コンター法とＸ線回折法の併用による大形軸受用浸炭鋼の三方向内部残留応力測定と固有ひずみ推定

In order to predict the failure of mechanical parts, it is necessary to understand the residual stress and its source, the inherent strain. In this study, the distribution of three-directional residual stress components was measured for a carburized18NiCrMo14-6 cylindrical roller test piece which has 80 mm diameter and 240 mm length using a method combining the contour method and the X-ray diffraction method (the extended contour method). Moreover, the inherent strain distribution was evaluated from measured residual stress by inverse analysis. First, it was shown that the distribution of three-directional residual stress components can be accurately reproduced using the extended contour method by numerical experiments of a carburized cylindrical specimen. Next, it was demonstrated that the distribution of three-directional residual stress components can be measured using general-purpose equipment by actually measuring the same type specimen. Furthermore, the inherent strain distribution was evaluated. Compressive residual stress and corresponding inherent elongation strain were detected in the carburized layer. In contrast, tensile stress and inherent shrinkage strain were determined in the layer just below the case. Finally, the factors that generate each inherent strain have been investigated by thermo-elastic-plastic analysis. Possible explanations are (i)the increase in transformation strain due to the change in carbon content, (ii)the delay in martensite transformation and (iii)the decrease in martensite transformation rate due to the decrease in the cooling rate at the core.