The role of pressure in carburisation: Crack dynamics in Type 316H austenitic stainless steel under CO2 environments

The oxidation and carburisation of steel in advanced gas-cooled nuclear reactors limit the life of key components during plant operation. To achieve a mechanistic understanding of the origin of carburisation phenomena, simulation or replication of service exposure conditions is needed. Laboratory tests are usually performed to accelerate processes such as crack initiation and growth in the material, and to simulate plant conditions as precisely as possible. This paper describes the results of carburisation data analysis of AISI Type 316H stainless steel under several CO2 gas exposure conditions. The behaviour of ex-service welded pipe components from advanced gas cooled reactors at service temperature and pressure is compared to a range of specimens loaded and cracked in air and under surrogate/simulated CO2 conditions. Both atmospheric pressure and pressurised CO2 gas (4.14 MPa) conditions were studied. Microstructural characterisation of the crack tip region and the bulk/matrix of the specimens was performed using advanced electron microscopy techniques. The regions ahead of the crack tips were systematically analysed as they have a potential to significantly influence crack propagation behaviour. This study shows a sig nificant effect of carburisation at the crack tip compared to the bulk of the material, but only for pressurised CO2 conditions, whereas atmospheric pressure CO2 conditions more closely resem bled the specimen exposed in air. Importantly, this study reveals that in simulated experiments, pressurised CO2 environments are essential to replicate the carburisation behaviour observed in ex-service conditions