Visualization of Hydrogen Damage and Aggregation for 22MnB5NbV Hot Stamped Steel

The microstructure, critical conditions of hydrogen damage shaped nuclear, visualization of hydrogen damage and hydrogen aggregation of 22MnB5NbV hot stamped steel were characterized and analyzed by scanning electron microscopy (SEM) and electron back-scatter diffraction (EBSD) analysis, as well as the electrochemical hydrogen charging and hydrogen microprint tests. A hydrogen damage evolution model was established to predict the evolution mode of hydrogen induced cracks. The nucleation and growth of hydrogen induced cracks were further observed through the hydrogen charging experiments to verify the prediction results of the model. It is found that the microstructure of 22MnB5NbV steel is basically composed of martensite, and the microstructure is relatively small. The average grain size is about 7.81 μm, and the proportion of small angle grain boundaries is up to 35.34%. The critical current density of hydrogen blister nucleation induced by 22MnB5NbV steel is 30 mA/cm² at 4 h hydrogen charging. The hydrogen atoms first accumulate in a large number inside the hydrogen blister, with the increase of hydrogen charging time, the hydrogen atoms recombine into hydrogen molecules, and then gradually diffuse outwards under the hydrogen pressure. After 10 h of hydrogen charging, the hydrogen atom aggregation trend disappears and is distributed around the hydrogen bubble in a dispersion form. The aggregation state of hydrogen bubbles exhibits directivity under prolonged electrochemical hydrogen filling, the multiple linear arrangement of hydrogen bubbles form inside the material, which is eventually connected as discontinuous hydrogen-induced cracks. The local softening occurs in the surrounding matrix after the high concentration hydrogen atoms depolymerizes in the inner wall of hydrogen bubble. Under the action of hydrogen pressure, hydrogen bubbling is prone to expand to one or both ends, forming hydrogen induced cracks.