PSI - Issue 42

Lisa Claeys et al. / Procedia Structural Integrity 42 (2022) 390–397 Claeys et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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a

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austenite α’ -martensite ε -martensite

25 µm

25 µm

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TD

60 µm

60 µm

RD

Fig. 3: EBSD phase maps of tensile specimens interrupted at an intermediate strain of 30% for (a) 304L ASS reference; (b) 304L ASS with H, (c) 18Mn-0.6C TWIP steel reference, (d) 18Mn-0.6C TWIP steel with H, twin boundaries are presented in white, excluded points in black The RD fracture surfaces of the hydrogen precharged tensile tested specimens are visualized on the SE images in Fig. 4. Due to the intrinsic low hydrogen diffusivity of austenitic steels, seven days of hydrogen charging did not suffice to reach a homogeneous hydrogen concentration through the thickness of the tensile specimens. A hydrogen affected zone is present at the edges of the fracture surface, while the center of the fracture surface is still characterized by ductile microvoid coalescence, similar to the entire fracture surface of the reference condition, due to the absence of an increased hydrogen concentration. In the hydrogen-affected zone, 304L ASS appeared to be prone to quasi cleavage fracture, whereas an intergranular fracture type was observed for the TWIP steel. A small fraction of the fracture surface of the TWIP steel showed quasi-cleavage fracture as well, cf. white arrows in Fig. 4b. Both fracture types are frequently observed in the presence of hydrogen. The quasi-cleavage fracture type requires accelerated and more planar deformation leading to stress concentrations at intersections of planar deformation features within grains where fracture can easily initiate (Martin et al. (2011)). Accelerated and more planar deformation was clearly observed on the phase map in Fig. 3b for 304L ASS. Moreover, the transformation to α’ -martensite is known to be a preferential damage initiation site. When austenite transforms to α’ -martensite, the hydrogen solubility drops by several orders of magnitude leading to a supersaturated matrix prone to crack initiation. These cracks typically initiate at the interface between austenite and α’ -martensite, contributing to a quasi-cleavage fracture surface (Zhang et al. (2010)). Transformation to ε -martensite, on the contrary, has the possibility to arrest crack propagation (Koyama et al. (2016)). Besides, density functional theory calculations showed that the hydrogen properties of ε -martensite are closer to the hydrogen properties of austenite (Hirata et al. (2018)). In the TWIP steel investigated in this work , ε -martensite indeed