The effect of tempering temperature on the coercive force of high-carbon steels measured at room temperature and a temperature above the curie point of cementite

The role of cementite in the formation of the H _c of carbon steels after quenching and tempering is determined based on analysis of dependences of the coercive force of У9A steel in the measurement temperature range from the temperature of liquid nitrogen to the cementite Curie point. It is shown that the key contribution to the formation of the maximum in the H _c (T _temp) dependence measured at room temperature is due to the magnetic hardness of cementite, whose maximum lies at T _temp = 500°C. The coercive force measured at 250°C is mainly related to the interaction of the domain walls of the ferrite matrix with nonferromagnetic inclusions of cementite, the maximum of which is at T _temp = 400°C, thus causing a shift of the maximum in the H _c (T _temp) dependence measured at the temperature above the cementite Curie point toward lower tempering temperatures.     

Magnetic properties of cementite and the coercive force of a Fe-5 at % C alloy after hardening and tempering

The role of cementite in the mechanisms of formation of H _c for specimens of a model high-carbon Fe-5 at % C alloy after hardening and tempering is determined from the analysis of the temperature dependences of the coercive force measured in the range from the temperature of liquid nitrogen to the Curie point for cementite. This analysis is based on the different characters of the temperature dependences of the contributions to H _c due to cementite as weakly magnetic inclusions at which the matrix’s domain walls are decelerated and cementite as a hard magnetic phase. It is shown that the magnetic hardness of cementite makes the main contribution to the formation of the coercive force in the region of medium and high tempering temperatures.     

On the causes of the formation of a maximum in the dependences of the coercive force of simple carbon steels on the tempering temperature

The dependences of the coercive force H _c of quenched specimens made of simple carbon steels on the tempering temperature T _t were measured at room and liquid-nitrogen temperatures. These studies showed that the formation of the character of H _c(T _t) dependences for the mentioned steels within the region of medium and high tempering temperatures is caused by the content of cementite and its magnetic hysteresis properties.     

Effect of Quenching and Tempering on the Coercive Force of Fe-5 at. % C Powders Sintered after Mechanical Alloying

The dependence of the coercive force of Fe-5 at. % C powders sintered after mechanical alloying on the tempering temperature and on the structural state of the alloy is studied. It is shown that a transition of cementite Fe₃C from a state with a distorted crystal lattice and a small value of H _c to a state with an equilibrium lattice and a high value of H _c of this phase may be one of the reasons for an increase in the coercive force in the range of tempering 300–500°C.     

The role of cementite in the formation of magnetic hysteresis properties of plastically deformed high-carbon steels: III. Magnetic properties of patented wire made of steel 25

Temperature dependences of the coercive force of mechanically alloyed cementite and wires made of patented steel 25 that were subjected to plastic deformation and subsequent annealing are presented. As is shown, cementite can be in both low-and high-coercivity states, for which the coercive force measured at room temperature is 80 and 240 A/cm, respectively; the coercive force measured at liquid-nitrogen temperature (?196°C) is 190 and 530 A/cm, respectively. Based on an analysis of the temperature dependences of the coercive force of deformed wires made of patented steel 25, the role of cementite in the formation of the coercive force of low-carbon steels was determined. It is shown that, in spite of a relatively small amount of pearlite, the contribution due to the magnetic hardness of cementite to the coercive force should be taken into account. In this case, the contribution due to the interaction of ferrite domain walls with weakly magnetic cementite inclusions is substantially lower.     

Influence of structural features of welded joints of low-alloy steels on magnetic and micromagnetic properties

Variations of magnetic (coercive force H _c , residual induction B _r , maximum magnetic permeability μ_max, maximum magnetic-permeability field H _μmax, saturation magnetization J _max) and micromagnetic (number of pulses N and RMS Barkhausen-noise amplitude U) characteristics along the parent metal-welded joint direction were studied for welds of steels 10XCHД, 15XCHД, 09Γ2C, and X70.     

An Automated High-Sensitivity Vibrating-Coil Magnetometer

A magnetometer allows the measurement of the magnetic moment of samples in a range from 1.3 × 10^–8 to 5.45 × 10^–4 A m². The range of variation of the magnetic field strength is from 0 to 56 kA/m; the temperature range is 20–700°C. Measurements are performed in an air atmosphere. The obtained data are automatically inputted into a PC. In addition, the magnetometer can measure the coercive force H _c and the destructive field H _cr. It can also be used to study the formation of thermoremanent and viscous magnetizations and measure the Curie-temperature spectrum and the blocking-temperature spectrum. Measurements of the field, temperature, and time dependences of the magnetic moment can also be performed by the researcher or by special software using a more complicated algorithm.     

