Examining the effect of the environment on the creep and creep-fatigue properties of type 316H stainless steel

Abstract

The United Kingdom's advanced gas-cooled nuclear reactors (AGR) contain many 316H stainless steel components that operate within high temperature carbon dioxide gaseous environments. Recently, cracking has been discovered within some of these components which could not be predicted by current assessment methodologies. Further examination has shown that this premature cracking is due to the interaction between stainless steel and AGR gas environment, resulting in significant carburisation which, has not previously been considered as a potential degradation mechanism. Gaining an understanding of how carburisation affects mechanical properties (under high temperature creep and fatigue conditions in particular) is essential, as it underpins the ability to accurately conduct component lifetime assessment. This is vital to substantiate a nuclear safety case and therefore this plays a crucial role in ensuring continued plant operation. In this thesis, the influence of AGR gas carburisation on the tensile, creep and fatigue properties of 316H has been investigated experimentally. Under laboratory test conditions, it has been found that carburisation generally results in a degradation of material performance (creep and fatigue life) at high temperatures, the extent of which has shown to be dependent on the applied loading scenario. This is driven by a substantial loss of surface ductility within the outer hardened layer and the subsequent premature initiation of widespread surface cracking. The reduced ductility is a consequence of the carburisation process that leads to significant intergranular carbide precipitation, weakening grain boundary strengths at the surface. The ability to detect carburisation on components in-situ would be of significant advantage to component lifetime assessment and this was explored through application of a magnetic non-destructive evaluation tool. Through experimental measurement and finite element analysis, it was found that the presence and severity of carburisation could be evaluated non-destructively under controlled laboratory conditions. Application of this technique to plant components was however concluded to invite significant challenges due to the inherent variability of carburised material across plant. The work of this thesis has significantly contributed to further the understanding of how carburisation may affect component lifetime with the findings directly influencing preliminary R5 structural integrity recommendations on how to treat carburisation in plant. Further testing is required to fully substantiate the effect of carburisation across a greater range of stresses and strains and recommendations for future research have been made.