Impact of Welding Residual Stresses (WRS) on the Risk of Fracture in The Brittle to Ductile Transition of Ferritic Steels – Large Scale Experiments

Abstract The primary and secondary PWR (Pressurized Water Reactor) circuits are a complex set of pressure-welded steel piping and components. As the second barrier to the spread of fissile materials is concerned, ensuring the integrity of these circuits is a major safety objective. It must cover all possible loading situations encountered in service (normal operation as well as accidental situations) and all possible failure modes, material functions and associated conditions of use. Fracture mechanics assessment in welded joints in these structures require particular attention. Indeed, welds may present defects (ex. lack of fusion) and generally have a lower fracture resistance than the base metal. Additionally, welding process generate Welding Residual Stresses (WRS) that might be considered in the assessment. Their potential impact on the risk of failure is directly related to material behavior. These WRS have been shown to have an impact on the risk of failure in the brittle domain where the behavior is quasi-elastic, but this impact appears to be overestimated in the upper shelf of the transition curve. Consequently, industry needs to develop criteria adapted to the problem of taking WRS into account in the analysis of fracture mechanics for fracture mechanics assessment, and to validate these criteria through experiments at different scales, then define the areas where these stresses must be taken into account or can be neglected. This paper present experimental results and first analysis of four points bending tests performed on large scale welded C-Mn steel pipe representative of the PWR secondary and auxiliary piping.