Development of Reduced Pressure Electron Beam Welding Process for Thick Section Pressure Vessel Welds

Abstract

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.

An evaluation of multipass narrow gap laser welding as a candidate process for the manufacture of nuclear pressure vessels

“…Recent developments in the development of Reduced Pressure Electron Beam (RPEB) welding have enabled large pressure vessels to be joined in a single weld pass, using a local sealing system, so that it is not necessary to place a large pressure vessel inside a vacuum chamber [1,16]. This approach offers a step change in productivity in comparison to arc welding processes in a way that multipass NGLW is unlikely to do, since thick sections (> ~ 100 mm) of material can be welded in a single weld pass, without the need to add filler material.…”

“…However, in recent years some attention has focused on the development of electron beam welding as a high productivity alternative to arc welding [1]. Electron beam welding offers the potential to complete steel welds in thicknesses exceeding 100 mm in a single weld pass, whereas the number of passes required with an arc welding process will be in the order of 100.…”

A perspective on welding technology challenges in the nuclear sector

“…The need to place large pressure vessels in a vacuum is a greater challenge and one that imposes costs, but the technology exists. Furthermore, work by TWI and partners on the development of localised vacuums and sealing systems, while welding under reduced pressures rather than high vacuums, is showing promise [18]. The application of preheat can also be difficult if a vessel is placed in a vacuum chamber.…”

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

“…A 160mm thick SA508-3 forging circumferential seam weld was manufactured, see Figure 2 and [1]. Run on and run off blocks were tack welded at each of the test-piece to provide restraint and prevent 'crocodile mouthing' of the joint.…”

“…Ferritic pressure vessel RPEB joints were tested in both the as-welded condition and after furnace Post Weld Heat Treatment (PWHT). Examples of metallurgical test results from the SA508-3 steel welds are provided in [1]. Although hard micro-structures are produced in thick EB welds due to rapid cooling rates, furnace PWHT results in good tempering and material toughness levels satisfying ASME Code requirements.…”