Defect identification in ferromagnetic steel based on residual magnetic field measurements

In this research, the variations of the residual magnetic field (RMF) and its gradients on the surface of 30CrNiMo8 steel specimens with two types of defect's shape were investigated by tensile tests. It was found that both the RMF and its gradient curves of the two defective specimens demonstrate similar evolution patterns in terms of shape and position during the loading process. The RMF gradients exhibit more pronounced characteristics than the RMF signals in the defect area, which makes it possible to capture the defect's information. Then some magnetic characteristic parameters are defined to characterize the quantitative relationship between the abnormal magnetic changes and the applied loads, as well as the defect characteristics. The characteristic parameters that reflected the degree of the abnormal changes are found to be quadratic polynomial correlated with the applied load. The characteristic parameters that reflected the acting range of the abnormal magnetic changes are capable of capturing the defect's location and shape. This research provides a potential possibility for quantitative defect identification for ferromagnetic steels by the RMF measurements.