PSI - Issue 68

Adam Ståhlkrantz et al. / Procedia Structural Integrity 68 (2025) 1051–1058 Author name / Structural Integrity Procedia 00 (2025) 000–000

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Global formability, ! = (1 + "#$ ) Local formability, = , " ! " " - Formability Index, . . = 1 ∙ !

(1) (2) (3)

Figure 3: Global and local formability analysis, A) shows the results from uncharged samples, B) shows the charged samples. The tensile data has been visualized and plotted in terms of ! and in Figure 3 for the uncharged specimens. The Figure clearly illustrates the differences between the five heat treatments by showing how they are most prone to deform in a tensile test. There is a striking difference between sample A (low ! , high ) and sample E, which has the highest amount of RA and exhibits more global deformation and less necking prior to fracture. In all samples, there is a drop in the true fracture strain ( TFS ) with charging. However, for the true strain at necking ( ! ) , a change was only seen in the four samples containing RA. Furthermore, when the drop in uniform elongation after charging with respect to RA is plotted (Figure 2B), it can be seen that there is a strong correlation between the drop in uniform elongation and the amount of RA. The results clearly indicate that the global ductility of the material is impaired more strongly by trapped hydrogen when RA is present, while the change in local ductility is the same for all specimens. The global ductility is controlled to a large degree by the increased strain hardening caused by the TRIP effect. We therefore pose the hypothesis that the mechanical stability of the RA has been impaired by the trapped hydrogen. The apparent effect on global ductility could then be caused by either the RA having transformed directly after charging due to a massive drop in stability, or that the martensite transformation proceeds more quickly with respect to strain due to a more moderate drop in ductility. Investigating the specific mechanism of trapped hydrogen on global ductility loss in AHSS is left as an avenue of future studies. It has been shown in this study that AHSS charged with hydrogen undergoes a significant change in material properties, which requires consideration in design and selection of the material. This further emphasizes the importance of better understanding the relationship between RA and hydrogen interactions in AHSS, both for material design and when choosing the material and heat treatment for specific applications.