13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Total hold time in ambient air, t (hours) Residual hydrogen content, CH, R (mass ppm) 5 mm 0.8 mm D' = 9.1 1013 m2/s CH, R = 5.1 mass ppm at t = 0 + Non-charged specimen: 0.13 mass ppm 1 10 100 1000 0 1 2 3 4 5 Figure 1. Residual hydrogen content as a function of total hold time in ambient air (Broken line shows the least-square fitting with Demarez et al.’s solution for the hydrogen diffusion from a finite cylinder [16]) 3.2. Hydrogen segregation in microstructure Figure 2 shows the result of HMT observation on a longitudinal section of a hydrogen-charged specimen. Numerous white spots in the photographs represent the silver particles to which the silver bromide particles were reduced by emitted hydrogen. The silver particles were mainly observed at the cross section of graphite nodules as exhibited in Figure 2(a). The particles were also observed along the cementite lamellae in pearlite colonies as shown in Figure 2(b), but were barely observed at ferrite. It is noted that in the non-charged specimens, no hydrogen emission was detected by the HMT. Figure 3 shows the HMT images on the fracture surface of the hydrogen-charged specimen. The HMT was applied right after the tensile test. As shown, hydrogen emission was observed not only at the graphite nodules, but also at the concaves apart from the graphite nodules. This result implies that a great amount of hydrogen exists at or near the graphite/matrix interface as well as in the graphite itself. (a) Hydrogen emission from graphite (b) Hydrogen emission from pearlite, HMT image (left); Etched microstructure at the same area (right) Figure 2. HMT images on a longitudinal section of the hydrogen-charged specimen
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