Study On Hydrogen Permeation Dynamics In Girth Joint Of Pipeline Steel Under High Pressure Hydrogen Gas Environment

The hydrogen coming from high-pressure transmission medium can penetrate into pipeline steel,which decreases ductility and toughness of the steel.The welded joint consisting of multipass weld and heat affected zone are affected by multipass thermal cycling.There are severe microstructure inhomogeneity and residual stress in the welded joint,and the former can easily affect the diffusion coefficient of hydrogen,while the latter means the existence of stress gradient.Both microstructure inhomogeneity and stress gradient could induce heterogeneous diffusion,which leads to local hydrogen enrichment thus enhance the risk of hydrogen induced failure.Therefore,it is necessary to study the formation of hydrogen enrichment in the girth joint of high pressure hydrogen pipeline,which is of great significance to control the hydrogen induced failure of the joint.In this thesis,high-pressure hydrogen permeation tests were conducted to determine the hydrogen permeation parameters in different regions of X80 steel welded joints,and finite element numerical simulation was used to calculate the welding residual stress.Thereafter,hydrogen penetration model was established considering the factors of microstructure inhomonegeity,welding residual stress and working stress.The hydrogen concentration distribution was finally obtained by hydrogen permeation dynamic simulation.The following results are acquired.(1)The high pressure hydrogen permeation tests of X80 pipeline steel welded joints were carried out under 1.2 MPa hydrogen gas environment by the modified two-cell device.The influence of microstructure on the hydrogen permeability was studied by the calculation of hydrogen permeation parameters.The results show that the diffusion coefficient increases along with the peak temperature,and the adsorbed hydrogen concentration and hydrogen solubility decrease gradually with the increase of the peak temperature.Compared with the heat affected zone,the base material has the largest diffusion coefficient.The diffusion coefficient of weld metal,obtained by different welding methods,increases gradually in the order semi-automatic welding,manual welding,and automatic welding.The second thermal cycle controllsthe hydrogen permeation parameters in the zones which experience a double thermal cycle,and the diffusion coefficient increases with the increase in the second peak temperature,but the adsorbed hydrogen concentration and hydrogen solubility decrease gradually.The difference of hydrogen permeation parameters is resulted from the difference of the microstructure.Less M-A constituent and lower hydrogen trap density are the main reason that result high hydrogen diffusion coefficient.(2)ABAQUS software was used to calculate the welding temperature field and welding residual stress field in the actual pipeline,as well as the coupling stress field of welding residual stress and the working stress induced by 12 MPa transmission gas.By contrasting the size and shape of simulation molten pool with those of actual weld fusion line,the simulation welding temperature field was corrected so as to ensure the correctness of simulation welding residual stress field and coupling stress field.The Mises stress is about 500 MPa in the completed butt joint weld and heat affected zone,and the axial stress is relatively higher than Mises stress and presents an obvious stress concentration.The backing welded zone in multipass welded joint is the stress concentrated area.The existence of working pressure increases the Mises stress level in the pipe joint,but the distribution of the stress is basically the same as that after the welding.(3)The hydrogen penetration process was simulated by ABAQUS,in which the parameters obtained in 1.2 MPa hydrogen permeation tests were used as the boundary or initial conditions.The stress fields without welding residual stress or with coupling stress field were considered in the simulation.The hydrogen concentration distribution in weld joint was finally acquired after the hydrogen diffusion reached a steady state.The results show that the degree of hydrogen enrichment is determined by the combined effects of microstructure and coupling stress.The effect of stress is more obvious than that of microstructure.The stress concentration area often presents hydrogen enrichment.The fusion zone of backing weld and the overlay area between the four and sixth pass in the six-layer and eight-pass welded joint often have high hydrostatic stress,and these two areas present prominent hydrogen enrichment phenomenon.