Influence of ageing on microstructure and mechanical properties of TP347HFG austenitic stainless steel

The paper presents the results of microstructural and mechanical investigation of long-term aged TP347HFG austenitic stainless steel. Ageing was performed at a time of up to 30 000 hours and the temperature of 600 and 650◦C. Ageing was proved to lead to the precipitation of secondary phase particles not only inside grains but also on the boundaries of grains and twins. The MX precipitates were observed inside the grains. However, M23C6 carbides and sigma phase precipitates were observed on grain boundaries. The changes in the microstructure of the examined steel translated into the mechanical properties, i.e. initially observed growth and then the decrease of yield strength and a gradual decrease in impact energy. The overageing process – a decrease in strength properties – was associated with the growth of the size of M23C6 carbides and the precipitation of the sigma phase. The reduction of impact energy in TP347HFG austenitic stainless steel was found to be associated with the precipitation of M23C6 carbides in the case of the 600◦C temperature, and the M23C6 carbides and sigma phase in the case of the 650◦C temperature. The rate of changes in the microstructure and mechanical properties depended on the ageing temperature.