Magnetic Residual Stress Monitoring Technique for Ferromagnetic Steels

The determination and control of residual stresses resulting from the intentional or unintentional thermal and mechanical loading of steels during their production or manufacturing process, as well as during their lifetime, is a challenge for both the scientific community and the relevant industries. Our team has developed a method and instruments for residual stress determination in ferromagnetic steels, based on the effect of localized strains on the magnetic differential permeability. The proposed method consists of determining the characteristic magnetic stress calibration curves in the laboratory, for the steel grade under examination, and correlating magnetic permeability with residual stresses either on the surface or in the bulk of the material. Magnetic permeability is determined by our new permeability sensors or by other classic permeability meters. Stress components are determined indirectly by strain monitoring using diffraction techniques, like X-ray or neutron diffraction for surface and bulk strain respectively. This way, the best uncertainty of the stress determination achieved has been in the order of 1%. In this paper, after introducing some of the most important details of our method, we illustrate the improvement of the sensitivity of the stress determination by implementing stress-strain dependence on bulk magnetic permeability, and then correlating these results with the neutron diffraction measurements, resulting in residual stress determination uncertainties better than 0.1%. The validity of these results is evaluated by microstructural Scanning Electron Microscopy studies and the superiority of the new method in terms of efficiency, cost, and applicability in industrial applications are discussed.