Hans Fostervoll

The Effects on Process Performance of Reducing the Pressure From 36 to 1Bar in Hyperbaric MIG Welding

Technology for remotely controlled (diverless) repair welding of subsea pipelines from 170 to 1000m water depth is being developed by StatoilHydro. The repair technology is based on a sleeve concept combined with MIG welding and the development is currently nearing completion. Technology for diver-assisted remotely controlled welding down to about 200m has been used in the North Sea for about twenty years. In order to reduce the use of divers, the deep water diverless technology is also being considered for use in shallow waters. The present work has been performed to investigate whether the deepwater welding procedure may also be used in shallow waters, and which modifications for the lower pressure conditions need to be made. Test welding has been performed in the pressure range from 36 to 1bar corresponding to 350 to 0m sea water depth to study the effect of ambient pressure upon the welding process behaviour and weld bead appearance and geometry. For the 12 o’clock welding position tested, welding parameters developed for deep water conditions also worked well for shallow water conditions down to about 2bar. It was also evident that the electrode polarity, which is negative for the deep water procedure, had to be changed to electrode positive for the lowest pressures, which coincides with conventional 1-atm MIG welding. Mechanical property testing and microstructure examinations revealed satisfactory results using the modified welding procedure.© 2009 ASME

Hydrogen Assisted Cracking in Welding of 13% Cr Supermartensitic Stainless Steels

Supermartensitic stainless steels are known to be prone to hydrogen induced cold cracking. Therefore, the objective of this work was to assess the susceptibility to hydrogen cracking in Gas Metal Arc Welding (GMAW) with use of matching base and filler materials (supermartensitic stainless steel), using the Instrumented Restraint Cracking (IRC) test. Root welding in the IRC test did not result in hydrogen induced cracking, neither for low nor high weld metal hydrogen content. Because of the martensitic transformation, the global residual stresses are very low after welding (below 100 MPa). Since the yield strength (Rp0.2% ) of the material is about 600–720 MPa, it implies that the IRC test method is not very suitable for supermartensitic stainless steels. However, by performing IRC test multi-layer welding, micro-cracks were found in the last pass. An increase in the weld metal hydrogen content resulted in reduced fracture stress and ductility, as observed in tensile testing of IRC test specimens directly after welding. Investigation of the fracture surfaces of the specimens with high hydrogen contents showed fish eyes, which are strong indications of hydrogen embrittlement. By performing heat treatment (225°C for 24 hours) of specimens with high hydrogen contents and subsequent tensile testing, the fracture stress and ductility were restored to the initial base metal level. Slow Strain Rate Testing (SSRT) with and without Cathodic Protection (CP) was performed on test specimens sampled transverse to the welding direction. CP has detrimental effect on the fracture stress and ductility due to the high weld metal hydrogen pick up.© 2003 ASME

Increased Robustness in Hyperbaric Welding of Subsea Pipelines by Introducing Copper Backing

Hyperbaric tie-in and repair welding of pipelines at the seabed is performed as full penetration TIG butt welding. Variations in pipe end fit-ups make the root pass welding challenging with relatively low robustness. The present investigation concerns with exploiting copper backing to increase the root pass welding robustness. Initial welding experiments are performed at 15.4bara ambient pressure with the TIG process based on the technique and procedures used by the Statoil PRS (Pipeline Repair System). The results demonstrate that significant increase in robustness can be achieved, which can be an important contribution in the strive for cost reductions. Mechanical testing and microstructure examination of the weld metal revealed no negative influence of the copper backing. Initial MIG root pass welding experiments at 11bara are also performed as a part of the development of hyperbaric MIG butt welding of pipelines. These results are promising and demonstrate that by exploiting copper backing can robust procedures be developed, which opens for deep water MIG butt welding in the future.Copyright

High Heat Input Welding of 12Cr-6Ni-2.5Mo Supermartensitic Stainless Steel

In conventional welding of 13% Cr supermartensitic stainless steels, the normal microstructure that forms on cooling is martensite. Although high heat input tends to give a certain coarsening of the final microstructure, the eventual accompanying loss in toughness is not known. The present study was initiated to examine the effect of heat input on weld metal and heat affected zone mechanical properties of a 12Cr-6Ni-2.5Mo grade. The results obtained showed that the notch toughness is low (25 J) and independent of heat input for the weld metal, while it is reduced with increasing heat input for fusion line and the heat affected zone locations. Subsequent post weld heat treatment gave a substantial increase in toughness for all notch locations. Based on these results, indications are that a specified maximum heat input is not applicable in welding of supermartensitic stainless steels, allowing more production efficient techniques to be used, both in longitudinal seam and girth welding.Copyright