PSI - Issue 68

Vahid Javaheri et al. / Procedia Structural Integrity 68 (2025) 1098–1104 V. Javaheri et. al , Structural Integrity Procedia 00 (2025) 000–000

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The freshly formed martensite (α′) becomes supersaturated with hydrogen, which is highly diffusible in this phase. This increased hydrogen mobility enables the redistribution of hydrogen to neighboring trap sites, particularly at the interphase boundaries that were previously γ/α interfaces. The presence of both the original and migrated hydrogen in these regions lowers the cohesive strength of the boundaries, as reported in previous studies (Chen et al., 2024; Liu et al., 2023; Zhang et al., 2024). This phenomenon is further exacerbated by the incompatibility between the hard, freshly formed α′ and the softer α/retained γ phases. During SSRT, these microstructural disparities result in high stress concentrations, which act as nucleation sites for hydrogen-induced cracking (HIC). The hydrogen-enhanced decohesion (HEDE) mechanism, commonly observed in intercritically annealed medium manganese steels, plays a critical role in this process. Furthermore, RA(γ) generally has a higher hydrogen solubility and higher desorption activation energy compared to its α/α′ counterparts (Chen et al., 2024). During plastic deformation and the accompanying martensitic transformation, significant amounts of hydrogen are expelled from the newly formed martensite due to its relatively lower hydrogen solubility. This leads to a localized redistribution of hydrogen to weaker microstructural regions, such as grain and lath boundaries, which promotes the formation of hydrogen-induced (micro)cracks. The HE mechanism observed in the studied medium manganese steel is summarized and illustrated in Figure 5 through three main steps: I. Hydrogen Accumulation – Initially, introduced hydrogen during charging primarily resides in the austenite (γ) grains and at interphase boundaries due to its higher solubility in austenite compared to ferrite (α) or martensite (α'). The microstructure of charged sample consists of a martensitic matrix (α) with approximately 40% RA(γ) dispersed in a lamellar morphology. After hydrogen charging, hydrogen atoms predominantly accumulate within the γ grains and at the interphase boundaries between γ and α and partially at the martensitic lath and block boundaries. II. Martensitic transformation – Upon plastic deformation, the RA(γ) undergoes a strain-induced transformation into fresh martensite (α′), which leads to two critical consequences. First, the mechanical contrast between the hard, freshly formed α′ and the softer α/retained γ creates significant stress concentrations at their boundaries (IIa). These stress concentrations act as preferential sites for crack initiation. Second, the hydrogen, which is initially supersaturated in the newly formed α′, rapidly diffuses to neighboring structures such as the α′ lath boundaries and grain boundaries (IIb). This migration of hydrogen weakens the cohesion of these boundaries, making them more susceptible to fracture. III. Hydrogen enhanced decohesion (HEDE) and crack initiation – The combination of high stress concentrations and increased hydrogen concentration at these boundaries weakens the atomic bonds (decohesion), leading to the nucleation of hydrogen-induced cracks (HICs). As plastic deformation continues, these HICs propagate, often nearly perpendicular to the loading direction, further compromising the material's structural integrity.

Fig. 5. The main mechanism of hydrogen embrittlement in the studied material