T. P. Tsar’kova

On the origin of essential differences in the coercive force, remanence, and initial permeability of ferromagnetic steels in the loaded and unloaded states upon plastic tension

The behavior of remanence Mr, initial permeability μ, and coercive force Hc of the steels during their plastic tension in both loaded and unloaded states upon “slow” and “rapid” loading is explained from a unified point of view. It is shown that (1) a sharp (severalfold) decrease in Mr and μ and a similar abrupt increase in Hc are caused by the same effect, namely the appearance of high residual compressive stresses in most grains along the force direction; (2) in the loaded state, Mr and μ slowly decrease and Hc similarly slowly increases with increasing degree of deformation ε due to a common effect of increasing dislocation density ρ with increasing ε; (3) the values of Mr, μ, and Hc in the transition region, in which ε ≤ εcr, substantially differ from each other upon “slow” and “rapid” loadings; (4) at ε ≥ εcr, the dependences of the Mr, μ, and Hc parameters on ε both in the loaded and unloaded states are parallel to each other.

Effect of tensile plastic deformations on the residual magnetization and initial permeability of low-carbon steels

The behavior of residual magnetization Mr, initial magnetic permeability μ, and coercive force Hc of steels subjected to plastic extension both in the loaded state and in the unloaded state during “slow” and “fast” loading is explained from a unified point of view. It is shown that, upon unloading, the appearance of residual compressive stresses along the direction of the applied force in a considerable portion of grains leads to an abrupt decrease in Mr and μ and to an equally sharp increase in Hc. A slow decrease in Mr and μ and an equally slow increase in Hc are observed in the loaded state with an increase in ε; this effect is caused by an increase in the dislocation density. The values of Mr, μ, and Hc in the transition region of ε ≤ εcr obtained under slow loading differ considerably from those obtained under fast loading. In loaded and unloaded states, the dependences of Mr, μ, and Hc on ε are identical at ε ≥ εcr. The results obtained will be important when the magnetic parameters analyzed in this study are used for nondestructive testing of steel structures that have plastic deformations.

Irreversible changes in the magnetization as indicators of stressed-strained state of ferromagnetic objects

The changes in the magnetic properties of low-carbon steels connected with reversible and irreversible magnetization reversal processes under the action of elastic stresses are studied. It is shown that, on the curve of return from the coercive force, the maximal irreversible increment in the magnetization corresponds to the zero values of applied stresses. The application of compressive stresses leads to a sharp decrease in the irreversible component of the magnetization increment. Tensile stresses influence slightly the proportion between reversible and irreversible processes. New possibilities for the multiparametric quasi-static evaluation of stressed-strained states of ferromagnetic objects are revealed.

Effect of Plastic Deformations and Heat Treatment on the Behavior of the Coercive Force under Load

Based on the measurements of the dependences of the coercive force of ferromagnetic steels on elastic tensile stresses, its minimum values and the positions of these minima have been found. Using the method of thermal neutron scattering, it has been experimentally proved that the stresses at which the coercive force is minimum is equal to the average value of the residual body stresses (internal stresses of the first kind). The latter were varied over wide limits with the aid of either a preliminary tensile plastic deformation to different degrees (steel St3) or using different tempering regimes after quenching (steel U8). In the first case, large residual compressive stresses arise in a significant part of grains along the direction of loading, which are caused by the anisotropy of Young’s modulus of iron. As a result of compensating these stresses by external tensile stresses, there appears a minimum of the coercive force, whose value is determined by an increase in the dislocation density at the stage of plastic deformation.

Effect of Plastic Deformations on the Dependence of the Remanence of Steels on Elastic Tensile Stresses

Maximum values of the remanence and the positions of the maxima, which correspond to the average value of body stresses (internal stresses of the first kind), have been found based on the measurements of the dependence of the remanent magnetization of ferromagnetic steels on the elastic tensile stresses. The internal stresses were varied over wide limits with the aid of different amounts of a preliminary plastic tensile deformation. The results obtained make it possible to use magnetic methods of measurements to determine residual stresses and control their changes during exploitation, which offers new possibilities for nondestructive testing of the stress-strain state of articles and constructions.

Choice of the parameters and algorithm for the magnetic hardness testing of thermally treated carbon steels by the method of regression modeling

The possibility of testing the hardness of quenched and tempered medium-carbon steels using their magnetic properties is studied. It is shown that the two-parameter method should be applied for the reliable estimation of the hardness. The coercive return magnetization (induction) and the coercive force must be used as the basic and subsidiary parameters of testing, respectively. It is established that the content of carbon in steel influences the sensitivity of testing. It is shown that the suggested magnetic hardness testing method can be practically implemented with the use of the modernized SIMTEST-2.10 system.

Measurement of the hysteresis characteristics of pipe steels under elastic and plastic tensile strain

The changes in the shape and characteristics of the major and asymmetric hysteresis loops of low-carbon pipe steels under the action of elastic and plastic tensile strains were studied. The hysteresis loop segments that experienced the most considerable transformations under the tension of a material were determined. The possibilities of the inspection of tensile strains by the coercive return magnetization and the magnetic field corresponding to a fixed magnetization on the descending branch of a hysteresis loop were shown.

The effect of the plastic tension of steels on the dependences of the coercive force on elastic compressive stresses

The effect of compressive stresses σ− on the coercive force Hc of annealed isotropic low-carbon steels, which were preliminarily subjected to plastic tension ɛpl+ of different values, has been studied. It was shown that the shape of hysteresis loops Hc(σ−) and their changes with increasing ɛpl+ are related to induced residual compressive and tensile stresses, which result from plastic tension, and also to the occurrence of plastic compressive deformations ɛpl− because of the Bauschinger effect. The causes of the openness of the first cycle of the Hc(σ−) dependence and also for the occurrence of maximum of the Hc(σ−) function in the ascending branch of this cycle are considered. A procedure for determining the residual tensile stresses, which result from the preliminary plastic tension, has been suggested and achieved experimentally. It was shown that the magnetoelastic change of magnetization, which is observed in the plastic compression, can serve as an indicator of these deformations σpl−.

Magnetoelastic effect in plastically deformed ferromagnetic steels in weak magnetic fields

A sharp decrease in the magnitude of magnetoelastic effect Mσ as a result of unloading after preliminary plastic extension has been revealed in ferromagnetic steels in weak magnetic fields. The physical origin of this effect and the dependence of Mσ on σ (in the elastic region) and on deformations ɛ (in the plastic region) are considered.

On the influence of plastic deformations of low-carbon ferromagnetic steels on the changes in the shapes of their hysteresis loops and the field dependences of the differential permeability

It is shown that inflections in the hysteresis loops of Ст3 low-carbon steel after its plastic deformation are determined by the appearance of a magnetic state of the easy-plane type in a part of the grains. The residual compressive stresses were found at which the maximum possible number of jumps of 90° domain boundaries (DBs) are observed. An expression for the energy of a domain with a 90° DB was obtained. It depends only on the field, the residual stresses, and the angle between the field and the chosen easy axis (EA) of a grain that is located in the easy plane (EP). This made it possible to find the conditions for the onset of irreversible jumps of the 90° DBs to the EP and back. As a result of the proposed theoretical model, the values of the fields of the differential permeability μd(H) peaks were determined, which are observed in positive and negative magnetic fields on the descending hysteresis-loop back. The ratios of the magnitudes of these peak were also found.