Modelling the Fabrication of a Pressure Vessel Toroidal Seal

Welding remains the key process in fabricating, as well as repairing, pressure vessel systems. Unfortunately, many manufacturing and through life service problems occur in or near welded regions. The through life integrity of welded components, e.g. distortion, fatigue, fracture, metallurgy and corrosion control remains a key challenge for structural integrity. The work reported in this paper investigates the complex manufacture and assembly of a pressure vessel toroidal seal. The seal is first created by depositing a series of austenitic welds onto a ferritic pressure vessel to build up a plinth, thus forming a transition weld. The transition weld is then subject to a high temperature Post Weld Heat Treatment (PWHT) to stress relieve the welds and to temper the HAZ in the welds. An austenitic toroidal ring is positioned onto the plinths and welded in place thus forming the toroidal seal. The seal is manufactured from a 347 austenitic stainless steel comprising good ductility and corrosion resistance. The manufacture of the seal is simulated using 2D axisymmetric finite element analysis that are available to support both the design and integrity analysis of welded components. As well as the inherent residual stresses associated with welding, additional effects are important, for instance the austenitic to ferritic transition welds between the vessel body and plinths. This paper presents a novel approach to the simulation of weld metal deposition that is ideally suited to the modelling of transition welds and subsequent PWHT. The plinth welds undergo PWHT and so the creep behaviour of the welds is simulated. A series of intermittent as well as end of manufacture PWHT’s are investigated. Additionally the choice of material hardening law for the austenic weldment is studied. The full manufacturing history of the seal is taken into account within the analysis including welding, component machining and component geometry fit up.