METAL DUSTING OF HIGH-TEMPERATURE ALLOYS

Metal dusting, i.e. disintegration into fine metal particles and carbo n. was induced on a selection of chromia forming high temperature allo ys in a flowing CO-H-2-H2O atmosphere in exposures at 650-degrees-C, 6 00-degrees-C, 500-degrees, and 450-degrees-. The materials were pretre ated by annealing in H-2 at 1000-degrees-C and electropolishing, this leads to large grain size and low surface deformation, both is disadva ntageous for formation of a Cr2O3 scale. The resistance to metal dusti ng is only dependent on the ability to form a protective Cr2O3 scale, thus the high Cr ferritic steels proved to be very resistant, the ferr itic steels with 12-13% Cr were less resistant. Due to the lower Cr di ffusivity in the austenitic steels, these were very susceptible, espec ially two alloys with about 30% Ni (Alloy 800, AC 66). The appearance of metal dusting was somewhat different for Ni-base materials but they were also attacked under pitting. The metal dusting is preceded in al l cases by internal carburization whereby the chromium is tied up, aft erwards the remaining Fe or Fe-Ni matrix can react to the instable int ermediate carbide M3C which decomposes to metal particles and carbon, in case of Ni-base materials a supersaturated solid solution of carbon is the intermediate.