Understanding the kinetics of sigma phase evolution in Super 304H using lab creep tested heats and long-term service aged components

Super 304H is an advanced austenitic stainless steels that is popularly used in superheater/ reheater sections due to its superior oxidation resistance and microstructural stability at higher operating conditions of ultra-supercritical plants. However, recent studies on this alloy have shown significant microstructural changes and associated degradation in creep performance in long-term creep tests. Changes to the microstructure during service and its effect on the long-term creep performance this alloy has not been comprehensively assessed. In this work, the microstructure of long-term service exposed Super 304H RH tubes (~99,600 hours at 596°C steam temperature) are documented, particularly focusing on the formation of sigma phase. The results for the ex-service material are compared to well-documented laboratory creep and aging studies to provide important perspective on improved life management practices for this mainstay advanced stainless steel.