13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Table 4. Vickers micro-hardness of PH-42 Supra steel non-nitrided specimen. Load 500g Vickers micro-hardness (mHV0.5) Conversion to Rockwell C hardness (HRC) 446.0 45.0 434.0 44.0 484.0 48.0 506.0 45.6 Average 454.2 45.6 3.2. Surface Roughness Before fatigue tests, the arithmetic average surface roughness Ra of plasma nitrided and non-nitrided specimens were measured, utilizing a profilometer Supertronic 25 Taylor Robson, with total sliding distance of 4.0mm and Gausian filter of 0.8mm [5]. The experimental results for three specimens and total average Ra are presented in Table 6 for 10 measurements on each specimen. The plasma nitrided specimens have increased the surface roughness Ra in relation to the non-nitrided specimens, but they both can be considered polished for their very low values. This is due to the formation of precipitates and nitrides on the nitriding layer [6]. Table 7. Arithmetic average surface roughness Ra of plasma nitrided and non-nitrided specimens. Specimen Ra (µm) Plasma nitrided Ra (µm) non-nitrided 1 0.31 0.14 2 0.44 0.11 3 0.29 0.08 Total average 0.35 0.11 3.2. Rotating Bending Fatigue Test Results The fatigue limits for as received (non-nitrided) and plasma nitrided specimens were obtained directly from the plotted Wöhler’s curve, seen in Fig. 5, which is the curve of fatigue life for the alternate bending stress tests versus total number of cycles to specimen fracture [7]. According to ASTM E466 standard recommendation, fatigue limit correspond to the maximum rupture stress for the number of cycles equal or superior to 106 cycles. The average values are presented in Table 5. Table 5. Fatigue Limit Stress for PH-42 Supra steel specimens plasma nitrided and non-nitrided. Average values obtained from the plotted Wöhler’s curve for fatigue life (1kgf/mm2 ≅ 10 MPa). Specimen Fatigue Limit Stress σfad (MPa) Ph-42 Supra steel plasma nitrided 710.0 Ph-42 Supra steel non-nitrided 440.0
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