Interaction Between Inclusions And Hydrogen In Pipeline Steel And Its Effect On HIC Sensitivity

With the increasing demand for oil and gas resources due to the development of national economy,the problem of hydrogen induced cracking（HIC）of pipeline steel caused by wet H₂S and other acidic media in oil and gas pipeline becomes more and more prominent.In this paper,the critical size of inclusions inducing hydrogen induced crack in steel is theoretically deduced,and the aggregation behavior of hydrogen atoms at the interface between different inclusions and matrix is theoretically calculated by using finite element method and first principles.Based on this,the inclusions in pipeline steel are modified by Ti/Mg composite deoxidation.The interaction mechanism between the modified inclusions and hydrogen and the mechanism of its effect on HIC sensitivity was systematically studied by scanning Kelvin atomic force microscopy.Based on the relationship between the concentration of solute atoms and stress,the critical size of hydrogen induced crack induced by typical inclusions in steel was theoretically deduced.The results show that the critical size of hydrogen induced crack caused by oxide inclusions is between0.14μm and 0.42μm,and Mn S has a larger critical size（2.52～2.6μm）;compared with Mn S,oxide inclusions in steel have stronger hydrogen capture ability and higher critical hydrogen concentration at interface.Based on the above theoretical calculation results,the inclusions in steel were modified by different Ti/Mg composite deoxidation treatments.The composition,size,distribution,and morphology of inclusions in the test steel were comparatively analyzed,as well as the internal relationship between inclusions and hydrogen diffusion and HIC sensitivity.It was found that when the Ti/Mg addition ratio was 4:1,more Ti addition was helpful to form more primary Ti oxide inclusions.At the same time,Mn S tends to precipitate near the oxide inclusions and wrap around the oxide inclusions to form a core-shell structure.Therefore,the refinement of Mn S and the increased small-size composite inclusions in the steel are realized,and the diffusion behavior of hydrogen in the steel is obviously hindered.In addition,the accumulation of reversible hydrogen atoms in the steel is reduced,and the HIC resistance of the pipeline steel is improved.In the Ti/Mg composite deoxidation modified inclusion test steels,the influence of inclusions on hydrogen microcosmic diffusion behavior was studied.The results show that hydrogen diffusion behavior in steel has periodic characteristics.After entering the steel,hydrogen will firstly distribute uniformly in the matrix,and then is gradually trapped by inclusions and accumulates around inclusions.The residual stress concentration area around the inclusion is the main site of hydrogen diffusion and accumulation in the steel,and it is also the most important channel for hydrogen diffusion into the steel and escaping from the steel surface.There is usually a large stress concentration around the oxide of long strip composite inclusions with large size,which can promote the local aggregation of hydrogen atoms and lead to hydrogen induced cracks.Based on hydrogen desorption equation of state and hydrogen thermal desorption experiment,it is found that all kinds of inclusions in Ti/Mg composite deoxidized steel have strong binding ability to hydrogen atom.Compared with oxide inclusions in steel,Mn S has weak binding ability to hydrogen atom,and its hydrogen desorption activation energy is about 71.6 k J/mol,while the hydrogen desorption activation energy of oxide in steel is in the range of 79～86.5 k J/mol.When Ti/Mg composite deoxidation is used to modify the inclusions in steel,the amount of hydrogen atoms trapped by oxides and Mn S inclusions in steel accounts for 51%of the total desorption hydrogen in steel,and the proportion of total irreversible hydrogen in steel reaches 58.8%,which effectively reduces the reversible hydrogen content in steel and improves the anti-HIC ability of material.