13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- Since the plasma nitriding process can use the hydrogen etching effect which can remove the surface oxide layer as well as surface contamination, the plasma nitriding process can form a nitrided and diffused surface layer for the stainless steel [19-22]. The plasma nitriding will improve the fatigue strength of the stainless steel but the plasma nitriding may cause the decrease of the fatigue strength in very high cycle fatigue life regime by an internal fracture because the plasma nitriding produces thin hardened layer and the fatigue failure of some surface hardened steel occurs at stress levels below the conventional fatigue limit in the life regime greater than 107 cycles [23]. However few studies on the fatigue life of plasma nitrided stainless steels have been carried out in the very high cycle regime. In the present study, SUS316 austenitic stainless steel was treated by the plasma nitriding under two nitriding times. Rotating bending fatigue tests were carried out for these nitrided specimens in order to investigate the effect of the nitriding time on the fatigue behavior in the high cycle and very high cycle fatigue regime. Fracture morphologies were observed using a scanning electron microscope (SEM) and the influence of the plasma nitriding process on the fatigue behavior of austenitic stainless steel was discussed. 2. Experimental Procedure The material used in this study was austenitic stain less steel, SUS316. The fatigue specimens were machined into the shape and dimensions as shown in Fig. 1. The round notch surface was polished with a grinder having a mesh size of #100. Notch radius was 7 mm and the stress concentration factor of the specimen was Kt=1.06. Figure 1. Shape and dimensions of specimen. The surface of the specimen to be exposed to plasma was cleaned using an ultrasonic bath in acetone. A vacuum chamber was pumped down to 50 mTorr and then back-filled with a gas mixture of NH3 and H2 up to 3 Torr (NH3:H2 mixing ratio; 4:1). The plasma nitriding process was immediately carried out with a pulsed dc potential at the designed time in the glow discharge of the plasma. In the present study, plasma nitriding was performed at 703 K for 15 hours or 25 hours. These nitrided specimens will be called N15 and N25, respectively. After the plasma nitriding, the vacuum chamber was pumped down to 50 mTorr and the specimens were furnace cooled to room temperature. Fatigue tests were carried out at room temperature by using a dual-spindle cantilever-type rotating bending fatigue-testing machine, which is the standard testing machine in the Research Group for Statistical Aspect of Materials Strength, Japan [24]. The stress frequency was 3150 rpm and the stress ratio R = –1. All of the fracture specimens were observed using SEM. The run-out number of cycles was N = 1.0×108 cycles. The fracture surface of the specimen was observed using SEM. A Vickers microhardness tester was used to measure the microhardness of samples. The parameters used in the microhardness test were 100 gf and a duration 15 s. The microstructure was observed using a cross sectional surface etched by Marble's regent (4.0 g CuSO4, 20 ml HCl, and 20 ml H2O).
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