Analysis Of Residual Stress And Hydrogen Diffusion In X70 Steel Spiral Welded Pipe

Spiral welded pipe is usually used to transport oil and natural gas.As a connection with high efficiency,simple equipment and low manufacturing costs spiral welding has great potential for application.However,in the welding process local,uneven heating and the nonlinear properties of materials will lead to residual stress,which affecting the integrity of the welding structure.In addition,the hydrostatic pressure gradient caused by the presence of residual stress causes the diffusion of hydrogen into the interior of the material.The accumulation of hydrogen leads to the increase of the hydrogen concentration in the stress concentration area,which leads to the increase of the stress gradient.The interaction between stress and hydrogen causes an increased risk of hydrogen cracking in the pipeline.Therefore,the interaction between stress and hydrogen will affect the safety of spiral welded pipe during service.On account of the above problems,the welding process of spiral welded pipe is simulated by finite element method.On the basis of the related reference,the numerical simulation is consistent with the experimental results.The effects of forming angle,the ratio between diameter and thickness,the welding heat input and preheating temperature have been investigated.Then the influence of residual stress on hydrogen diffusion,the effects of forming angle,the welding heat input,the preheating temperature and grain boundary have been investigated,which provides a theoretical basis for the safety of X70 steel spiral welded pipe.The main work and conclusions are as follows:(1)The numerical simulation results are in good agreement with the experimental data,which proves the feasibility of the finite element numerical simulation.A maximum stress occurs at the junction of the heat affected zone and the matrix metal,this is because the rapid increase of welding temperature in the welding process leads to large temperature gradient in the weld toe,where the shrinkage tendency and rate of the filled metal is large,resulting in a larger stress concentration.(2)With the increase of the forming angle,the hoop stress decreases,the axial stress increases,and the optimal forming angle is about 42 degree.With the increase of the ratio between diameter and thickness,the hoop stress increases and the axial stress decreases.The residual stress decreases with the increase of preheating temperature and welding heat input in a certain range.(3)The peak concentration of diffused hydrogen in the residual stress state is more than 100 times higher than that in the non-stress state,and the existence of the residual stress has a significant effect on the concentration and distribution of the diffused hydrogen.And the concentration of diffused hydrogen in the non-stress state is decreasing from the inner wall to the outer wall in the thickness direction.Under the stress state,the concentration of hydrogen in the stress concentration region is the highest.(4)With the increase of forming angle,preheating temperature and welding heat input,the concentration of diffused hydrogen decreases.The change of the residual stress in the welded joint results in the change of diffusion hydrogen concentration,but the distribution state remains unchanged.(5)There are two ways of hydrogen diffusion in the material microstructure.In the early stages of simulation,hydrogen atoms start diffusing through the boundaries.As hydrogen atoms diffuse through the boundaries of the microstructure,the hydrogen concentration in boundaries continues enhancing to a considerable high level before it can start diffusing into the grains...