Optimization of INCONEL® Alloy 740 for Advanced Ultra Supercritical Boilers

INCONEL (R) alloy 740 was developed as a material to enable design of coal-fired boilers capable of operating with 700 degrees C steam temperature and high pressure. This material was successful in meeting the targets of the European program which called for its development; these targets included a stress rupture requirement of 100,000 rupture life at 750 degrees C and 100 MPa Stress, as well as metal loss of less than 2mm in 200,000 hours of superheater service. While the originally developed INCONEL (R) alloy 74Q was successful in meeting these demands, thick section fabrication posed weldability challenges. Specifically, grain boundary microfissuring occurred in the heat affected zone (HAZ) of the base metal. Slight modification of the composition has produced a variant which possesses significantly improved resistance to HAZ microfissuring, as well as improved theimal stability while maintaining desirable properties. This paper describes the process utilized in formulating this development, and presents properties of material produced within the formulated composition range. Stability of the respective microstructure of two presented composition ranges is compared as well,