Effect of a Pressurised CO2 Environment on Deformation and Failure Behaviour of Type 316H Steel Under Creep Conditions

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Abstract

Stainless steel components in the UK advanced gas-cooled reactors (AGR's) are known to undergo microstructural changes near the surface due to the reactor carbon dioxide environment. Carburisation tends to increase the material's elastic modulus, yield strength and creep resistance and decrease creep ductility. Over the years of operation, small microcracks have been discovered in bifurcation welds that been carburised. R5, the current assessment procedure used for structural integrity assessments, does not consider the effects of carburisation, hence fails to predict creep/creep-fatigue crack initiation. This has led to a simplified approach for assessing such structures. The simplified approach suggests treating the structure as globally homogeneous but with reduced ductility. However, an alternative approach would be to treat the carburised component as a bi-metallic structure consisting of two homogeneous materials with different material properties. This work presents a detailed insight of the effects of carburisation on creep strain and creep damage accumulation, and assesses the validity of the proposed approaches by analytical and finite element modelling. Analytical models have shown that carburisation will lead to increased stress in more creep resistant regions resulting in reduced stress in the bulk section. An analytical solution for a pressurised cylindrical pressure vessel is presented where graded material properties can be implemented for creep lifetime assessment. At the steady state carburisation provide the benefit of reduced creep strain rate and reduced stress triaxiality for internally pressurised carburised cylinders. However, results show that for a small area fraction carburised carburisation cannot extend the time for damage initiation. Finite element models have been used to assess the effects of material being carburised before and after the load application and the impact of bi-metallic assumption. The graded material properties of the carburised layer were found using artificial neural networks that were trained by feeding results from a large-scale Abaqus parametric study. Research has shown material carburisation before the load application can result in either a conservative or non-conservative assessment. Carburisation after the load application shows that the proposed homogeneous assumption will result in a conservative assessment.

Keyword(s)

316H; carburisation; creep; high temperature; lifetime assessment

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