dislocations. In addition, the activation energy ΔH of DM steels during additional tempering between 620 °C and 700 °C is slightly higher than that of the diffusion of usual alloying elements (e.g. Cr, V, Mo and W) in ferrite, which may be attributed to a higher Mn content in DM steels that dissolves into carbides and retardsthe coarsening of carbides at high temperature.