Defects in the UK’s AGR nuclear reactors have been historically found in superheater regions of the boilers. These components are fabricated from type 316H austenitic stainless steel and operate in carbon dioxide gas coolant environments under creep conditions, at temperatures up to 550°C. As a result, some components maybe carburised throughout their life resulting in the formation of a hardened outer surface layer. This layer results from interstitial carbon diffusion and is thought to impact on the creep, creep-fatigue and fracture properties of 316H. Carburisation is currently unaccounted for within high temperature structural integrity assessment procedures. It is essential that carburisation and resulting damage mechanisms are well understood in order to accurately predict the failure of components.
This paper aims to investigate the effect of AGR gas carburisation on the creep and fracture properties of type 316H stainless steel. Specimens have been preconditioned within a simulated AGR gas environment. The presence of carburisation has been confirmed through metallographic examination, hardness testing and surface analysis techniques. A series of constant load high-temperature creep tests have been conducted on preconditioned specimens. Compared to as-received material, carburised specimens displayed a significant reduction in creep rupture time with cracking of the outer carburised layer initiating creep crack growth. This phenomenon is seen to occur at very low strains and has been confirmed through interrupted creep testing. The substantial reduction in creep rupture time is postulated to result from embrittlement of the carburised material owing to strong precipitation of carbides along grain boundaries. It is concluded that carburisation can lead to a severe reduction in creep rupture life in test conditions; the possible implications of this with regards to plant conditions are discussed.