SCALING OF AUSTENITIC STAINLESS-STEELS AND NICKEL-BASE ALLOYS IN HIGH-TEMPERATURE STEAM AT 973-K

Scaling of commercial high-strength austenitic stainless steels and nickel-base alloys, possible candidates for ultra-supercritical (USC) boiler-tube materials, was studied in atmospheric steam at 973 K for 1000 hours. For steels of more than 22 wt% chromium, uniform formation of protective Cr2O3 resulted, and scaling was negligible. For steels of 20 to 22 wt% chromium, duplex-type oxide scale with uniform healing was observed, and scaling was slight. For steels of less than 20 wt% chromium, fine-grained steels had a better scaling resistance than the coarse-grained steels. For steels such as 321H (UNS(1) S32109), 347H (UNS S34709), Esshete 1250, and AN31, inner scale was irregular because of local healing at or near grain boundaries of the base metal. For 12R72V and 17-14CuMo, the inner scale was uniform and no local healing at grain boundaries occurred. This different healing behavior was observed for both coarse- and fine-grained steels as well. For fine-grained Esshete 1250 and 347H, healing was completed uniformly after oxidation for 500 hours and growth of the inner scale was suppressed thereafter; for fine-grained 12R72V, the inner scale continued to thicken with time and uniform healing was not established. For 12R72V, grain boundary precipitation of the base metal was heavy, which may explain the absence of uniform healing for 12R72V.