Coercive Force as a Function of Uniaxial Stresses (Part 2)

The relative anisotropy of the coercive force is investigated as a function of the tensile stress. A generalized calculated curve with coefficients averaged over all the low-carbon steels tested is constructed. A mechanism for determining the effective magnetostriction λ ₁₀₀ ^* and λ ₁₁₁ ^* from the field dependence λ(H) is proposed. The magnetoelastic sensitivity ΔH _c coefficients are determined as functions of the magnetostriction λ ₁₀₀ ^* .     

The role of cementite in the formation of magnetic hysteresis properties of plastically deformed high-carbon steels: I. Magnetic properties and structural state of cementite

Magnetic properties of cementite after strong plastic deformations and subsequent annealing in a broad range of temperatures are studied. The plastically deformed cementite is shown to exist in a soft (H _c ≈ 80 A/cm) state; the annealed cementite, in a hard (H _c ≈ 240 A/cm) state. The nature of the cementite’s soft and hard states is discussed. The field dependence of the cementite’s magnetostriction is measured. The longitudinal magnetostriction of the polycrystalline cementite’s saturated state is shown to be negative and approximately four times smaller than iron’s magnetostriction in the saturated state.     

Evaluation of the Current Condition and the Residual Lifetime of Forming Rolls Using the Magnetic Method of Nondestructive Testing Based on the Coercive Force

The magnetic method on the basis of the coercive force H_c is proposed to use for estimating the current condition and the residual lifetime of forming rolls. Sixty rolls have been tested. It is shown that H_c increases almost twice in exploiting the roll from the beginning of the operation to reaching the limit of its lifetime. Results of observation of the degree of nonuniformity of the H_c distribution over the roll surface in the operation process are given. The results of investigation make it possible to optimize the path of each roll through the mill stands in order to avoid the stress concentration at the same depth of the roll operating (magnetic) coating in the roll change process. Using the magnetic testing method based on H_c makes it possible effectively to estimate the current condition of accumulation of the fatigue in the roll operating (magnetic) coating.     

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

The behavior of residual magnetization M _r , initial magnetic permeability μ, and coercive force H _c 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 M _r and μ and to an equally sharp increase in H _c . A slow decrease in M _r and μ and an equally slow increase in H _c 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 M _r , μ, and H _c in the transition region of ε ≤ ε_cr obtained under slow loading differ considerably from those obtained under fast loading. In loaded and unloaded states, the dependences of M _r , μ, and H _c on ε are identical at ε ≥ ε_cr. 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.     

Magnetic Measurements of Stressed-Strained States and Remaining Service Lives of Steel Structures in Hoisting Machines and Pressurized Vessels

A technique of magnetic diagnostics has been developed on the base of correlations between physico-mechanical properties of ferromagnetic materials and their coercive force H _c. Examples of practical utilization of the technique in assessing conditions of bridge cranes and oxygen cylinders are given. By solving the inverse problem, it is possible to diagnose the stage of transition to the yield region on the base of measurements of the maximal and average coercive force, which enables one to diagnose the predestruction condition of gas cylinders. Criteria for rejecting gas cylinders based on results of statistical analysis have been established.     

Estimation of the State of Metal of Blast-Furnace Jackets and Hot-Blast Stove Casings by a Nondestructive Testing Method

The possibility of using the technique of magnetic testing of the stress-strain state of metallurgic structures is investigated. This technique is based on the correlation dependence between the physico-mechanical properties and the coercive force H _c . Examples of the practical use of the technique for estimating the state of blast-furnace jackets and hot-blast stove casings are given. Trends of variation of the state of metal during a long time period are established.     

The role of cementite in the formation of magnetic hysteresis properties of plastically deformed high-carbon steels: II. Magnetic properties of patented wire made of steel 70

It is shown that, during discussion of the mechanism reliable for formation of the coercive force in high-carbon steel, in addition to the interaction between the domain walls and weakly magnetic cementite inclusions, the contribution from the cementite’s magnetic hardness should be considered. With the use of the temperature dependence of the coercive force in deformed and annealed specimens, the contribution of each of the considered mechanisms to the specimens’ coercive force is estimated.     

