Investigating the Effect of Creep Properties Mismatch in Very Thin Pipes Within High-Temperature Facilities

Most industrial structures are affected by material mismatch effects, due to the design necessity that leads to the use of dissimilar materials like welding of different parts. In other circumstances, this mismatch is introduced by material transformation like radiation embrittlement, hydrogen attack or carburisation, which can drastically change the material response of a restricted area of the component. Such an outcome can have an unpredicted effect on the behaviour and endurance of the component. Carburisation has been identified within the thin stainless steel pipes of UK Advanced Gas-cooled Reactors (AGR). This carburisation is known to affect crack initiation and creep-fatigue properties, ultimately impacting on service life. The current assessment procedure for UK AGRs has several limitations when addressing carburisation and is believed in some circumstances to be overly conservative and in other conditions non-conservative. The work of this study aims to aid in clarifying the effect of creep properties mismatch due to carburisation in thin pipes within such high-temperature facilities. A numerical study is undertaken to investigate the effect of creep properties mismatch in a thin pipe subjected to a combination of primary (load controlled) and secondary (displacement controlled) cyclic loading. In order to perform an extensive parametric study, a special numerical procedure based on the finite element commercial code Abaqus is used to predict the cyclic behaviour of the structure. The effect of creep properties mismatch on global shakedown and creep ratcheting will be investigated providing new insight in the field of structural integrity of pressurised components.