High-temperature softening mechanism and kinetic of 4Cr5MoSiV1 steel during tempering

The high-temperature softening mechanism and kinetic of 4Cr5MoSiV1 steel during tempering from 500 to 700 °C for different periods are systematically investigated by scanning electron microscope (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and x-ray diffraction (XRD). The obtained results show that a sharp decrease in hardness occurs in the first 10 min of tempering, and the softening rate increases significantly with increasing tempering temperature. It exhibits the secondary hardening when tempered at 500 °C for 2 h, which may be triggered by Guinier-Preston (G.P.) zone. That is actually the desolvation precipitation of clustering of Cr and C. High-temperature softening mechanism of the hot working die steel is that the tempered martensite with high dislocation density and a high carbon super saturation undergoes local recrystallization, precipitation of alloying carbides, the growth of grains and second phase at tempering 560, 600 and 640 °C, respectively. In addition, the activation energy of 4Cr5MoSiV1 steel during tempering between 560 °C and 700 °C is slightly higher than that of the diffusion of alloying elements in ferrite, which may be ascribed to a high alloying content and high quenching temperature.