Microstructural evolution of SIMP steel during martensitic transformation

SIMP steel, a reduced activation ferritic/martensitic heat–resistant steel, is a promising candidate for structural applications in future nuclear fusion reactors. Understanding the evolution of microstructure in SIMP steel is of practical importance for improving the structural integrity of components made from this material. In this work, we study the effects of cooling rate on the evolution of microstructures in the SIMP steel, including martensitic transformation and indentation deformation. At low cooling rates, the martensitic transformation exhibits characteristics of multi–step transformation, while these characteristics disappear at high cooling rates. A modified Koistinen-Marburger model was introduced to analyze the temporal evolution of martensite in the SIMP steel at stage IV under three different cooling rates (0.5, 1, and 5 K/s). The characteristic undercooling temperature at stage IV follows a power-law dependence of the cooling rate with a power index of 0.17. The dependence of indentation hardness on the characteristic block width exhibits two distinct regimes: a Hall-Petch-like relationship for block widths less than or equal to 4.1 μm, and a possible inverse Hall-Petch-like relationship for block widths larger than or equal to 4.1 μm.