An inclusion modification approach to improve hydrogen embrittlement resistance: Trace Mg addition facilitating MnS dispersion for enhanced deep hydrogen trapping

This study investigated the effects of substituting conventional Ca treatment with trace Mg treatment (0.005 wt%) on the hydrogen trapping behavior and hydrogen embrittlement (HE) susceptibility of X70 pipeline steel. The strategy significantly decreased HE susceptibility from 44.0 % to 21.3 %. The results revealed that Mg-modified inclusions MgO·xAl₂O₃ do not exhibit strong hydrogen trapping capabilities. Instead, they promote the uniform distribution of hydrogen in steel and delay cracking by forming finely sized (< 2 μm) and massively dispersed MnS, MgO·xAl₂O₃·MnS·TiN·NbN inclusions, which replace coarse CaS·Al₂O₃ and (Ca, Mn)S inclusions prone to HE. Additionally, grain refinement, the uniform distribution of the specific {111}//ND grain orientation, and an increased proportion of high-angle grain boundaries (HAGBs increased from 38.9 % to 45.1 %) contribute to improved HE resistance. Notably, the MnS/matrix interface exhibits the strongest hydrogen trapping capacity, as evidenced by a hydrogen segregation energy of −0.821 eV obtained through first-principles calculations and a potential difference variation of 16 mV reflecting the hydrogen trapping level, which is double the variation (8 mV) observed at the CaS/matrix interface in Ca-treated steel.