TWI Industrial Member Report Summary 660/1998
L S Smith and M F Gittos
Given the increasing depths and severity of the operating environments experienced in modern oil field exploration, titanium alloys offer significant advantages for hydrocarbon production. The low density and high specific strengths can lead to weight savings, and high corrosion resistance in a range of media may reduce reliance on protection methods. Traditional applications of titanium alloys have not necessarily required the achievement of high joint completion rates or productivity, but have instead placed the emphasis on quality. In newer applications, such as offshore pipework for which titanium alloys must compete with established ferrous alloys, the availability of appropriate high productivity welding technologies may determine the extent to which titanium alloys are utilised.
High energy keyhole welding processes, such as laser and electron beam (EB) welding, offer the potential to increase productivity appreciably beyong that gained using established TIG welding methods. Thick sections can be welded autogenously in one pass with minimal joint preparation, saving time and negating the need for consumables. An alternative technique, keyhole plasma welding, can offer further advantages over these processes. Cooling rates will be slower, leading to the formation of more favourable weld microstructures. Furthermore, this process may give greater flexibility and certainly involves much lower capital outlay. Reduced pressure EB (RPEB) welding techniques have been developed which may be suitable for welding large titanium alloy pipelines in-situ, in that welding can be performed in a local environment which seals to the workpiece, rather than the fully enclosed chamber required for the conventional EB method. Rotary friction welding processes may also offer a rapid means of joining axially symmetrical components, such as pipe.