Study On Hydrogen Induced Cracking And Hydrogen Permeation Behavior X70 MOS Steel With Trace-Mg Treatment

In this paper,the microstructure,inclusions and precipitates of tested steels with trace-Mg treatment were observed and analyzed by optical microscope（OM）,field emission scanning electron microscope（FE-SEM）,field emission transmission electron microscope（TEM）and X-ray energy dispersive spectrometer（EDS）.The HIC susceptibility of tested steels were evaluated by NACE TM 0824-2016 standards,and the hydrogen embrittlement（HE）susceptibility was evaluated by pre-charged hydrogen tensile test.The generation,adsorption,diffusion,and capture behavior of hydrogen atoms in the tested steel were clarified by electrochemical methods such as electrochemical cycle charging and discharging hydrogen.The effect of trace-Mg treatment on the modification of inclusions and the susceptibility of HIC and HE were discussed.In addition,the influence of cyclic load frequency on the generation,adsorption,diffusion kinetic behaviors of hydrogen atoms was preliminarily discussed and its mechanism was revealed.The main conclusions are as follows:（1）With trace-Mg treatment,on the one hand,the microstructure of the tested steels was obviously refined and the bainite content increased;on the other hand,both the number and the chemical composition of inclusions also changed.In addition,the precipitates tended to decrease and grow up.Most composite inclusions were of“core”of Mg O·Al₂O₃ and“shell”of MnS and/or TiN structure and appearing dispersed distributed,these more refined,spheroidized and dispersed“core-shell”structure inclusions.These inclusions could provide more hydrogen trapping sites and compensate the adverse factors caused by the reduction of precipitates in tested steels,so that Mg4tested steels with 4 ppm Mg addition had the lowest J_∞L and D_app and highest C_o and N_T,which suggested the highest hydrogen trapping efficiency and highest saturated hydrogen concentration.Through improving hydrogen trapping efficiency,obtaining the best HIC resistance.（2）It was proved by electrochemical tests that trace-Mg treatment not only promotes the precipitation of hydrogen atoms on the surface of the sample,but also promotes the adsorption process and inhibits the desorption reaction,resulting in an increasing in the concentration of hydrogen atoms at the subsurface.However,due to the improved hydrogen capture ability of the’core-she’structure inclusions in the steel,the HE susceptibility was significantly improved.（3）When the alternating load frequency increases from 0.01 Hz to 1 Hz,the kinetic parameters of hydrogen precipitation,adsorption and permeation of tested steel in wet H₂S environment showed a V-shaped or anti-V-shaped change trend,with the critical frequency is 0.25 Hz,which is the result of the interaction of samples surface and the diffusion rate of cathode reactants in corrosive medium.