Metal dusting of heat-resistant alloys

Metal dusting leads to disintegration of such alloys as iron and nickel-based into a “dust” of particulate metal, metal carbide, carbon, and/or oxide. It occurs in strongly carburising environments at 400-900°C. Literature survey has shown that alloys behave differently in metal dusting conditions based on their composition and the environment. Metal dusting mechanisms for iron and nickel-based alloys have been proposed but, nevertheless, have not been agreed upon and numerous modifications to them have been suggested. Further adding to the complexity, the mechanisms were found to have differed due to operating condition alterations. In view of that, this research was carried out to gain a better understanding of metal dusting process(s) by evaluating the performance of heat-resistant alloys, namely KHR35C HiSi© (HP), KHR45A LC© (35Cr-45Ni), and UCX©, in metal dusting conditions. HP, which is an iron-based alloy, was modified by adding more silicon in order to improve its resistance through the development of SiO2 at the surface. The carbon content in the nickel-based alloy, 35Cr-45Ni, was lowered to delay the attack onset by accommodating more diffused carbon. UCX©, however, has the highest nickel and chromium levels. The alloys were exposed to a gas containing 80 vol% CO+20 vol% H2 at 650, 750, and 850ºC for 100, 500, and 1000h. Analyses including visual inspection, XRD, and SEM/EDX revealed that the alloys suffered localised attacks at the three temperatures but to varying degrees and in different shapes. In general, the attack initiated at the matrix rather than the primary carbides and also progressed through the matrix. Increasing the exposure temperature caused less carbon deposition and more oxides formation on the alloy surfaces leading to a reduction in the attack aggressiveness. UCX© exhibited the highest resistance to metal dusting whilst HP suffered the severest attack. The presence of high concentrations of chromium at the surface catalysed a quick formation of Cr2O3 scale that reduced the extent of metal dusting. Also, the increase in nickel content might have slowed down the carbon diffusion into the alloy. In addition, the presence of other oxide and carbide-forming elements such as silicon and tungsten might well have enhanced the alloy performance. Diffused carbon binds with free tungsten, niobium, and chromium to form carbides prolonging the incubation period prior to the attack initiation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo