Evolution of the Microstructure and Hardness of Fe-8Cr-2.1Mo-Si-V Die Steel at Different Cooling Rates after Hot Deformation

Abstract

In this study, the effect of microstructure evolution, especially that involving the proeutectoid carbides, on hardness distribution of ledeburite steel containing 8% chromium content was investigated at different cooling rates after hot deformation. The continuous cooling transformation was carried out using a fully automatic transformation measuring apparatus under cooling rates in the range of 0.03-80 °C/s. The transformation temperatures, Ac1, Ac3 and Ms, were measured by dilatometry. A nanoindentation test was employed to evaluate the mechanical properties of the cooled specimens. The evolution of microstructure indicated that the proportion trend of the precipitated carbides was dependent on the cooling rates. In addition, a mixture of pearlite and proeutectoid carbides was observed under slow cooling rates in the range of 0.03-0.2 °C/s, for which only a martensite transformation took place in austenite, and the proeutectoid carbides precipitated negligibly at cooling rates greater than or equal to 10 °C/s. The results also showed that the nanohardness significantly decreased and then slowly increased to a stable value with increasing cooling rates, which was similar to the content trend of the carbides that precipitated during the cooling process.