Evolution of the Stress-Induced Residual Magnetic Field of X70 Pipeline Steel With Multiple Corrosion Defects

The aim of this research is to investigate the correlation between the residual magnetic field (RMF) and multiple corrosion defects in ferromagnetic steels. Specimens of X70 pipeline steel were machined into standard bars with a single corrosion defect, double corrosion defects and triple corrosion defects, respectively. Tensile tests were carried out to detect the RMF signals on the surface of these specimens. The variations of abnormal magnetic changes of the RMF signals with the external loads were investigated and the results showed that the tangential component and the normal component of the RMF signals of three defect patterns presented different shapes. It was found that the RMF signals were capable of capturing the location and the number of defects in pipeline steels. The peak values of the tangential component and the slopes of the normal component were not influenced by the defects number. This research will promote the investigation on the corrosion defect cluster of ferromagnetic steels based on the metal magnetic memory (MMM) technique.

Lift-off Invariant Inductance of Steels in Multi-Frequency Eddy-Current Testing

Eddy current testing can be used to interrogate steels but it is hampered by the lift-off distance of the sensor. Previously, the lift-off point of intersection (LOI) feature has been found for the pulsed eddy current (PEC) testing. In this paper, a lift-off invariant inductance (LII) feature is proposed for the multi-frequency eddy current (MEC) testing, which merely targets the ferromagnetic steels. That is, at a certain working frequency, the measured inductance signal is found nearly immune to the lift-off distance of the sensor. Such working frequency and inductance are termed as the lift-off invariant frequency (LIF) and LII. Through simulations and experimental measurements of different steels under the multi-frequency manner, the LII has been verified to be merely related to the sensor parameters and independent of different steels. By referring to the LIF of the test piece and using an iterative inverse solver, one of the steel properties (either the electrical conductivity or magnetic permeability) can be reconstructed with a high accuracy.

Lift-off Invariant Inductance of Steels in Multi-Frequency Eddy-Current Testing

Eddy current testing can be used to interrogate steels but it is hampered by the lift-off distance of the sensor. Previously, the lift-off point of intersection (LOI) feature has been found for the pulsed eddy current (PEC) testing. In this paper, a lift-off invariant inductance (LII) feature is proposed for the multi-frequency eddy current (MEC) testing, which merely targets the ferromagnetic steels. That is, at a certain working frequency, the measured inductance signal is found nearly immune to the lift-off distance of the sensor. Such working frequency and inductance are termed as the lift-off invariant frequency (LIF) and LII. Through simulations and experimental measurements of different steels under the multi-frequency manner, the LII has been verified to be merely related to the sensor parameters and independent of different steels. By referring to the LIF of the test piece and using an iterative inverse solver, one of the steel properties (either the electrical conductivity or magnetic permeability) can be reconstructed with a high accuracy.

Quantitative Stress Evaluation and Defect Identification in Ferromagnetic Steels Based on Residual Magnetic Field Measurements

In this paper, the correlation between the residual magnetic field (RMF) and the applied load is investigated. Tensile tests were carried out to measure RMF signals on the surface of 30CrNiMo8 steel specimens with three types of machined defect shapes. Results show that the RMF curves of the three different defective specimens demonstrate similar overall evolution patterns during the loading process, while the magnetic signals exhibit noticeable differences in the defect area. It suggests that the profiles of the stress-induced RMF curves are strongly dependent on the defect’s shape, notch width, and load level. An improved method is proposed to extract some quantitative characteristic parameters from the magnetic signals. The characteristic parameters that reflect the fluctuation degree are in quadratic polynomial relation with the applied load, which can be potentially used to evaluate the applied load acting on a ferromagnetic material with a macro defect. The characteristic parameters that reflect the acting range seem to be independent of the applied load, and the normal ones are capable of capturing the defect’s location and shape. This paper presents a supplement for quantitative defect identification for discontinuities in ferromagnetic steels by RMF measurements.

Магнитные превращения и полиморфный переход ферромагнитных сталей при ударно-волновом нагружении

A.M. Molodets A.A. Golyshev, A.N. Emelyanov, A.A. Kozlov The method for recording magnetic transformation is presented for ferromagnetic steels under shock-wave loading. The operation of the magnetic transformation gauge is considered under shock-wave loading at the polymorphic transition pressure of ARMCO iron and 15Kh2NMFA (in Russian) nuclear reactors case steel. It is shown that the pressures of magnetic transformations of steels correlate with the pressures of polymorphic transitions of these shocked steels. Comparison of gauge readings and parameters of polymorphic transitions was performed for iron and steel during shock-wave compression and subsequent unloading

MAGNETIC METHODS OF EVALUATING ELASTIC STRESSES IN FERROMAGNETIC STEELS (REVIEW)

The paper describes the magnetoelastic effects and the existing concepts of the formation of magnetic textures under the action of elastic stresses in ferromagnetic materials. The possibilities of using magnetic parameters to assess the acting stresses in structural steels are shown: the assessment of the value of uniaxial compressive stresses is practically not difficult, however, there is a problem of assessing tensile stresses in low-alloy steels due to the ambiguous dependence of the magnetic characteristics on elastic tensile deformation. Possible reasons for this ambiguity are discussed, and methods for solving this problem using the anisotropy of the coercive force and the parameters of the magnetic rigidity spectra are shown. The possibilities of evaluating the acting stresses in multilayer ferromagnetics based on the field dependences of the differential magnetic permeability are considered. Papers on the study of the influence of complexly stressed states on the magnetic characteristics of ferromagnetic materials are discussed.

Application of Metal Magnetic Memory Testing for Quantitative Evaluation of Stress Concentration in Ferromagnetic Steels

The objective of this research is to develop a quantitative algorithm between the magnetic parameter bm and the stress concentration factor in ferromagnetic steels. Specimens of U71Mn steel were machined into smooth plates with artificial defects (circular holes and rectangular holes) to represent various stress concentration degrees. Variations of the magnetic parameter bm with the stress concentration factor and the defect area were studied, respectively. The magnetic parameter bm of the U71Mn steel specimen was compared with that of the X80 steel specimen. Quantitative correlations of the magnetic parameter bm with the stress concentration factor and the defect area were investigated. The results show that the magnetic parameter bm is exponentially varied to the stress concentration factor and the defect area. The quantitative algorithm between the magnetic parameter bm and the stress concentration factor is influenced by the defect shape and the material type. An equation was proposed to evaluate the defect area based on the magnetic parameter bm.