Evolution of the microstructure and mechanical properties of HR3C austenitic stainless steel after ageing for up to 30,000 h at 650–750 °C

After long-term ageing of heat resistant austenitic stainless HR3C steel at 650, 700, and 750 °C for up to 30,000 h, microstructural investigations, identification of precipitates, and testing of mechanical properties were conducted. In the as-received condition, the steel had austenitic microstructures with annealed twins and numerous, large primary NbX and also Z-phase precipitates. Long-term ageing of the steel mainly resulted in numerous precipitation processes: at the grain boundaries, M23C6 carbides and σ phases occur, whereas inside the grains, Z-phase precipitates, M23C6 carbides, and σ phases are observed. The amount and size of σ phases in the structure of the HR3C steel depend on the time and temperature of ageing. The precipitation processes and coarsening decreased the plastic properties and impact energy and resulted in alloy overageing, and the yield strength and tensile strength after 30,000 h ageing were similar to the properties of the as-received steel. In the steel, σ phase precipitates had only a small effect on the toughness, and the dominant impact was shown by M23C6 carbides.