PSI - Issue 42

Guocai Chai et al. / Procedia Structural Integrity 42 (2022) 155–162 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 3. in-situ SSRT stress versus strain curves, (a) without hydrogen charging; (b) with hydrogen charging.

Table 2. Tensile properties of the material from in-situ SSRT; R p0.2 -0.2% proof strength,  f -failure strain,  nec -strain at necking point,  f -  nec - plastic strain during necking, Z-fracture surface area reduction Material R p0.2  f  nec  f -  nec  MPa % % % % As-delivered 976 6.40 2.40 4.00 66.0 Tempered 950 7.95 3.71 4.24 70.4 As delivered+H 948 3.56 2.67 0.89 26.9 Tempered+H 931 5.26 3.18 2.08 46.9

As shown in Table 2, the influence of hydrogen charging on homogeneous plastic deformation or strain (up to  nec ) is relatively small. On the other hand, hydrogen charging significantly reduces necking strain (  f -  nec ), about 77.7% for the as-delivered material, and about 51% for the tempered material. The results can be explained that hydrogen embrittlement is a localized cracking behavior with a high crack propagation rate by cleavage or intergranular cracking. This leads to a small fracture area reduction. With increase of necking strain in the tempered material, occurrence of more plastic deformation indicates a decrease in susceptibility of hydrogen embrittlement in the material. Necking strain can therefore be used as an indicator to susceptibility of hydrogen embrittlement. 3.3. Influence of tempering and hydrogen charging on fracture and hydrogen embrittlement mechanism Fig. 4 shows the fracture behavior of the materials after the in-situ SSRT. Fig. 4a and 4d show the fractures of the tempered reference material without hydrogen charging. It is a typical cup-cone fracture with nice dimples, indicating a good ductility. The as-delivered reference material has a similar behavior. Fig. 4b and 4e show the fracture behavior of the as-delivered material with hydrogen charging. Several brittle fracture areas (HE in Fig. 4b) near the specimen surface can be observed. The middle part of the fracture is the remaining fracture with dimples (ductile in Fig. 4b). Fig. 4e shows the details in one brittle fracture area. Intergranular cracking, IG, is dominant, although transgranular cleavage cracking, TG/CL, micro/nano voids, MVC, and quasi-cleavage, QC, can also been observed. HEDE is the main mechanism in the as-delivered material. Fig. 4c and 4f show the fracture behavior of the tempered material with hydrogen charging. Several HE areas can be observed but with smaller sizes (Fig. 4c) comparing those in the as delivered material (Fig. 4b). Intergranular cracking, IG, still appears, but transgranular cleavage cracking, TG/CL, becomes dominant now (Fig. 4f), and amount of MVC also increases, indicating a transition of mechanism from HEDE to HELP. According to Wasim et al. (2021), this transition depends on amount of hydrogen in the material. HELP can transit into HEDE when hydrogen concentration in the material increases to a critical hydrogen concentration. In this study, hydrogen charging process for these two materials is same, hence hydrogen charging rate in the materials should be similar. However, the distribution of hydrogen in the materials can be different due to the