13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 3. Experimental Results and Discussions 3.1. Characterization of hardened layer Figure 2 shows the cross sectional surfaces subjected to the two different nitriding processes, using Marble's regent as an etchant to reveal the microstructure. The unetched surface layers, which are well-known as "white layer", were formed at both specimen's surfaces. The white layer for the N25 was slightly thicker than that for the N15. However, the white layer was thin and the thickness were about 20 µm for the N15 specimen and 30 µm for the N25 specimen, respectively. In both cases, the microstructure and the grain size below the white layers were no difference from the as-received series and the diffusion layer could not be identified by etching. So, the difference between the as-received and nitrided series was only forming of the surface white layer. Figure 2. SEM images of plasma nitrided SUS316 stainless steel etched by Marble regent: (a) N15 and (b) N25 specimens. Figure 3 shows the micro-Vickers hardness distribution of nitrided specimens on the cross sectional surface. The hardness of the as-received specimen was uniform from the surface to the core and the 30 points average value was about 331 HV. Compared with the virgin hardness, the hardness increased near the surface. The highest hardness values, 985 HV for the N15 specimen and 1041 HV for the N25 specimen, were measured at the surface, which is approximately three times of the virgin hardness of 331 HV. However, the hardness decreased steeply toward the center and the hardness at 50µm deep was almost same as virgin hardness for both nitrided specimens. Therefore, the thickness of the hardened layers, defined as a distance from the surface to the unhardened matrix, was less than 50 µm for both nitriding times. Since the thickness of white layer as shown in Fig.2 was about 20 - 30µm and the half size of indentation for 331 HV was about 12µm, the thickness of the diffusion layer was less than about 10µm. In general, nitriding process produces a compound layer referred to as the white layer, and a diffusion layer below the compound layer. The former is a thin layer with high hardness but brittle [25]. As a result, the fatigue strength sometimes are degraded by the brittle layer. The latter is a relatively thick hardened layer. In the diffusion layer, the hardness gradually decreases gently toward the core. The depth of diffused zone is generally more than few hundred micrometers [13-18]. The depth of the diffused zone is important to improve the fatigue strength as well as the hardness [14]. In this study, however, no obvious diffusion layer was observed on the etched surface, and only the white layer was observed at the surface. Moreover, the hardened layer was quite shallow and the thickness was less than 50 µm which included the thickness of the compound
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