Development of Residual Stress Profiles for Defect Tolerance Assessments of Thick Section Electron Beam Welds

Abstract

This paper describes the results of weld model analysis and deep hole-drilling measurements undertaken to evaluate residual stress distributions in austenitic and ferritic steel thick section electron beam welds. The work was undertaken in support of a Rolls-Royce and TWI development programme in the UK, for a Reduced Pressure Electron Beam (RPEB, 0.1 to 1mbar) welding process using a mobile local vacuum seal for the manufacture of thick section pressure vessel and pipe welds for nuclear power plant applications. Measurements were undertaken on representative mock-ups including a 160mm thick SA508-3 forging circumferential seam weld, in both the as-welded and furnace post weld heat treated condition. A 316L stainless steel plate butt weld and a 304L pipe girth weld of 80mm and 36mm thickness respectively were also analysed. There is now considered to be sufficient understanding of the residual stress fields generated by thick Electron Beam (EB) welds, to propose through thickness ‘upper bound’ R6 Level 2 stress profiles for use in defect tolerance assessments. The intention is to incorporate residual stress profiles of this type into the R6 structural integrity assessment procedure, following peer review. This would represent a significant step forward in demonstrating technology readiness for plant applications. It is also anticipated that an ASME Code Case will be drafted and proposed for the RPEB welding process. EB welding is a relatively low heat input process, compared with a multi-pass arc weld, such that the fusion zone and heat affected zone are narrow. The centre of an EB weld is the last region to solidify and cool-down, so typically there is a high degree of restraint to weld metal contraction, thereby generating a highly tri-axial yield magnitude tensile stress state at the mid-thickness location. The stress components acting in the longitudinal welding direction and through-thickness orientation tend to be large in the centre of EB welds of high aspect ratio (depth / width). By contrast, lower stress levels are produced on the surfaces acting transverse to the weld plane compared to conventional multi-pass metal arc welds. The transverse stress component is most likely to be required for the assessment of any postulated EB welding defects. The residual stress field decays rapidly with distance from the EB joint into the adjacent parent metal. Symmetrical stress distributions are typically generated in a 1-pass EB plate weld and stress fields are characteristically sinusoidal of wavelength between 1 and 4 times the section thickness.