The Creep Ductility and Fracture of Carburised Alloy 800H at High Temperatures

Abstract

The conjoint action of stress and environmental degradation in high temperature process plant increasingly determines the material selection, process efficiency and economic viability. Degradation through carburisation is evidenced at the microscopical level in high temperature reactors and reformers and, at a much greater level, in many coal conversion processes and ethylene pyrolysis furnaces. Metallic materials utilised in such processes and coming into contact with the carburising atmospheres are normally operating under stress in the high temperature creep regime. One commonly used metallic structural material is the austenitic Fe-Ni-Cr Alloy 800 H and, as it has been established that this alloy’s room temperature ductility is destroyed through carburisation (1–3) while its creep ductility at the very high temperature of 1000°C is, if anything, enhanced (4), it is necessary to account for such adifference and, equally importantly, to assess the behaviour at intermediate temperatures. A temperature more consistent with coal conversion processes, 800°C, is chosen as the main focal point of the present study but it is recognised that ductile/brittle transitions need to be discovered for thermal cycling characterisation and results from tests down to 600°C are added for comparison.