PSI - Issue 14

Takuya Yoshimoto et al. / Procedia Structural Integrity 14 (2019) 18–25 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Table 2. Mean graphite diameter, RRA and hydrogen content. Material Mean graphite diameter (µm) RRA

Hydrogen content (mass ppm)

S L S U M

11.4 13.8 21.0 28.5

0.77 0.38 0.48 0.43

2.3

34.8 31.3 39.7

L

Fig. 7. Relationship between RRA and mean graphite diameter.

3.3 Relationship between hydrogen content and ductility loss

Figure 8 shows the relationship between the hydrogen content of tensile-fractured specimens and the mean graphite diameter. The hydrogen contents of the square-shaped thin chips, shown in Fig. 5, are also plotted on Fig. 8. Although the hydrogen contents of tensile-fractured specimens were different from those of the chips owing to difference in charging condition, they also showed a dependence on the graphite size similarly to the chips. In comparison between Fig. 7 and Fig. 8, when the mean graphite diameter became over the critical value, the RRA changed significantly as well as the hydrogen content. Figure 9 shows the relationship between the hydrogen content and RRA. These figures mean that the RRA correlates strongly with the hydrogen content and the graphite size is nothing more than a factor affecting the hydrogen content. In short, a dominant factor affecting the severity of hydrogen embrittlement of ferritic DCI is not graphite size but the hydrogen content. From the viewpoint of practical design, nevertheless, it should be noted that the graphite size influences the hydrogen embrittlement of DCI indirectly yet significantly. The hydrogen content and the ductility loss due to hydrogen were large when the mean graphite diameter was over 13 µm. In contrast, the hydrogen absorption capability was reduced, when the mean graphite diameter became lower than 13 µm, and the hydrogen effect decreased markedly with reduction of hydrogen content. Accordingly, it is concluded that to control the mean graphite diameter lower than 13 µm can be a design guideline for microstructure of DCI which resistant to hydrogen.