Polyxeni Vourna

On the Universality of the Dependence of Magnetic Parameters on Residual Stresses in Steels

A method for the monitoring of residual stress distribution in steels has been developed based on non-destructive magnetic permeability measurements. The dependence of differential permeability on residual stresses induced through a controlled process of applied tensile and compressive stress in the elastic region, of all three zones of the welded metal, yields the magnetic stress calibration curve (MASC). MASC is obtained on flawless welded steel plates and can be measured for any grade of ferromagnetic steels. A surface MASC correlates the magnetic permeability with the spatial stress distribution, as determined by the X-Ray Diffraction Bragg-Brentano diffraction. A bulk MASC correlates the bulk magnetic permeability with residual stresses, as determined by the neutron diffraction. The resulting calibration curves, obtained for several grades of ferromagnetic steels, have a sigmoid shape but are unique for each grade of steels. Normalizing the magnetic permeability and the stress values against the differential permeability measured at the yield point and yield stress, respectively, the dependence of the local magnetic permeability on residual stresses for all different tested grades of steels results in a universal curve relating magnetic and elastic properties of steels at the macroscopic level.

Correlation of Magnetic Properties and Residual Stress Distribution Monitored by X-Ray and Neutron Diffraction in Welded AISI 1008 Steel Sheets

In this paper, two identical American Iron and Steel Institute 1008 steels sheets were welded together in a butt joint configuration, with the welding line direction parallel to the rolling direction. The sheets were welded using the electron-beam welding technique. Stress distribution was monitored in the surface and the bulk of the welded samples using X-ray Bragg-Brentano and neutron diffraction methods, respectively. Accordingly, surface and bulk magnetic properties, namely magnetic Barkhausen noise and quasi-dc permeability, were monitored in the same area. Both surface and bulk magnetic measurements exhibit good agreement with the surface and bulk stress measurements obtained by the diffraction methods, after a proper calibration procedure. Complementary measurements with scanning electron microscopy and Vickers microhardness tester provide similar good agreement between the microstructural characterization and the residual stress distribution.

Residual Stress Analysis in Nonoriented Electrical Steel Sheets by Barkhausen Noise Measurements

The nondestructive magnetic Barkhausen noise (MBN) technique has been used to study the effect of residual stress in nonoriented electrical steels. The MBN response is presented and evaluated with respect to thermomechanical treatments. The ensuing microstructure was studied using scanning electron microscopy, transmission electron microscopy, and hardness measurements. The stress-dependent MBN measurements exhibit good correlation with XRD stress measurements made by the d - sin2 ψ method, using Cr-Ka radiation, and (211) reflections. It has been found that there is a good correlation between MBN response and variations in microstructure, which affects mechanical properties.

Dependence of Magnetic Permeability on Residual Stresses in Welded Steels

The aim of this paper was to further broaden current knowledge of magnetic properties’ dependence on welded steels’ stress state. For this reason, commercial micro-alloyed steel was welded using tungsten inert gas as the welding technique. Magnetic stress calibration curve (MASC) was a useful laboratory tool for evaluating the material’s residual stress distribution with the aid of magnetic permeability values. X-ray and neutron diffraction techniques were used for determining surface and bulk residual stress values, respectively. The overall measurement results have shown that the residual stress determined by the magnetic method was in good agreement with the diffraction results. In order to investigate the potential utilization of the magnetic method in evaluating residual stresses, the magnetic calibration curves of various ferromagnetic steels’ grade were examined. The normalization of these curves revealed the existence of a unique curve, which suggested the existence of a universal relationship between the intrinsic magnetic properties and the internal elastic mechanical characteristics of steels. Further experimental investigations are required to validate the preliminary results and to verify the presence of a unique normalized magnetic stress calibration curve.

A New Magnetic Method for Stress Monitoring in Steels

Technology developments in analytic techniques as well as in data acquisition and management are already revolutionizing the ways that steel industry assets are managed. Their adoption in several fields of steel operations promises to a competitive advantage and sustainability to the steel industry. Real-time monitoring of steel production lines as well as steel structures using non destructive inspection is such a field. In this paper we present a sensor developed in our laboratory which when used in the non-destructive evaluation procedure also described in this work, it offers real-time monitoring of the structural integrity of magnetic steels, going beyond the state of the art by allowing for the correlation of macroscopic magnetic and magnetoelastic parameters to the total of the hydraulic and residual stresses as a function of position.