13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of Ultrasonic Nanocrystal Surface Modification to the Fatigue Properties of S45C with Different Surface Hardness Bo Wu1,2, Jianxun Zhang1, Young-Shik Pyoun3, Ri-ichi Murakami2* 1 State key laboratory for mechanical behavior of materials, Xian Jiaotong University, Xian 710049, People’s Republic of China 2 Department of Mechanical Engineering, The University of Tokushima, 2-1 Minami-josanjima-cho, Tokushima, Japan 3 Institute for Manufacturing Systems Technology, Sun Moon University, Asan, Choongnam, Republic of Korea * Corresponding author: murakami@me.tokushima-u.ac.jp Abstract The Ultrasonic nanocrystal surface modification (UNSM) was employed to enhance the fatigue strength of S45C steel with different pre-plasma nitriding (un-nitrided, nitriding 8 h and 48 h) by improving the surface roughness and producing nanocrystalline on the sample surface. The parameters of UNSM were two different strike numbers, 34000 mm-2 and 68000 mm-2. Different experimental processes including microstructure observation, microhardness, roughness and X-ray diffraction measurements have been performed to characterize the treated surface of specimens. The results shown that after UNSM treatment an improvement can be obtained for surface microhardness, surface roughness, and compressive residual stress. Under the same UNSM conditions more surface defects can be produced with the increasing strike number and with increase of surface hardness the effect to surface roughness decreased. For the higher hardness surface sample, smaller nanocrystalline can be obtained compared with other sample through XRD results. Higher compressive residual stress was found for un-nitrided S45C steel with increasing strike number, but a decrease was found for the nitriding sample. Rotating bending fatigue test was carried out to evaluate the effect UNSM to the fatigue strength. An improvement of fatigue strength for un-nitrided and nitriding 8 h sample was found and the surface defects were the main reason inducing the fatigue failure. However, for N48 samples, no improvement of fatigue strength as little effect of UNSM on the sub-surface crack induced by inclusion. Keywords UNSM, Plasma nitriding, Surface Hardening, Fatigue 1. Introduction Ultrasonic nanocrystal surface modification (UNSM), as one of severe surface plastic deformation (S2PD) methods to improve material surface properties, has been used on many materials treatments [1-5]. The properties of wear [4, 5], fatigue [1-3] of materials can be improved as the reason of high surface hardness and compressive residual stress produced by the UNSM treatment. In the study of Cao [2], with the increase of strike number higher fatigue strength can be obtain and small fatigue crack can be restrained as the compressive residual stress produced by UNSM treatment. Suh’s study [3] shows that the grain in the top surface zone can be refined into nano-size and the depth of gradual grain refined layer about 100 µm can be produced by UNSM treatment with the applied static force of 100 N. Amanov [5] has found that high impact load can get a lower friction and well wear characteristics and smaller nano-grain size can be obtained at the same time. So we can see that the processing parameters (the applied static force, the strike number per mm-2, the size of tip ball) can be controlled accurately and all of these parameters have a close connection with the characters of specimen’s surface zone after the UNSM treatment. Also the structure of the material receiving surface treatment plays an important role for the formation of microstructure in the surface processing layer. Zhou [6] showed that the SMAT steel sample has a thicker refined layer (with grain size < 100 nm) compared with pure Fe. Cho [7] and Lee [8] found that with more hard
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