High-Temperature Performance of Alloy 800HT: Effect of Ageing Treatment

Abstract This paper investigates the effect of ageing treatment on the high-temperature behaviour of the iron-nickel-chromium alloy, Alloy 800HT, focusing on its tensile, creep, and creep-fatigue performance at elevated temperatures. Alloy 800HT has been widely used in the petrochemical industry for decades due to its excellent resistance to carburisation, oxidation, and corrosion in high-temperature environments. Its potential as a candidate structural material for future high-temperature gas-cooled fission reactors has stimulated further research to better understand its high-temperature mechanical behaviour. While extensive data exist for Alloy 800H particularly regarding conditions relevant to the petrochemical plant environment and loading, information specific to certain variants suitable for nuclear industries, such as INCOLOY 800HT or VDM® Alloy 800H/HP, remains relatively scarce. In this study, ageing heat treatment was conducted on Alloy 800H/HP prior to high-temperature mechanical testing. The results demonstrated that ageing the material at the same temperature as the subsequent high-temperature mechanical tests for 100 hours can significantly influence its mechanical response. This behaviour is attributed to a combination of the loss of solid-solution strengthening—caused by matrix carbon depletion during carbide precipitation—and the precipitation of intra- and intergranular carbides.