13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- Fe. The microstructure was composed of a ferrite/pearlite matrix in conjunction with spheroidal graphites. The volume fraction of each phase, which was measured at a mirror-finished section, was 21 % for pearlite, 15 % for graphite and bal. ferrite. The average value of the Vickers hardness, HV, measured with a load of 9.8 N, was 169 in the ferrite section and 217 in the pearlite section. The round bar specimens, having a diameter of 5 mm and a gage length of 30 mm, were used for the tensile tests. The surface of the specimen was finished by polishing with emery papers and then by buffing with an alumina paste. 2.2. Hydrogen-charging, TDS analysis and microscopic visualization of solute hydrogen The specimens were charged with hydrogen by soaking them in an aqueous solution of 20 mass% ammonium thiocyanate at 313 K for 48 hours. The hydrogen-charged specimens were re-polished before tensile tests to remove the corrosion layer produced by the charging. A number of studies (cf. [9-12]) have reported that cast irons as well as steels are vulnerable to surface cracking due to hydrogen-charging. To confirm the presence or absence of a crack in the specimen after the charging, the surface was mirror-polished with buff and then etched with nital. In this study, no crack was detected on the specimen surface. The hydrogen content in the specimens was measured by the thermal desorption spectroscopy (TDS). Circular disks of 0.8 mm-thick were sliced from the specimen cross section and used for the TDS analysis. The measurements were carried out up to a temperature of 873 K (600 °C) at a heating rate of 0.5 K/s. The hydrogen emission from the cast iron was visualized by using the hydrogen microprint technique (HMT) [13-15]. 2.3. Tensile test Displacement controlled tests were carried out with three different crosshead speeds (CHSs) of 50, 1 and 0.02 mm/min by using a hydraulically-controlled testing machine in ambient air. By assuming that crosshead displacement is equal to the elongation of the test section, the strain rate ε for each CHS is rendered as 2.8×10−2, 5.6×10−4 and 1.1×10−5 s−1, respectively. During the experiment, strain was measured with a clip gauge extensometer. 3. Results and discussion 3.1. Hydrogen content in specimen and the desorption behavior Figure 1 displays the residual hydrogen content, CH, R, as a function of the total hold time after hydrogen-charging, Δt. The content decreased gradually while the charged specimen was exposed to ambient air. After 200 hours, the content reduced to the same level of hydrogen content as in the non-charged specimen. From the reduction of hydrogen content, an apparent hydrogen diffusion coefficient at room temperature, D', was estimated by means of Demarez et al.’s solution for the hydrogen diffusion from a finite cylinder [16]. The least-square fitting rendered the coefficient to be D' = 9.1 × 10−13 m2/s, which is relatively smaller than the diffusion coefficient in steels. In the hydrogen-charged specimens, all the tensile tests were initiated within 2 hours after hydrogen-charging, and they were finished within 3 hours after the initiation of the test; i.e. around 3-5 mass ppm of hydrogen seemed to be present in the specimen during the tensile test.
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