Long-term evolution of sigma phase in 304H austenitic stainless steel: Experimental and computational investigation

Abstract

The long-term precipitation of the sigma phase in a 304H austenitic stainless steel, which had been used for an oil refinery system at 720 C-omicron for 18 years, was investigated in terms of both the base and weld metals using experimental and computational methods. The microstructure was quantitatively analyzed using the electron backscattered diffraction (EBSD) technique, which clearly revealed a substantial amount of sigma phase formed at the expense of delta ferrite during the service. Whereas the amount of sigma phase precipitated after service corresponded to the fraction of the initial delta ferrite in the base metal, the fraction of the sigma phase significantly exceeded the initial amount of delta ferrite in the weld metal. The simulation of long-term precipitation kinetics using MatCalc indicated that the nucleation of the sigma phase occurred only in the delta ferrite in both the base and weld metals. The subsequent growth of the sigma phase after the nucleation in delta ferrite was predicted to actively occur in austenite during the service. The simulation results also implied that the precipitation behavior of the sigma phase was very sensitive to the Cr content.