The microstructural development of type 321 Austenitic Stainless Steel with long term ageing

Austenitic stainless steel is important in the power generation industry where it is expected to be in service at high temperatures for extended periods of time. Work carried out on the microstructural development of two 321 stainless steel samples has shown that there are complex phase changes that can take place in this alloy. Although the alloy is expected to be fully austenitic at room temperature there is a fraction of ferrite present in the as-received materials. High temperature XRD has shown that this ferrite phase can be dissolved at temperatures between 800 and 900°C but precipitates on cooling at temperatures below 200°C. Due to the low temperature of formation, similarities in chemistry and orientations relationships indicate that the ferrite is forming in a displacive manor from the austenite grains. Thermal ageing at 750°C has been carried out up to times of 15,000 hours and the microstructural changes quantified. The fraction of sigma phase and ferrite increases with ageing time with a corresponding decrease in austenite fraction. This change in the microstructure is postulated to be caused by the changes in the matrix chemistry due to the formation of second phases particles.