The effects of heat treatment temperature on the precipitation behavior of χ phase and impact toughness in a UNS S38815 stainless steel

The effects of heat treatment temperature on the microstructure and impact toughness of a UNS S38815 high-Si stainless steel were investigated by means of SEM and EBSD. The microstructure composed of matrix austenite (γ), ferrite (δ) and intermetallic chi (χ) phase precipitates. 800 °C heat treatment led to a eutectoid decomposition of δ phase, which fully converted into island-like χ phase particles and secondary austenite (γ2) grains. The γ2 grains can be swept away by the growth of their adjacent primary γ grains. As the increase of the temperature, the χ phase particles coarsened at 1020 °C and completely dissolved at 1120 °C, resulting a microstructure only composed of δ and γ. The further increase of heat treatment temperature to 1200 °C produced a microstructure with >40% δ phase. The orientation relationship (OR) analysis shows that χ phase particles and γ2 grains adopt the cube-on-cube and the Kurdjumov-Sachs (K-S) ORs respectively from their parent δ grains, resulting in a K-S OR between χ particles and γ2 grains. χ phase particles may also precipitate at the interfaces of δ/γ or γ/γ without any specific OR with them. The impact absorption energy of samples strongly depended on the precipitation of χ phase, showing the worst toughness in the 800 °C heat treated sample and the best toughness in the 1120 °C heat treated sample (free of χ phase). The strip distribution of δ along ED direction induces anisotropy in toughness.