Effect of PWHT of X6Cr-NiNbN 25-20 steel on the evolution of the sigma phase during creep exposure at 700 °C

Abstract

Superheaters and reheaters made of heat-resistant austenitic steels are being replaced in existing fossil fuel power plants to operate at Advanced Ultra Super Critical (A-USC) parameters of steam. Newly developed and used high nickel content austenitic steels, such as in X6Cr NiNbN 25-20, are predicted to significantly suppress the precipitation of a hazardous s-phase during long-term creep exposure. The sphase causes premature failure of the component during creep, which is usually supported by a local change in hardness in the case of welded assemblies. Experimental results show that PWHT significantly improved reduction of area. However, for the PWHT specimen with a shorter exposure time of 15,712 hours, there was an acceleration in the precipitation of sigma-phase particles during creep at 700 degrees C compared to the specimen in the AW state, after 20,557 hours.