The possibility of testing the structural state of highly-deformed high-carbon steels by the magnetic method

The influence of cold plastic deformation on phase transformations and on the magnetic properties of powder specimens of the Fe-5-at. % C alloy modeling U12 high-carbon steel has been investigated in the context of nondestructive testing. It has been shown that under high plastic deformation dissolution of cementite Fe3C occurs by the scheme -Fe + Fe₃C -Fe + (Fe₃C)_d + -Fe + Am(Fe-C). An increase in the rate of plastic deformation of powders decreases the coercive force from 50 to 13 A/cm due to the low coercivity of the #x03B1;-Fe and Am(Fe-C) phases. The structural state of high-carbon steels following a strong cold plastic deformation can be determined by measuring two magnetic parameters: the coercive force, and the temperature dependence of differential magnetic permeability.Translated from Defektoskopiya, Vol. 40, No. 7, 2004, pp. 42–52.Original Russian Text Copyright © 2004 by A.I. Ulyanov, Arsenteva, Zagainov, A.L. Ulyanov, Elsukov, Dorofeev.     

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 H _c of annealed isotropic low-carbon steels, which were preliminarily subjected to plastic tension ? _pl ⁺ of different values, has been studied. It was shown that the shape of hysteresis loops H _c (σ^?) and their changes with increasing ? _pl ⁺ are related to induced residual compressive and tensile stresses, which result from plastic tension, and also to the occurrence of plastic compressive deformations ? _pl ^? because of the Bauschinger effect. The causes of the openness of the first cycle of the H _c (σ^?) dependence and also for the occurrence of maximum of the H _c (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 σ _pl ^? .     

Effect of heat treatment on the microstructure and properties of Ni based soft magnetic alloy

A Ni‐based alloy was heat treated by changing the temperature and ambient atmosphere of the heat treatment. Morphology, crystal structure, and physical performance of the Ni‐based alloy were characterized via SEM, XRD, TEM, and PPMS. Results show that due to the heat treatment process, the grain growth of the Ni‐based alloy and the removal of impurities and defects are promoted. Both the orientation and stress caused by rolling are reduced. The permeability and saturation magnetization of the alloy are improved. The hysteresis loss and coercivity are decreased. Higher heat treatment temperature leads to increased improvement of permeability and saturation magnetization. Heat treatment in hydrogen is more conducive to the removal of impurities. At the same temperature, the magnetic performance of the heat‐treated alloy in hydrogen is better than that of an alloy with heat treatment in vacuum. The Ni‐based alloy shows an excellent magnetic performance on 1,373 K heat treatment in hydrogen atmosphere. In this process, the µ_m, B_s, P_u, and H_c of the obtained alloy are 427 mHm^?1, 509 mT, 0.866 Jm^?3, and 0.514 Am^?1, respectively. At the same time, the resistivity of alloy decreases and its thermal conductivity increases in response to heat treatment.     

Relation between the Residual Magnetization and Change in Magnetization on Steel and Alloy Return Curves

The relation between the residual magnetization M _r and change in magnetization on the return curves as a function of the coercive force is studied experimentally for heat-treated steels of different chemical compositions and a number of polycrystalline iron–nickel–cobalt alloys. It is shown that in the quenched condition the ratios M _s /M _r and M _r / are almost constant and close to 2 and 4, respectively, for all investigated steels regardless of their chemical composition. For highly tempered steels and annealed alloys these ratios represent structurally sensitive quantities depending on the critical fields in the material and characterize the relation between reversible and irreversible remagnetization processes. A model interpretation of the results obtained is given.     

About the Formation of the Second Harmonic Component of Magnetization under Weak Alternating Exciting Fields

Experimental data on the regularities of formation of a second harmonic component of the output emf of a pickup coil, which is generated during remagnetization of a steel specimen by a weak sinusoidal low-frequency exciting magnetic field accompanied by simultaneous biasing of the specimen with a DC (biasing) magnetic field, are reported. In this case, a weak field is understood to be a field whose peak strength H _m is much smaller than the coercive force of the specimen being remagnetized. It is shown that these regularities match those that follow from the assumption that the principal contribution to the formation of this harmonic component is due to the nonlinear character of the dependence of the reversible magnetization component (within the minor steady-state remagnetization loop obtained) on the magnetizing force.     

Effect of Magnetic Parameters and Dimensions of Uniformly Magnetized Bodies on Inductive Coercive Force

From the Fröhlich approximation of changes in the magnetization of a ferromagnetic material on the descending branch of the hysteresis loop, an analytical expression for the inductive coercive force H _B of a homogeneously magnetized body has been derived. Effects of magnetic parameters of the material and demagnetizing factor on H _B have been studied.