Study On Residual Stress Of Three-roll Continuous Rounding Process For Large Thin-walled Tube

Large-scale thin-walled pipe fittings are often used in the mining and transportation of mineral resources.With the increase of oil and gas consumption,new transportation routes continue to be constructed,and the requirements for large-scale thin-walled pipe fittings are increasingly strict.The residual stress in the pipe fittings will have a negative impact on the fatigue strength,stress corrosion resistance and service life of the pipe fittings.If the residual stress around the pipe is not uniformly distributed,the ovality of the pipe fittings will even increase with the release of the residual stress.Rounding process is one of the most important processes before pipe fittings are put into production,it is necessary to study the residual stress generated by the rounding process.In this paper,based on the springback theory of plane bending with small curvature and the unified curvature theorem of reciprocating bending,the springback analysis of the circumferential reciprocating bending process of pipe fittings is carried out.Based on the elastic-plastic theory,the residual stress distribution after the elastic-plastic deformation,unloading and springback in the circumferential direction of the pipe is analyzed,and the residual stress distribution function after repeated loading and unloading and springback is derived,which provides theoretical guidance for the analysis of the ovality and residual stress after the three-roll continuous rounding process.The finite element simulation of the three-roll continuous rounding process is carried out by ABAQUS,and the stress distribution in the circumferential,axial and radial directions of the pipe fittings in the loading stage,the rounding stage and the unloading stage,as well as the residual stress distribution of the pipe fittings after complete unloading and springback are analyzed.According to the finite element simulation results,the effects of the reduction,the number of reciprocating bending,and the initial ovality of the pipe on the ovality and residual stress of the pipe after rounding are analyzed.The roundness results at different positions of the pipe fittings and the reasons for the differences in the roundness results are also discussed.In this paper,the roundness experiment is designed.The possible influence of the pre-sequence process on the rounding result is removed.The ovality and residual stress of the pipe after rounding are tested by using the XRD ray stress instrument and the coordinate measuring instrument.The test results are compared with the finite element simulation results and the factors that may cause the deviation of the experimental results are discussed.The final conclusion is that the three roll continuous straightening process can effectively reduce the ellipticity and residual stress of the pipe fittings and make the distribution of residual stress more uniform by controlling the reduction amount and the number of reciprocating bends.The variation rule of ovality and residual stress of pipe fittings after rounding with various parameters obtained in this paper has guiding significance for the actual production process of large thin-walled pipe fittings.Different rounding parameters can be designed according to the specific use conditions of pipe fittings,so as to improve the performance of pipe fittings and prolong the use time.