13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Very High Cycle Fatigue Behavior of Plasma Nitrided 316 Stainless Steel Daisuke Yonekura1,*, K. Ozaki1, R. Shibahara1, Insup Lee2 and R. Murakami1 1 Department of Mechanical Engineering, The University of Tokushima, Tokushima 770 8506, Japan 2 Department of Advanced Material Engineering, Dongeui University, Busan 614 714, Korea * Corresponding author: yonekura@tokushima-u.ac.jp Abstract In this study, plasma nitriding was performed for 316 stainless steel under 2 different processing time, 15hours and 25hours. Cantilever type rotational bending fatigue tests were carried out using the nitrided and non-treated samples in high cycle and very high cycle fatigue life regime in order to examine the influence of the plasma nitriding on the fatigue properties in the both fatigue life regimes of 316 stainless steel. As a result, the fatigue strength was improved by the plasma nitriding and the shape of the S-N curves was a asymptote shape with a fatigue limit up to N = 108 cycles. The fatigue cracks initiated from specimen surface in all specimen. However, the improvement of fatigue strength by plasma nitriding and the influence of nitriding time on the fatigue strength was small because the nitriding layer was quite thin, especially in the diffusion layer. For this reason, it is believed that the nitrided layer could not affect significantly the fatigue strength. Keywords Very high cycle fatigue, Plasma nitriding, Stainless steel, Cantilever-type rotary bending fatigue test. 1. Introduction Surface treatment is widely used to improve fatigue, wear and corrosion resistance of various industrial products. There are many surface treatment methods, such as carburizing, induction hardening, shot peening and physical vapor deposition coating etc. These treatments form a hardened surface layer with compressive residual stress, and therefore, the fatigue properties are improved by the surface layer [1-9]. Nitriding is one of the surface treatment methods. Nitriding produces a nitrogen diffused surface layer with a compound layer which is formed by reaction between nitrogen and nitride-forming elements in a material such as aluminum, chromium, molybdenum and titanium. Good fatigue and wear resistance are obtained by the nitrided surface layer. However, it is well known that the nitriding treatment is difficult to apply to stainless steel because the stainless steel has a passivation film on the surface which hinders the diffusion of nitrogen into the material. Therefore, the removal of the passivation film is required when the nitriding treatment is applied to the stainless steel. There are three major methods to perform the nitriding process; gas nitriding, salt-bath nitriding and plasma nitriding processes. In particular, plasma nitriding process is a very attractive method to improve the surface properties for various engineering materials due to its advantages, such as low emission of toxic gases, low maintenance cost and low pollution, compared with conventional gas or salt bath nitriding process [10]. Plasma nitriding process has many parameters: the nitriding temperature, the gas mixture and the time duration [11-12]. These parameters strongly affect the material properties of nitrided samples. Many researchers have reported the influence of the parameters on the fatigue properties using various steels and have also reported that the fatigue limit can be improved by the plasma nitriding. In particular, the fatigue limit increases with increasing the processing time under cyclic bending stress due to the thick hardened layer because the fatigue cracks initiate from internal defects such as inclusions or cavities or from the interface between substrate and nitriding layer [13-18].
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