Investigation On Hydrogen Induced Stress Corrosion Cracking Of X80 Pipeline Steel

Among various oil and gas transportation modes,due to the unique advantages,pipeline transportation is widely used.With the rapid development of oil and gas construction,oil and gas pipeline transportation has its unique advantages and irreplaceability.Pipe steel is developing towards high steel grade.As a new generation of high strength and high toughness pipeline steel,X80 pipeline steel is increasingly used.However,working in the complex conditions,the pipeline steel is subject to the dual coupling of hydrogen diffusion and stress,and the possibility of fracture failure of the pipe body is much higher than that of a single stress condition.The stress state,hydrogen diffusion state and microstructure of the material are three important factors for hydrogen induced stress corrosion cracking.Corrosion cracking of pipeline steel causes oil and gas leakage,which may cause accidents such as fire disaster and explosion,which poses a threat to the safety of pipelines.Therefore,it is necessary to study the hydrogen induced stress corrosion cracking behavior of X80 pipeline steel.The main research contents and research results of this paper are as follows:(1)Based on ABAQUS,a single notched tensile specimen model is established to simulate the hydrogen diffusion behavior of X80 pipeline steel under the load.Based on Fick’s second law,model couples the stress-diffusion sequentially to obtain the influence of stress on hydrogen diffusion.The hydrogen distribution and the concentration of hydrogen along the crack ligament are extracted from the simulation results.The results show that the distribution of hydrogen and the distribution of hydrostatic stress tend to be the same,and the maximum concentration of hydrogen appears in a distance from the crack tip,which is consistent with the conclusions of similar test results.The equivalent plastic strain value of the tip path was extracted.The distribution of the lattice hydrogen concentration and total hydrogen concentration under different loads were summarized.(2)Aiming at the influencing factors such as diffusion time,initial hydrogen concentration and initial tip length,the hydrogen diffusion law study is carried out.The results indicate that the hydrogen atom concentration increases rapidly in the initial stage of diffusion.The diffusion rate of hydrogen atoms slows down as time goes on,and the content tends to be stable.The initial hydrogen concentration and the initial tip length mainly affect the steady-state hydrogen concentration,showing a positive value.The two factors have a small effect on the distribution law of hydrogen,and the peak position along the crack tip path of the hydrogen concentration has microscopic change.(3)The Cohesive Zone Model(CZM)is used to simulate the role of hydrogen diffusion playing in corrosion crack propagation.The Load-Displacement relationship of various forms of CZM is summarized,and consider the hydrogen influence on the Traction-Separation Law(TSL)parameters.The influence of hydrogen concentration on the maximum stress and cohesive energy is investigated.The polynomial CZM constitutive equation affected by hydrogen diffusion is established.The three-point bending finite element model of X80 pipeline steel was established,and the numerical simulation of CZM crack propagation in ABAQUS was carried out using the hydrogen-influencing finite element subroutine VUMAT.The simulation results were fitted with the existing experimental data to obtain the cohesive parameter values.The energy value is 2.29 times of the cohesive energy value after hydrogen corrosion.The ability of the material to resist damage was weakened by hydrogen,and the damage of the crack tip is accelerated.(4)Taking the experimental results of fracture toughness of X80 pipeline steel as the basic data,analysis the factors that influence on the hydrogen induced stress corrosion cracking behavior of pipeline steel.The hydrogen coverage,load and notch size are taken as the main influencing factors to study the law of hydrogen induced stress corrosion cracking.The hydrogen concentration and the cohesive force unit related parameters are extracted from the simulation results,the hydrogen diffusion under different conditions is considered and the change of the cohesive parameters are obtained.Simultaneously,the different width hydrogen induced stress corrosion cracking model was established,which is compared with the 2D model.The effect of thickness on the hydrogen diffusion in the X80 pipeline steel and the cracking of the pipeline steel were analyzed.The paper closely links hydrogen diffusion with cracking.The stress corrosion cracking law of X80 pipeline steel under hydrogen environment is analyzed,and the influence of various related parameters on it is studied,which provides a certain theoretical basis for the prevention of corrosion cracking.