Investigation on the effect of heat affected zones overllaping in multipass TIG welding of austenetic stainless steel pipes

Multi-pass Tungesten Inert Gas (TIG) welding is one of the popular weld processes employed to join austenitic stainless steel pipes in industrial sites like refineries and petrochemical industries. Since experiencing intensive temperature cycles and residual stresses as a result of high heating, Heat Affected Zone is a vulnerable region which is threatened by various types of cracking's and defects in welding. In the pipeline designs and fittings because of different reasons, in some cases, the distance between two consecutive multi-pass butt welds are too much small that generated HAZ's from welds, may be overlapped in the tiny pipe located in the middle of two other parts. Reports reveal that there are leakages from these HAZ's overlapping sections. The aim of this study was to investigate the influence of overlapping of HAZ's, generated from two consecutive close welds, on the mechanical behavior of TIG welded 304L austenitic stainless steel pipes with different outer diameters and wall thicknesses. In this investigation, four pipes are selected to be studied. Two pipes with the same outer diameter of 2" but different schedule numbers of 40 and 80. And two the other pipes have the outer diameter of 4" and schedule numbers of 80 and 120. In the fist step, a 3D finite element model was created for multipass welding mentioned four pipes. Considering the thickness of pipes, number of applied weld passes were chosen (2,3,4 and 6 passes). all the analysis has been done in ABAQUS software. Base on FEA, the transient temperature distributions and both axial and hoop residual stress distribution on inside and outside of pipes in different orientations were plotted for all the pipes. The results show that the temperature and residual stresses distribution are constant in different orientations through the pipes (90°.180°.270° from the welding start point). For all the samples, on the outer surface, axial residual stress are compressive in fusion zone and by getting away from the FZ it is changing to tensile situation. The reverse situation can be seen in the inside surface where the axial residual stress is tensile in the FZ and growing compressive by distance from FZ. For the hoop stress at the weld zone and its vicinity large tensile stresses are produced in outer and inner surfaces. By studying Temperature cycles, the length of Heat affected zones are identified for multi-pass TIG welding of pipes with different number of welding passes. In the next step, these obtained lengths were applied in experimental tests, for each size of pipes, a small pipe with a length equals to two times the length of related measured HAZ length was provided. Then, these small pipes were TIG welded to two other pipes with the same sizes but larger lengths from both-sides. The situation of welding (number of passes, current, voltage and speed of welding for each pass) was same as the finite element analysis. In the next step, mechanical testes were carried out to investigate the behavior of the material under HAZ's overlapping. Over-ally it can be concluded that HAZ's overlapping is not a desirable situation and it should be better to avoid too close butt welds in the multi-pass TIG welding of austenitic stainless steel pipes.