- Develop stable welding conditions for SA508 pressure vessel steel and determine the thickest sections consistently addressable in material greater than x 100mm and approaching 200mm
- Demonstrate transferability from flat plate to circumferential girth welds
- Contribute to demonstration of viability for EB welding of a steel pressure vessel >1.5m diameter, >100mm wall thickness and >50 tonnes in weight) without the economic constraints of a large and expensive vacuum chamber
- Evaluate and document the weld integrity
The project will address the technology gap which prevents uptake of EB welding for very large assemblies. Out-of-vacuum-chamber EB welding will be further developed and them deployed to demonstrate the viability of the process beyond the laboratory and in doing so remove barriers to entry for prospective users.
The primary barrier to industrial use in this type of application is the stability of the EB welding process when very thick sections are to be joined. The major focus of this project will be weld development tasks in which linear test welds will be made, followed by circumferential girth welds, with the aim of ensuring a stable and repeatable process in thick section steel, greater than 100mm and approaching 200mm in thickness. Welding will be performed using TWI’s proprietary reduced pressure EB gun technology which is the enabler to welding in poor quality vacuums while achieving good quality joints. Welds will be made in SA508 pressure vessel steel, to establish stable process conditions, in wall thicknesses approaching 200mm.
Ultimately the established process will be applied on-site to very large forgings, approaching 50 tonnes in weight, to demonstrate viability. These activities sit in the region of technology readiness levels 4 to 6 - the well-defined technology ‘valley of death’ and hence the requested funding is required to address the significant challenges foreseen.
Relevant Industry Sectors
Electron beam (EB) welding is predominantly restricted to use in small to medium sized fabrications because the whole welded assembly and any required tooling must be contained within a vacuum chamber; as the chamber increases in capacity, it and the associated pumping plant become the highest cost elements of the machine. Capital investment can be high for large vacuum chambers and hence medium to large EB welded assemblies tend to exist predominantly in the very high value adding, high integrity, aerospace and nuclear power manufacturing industries.
Use of local vacuum containment, only where vacuum is needed to ensure beam quality and a clean environment around the weld zone, has been demonstrated by TWI and others but remains largely developmental in nature. First generation commercial equipment is now available but not deployed. Local vacuum systems reduce the capital investment required to access EB welding and could bring major productivity benefits. In a thick section pressure vessel welding floor-to-floor times may be reduced from months to days when moving from narrow gap TIG to EB. The removal of pre-heat, statutory lay down periods and interpass inspection are a major contributor, none of which are required when using autogenous single-pass EB welding.
Electron beam welding may be used to weld very thin or very thick sections in most metals, examples in excess of 200mm are often shown; the very narrow, high aspect ratio of the weld zone is unique among fusion processes at these thicknesses and brings notable benefits. Industrial usage is normally limited to around 50mm-75mm penetration, above this process stability may be maintained but it becomes a greater challenge. Stable and repeatable EB welding condition for very thick section power industry steels must be determined to remove barriers to successful commercial uptake.
Addressing these primary barriers will allow prospective users to assess the process in terms of: i) technical viability – upscaling, quality attainable and appropriate inspection, ii) code compliance - especially important in pressure vessel application and iii) economic justification - the benefits of removing auxiliary tasks may be significant.
The pressure vessel field is as large as the entire aerospace EB welding market. Should EB welding displace only a small proportion of the narrow gap TIG and submerged arc welding currently employed it would expand EB welding deployment and accessibility significantly.