Study On Burn-through Criterion Of X70 Steel High Pressure Gas Pipeline In-service Welding

In service welding test and SYSWELD simulation were used to study the burn through criterion of X70 steel high pressure gas pipeline in-service welding. The X70 pipeline steel in-service welding was carried out experimentally. The high temperature mechanical properties of X70 pipeline steel was studied and the parameters of the double ellipsoid heat source model were adjusted. The effects of radial deformation on the burn through during in service welding were studied. The influence of pool size on residual strength and radial deformation were simulated by SYSWELD. The in-service welding residual strength was studied by using Residual strength theory and transient equivalent method and the results of in-service welding experiment and numerical simulation.X70 pipeline steel high temperature mechanical test shows that when the temperature is 700℃,the strength is 250 MPa,while the strength decreased to 102 MPa when temperature increased to 800℃,when the temperature increasing from 800℃ to 1300℃,the strength reduce from 102 to 19 MPa. The radial deformation was measured by the in-service welding radial deformation experiment, which reveals that the nature of radial deformation is pressure and welding temperature field accelerates its development. The results of radial deformation experiment and numerical simulation shows that, the burn through has a critical radial deformation value which owns time effect feature.The in service welding heat source model was adjusted. The heat source model sensitivity formula was acquired through the fitting of the calculation results of heat source. The error between verification calculation and regression results is less than 10%. The sensitivity formula was improved according to the characteristics of the in-service welding, and the heat parameter prediction formula that determined by the shape of molten pool and the welding speed was got. The error of width and depth are 5% and 10% respectively by using this formula.When the pressure in the pipeline was increased from atmospheric pressure to 4MPa the radial deformation was increased gradually when the pressure is between 4MPa and 6MPa, the radial deformation was reduced with the increase of pressure. The deformation of inner wall point of vicinity weld zone and in-service welding repair of 150 seconds showed that, the longer repair time resulted in the increase of deformation. Therefore the shorter track and discontinuous welding are helpful to reduce the occurrence of burn through. The radial deformation was increased with the increase of the pipe diameter at a constant thickness, the radial deformation will be reduced,and when the wall thickness increased the possibility of burn through was reduced. The size of the pipe diameter in the early stage of the welding repair didn’t change the trend of the increase of the deformation,and when the wall thickness is bigger than 6mm,the increases of wall thickness no longer has a significant impact on the radial deformation of the inner wall point.The burn through criterion based on radial deformation was studied systematically, and test verification is carried out, the results showed that 0.1 times of the value of the wall of pipeline used as a criterion with poor accuracy, the deformation criterion was improved, by regarding the instantaneous maximum radial deformation as criterion. The improved criterion improved was refined by considering the pool size effect, which can predict the burn through exactly.The pool sizes that used to calculate the stress of the molten pool should be based on the actual data of the molten pool. When the area of the Equivalent defect increased to 1.1 times the bearing capacity decreased from 3.38 MPa to 0.5MPa,which showed that the bearing capacility was decreased significantly with a slightly increase of molten pool area,the pool length had little effect on the residual strength,the pool depth and pool width have significant effect on the residual strength,and the change of penetration greater affected the distribution of the high temperature area. The pool size effect has a great influence on the residual strength calculation,and the influence to the radial deformation need more time to reflect.The equivalent method of defect,the numerical simulation results and high temperature mechanical properties research data were combined to establish residual strength burn through criterion of in service welding,the results of equivalent residual wall thickness and pressure of safety welding were got by the criterion,the criterion was improved according to the pool size effect,the improved criterion can predict the burn through betterly.The DNV-RP-F101 is suitable for estimating the residual strength of X70 pipeline that containing volume defect. The residual strength of the forward half pool was studied by the method of defect equivalent,and the results are more accurate.