Factors Affecting the Oxidation and Carburisation Behaviour of an Austenitic Stainless Steel Used in the UK Advanced Gas-Cooled Reactors

A significant number of stainless steel components within the boilers of the UK advanced gas-cooled reactor (AGR) plants are subjected to oxidation, carburisation and other changes in the underlying microstructure of the material during operation. This results from exposure to the pressurised CO₂-based primary circuit coolant at temperatures from about 500 to 650°C. It is believed that there is a synergistic relationship between the pressurised CO₂ coolant environment and creep-fatigue initiation and cracking. Devising and implementing an evaluation methodology to account for oxidation and carburisation to enable conservative lifetime assessments is essential to current and future plant safety. Therefore, the development of a new and fundamental understanding of environmentally assisted degradation and failure mechanisms is required. It has been postulated that the mechanism underlying the initiation of cracks is carburisation associated with the presence of a duplex oxide layer. In this study, the material-environment interaction for Type 316H stainless steel under simulated AGR conditions has been investigated to increase the understanding of the combined effects of stress, strain and surface preparation, for example, on oxidation and cracking behavior. Experimental data are presented which show that surface deformation promotes the formation of a thin, protective oxide scale, which does not protrude along the grain boundaries, whereas a deformation-free surface leads to the formation of thick duplex oxide layers as well as intergranular oxide penetration. Furthermore, an increased surface hardness due to carburisation has been observed for the undeformed surface only, suggesting that carburisation occurs at an early stage on a chemically treated surface. It is found that when the substrate is plastically deformed and under the effect of active stress, the thin oxide on the mechanically deformed surface can be disrupted, resulting in similar behaviour to a chemically treated surface with no deformation.