Properties of cold-work tool steels are primarily determined by the nature of carbides in the microstructures. Therefore, it is of crucial importance to know the behavior of carbide formation with respect to the processing condition. In this study, the fabrication and heat treatment processes of a commercial 8%Cr cold-work tool steel were simulated. And the evolution of various carbides was investigated following the full processing route.
Small cast coupons were obtained by plasma arc remelting of the commercial alloy. High-temperature compression was performed to simulate the hot working process. Then, the specimens were subjected to spheroidization annealing followed by quenching and tempering heat treatments. The characteristics of the carbides were analyzed in every step of the processing route by using SEM, TEM, EDS and EBSD techniques.
It was revealed that (Fe,Cr)-rich M7C3, Nb-rich MC and Mo-rich M₂C carbides formed during the solidification. The crystallized M7C3 and MC existed through the whole processing route, while M₂C was dissolved in the matrix after the compression at 1180℃. M7C3, (Fe,Cr)-rich M23C6 and Mo-rich M6C were precipitated after spheroidization annealing at 870℃. Although M₂C and M6C were not thermodynamically equilibrium phases in the given composition, they were formed due to the segregation of Mo. The M23C6 and M6C were dissolved after quenching from 1030℃, while fine M23C6 was precipitated again after tempering at 520℃.