13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- phase smaller sized grain can be induced due to the easier formation of dislocation around the hard phase. Although a few studies have been conducted on the UNSM treatment with many kinds of materials, few studies about the UNSMed sample with different surface hardness were found. In this paper, the specimens of S45C steel with different surface hardness introduced by pre-surface treatments were processed by the UNSM treatment under the same conditions. The surface morphology, microstructure, residual stress, microhardness and fatigue properties were investigated and the effect of different pre-hardening surface to the UNSM treatment on the surface properties was discussed. 2. Experiment 2.1. Materials preparation The material used was a medium-carbon steel S45C shaft with a chemical composition (in wt.%) of 0.45%C, 0.15%~0.35%Si, 0.6~0.9%Mn, ≤0.03%P, ≤0.035%S, ≤0.3%Cu, ≤0.2%Ni, and ≤0.2%Cr, balanced with Fe. To test the fatigue properties, the specimens were machined to the dimensions shown in Fig. 1, resulting in a stress concentration factor, Kt, of approximately 1.08. The specimens were first quenched (austenitized, 1113 K, 150 min; oil quenched, room temperature, 60 HRC) and tempered (tempered, 773 K, 270 min, 31 HRC). The specimens were then polished by using sandpaper from grade 400 to grade 2000. Subsequently, the specimens were nitrided in a mixture gas (40%N2-60%H2) and at 400 Pa. Two nitriding time durations, 8 h (N8) and 48 h (N48), were used to obtain different nitriding properties. Figure 1. Dimension of the test specimen for S45C steel. 2.2. Surface treatment After plasma nitriding, the surface layer (approximately 12 µm) was removed to avoid the effect of the nitriding layer on the subsequent UNSM treatment. The UNSM technique details have been previously published. An ultrasonic generator (a 30 µm amplitude and 20 kHz) and air compressor devices were used to produce a static force with a dynamic load to strike the material surface with a tungsten carbide (WC) ball (the diameter: 2.4 mm). Accurate surface strike numbers can be obtained by calculating the speed of the surface being treated when the specimen is rotated by a turning lathe. Thus, it is easy to process the material surface and control the static loads and strike numbers. Throughout the process, cold liquid was used to avoid high specimen surface temperatures. Table 1. Specimen treatment conditions Specimens Untreated U1 U2 N8 N8U1 N8U2 N48 N48U1 N48U2 34000 mm-2 − Y − − Y − − Y − 68000 mm-2 − − Y − − Y − − Y To study the effect of strike number on the fatigue properties of the nitrided sample, two treating
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