Metal Magnetic Memory Inspection of Q345 Steel Specimens with Butt Weld in Tensile and Bending Test

Metal magnetic memory (MMM) method is a non-destructive testing method based on the analysis of self-magnetic-leakage field (SMLF) distribution on components’ surfaces. The MMM method can determine stress concentration zones, imperfections, and heterogeneity in the microstructure of the material and in welded joints. In order to study the magnetization of defective and non-defective butt welded Q345 steel specimens under tensile and bending loads, the normal component of the SMLF (H_p(y)) field values were measured. The results demonstrate that H_p(y) field values and gradients are effective in capturing different stress states under tensile and bending loads. The distribution of H_p(y) field values for flexural tests are quite different from that of tensile tests. The gradient values can be used to determine the degree of stress concentration. In addition, three characteristic parameters were calculated. All three parameters can predict failures with early warnings. Specifically, whether the specimen is in tensile stress state or in compressive stress state can be distinguished by the average value of H_p(y) field area. The quality of the butt weld can be judged by the magnetic index (m). The judging criteria can be a significant complement to the inspection of welded joints using MMM method. Further research could help to validate the judging criteria and analyse the factors affecting the accuracy of the predictions.