Comparative study on the hydrogen-induced cracking behavior of Al-Si- and Al-Fe-coated 31MnB5 martensitic steels

In this study, the hydrogen embrittlement (HE) resistance of Al-Si- and Al-Fe-coated high-strength martensitic steels was investigated by controlling the austenitization dew point and diffusible H content, with emphasis on crack propagation behavior. Under identical austenitization conditions (900 °C, 2 min, and dew point 16.7 °C), the Al-Si-coated specimen exhibited lower HE resistance than the Al-Fe-coated specimen due to its higher absorbed diffusible H content. However, under conditions of nearly identical diffusible H contents (∼0.53 wt ppm), the Al-Fe-coated specimen showed inferior HE resistance. In the Al-Si-coated specimen, vertical cracks originating in the intermetallic compound (IMC) layer were arrested at the Kirkendall void (KV) band and subsequently propagated horizontally along the band at 2% strain. With increasing strain, deformation bands formed along {110} planes in the α -Fe layer beneath the KV band, eventually evolving into vertical cracks that propagated into the substrate. These observations clearly demonstrate that crack deflection along the KV band suppressed vertical crack propagation, thereby enhancing HE resistance. In contrast, in the Al-Fe-coated specimen without the KV band, IMC-originated vertical cracks readily penetrated through the α -Fe layer into the substrate. These cracks exhibited mixed intergranular and transgranular propagation, with local alignment along {110} planes and delamination along the α -Fe/substrate interface, thereby resulting in lower HE resistance.