A reduced pressure electron beam (RPEB) process is being developed in the UK by ‘The Welding Institute’ (TWI) for the manufacture of thick section plate, forging and pipe welds. Potential gains include minimal distortion, fewer weld defects and rapid production times, compared to conventional arc welding processes. To date, RPEB welding equipment and parameters have been successfully developed and applied in a sealed chamber under partial vacuum (∼1mbar), to produce 1-pass seam welds in low alloy steel plates and forgings. Rolls-Royce commissioned TWI to produce RPEB welds in SA508 Grade 3 Class 1 steel forgings of between 100mm and 160mm thickness, as part of a feasibility study for nuclear pressure vessel applications. This paper presents results of micro-structural examinations, material property tests and residual stress analyses of RPEB welds in SA508 Grade 3 steel forgings, both in the as-welded condition and after post weld heat treatment (PWHT). This data was required in order to assess the structural integrity of the weldment. A narrow uniform fusion zone approximately 10mm wide and 3mm deep heat affected zone (HAZ) was produced. High hardness levels were measured in the weld HAZ, but the application of PWHT at 600°C had a beneficial tempering effect, reducing the maximum hardness to below 300Hv. A 3D finite element model was used and deep hole-drilling measurements were independently performed, to determine welding residual stress distributions. In both cases yield magnitude tri-axial tensile stresses were evaluated in the centre of the weld and adjacent HAZ, of up to +600MPa. This result is as expected since the centre region is the last to solidify and cool, with a high degree of restraint to weld shrinkage. However effective stress relaxation occurs during PWHT, mainly due to creep, reducing the maximum residual stress to about 100MPa (or 20% yield strength). This feasibility study has shown that RPEB welding is a viable method for fabricating large pressure vessels in low alloy steels. Sound joints can be produced in sections up to 160mm thick in a vacuum chamber. Further development work is being done by TWI in order to apply the technique out-of-chamber using a local vacuum seal.
Rolls-Royce plc is conducting work to investigate the feasibility of using Reduced Pressure Electron Beam Welding (RPEB) for thick section welded joints in power plant construction. As part of the work, simple specimens have been manufactured at TWI ltd in order to develop welding parameters and conditions and to examine the achievable weld quality. Previous work in this project has shown good correlations between measured and predicted stresses in RPEB welds in ferritic components [5,6]. This paper describes Finite Element (FE) modelling that was carried out to try to predict the residual stress field generated by the welding process in three of the specimens. The first specimen that was modelled was a full penetration butt weld in 80 mm thick Type 316L plate (W17). The other two models were of circumferential butt welds in 14 inch nominal diameter Type 304L pipe. The first pipe model (W20) was a single pass, 360° weld, while the second (W22) featured a slope-up and slope-down each lasting for 16° either side of a 360° full penetration weld, giving a total weld of 392°. The modelling was carried out in Abaqus [1] using a DFLUX user subroutine to model the welding heat input as a cylindrical heat source, due to the reduced pressure during specimen manufacture, only radiation heat losses were considered. The built-in Chaboche mixed hardening model was used for both materials during the structural analysis. The residual stresses predicted by the FE modelling have been compared with the results of Deep Hole Drilling (DHD) that was carried out on the equivalent specimens. Full details of the measurements are reported in [4].
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