PSI - Issue 68

Renata Latypova et al. / Procedia Structural Integrity 68 (2025) 1115–1120 R. Latypova et al./ Structural Integrity Procedia 00 (2025) 000–000

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Figure 2. H concentrations of the materials with 0 - 4 h H-charging.

The same phenomenon is observed in the EP tests. When C occupies the dislocations, fewer H traps remain available resulting in faster H diffusion in T50 – T150 materials. In addition to C segregation, the lower H concentration and faster diffusion in T250 may also be caused by the absence of RA or its transformation to cementite, which in this case does not provide sufficient H-trapping. However, the effect of RA is likely negligible given its concentration is less than 1%, and it has been reported that RA might not be activated as an H-trap with electrochemical H-charging unless tensile loading is applied (Vander Vennet et al., 2022). It is possible that some H-trapping is occurring at the interface of RA and martensitic matrix (Chan et al., 1991; Malitckii et al., 2019). The EP curves and corresponding H diffusion coefficients (D) are presented in Figure 3. TFT test results show that the T250 material exhibits a significantly higher threshold stress than the other materials, indicating superior resistance to HE. In contrast, DQ, T50, and T150 exhibit lower threshold stress levels with considerable overlap among them, suggesting no substantial difference in their HE resistance.

Figure 3. (a) EP curves and (b) H diffusion coefficients of test materials.

Dislocations play an important role in various proposed HE cracking mechanisms, such as H Enhanced Decohesion (HEDE), H Enhanced Localized Plasticity (HELP), and Adsorption-Induced Dislocation Emission (AIDE) (Venezuela et al., 2016). In the case of T250, C diffusion is pronounced, leading to higher concentration at dislocations, locking them, and restricting their mobility as well as further H uptake. This restriction in dislocation mobility most likely causes the shift in the cracking mechanism in T250, which contributes to a higher threshold stress level. For example, in low-C boron-alloyed DQ steel, HE susceptibility has been alleviated with 100 °C tempering and cracking mechanisms shifted from mixed transgranular-intergranular to transgranular quasi-cleavage (Zhang et al., 2018). For automotive steels, HE susceptibility shows mixed results after LTT, which simulates paint baking treatment. For