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 1. Chemical compositions. C Si

Mn

P

S

Cu

Mg

Material S Material M Material L

3.61 3.67 3.68

2.04 2.28 2.16

0.29 0.29 0.29

0.025 0.027 0.024

0.01 0.01 0.01

0.01 0.02 0.01

0.038 0.034 0.035

2.2 Hydrogen charging and measurement of hydrogen content

In order to conduct hydrogen charging, tensile specimens were exposed to 100-MPa hydrogen gas at 85 ºC for 452 h to obtain a uniform distribution of hydrogen. This specimen is hereafter called H-charged specimen , while the specimen without hydrogen charging is called uncharged specimen . The hydrogen content of a specimen was measured by a thermal desorption analyzer (TDA). The temperature was raised up to 700 ºC at a rate of 100 ºC/h. In order to evaluate the effect of graphite diameter on the hydrogen absorption behavior, square-shaped thin chips with a thickness of 1 mm were machined from the various positions of three kinds of as-cast blocks. These samples were charged with hydrogen by soaking in an aqueous solution of 20 mass% ammonium thiocyanate at 40 ºC for 168 h. The hydrogen content was also measured by TDA.

2.3 Tensile tests

The tensile tests were carried out using H-charged specimen and uncharged specimen. All tests were performed in air at room temperature. The crosshead speed was 0.02 mm/min. 3. Results and discussion

3.1 The effect of graphite size on the hydrogen absorption capability

Figure 5 shows the relationship between the hydrogen content of these thin chips and mean graphite diameter. The mean graphite diameter was estimated by image analysis on the cross section at the middle in thick of chip. The hydrogen contents of these chips were ranged from 7.8 mass ppm to 65.9 mass ppm, and they were dependent on the graphite size. When the graphite diameter was less than approximately 13 µm, the hydrogen content was approximately 10 mass ppm regardless of graphite size. On the other hand, when the graphite diameter was larger than 13 µm, the hydrogen content was approximately 60 mass ppm and then it became almost constant irrespective of graphite diameter. In other words, there was a critical graphite diameter of about 13 µm that significantly increased the hydrogen absorption capability. So far, some papers about the hydrogen absorption behavior of ductile cast iron were published. Takai (Takai et al. (2002)) reported that most hydrogen in hydrogen-charged ferritic pearlitic DCI exists in the graphite-matrix interface using secondary ion mass spectrometry (SIMS). In addition, Matsunaga (Matsunaga et al. (2013); Matsunaga et al. (2014)) found out that a large amount of hydrogen was stored inside the graphite nodule as well as the graphite-matrix interface using hydrogen microprint technique (HMT). Although several mechanisms have been proposed as just mentioned, the results shown in Fig. 5 mean that the hydrogen absorption behavior of DCI cannot be explained by a single mechanism and there seem to be different two kinds of mechanism, depending on the graphite size. In order to understand the hydrogen absorption mechanism of DCI, a further thorough research is necessary, and therefore, the hydrogen absorption mechanism is not discussed in this paper.