Metal dusting of high temperature alloys

Abstract

AbstractMetal dusting, i.e. disintegration into fine metal particles and carbon, was induced on a selection of chromia forming high temperature alloys in a flowing CO‐H2‐H2O atmosphere in exposures at 650°C, 600°C, 500°, and 450°C. The materials were pretreated by annealing in H2 at 1000°C and electropolishing, this leads to large grain size and low surface deformation, both is disadvantageous for formation of a Cr2O3 scale. The resistance to metal dusting is only dependent on the ability to form a protective Cr2O3 scale, thus the high Cr ferritic steels proved to be very resistant, the ferritic steels with 12–13% Cr were less resistant. Due to the lower Cr diffusivity in the austenitic steels, these were very susceptible, especially 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 all cases by internal carburization whereby the chromium is tied up, afterwards the remaining Fe or Fe‐Ni matrix can react to the instable intermediate carbide M3C which decomposes to metal particles and carbon, in case of Ni‐base materials a supersaturated solid solution of carbon is the intermediate.