Precipitation study in Ti-stabilised austenitic stainless steel after 207,000 h of service

The article presents the results of research on the precipitation processes in austenitic T321H steel in the as-received state and after around 207,000 h of service under the creep conditions at the temperature of 540 degrees C. The tests performed on the T321H steel showed that during long-term service the precipitation of G phase and sigma phase occurred and those phases precipitated at the cost of M23C6 carbides. The preferential place of precipitation of the phases was the boundaries of grains, and in the case of sigma phase - the triple points at grain boundaries. After long-term service, the sigma phase was the dominant precipitate in the examined steel. Inside the grains, both in the as-received state and after long-term service, the TiX and TiC precipitates were observed. The following precipitation sequence for austenitic T321H steel can be suggested: MX -> MX + M23C6 -> MX + M23C6 + sigma phase -> MX + M23C6 + sigma phase + G phase