Evaluating the Stress State of Steels by Simultaneous X ray Diffraction and Magnetic Barkhausen Noise Measurements during Four point Bending Tests

Almost all manufacturing operations and subsequent treatments induce residual stresses which affect both the succeeding manufacturing steps and the final product performance. Therefore, evaluation of residual stresses has become an important part of materials science and a number of different methods have been developed. The most widely used X-ray diffraction (XRD) method has been proven to give satisfactory results even with the simple sin2Ψ-method. The magnetic Barkhausen noise (MBN) measurement technique, on the other hand, might offer much quicker non-destructive evaluation of residual stresses. However, MBN technique requires a proper calibration for quantitative results, which generally are not as repeatable and reliable as XRD. Almost all of the previous studies’ calibration procedure consisted of collecting MBN data during a simple tension test. Nevertheless, microstructure related differences between the calibration and test samples and inhomogeneous flow specifically at the plastic region affect the validity of this procedure. In this study, simultaneous X-ray diffraction (XRD) and MBN measurements were conducted during a four-point bending test in order to improve the reliability of the calibration. Moreover, this new procedure enables comparing two different techniques for the evaluation of residual stresses. After a proper calibration, the results of MBN technique agree well with XRD based results in the elastic region. With increasing bending stress the average length of 180o domain walls increase, causing a higher MBN activity. In the plastic region the results do not agree well since the increasing defect density during plastic deformation also influences the domain wall motion. The mean free path of domain wall motion decrease as the defect density increases; therefore MBN activity decreases. At the same time XRD peak widths increase due to increased density of defects. For a better correlation for plastically deformed samples the influence of both the density of 180o domain walls and the mean free path of domain wall motion should be considered quantitatively