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It has been identified that in-service carburization of austenitic stainless steels in the UK Advanced Gas Cooled Reactor (AGR) fleet can impact upon creep-fatigue crack initiation, which is assessed using the R5 Volume 2/3 assessment methodology.Material properties for the carburized layer have been derived through testing of preconditioned specimens. These properties have been used to propose a simplified assessment methodology for the treatment of creep-fatigue crack initiation in carburized specimens. The methodology accounts for the negative impacts of carburization, such as the reduced creep ductility, but does not benefit from the potentially positive impacts, such as the increased creep deformation resistance.At relatively low strain ranges, typically seen by plant components, these simplifications generally result in a very conservative creep-fatigue lifetime prediction. However, at short dwell times (in load control) at high strain ranges (in strain control) the methodology has the potential to be nonconservative when the stress mismatch between the carburised layer and bulk material is severe due to differing cyclic stressstrain properties. Therefore pragmatic limits of application must be applied to the simplified approach. This paper explores these limitations of the simplified assessment advice, the impact on plant assessments and how current research is looking at how less conservative assessment methodology can be developed.

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The R5 procedures have been developed within the UK power generation industry to assess the integrity of nuclear and conventional plant operating at high temperatures. Within R5, there are specific procedures for assessing creep-fatigue crack initiation in initially defect-free components (Volume 2/3) and procedures for assessing components containing defects (Volume 4/5). Recent developments in the R5 Volume 2/3 procedure for assessing creep-fatigue initiation in defect-free components include improved methods for predicting creep damage and a new approach for assessing weldments. The improved methods for predicting creep damage include the option of using the stress-modified ductility exhaustion (SMDE) approach as an alternative to the existing ductility exhaustion approach, together with improved methods for assessing compressive dwells. The new approach for assessing weldments involves splitting the existing Fatigue Strength Reduction Factor (FSRF) into a Weldment Endurance Reduction (WER), which accounts for reduced fatigue endurance due to weld imperfections, and a Weldment Strain Enhancement Factor (WSEF), which accounts for material mismatch and local geometry effects. This paper briefly outlines the current R5 Volume 2/3 procedures and then focuses on these two significant recent developments in the procedures.

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