Research On Strain Distribution Characteristic Caused By Depression In Weld Of X80 Steel Pipe

High-grade steel pipes are susceptible to external mechanical damage to the safety of pipelines during construction and service.Especially when sag damage occurs in the welds of steel pipes,cracks are easily generated at the weld toes,which seriously affects the safe service of pipelines.Based on the engineering service conditions,finite element simulation and sag pressing test were used in this paper to study the effect of the strain distribution of spiral welded pipelines on the mechanical properties.The research resu lts can provide theoretical support and reference basis for the evaluation of pipeline service safety.X80 pipeline steel pipe was taken as the research object,the finite element simulation software was used to simulate the influence of the parameters of the indentation diameter,indentation depth and internal operating pressure on the strain field of the depressed pipeline.The strain evolution and distribution characteristics of the X80 pipeline steel under the sag state of the single pipe body and the s piral weld zone were compared.Distribution characteristics of strain evolution in the depressed state of the spiral weld zone was analyzed.The study found that the internal pressure of the simple sag pipeline plays a leading role in the sag process.The greater the internal pressure,the smaller the depth of the pipeline pressure;the internal pressure is positively correlated with the equivalent plastic strain,and the diameter and strain of the applied trace are negative Related related trends.When the depth of the depression is small,the presence or absence of internal pressure has little effect on the strain of the pipeline.For the weld depression,the diameter of the applied traces is negatively correlated with the depth of depression,and the internal pressure of the pipeline is significantly negatively correlated with the depth of depression.When the depth of the depression is less than 3% of the pipe diameter(OD),the maximum strain is in the weld zone.As the depth of the depression continues to increase,the maximum strain appears in the heat-affected zone as the depth of the depression continues to increase.In order to better reveal the strain law of the material in the process of strain change during deformation of the material,the digital pre-speckle three-dimensional full-field strain measurement and analysis system(XTDIC)was used to study the dynamic s train transfer mechanism for the depression prefabrication and tensile tests of X80 spiral welded pipes.A strain hardening model based on the weld zone of the pipe body was established,and the strain distribution characteristics of the complete spiral we lded pipe depression were discussed in combination with dislocation configurations in different regions.It is found that the experimental phenomenon is consistent with the simulation results,which indicates that the established model can effectively eval uate the strain distribution and change process of the depressed pipeline.The experimental study found that due to the large plastic deformation of the material during the simple depression deformation,the grains at the bottom and side walls of the depre ssion area were elongated along the direction of maximum deformation due to the large plastic deformation of the material during the simple depression deformation.The dislocation density increased and interactions produced strain in the depression pipe,r esulting in strain hardening of the depressed pipeline,thus improving the pipeline strength.When the weld pipe undergoes significant deformation,the maximum strain gradually shifts from the weld zone to the heat-affected zone,and finally cracks are produced at the weld toethe strain concentration in the heat-affected zone continues to generate cracks at the weld toe.It is considered t hat the deformation of the weld seam after the deformation occurs typically Transformation Induce Plasticity,affected by plastic deformation.It increases the residual austenite and induces transformation to martensite,which increases the hardness and pr oduces strain hardening.With the increase of the depth of the depression,the dislocation density zone increased at the grain boundaries of the weld and the heat-affected,the dislocations continued to slip and accumulate,and a typical dislocation cell-like substructure appeared in the heat-affected zone,reducing uniform plasticity deformability with the increase of the depth of the depression.