STUDY ON σ PHASE PRECIPITATION OF HR3C STEEL USED IN ULTRA-SUPERCRITICAL BOILER

HR3C steel is a new type of austenitic heat-resistant steel which has been widely used for superheater and re-heater tubes in the ultra-supercritical (USC) boiler. The mechanical properties of HR3C steel were dependent on the stability of the microstructure, particularly the large amount of precipitates formed during service. The precipitation of sigma phase in HR3C steel during long-term aging for 2000 h at temperature of 700 and 750 degrees C was investigated by OM, SEM and TEM. The phase calculation method was applied to understand the factors influencing the precipitation. After 1000 h of the aging duration, irregular mass second phase was found to precipitate at the grain boundary, followed by the subsequent increasing and coarsening with time. The constituent elements of the phases were determined as Fe and Cr through SEM equipped with EDS. Further SAED analysis results led to the confirmation that these phases were basically sigma-FeCr compound. Combined with the prediction made through New PHACOMP method, the microstructure or phase structure in initial state may affect the subsequent precipitation behavior.