A. I. Ul'yanov

Magnetic acoustic emission in ferromagnetic materials. 3: Effect of structural changes on magnetic acoustic emission (review article)

This part of the review is devoted to effects of structural changes in multiply alloyed ferromagnetic materials on parameters of magnetoelastic acoustic emission.

Effect of Pore Shape and Pore Orientation on Internal Demagnetization Factor of Porous Ferromagnetic Materials

Taking as examples toroidal model samples with equal values of porosity, but with pores of different shapes, we studied the effect of porosity, pore shapes, and applied magnetic field on the internal demagnetization factor for porous magnetic materials. An empirical formula has been obtained for estimating the internal demagnetization factor of porous magnetics.

Formation of chromium-alloyed cementite in the process of mechanosynthesis and subsequent annealings

Mössbauer spectroscopy and magnetic measurements have been used to investigate the formation of phases in the process of mechanosynthesis of a nanocomposite of composition (Fe0.93Cr0.07)75C25 in a planetary ball mill with the subsequent annealings at various temperatures. It has been shown that, at the initial stage of the mechanosynthesis, an amorphous Fe-C phase is formed. Later, an amorphous Fe-Cr-C phase is formed and, in the process of the subsequent mechanosynthesis, an alloyed cementite is formed from it. It has been found that, due to the products of the pickup from the milling balls made of ShKh15 steel, the changes in the initial composition (Fe0.93Cr0.07)75C25 of the powders that reach 5 wt % have almost no effect on the phase composition, specific saturation magnetization, and the coercive force of samples annealed at 800°C for 1 h.

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 Hc 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 Hc 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.

Effect of Structure and Phase Composition on Coercive Force and Saturation Magnetization of Mechanically Alloyed Fe100 – xCx (x = 25, 32) Powders

Mechanical fracture of iron and graphite powders leads to formation of the amorphous Fe–C phase and subsequent precipitation of the Fe3C carbide in the Fe(75)C(25) system and Fe3C→Fe7C3 carbide in the Fe(68)C(32) system, which are in strongly strained states. After annealing, carbides with undistorted structures are formed, and their content is higher. On the initial stage of this mechanical alloying, the magnetic characteristics of these structures are determined by their defectiveness, and on the subsequent stages of fracturing and annealing by the types, quantities, and structural conditions of the resulting carbide phases.

Role of Chromium in the Formation of Phases in Mechanically Synthesized Alloys of Cementite Composition Alloyed with Chromium and Nickel

Phase transformations and aspects of alloying the phases of alloys of cementite composition, alloyed with chromium and nickel in the state immediately after mechanical synthesis and subsequent annealing, are studied. It is shown that cementite after mechanical synthesis is basically alloyed with chromium, while the amorphous phase is alloyed with chromium and nickel. Cementite produced upon the crystallization of the amorphous phase at Tann = 300°C is enriched with nickel. At medium temperatures of annealing, the cementite regions with the highest nickel content decompose to form para- or ferromagnetic austenite.

Effect of heat treatment on the magnetic behaviour of powder steel 50Ni2Mo under uni-axial tension:

In this study, investigation results on the effect of heat treatment conditions on the mechanical properties, internal stresses, magnetic characteristics, and electrical resistivity of powder steel 50Ni2Mo are presented. Coercive force and maximum magnetization are shown to be applicable as magnetic test parameters to reveal underheating for quenching. Heating for quenching above the critical point A c 3 up to 910°С has an insignificant effect on the mechanical characteristics of the steel tested. It has been demonstrated that the use of the coercive force measured on minor magnetic hysteresis loops is preferable when estimating the level of applied tensile stresses in heat-treated powder steel 50Ni2Mo.

Magnetic Properties of Mechanically Alloyed and Annealed Powders Fe100 – XCX (X = 5 and 15 at.%)

The effect of the structural state and the phase composition on the magnetic hysteresis properties of Fe(95)C(5) and Fe(85)C(15) powders after mechanical alloying and annealing is investigated. It is shown that in the initial stage of grinding in a ball planetary mill the coercive force of the Fe–C powders is determined by the degree of defectiveness of the α-Fe phase and the relative volume and dimensions of nonmagnetic graphitic inclusions in iron powder particles. As the grinding time increases, the α-Fe phase goes over into a nanostructure state and carbon from graphite inclusions goes over into an amorphous Am(Fe–C) phase. After annealing on the temperature interval 300–600°C, the amorphous Am(Fe–C) phase is transformed into Fe3C ferric carbide. The coercive force of the Fe3C phase strongly depends on the degree of distortion of its crystal lattice and amounts to ∼ 80 A/cm for the phase with distorted lattice and ∼ 240 A/cm for the phase with equilibrium undistorted lattice. The magnetic characteristics of the powder after mechanical alloying and subsequent annealing are determined by the type, amount, and structural state of the phases containing in the samples.

Elastic and magnetoelastic properties of an iron-carbon powder steel

The resonance frequency, internal friction, and emf of the double conversion are measured in samples of ZhGr steels of the variable density and carbon content by means of the resonance electromagnetic-acoustic conversion method. For the contactless excitation, the close correlation between the density (porosity) and the resonance frequency of the ultrasonic vibrations is obtained. It is shown that the magnetic state of the samples affects the internal friction. The dynamics of the field dependences of the emf is investigated with allowance for the internal friction in variation of the carbon content. This makes it possible to determine the optimum interval of the field for estimating this parameter.

Measuring the carbon content in sintered steels by the magnetoelastic acoustic-emission method

ConclusionsIn magnetic reversal of sintered steel specimens the parameters of the magnetoelastic acoustic-emission signals occurring depend upon the carbon content and the degree of porosity. The parameter most sensitive to the structural condition of the sintered steels, the total number of pulses measured per unit of time, is nine times more sensitive in relation to variations in carbon content than to changes in porosity within the tolerance limits for the investigated types of steel, which may be used for nondestructive inspection of the structure of sintered steels.