Influence of thermo-mechanical treatment in austenitic and ferritic field condition on microstructural and mechanical properties of reduced activation ferritic-martensitic steel

Abstract

Abstract 9Cr-1W-0.06Ta reduced activation ferritic-martensitic (RAFM) steel has been investigated in normalized and tempered (N + T) condition and in thermo-mechanically treated (TMT) in austenitic and ferritic field conditions. RAFM steel in N + T condition has a tempered martensitic structure (body-centred tetragonal, bct). It is unstable and will transfer into a stable body-centered cubic (bcc) in the structure during a long period subjected to high temperatures. To make it stable structure, the steel was subjected to austenitization at 1423 K for 10 min, and then cooled in still air (≈−1 K s−1) to 973 K and this temperature was maintained for 2 h in furnace to fully convert austenite into ferritic phase then the steel was subjected to 25% reduction in thickness by hot rolling. Optical, scanning and transmission electron microscopic investigations have been carried out to assess the microstructural changes of the steel N + T and TMT conditions. Hardness, tensile and creep studies are carried out and the results were correlated with the microstructural studies. TMT processed steel resulted in coarser prior austenite grains and exhibited ferrite in phase with fine distribution of M 23C6 and MX precipitates whereas N + T condition is subjected to tempered martensitic structure with coarser M 23C6 and MX precipitates. Even though ferrite phase present in TMT processed steel it exhibits higher tensile and creep strengths due to the presence of high dislocations and finer distribution of precipitates than the N + T condition.