Austenite formation in 9Cr–1Mo type power plant steels

Abstract

Modern power plant steels are post-weld heat treated at temperatures in excess of 700°C. Some of these steels are richly alloyed with elements such as nickel or tungsten and have been reported to exhibit unexpected changes in strength as a function of heat treatment and solute concentration. It is demonstrated here that the changes are a consequence of solute induced variations in the stability of austenite in the temperature range over which heat treatments are conducted. The evidence for this comes from direct experimental measurements using dilatometry and from phase diagram calculations. Excessive alloying with nickel is found to lead to the unintentional formation of austenite during post-weld heat treatment, whereas variations in the concentrations of ferrite stabilising elements render the alloys incapable of becoming fully austenitic. This in turn leads to quite large variations in the mechanical properties of the final component.