The combined effect of molybdenum and nitrogen on the fatigued microstructure of 316 type austenitic stainless steel

Introduction The mechanical properties of austenitic stainless steels, including the low cyclic fatigue behavior have been a subject of numerous studies [1-8]. Nitrogen addition, in general, increases the fatigue strength by decreasing the stacking fault energy of austenitic stainless steels [6]. The stacking fault energy has a strong influence on the cross slip difficulty, thus it affects the saturation dislocation structure formed by the cyclic deformation. However, several studies [9-12] reported that the effect of nitrogen on the dislocation distributions in the deformed microstructures can not be explained by a decrease in the stacking fault energy alone. Swann [9] observed the microstructure of elongated Cr-Ni austenitic stainless steels and suggested that nitrogen atoms associated with the chromium atoms interact with dislocations. Douglass et al. [10] proposed that nitrogen atoms are attracted to the short range ordered Fe/Cr regions and this ordering leads to a change in the dislocation structure. Recently, using X-ray absorption fine structure measurement, Oda et al. [11] suggested the Cr-N interstitial-substitutional (I-S) complex in a 15Cr-15Ni austenitic stainless steel plays an important role in work softening in the low cycle fatigue test. Hence, there is some disagreements on the effect of nitrogen on the dislocation structure of the cyclic deformed austenitic stainless steels. Recently, Vogt et al. [6] reported that nitrogen addition improves the low cycle fatigue resistance of 316 type austenitic stainless steel. They attributed this effect to the reduction of the stacking fault energy by the addition of nitrogen. More recently, Hirukawa et al. [13] confirmed that additions of more than 0.2 wt.% nitrogen to a 316 stainless steel decrease the fatigue crack growth rate and increase the fatigue threshold. In order to understand the origin of the nitrogen effect, Hirukawa et al. [13] measured the fatigues properties of Mo-free 316 stainless steel and reported that the effect of nitrogen addition is not noticeable when Mo is removed from the 316 type stainless steel. Since the role of Mo was not considered in the most of previous studies, the present study aimed at understanding the combined effect of Mo and N on the fatigue properties of 316 type austenitic stainless steels. For this purpose, we have investigated the fatigued microstructure and the distribution of nitrogen by a transmission electron microscope (TEM), a conventional atom probe (1DAP) and a three dimensional atom probe (3DAP).