Internal oxidation and carburisation of heat-resistant alloys

The commercial alloys Nicrofer-HT, Alloy 800 and Type 304 stainless steel have been exposed under thermal cycling conditions to CO-CO₂ gas mixtures at temperatures of 650-750 °C. Thermal cycling led to repeated scale spallation which accelerated chromium depletion from the alloy subsurface regions. Subsequent dissolution of carbon and oxygen into the alloys led to extensive internal precipitation of carbides and oxides. The large volume fractions of carbide and oxide left small quantities of iron-nickel-rich metal. The in situ oxidation of internal carbides in the stainless steel led to large volume expansions and the development of mechanical stress. This was increased during thermal cycling, leading to disintegration of the surface regions. Temperature and surface treatment were both found to be significant factors in the resistance of alloys to the CO-CO₂ atmosphere.