Simulation and analysis of temperature field for in-service multi-pass welding of a sleeve fillet weld

The multi-pass fillet welds of sleeve were welded onto the in-service pipeline, during which the natural gas was substituted by flowing water as the cooling medium. In accordance with the practical welding process, the parameters of double ellipsoid heat source for each bead were determined by a fitting method for heat source, and the molten pool morphology simulated was coincided well with that of welded fillet bead. Using the heat source adjusted above, the processes of in-service welding were simulated by finite element method (FEM), and the temperature fields of multi-pass fillet welds were obtained. Two kinds of methane gas with different pressure and flowing rate (1.6 MPa, 10 m/s and 4 MPa, 20 m/s) were introduced to investigate the impact of cooling media on thermal cycle properties. The results show that the heat transfer condition has a significant effect on thermal cycle characteristic at the inner wall of pipelines, but a less effect on that at the outer surface of pipelines. For the different pass sequence, it is found that a faster cooling rate will appear if a fillet weld is as the first pass, compared with the situation when a surfacing bead on the pipeline is as the first one. For in-service welding of sleeve-repair, it should be concerned that the effect of pass sequence on thermal cycling characteristics.