Investigation on the correlation between grain boundary character distribution and precipitate behavior in T91 ferritic/martensitic steel

Abstract

In the present study, the relationship between microstructure, grain boundary character distribution (GBCD) and precipitate behavior in T91 steel was investigated via a series of thermomechanical treatments (TMTs). The experimental results revealed that subjecting the steel to cold-rolling (CR) with a deformation of 15–30% followed by tempering at 750 °C for 1 h caused the microstructure of T91 steel to transition from the typical lath martensite structure to a uniform polygonal ferrite, resulting in a change in the GBCD. Specifically, the proportion of random grain boundaries for the ferritic structure was significantly higher than that for the martensitic structure. The M23C6 carbide appeared as irregular polygons and precipitated within the ferrite grains, while it exhibited a rod-like shape and precipitated at the martensitic boundaries. Additionally, the orientation relationship (OR) between M23C6 carbide and the adjoining matrix transformed from the double Kurdjumov-Sachs (DKS) relation to (112) M23C6 // (110) ferrite and 201¯M23C6 // [012] ferrite. The change of OR resulted in a higher coarsening rate of M23C6 carbides precipitated in ferrite compared to those precipitated at martensitic boundaries.