Influence of Segregation of a Large Forged Steel Product on X-Ray Stress Measurement by the Cosα Method

Surface treatments involving residual stresses are an effective method to increase the fatigue strength of forged steel products. Therefore, X-ray stress measurement is important to evaluate the fatigue strength of these materials. For large forged steel products, regions where alloy elements are segregated cannot be avoided. Furthermore, because of the large size of these products, it may be difficult to measure their residual stresses in a laboratory. Nevertheless, compact and portable X-ray equipments operating based on the cosα method using a two-dimensional detector, are appropriate for the measurement of the residual stresses at the manufacturing sites. Although it is known that carbon and alloy elements concentration influence X-ray stress measurement, there is little research about the influence of segregation in X-ray stress measurement using the cosα method. This study evaluated the differences in X-ray stress results using the cosα method in segregation regions of large forged products made of CrMo steel, under tensile stress. It was confirmed that the differences between the X-ray stress and nominal stress increased with increase of carbon concentration in segregation, and crystal grains in which coarse and fine grain size coexist worsen the linearity of the cosα diagram. Because the level of carbon concentration and grain size varies at each X-ray irradiation points, we proposed an averaging method of X-ray stresses on plural measurement points as an effective way to avoid the effect of segregation.