ON THE MECHANISM OF CATASTROPHIC CARBURIZATION - METAL DUSTING

Metal dusting is a disintegration of metals and alloys into small metal particles and carbon (graphite) occurring at carbon activities of a(C) > 1 in a range of intermediate temperatures 400-800-degrees-C. The phenomenon was simulated in CO-H-2-H2O atmospheres at 650-degrees-C. For iron and low alloy steels a mechanism was confirmed in which the unstable carbide M3C is an intermediate, which decomposes according to M3C = 3M + C, the metal particles serving as catalysts for further coke deposition. According to thermodynamic considerations this mechanism might be suppressed by alloying with Ni or Mn. Exposures with Fe-Ni alloys, however, showed that at high Ni contents another mechanism applies, the disintegration of a supersaturated solid solution. Also Fe-Mn alloys were susceptible to metal dusting after Mn depletion of the surface-near region by selective Mn oxidation; similar behaviour is to be expected for Fe-Cr alloys after selective Cr-oxide or Cr-carbide formation. Thus, in principle no alloys are resistant against metal dusting if no protective oxide layer is formed.