Hydrogen embrittlement susceptibility of additively manufactured high-strength low-alloy AISI 4340 steel

Hydrogen embrittlement (HE) poses a significant challenge for high-strength steels. Although HE of wrought steels has been extensively studied, it remains limited in steels processed by additive manufacturing (AM). The present work (i) compares the HE susceptibility of AISI 4340 ultra-high-strength steel fabricated by selective laser melting (SLM) with its wrought counterpart; (ii) investigates the predominant factors and possible HE mechanisms in the AM-fabricated material; and (iii) correlates microstructures produced with different SLM processing parameters to HE susceptibility of the steel. Generally, conventionally processed AISI 4340 steel is used with a tempered martensitic structure to ensure the ultrahigh strength, and therefore is susceptible to HE. In contrast, SLM-fabricated 4340 exhibits a uniform, refined bainitic microstructure. How this change of microstructure influences the HE susceptibility of the steel is unknown and needs investigation. Our results demonstrate that, at the same level of strength, the SLM-fabricated 4340 steel exhibits significantly lower HE susceptibility than its wrought counterpart. The SLM-fabricated steel showed a higher hydrogen diffusion rate. Furthermore, the refined microstructure of the SLM-fabricated steel contributes to enhanced ductility, even with hydrogen. These findings indicate that AM of high-strength steels has strong potential to improve HE resistance, providing a pathway to solve this long-term problem. This study highlights the critical role of microstructure in influencing HE and offers valuable insights for developing steels for hydrogen applications.