13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Hence, the tensile and compression stresses on the specimen surface were calculated by the equation, 3 D Pa 160 π σ = (kgf/mm2) (1) where D (= 7mm) is the diameter of the useful region in the specimen, P (kgf) is the applied load on the plate at the load arm end and “a” is the distance from the point of bending support (ball bearing) to the point of load application. For the nitrided specimens, the load varied from 4.8 to 8.0kgf and for non-nitrided specimens the load varied from 2.72 to 5.2kgf. The electric motor rotating velocity was fixed and equal to 3000 rpm. The fatigue tests were initiated for high applied load until the specimen rupture. This high load was gradually reduced for the following specimens till obtaining the fatigue limit for the material. The fatigue limit was defined as the maximum stress for fatigue life of 106 cycles. 3. Results and Discussions 3.1. Vickers Micro-Hardness Tables 2 and 3 shows the average Vickers micro-hardness (mHV), using indenter load of 500g, for the PH-42 Supra steel fatigue specimen plasma nitrided at two distinct regions: measured on the plasma nitrided surface layer of 0.3mm, Table 2, and at the substrate or bulk of material, Table 3. In addition, the correspondent Rockwell C hardness conversion is also presented. In Table 4, the average Vickers micro-hardness measured at surface, indenter load of 500g, for the non-nitrided specimen of PH-42 Supra steel fatigue specimen is presented. The nitriding process has increased the substrate material hardness from 454 to 885 mHV as can be observed from Table 3 and 4. Table 2. Vickers micro-hardness of nitrided surface layer of PH-42 Supra steel nitrided specimen. Load 500g Vickers micro-hardness (mHV0.5) Conversion to Rockwell C hardness (HRC) 861.22 65.9 900.56 67.0 893.77 66.8 Average 885.2 66.6 Table 3. Vickers micro-hardness of substrate of PH-42 Supra steel nitrided specimen. Load 500g Vickers micro-hardness (mHV0.5) Conversion to Rockwell C hardness (HRC) 680.41 59.2 588.70 54.6 599.54 55.2 727.31 61.2 Average 648.99 57.7
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