Precipitates change of ferritic-martensitic 11 % chromium steel under short-term creepAusscheidungsveranderungen von ferritisch-martensitischem 11 % Chromstahl beim Kurzzeitkriechen

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 degrees 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.