13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- of detrimental flows, e.g. nonmetallic inclusions and small fatigue cracks, in the real components [9]. In this study, for a more extended understanding of the defect size dependence on the fracture toughness of H-precharged high strength steels, tensile tests of SAE52100 were carried out using specimens having various shapes of artificial surface defect with sizes of about 35 ~ 500 μm. The smooth specimens were also tested to investigate the fracture toughness for subsurface nonmetallic inclusions with sizes of about 10 ~ 30 μm. 2. Experimental 2.1. Material and specimen A commercial grade SAE52100 bearing steel was used. The chemical composition in mass% is 1.00 C, 0.26 Si, 0.38 Mn, 0.013 P, 0.001 S, 0.10 Cu, 0.08 Ni, 1.40 Cr, 0.03 Mo, 0.0016 Ti, 0.0005 O and bal. Fe. The steel was heat-treated so that he average Vickers hardness, HV, measured with a load of 9.8 N had 712. Figure 2 shows the shape and dimensions of specimen. In some specimens, the following four types of artificial defects with various sizes were introduced onto the specimen surface; (i) a rectangular slit made by the focused ion beam (FIB) technique, (ii) a semi-circular slit made by the electrical discharging, (iii) a drilled hole which has crack-like thin slits made by the FIB technique and (iv) a hole made by the electrical discharging having the FIB slits. Figure 3 shows the shapes and dimensions of the defects, which are designated as Type A ~ D, respectively. 2.2. Hydrogen precharging (H-precharging) and Tensile testing The specimens were charged with hydrogen by soaking them in an aqueous solution of 20mass % ammonium thiocyanate at 313 K for 48 hours. Displacement controlled tests were carried out under a crosshead speed V of 1 mm/min corresponding to the strain rate ε of about 6.5 × 10−5 s−1 by using a hydraulically-controlled testing machine in ambient air. All the tensile tests were initiated within 2 hours after the charging. The time after the H-precharging means the total hold time in ambient air. 2.3. Measurement of hydrogen content After each tensile test, the residual hydrogen content CH, R in the specimen was measured by the thermal desorption spectroscopy (TDS). Right after the final fracture of the specimen, circular disks of 2.0 mm-thick were sliced from the broken specimen and used for the TDS analysis. 2.4. Characterization of fracture origins and nonmetallic inclusion rating The size of the defect was represented by the area , where area denotes the area of the domain defined by projecting the crack or defect on a plane normal to the cylindrical axis of the specimen [7, 8, 10]. KTH, MPa Yield strength, MPa Figure 1. Schematic relationship between the threshold stress intensity factor KTH of a steel in hydrogen gas as a function of the yield strength σYS [4]
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