Study Of The Influence Of Residual Stress On Hydrogen Diffusion Of Typical Welded Joints

Flat butt welding is commonly used to construct large welded structures,such as ships.Under high temperatures during the welding process,the hydrogen-containing compound in the arc welding decomposes into monoatomic hydrogen,which is dissolved in the molten pool in large amounts.High-strength steel,widely used for large ship structures,is sensitive to welding residual stress.Non-uniform welding residual stresses in such structures could facilitate the diffusion and accumulation of hydrogen in the steel,leading to excessive hydrogen localized at the welded joints.This behavior can cause hydrogen embrittlement,threatening the safety and reliability of ship structures.Therefore,it is necessary to study the residual stress and hydrogen diffusion under the influence of the residual stress.Two kinds of butt welded thick plate models I and II with different yield strength and geometric dimensions are studied in this paper.Firstly,the residual stress of butt welded thick plate model I is studied,then the residual stress of butt welded thick plate model II of high strength steel and the influence of residual stress on hydrogen diffusion are analyzed.Finally,the tissue heterogeneity and other factors which affect the hydrogen diffusion are analyzed.The main research contents and conclusions are as follows:(1)Based on the basic theory of thermal elastic-plastic theory and the secondary development technology of finite element software ABAQUS,the residual stress of butt welded thick plate model I of high-strength steel is numerical simulated,and the results are compared with the test results in the literature.The results show that the numerical simulation results are in good agreement with the test results,and there are large welding residual stresses in the weld and heat affected zone.With the increase of the distance from the weld,the welding residual stress decreases gradually.(2)The hydrogen diffusion coefficient of the samples in each zone of the butt welded thick plate model II of high strength steel under different tensile stress are measured with the electrochemical hydrogen permeation method,and the hydrogen concentration in the weld zone is measured by the thermal conductivity method,which provided the parameter basis for the simulation of hydrogen diffusion of model II under the influence of welding residual stress.Based on the measured hydrogen diffusion parameters,the hydrogen concentration distribution of model II without the influence of residual stress is simulated,and the influence of the tissue heterogeneity of the welded joint is ignored.The results showthat the hydrogen is uniformly distributed in the welded joint without the influence of residual stress.(3)Based on the theory of residual stress-induced hydrogen diffusion,the welding residual stress of butt welded thick plate model II is analyzed with the finite element method of residual stress analysis of high-strength butt welded thick plate model I.The residual stress field is taken as the predefined field,and the sequential coupling calculation of hydrogen diffusion is carried out for model II in combination with the relevant parameters of hydrogen diffusion of model II which determined by hydrogen diffusion test,with the tissue heterogeneity is ignored.The results show that the hydrostatic stress affecting hydrogen diffusion exhibits a symmetric bimodal distribution,with the maximum value appearing near the weld toe,corresponding to tensile stress.The concentration of diffusible hydrogen is also high in the region with large tensile stress,and the distribution trend of the concentration of diffusible hydrogen is consistent with that of hydrostatic stress.Therefore,the residual tensile stress in the welded joint could facilitate the diffusion and accumulation of hydrogen in the welded joint.(4)The influence of the tissue heterogeneity of high strength steel butt welded thick plate model II on hydrogen diffusion is analyzed.The results show that the tissue heterogeneity of welded joint has a significant effect on hydrogen diffusion.The maximum of hydrogen concentration obtained with the tissue heterogeneity is considered is far greater than that obtained without considering the tissue heterogeneity.The change of hydrogen concentration is more obvious when the influence of the tissue heterogeneity is considered.(5)With the increase of initial hydrogen concentration applied in the weld,the maximum hydrogen concentration after diffusion in model II increases linearly.The change of welding sequence has little effect on hydrogen diffusion,and the distribution curves of hydrogen diffusion are basically in coincidence under the two welding sequences.