Discontinuous grain growth in an equal-channel angular pressing processed Fe-9Cr steel with a heterogeneous microstructure

Abstract

A particle-containing Fe-9Cr steel was processed by six passes of equal-channel angular pressing (ECAP) using route Bc to produce an ultrafine-grained (UFG) microstructure with a heterogeneous distribution of high-angle boundaries (HABs) and low-angle boundaries (LABs). Annealing was carried out on the ECAP-processed Fe-9Cr from 500 to 700 °C, for up to 48 h. It was found that the UFG microstructure was stable up to 550 °C. Discontinuous grain growth was first observed in the sample annealed at 600 °C, accompanied by an increase in LAB fraction. The discontinuous grain growth became evident during high temperature annealing (650–700 °C), leading to a reduction in LAB fraction and a significant increase in grain size. Grain growth was the main reason for the decline in hardness. It is proposed that in the current heterogeneous microstructure, grain growth initially occurs in regions of dominantly HABs, and subsequently takes place in regions that mainly consist of LABs, thereby forming a bimodal microstructure. Textures of the annealed structure were shown to be similar to those of the ECAP processed structure. No differences in textures were found between abnormal grains and the matrix in the annealed structure in current case.