Study On Residual Strength Method Of Burn-through During In-service Welding On X70 Pipeline Steel

In-service welding plays an increasingly important role with the improvement of pipeline network.Compared with common welding,in-service welding has the fatalness of burnthrough.The reason of burn-through during in-service welding is that the metal below welding pool has the lower strength because of high temperature,which cannot bear the stress from pipeline inner pressure.The method to judge the possibility of burn-through was indirect in early studies,such as highest pipe inner temperature and “Equivalent molten pool” calculation.There were some limitations and inaccuracy by these indirect methods.This paper establishes the strength field mathematical model of in-service welding according with temperature field and relationship between temperature and strength.And then we can get the residual strength of the welding pipeline by three-dimensional integration of the strength field with proper integral region.And we can judge the pressure capacity by the residual strength.In this paper,we use two methods to calculate the residual strength,which are analytic method and finite element simulation.The accuracy of this method is based on the choose of welding heat source model.We choose the double-ellipsoid heat source as in-service welding heat source after comparison.The calculation by analytic method is that double ellipsoid heat source is look upon as a point heat source in the double ellipsoid region.And we can get the temperature field of the double-ellipsoid heat source through integration of the temperature increment caused by the point heat source.And then we can establish the strength filed according with the temperature field.The residual strength can be calculated by integration of the strength field.The calculation by finite element simulation is to simulate the temperature field of in-service welding by SYSWELD.And we can get the volume of 3D element at small temperature ranges.The residual strength can be calculated by three-dimensional integration and volume fraction method.The integral region has a great impact on residual strength calculation.During in-service welding the temperature filed and the pipe thickness has great relation with burn-through.The parameter of highest temperature of inner pipeline can be chosen as reference of the integral region because the highest temperature also has the great relation with temperature field and pipe thickness.Taking the highest temperature of inner pipeline as the reference,we can select the appropriate standard temperature.The integral region is chosen the region which has the higher temperature than standard temperature.We got the proper integral region by verification.The specific chosen principle is that we used highest temperature of inner pipeline at 980℃ and the 6.4mm thickness as the standard of measurement.When the pipeline is thinner than the standard thickness or the temperature of inner pipe is lower than the standard temperature,we should choose the 3/4 of the highest inner temperature as the integral region.When the pipeline is thicker than the standard thickness and the temperature of inner pipe is higher than the standard temperature,we should we should choose the 2/3 of the highest inner temperature as the integral region.After comparison with the “Equivalent molten pool” method,the residual strength method can guarantee the safety of in-service welding and can be more accurate in terms of prediction the burn-through.The simulation of welding process has also been worked in this paper.According to the stress field simulation results,we can see that the biggest plastic strain is near to the fusion-line.And the most dangerous burn-through position is at the rear of the maximum position of the melting depth.