Guidance on Treatment of Welding Residual Stresses in Design Evaluations Using Section III, Division 5 Rules

Abstract Current ASME Boiler and Pressure Vessel Code Section III, Division 5 rules for design and construction of high temperature nuclear reactors do not consider weld residual stresses in the design evaluations. These rules essentially assume that the selection of weld wires and welding process produce ductile welds and subsequent load cycling and creep reduce residual stresses. However, welding residual stresses have been identified as an important contributing factor for premature in-service degradation and cracking of components in various nuclear reactor fleets. One specific critical failure mode is stress relaxation cracking which occurs due to the enhanced creep crack growth in certain materials caused by relaxation of weld residual stresses in components operating in high temperatures. The UK standard R5 addresses this issue by incorporating welding residual stresses in the structural integrity assessment procedure. Consistent with Volume 2/3 of R5, this paper recommends incorporating the effects of weld residual stress in the creep-fatigue damage evaluation of ASME Section III, Division 5 rules when these stresses are non-negligible or not mitigated by post-weld heat treatment. For creep-fatigue evaluations, Section III, Division 5 and related nuclear code cases offer three analysis methods: (i) design by elastic analysis, (ii) design by elastic-perfectly plastic analysis, and (iii) design by inelastic analysis. The paper presents potential methods for integrating weld residual stress effects into creep-fatigue damage evaluations for all three analysis methods. Finite element simulation of welding residual stresses is performed on a component with available test data found in literature. Various approaches are explored to assess the effect of welding residual stresses on enhanced creep damage. The validity of these approaches is then evaluated through comparison with the base case inelastic analysis results.