Abstract

Stainless steel components in the UK’s Advanced Gas-cooled Reactors can 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. However, the ductility of the material tends to be reduced. This paper assesses the effect on the component response of the changes in local elastic, plastic and creep material properties resulting from carburisation. A pressurised carburised cylinder is modelled analytically as a set of concentric, creeping homogeneous cylinders. The results show a reduced overall deformation rate in the steady state as expected. For completeness, it is shown quite generally that steady-state deformation in a creeping component is reduced by locally increased creep resistance. The cylinder model, however, shows that the locally increased creep and plastic resistance leads to higher stresses in the carburised region than in a homogeneous cylinder, particularly during the transition before steady-state conditions are established. The transient period before steady state creep is then examined in more detail numerically by allowing the material properties to change due to carburisation during the transient phase. This leads to reduced stresses in the carburised region. The influence of the results on deformation, triaxial stress fields and associated creep damage distributions are examined and used to provide guidance on both component assessment and on evaluation of carburised material properties from deformation measurements on test specimens.

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