Precipitates change of ferritic‐martensitic 11 % chromium steel under short‐term creep

Abstract

A ferritic‐martensitic (FM) 11 % chromium steel with final heat treatment was subjected to a short‐term creep test at a stress of 150 MPa and 600 °C for 1100 h in order to study the change of precipitates in the steel during the creep test. Except for Nb‐rich metall carbides (MC, M23C6) and Laves phases, Fe‐W‐Cr‐rich M6C (based on Fe3W3C) carbides forming during the creep test were also identified in the crept steel by electron diffraction and x‐ray diffraction in combination with energy dispersive x‐ray analysis of extraction carbon replicas. The identified M6C carbides have a fcc crystal structure, a metallic element composition of approximately 44Fe, 32 W, and 20Cr in atomic %, and large sizes ranging from 100 nm to 300 nm in diameter. The M6C carbides are a dominant phase in the crept steel. M6X precipitates are generally not easy to form during high temperature creep, even if it is a long‐term creep, in ferritic‐martensitic 9–12 % chromium steels with a final heat treatment. The present work provides the evidence for the M6C carbides forming during short‐term creep in ferritic‐martensitic high chromium steels. The formation of the M6C carbides was discussed.