Correlation of Microstructural and Mechanical Properties with Residual Stresses in Multi-pass Girth Welding of 56-inche Steel Pipe of Grade API X-70

The weld residual stresses decrease the design stress in high-pressure large-diameter gas transportation pipelines. In this paper، two API X70 steel pipes (with spiral seam weld) with an outside diameter of 56 inch and a wall thickness of 0. 780 inch were first girth welded (according to NIGC code). Next، hole drilling tests were conducted for strain measurement on the surfaces of the pipes. The values of residual stresses on the internal and external surfaces of the pipe were then determined from the measured strain data. To ensure the integrity of the seam weld، different experiments، including chemical analysis، metallographic observation، tensile، and impact tests were performed. The experimental data showed that the maximum tensile residual stress (of the order of 318 MPa) was located on the centre line of the seam weld on the pipe outer surface alongside with the pipe hoop direction. This was more than 60 percent of the yield strength of the seam weld. In the welded zone، micro-alloys content reduced around 75% and un-tempered martensite was observed on the outer weld pass. The carbon content increase resulted in an increase in martensite volume fraction، and thus reduced the residual stress in this region. On the other hand، this effect reduced the material strength. The reduction in material yield strength in the weld caused the increase in residual stress with respect to static yield strength of the pipe. In addition، the minimum Charpy fracture energy (of the order of 45J) was observed in the pipe hoop orientation in the weld center line. Therefore، the external surface of the pipe (weld center line) in its hoop direction with the maximum residual stress، maximum martensite content، minimum micro-alloyed contents، minimum yield strength، minimum elongation and minimum Charpy energy is the critical point in the structure.