ON THE USE OF ASYMMETRIC MAGNETIZING FIELDS FOR MEASURING THE MAGNETIC CHARACTERISTICS OF MATERIALS

I. G. Leshchenko, V. Yu. Sosnin

(Presented by the scientific-technical seminar of the Department of Information-Measuring Technology)

Quality control of ferromagnetic materials and products by dynamic electromagnetic characteristics is at present carried out, in the overwhelming majority of cases, with the sensors supplied by a symmetrical current. In this case it is necessary to determine certain interrelations between the properties being monitored and the magnetic characteristics, and to choose a method for measuring the latter from the standpoint of accuracy, reliability, and simplicity of control.

Work carried out by the authors on the use of asymmetric and pulsating supply of sensors makes it possible, already at the first stage of research, to recommend the indicated method for quality control of products made of various ferromagnetic materials by dynamic magnetic characteristics.

1. Supplying an electromagnetic sensor with single-half-period or double-half-period current (in the prescribed-field-strength mode) or with voltage (in the prescribed-induction mode).

The material or product being inspected is remagnetized along a partial cycle (Fig. 1, curve 1). When the emf of the secondary winding of the sensor is detected, the mean value of the emf is proportional to the difference between the maximum induction \(B_m\) and the residual induction \(B_r\).

For single-half-period rectification,

\[ E_{2\mathrm{cp}}=\frac{1}{T}\int_{t}^{t+T/2} e_2\,dt = f w_2 S (B_m-B_r). \tag{1} \]

Fig. 1

In view of the small losses for remagnetization along a partial cycle, it proves possible, with an insignificant expenditure of power, to obtain half-...

take into account the saturation induction in the material being inspected. In this case expression (1) can be rewritten in the form

\[ E_{2\mathrm{avg}}=f\omega_2 S(B_s-B_r). \tag{2} \]

For many structural steels there is a quite definite relationship of \(B_s\) and \(B_r\) with their chemical composition, structure, and technological characteristics, which provides grounds for using a simple measuring and magnetizing circuit to inspect the named properties by the value of the average e.m.f. of the sensor secondary winding. In particular, experimental results for the dependence of \(E_{2\mathrm{avg}}\) on the hardness of tubes made of 30KhGSA steel, with half-wave and full-wave supply of feed-through sensors, were analogous to inspection by dynamic coercive force when supplied with a symmetrical magnetizing current [1].

Fig. 2.

1. Supplying the sensor with an alternating sinusoidal current, one half-wave of which is regulated in amplitude by the resistance \(R_1\), according to the circuit in Fig. 2.

In this case the material or product being inspected is remagnetized along partial asymmetric cycles 2 and 3 (Fig. 1). When remagnetized along partial cycle 2, the mean value of the rectified e.m.f. of the sensor secondary winding is proportional to the maximum induction and, if necessary, to the saturation induction, which depends unambiguously on the chemical composition of the material.

Remagnetization along partial cycle 3 makes it possible to inspect various properties of ferromagnetic materials by the dynamic breaking magnetic-field strength \(H_p\). It has been shown experimentally that \(H_p\) has the same relationship with the structural-mechanical properties as the dynamic coercive force \(H_c\). It is known that \(H_c\) is unambiguously related to many structural-mechanical properties of structural steels (hardness, quenching temperature, structure of the surface layer of products, etc.). Thus, by measuring the amplitude of the regulated half-wave of the current feeding the sensor, one can judge the magnitude of \(H_p\) and, consequently, the inspected properties of materials and products.

The use of a magnetizing-coil supply by an alternating current asymmetric in amplitude makes it possible to oscillograph the dynamic initial magnetization curve of ring and open specimens. The basic block diagram of oscillographing is given in Fig. 3. With the aid of the resistance \(R_1\), one half-wave of the sinusoidal current is varied until the amplitude of this half-wave reaches the value corresponding to the field strength \(H_p\). When the field strength is reduced from the value \(H_p\) to zero, the test specimen \(IO\) is demagnetized, its residual induction becomes equal to zero, i.e., the partial cycle (Fig. 1, curve 3) passes through the origin. When the second half-wave of current passes through the magnetizing coil, the specimen is magnetized along the initial dynamic curve. As an indicator of the passage of the partial cycle through the origin, a phase-sensitive controlled rectifier \(UV\) (Fig. 3) is used; in addition, the method of commutation of the full and partial remagnetization loop, the method of light marks, etc., may be used. The light marks are produced with the aid of the circuit for forming illumination pulses \(FI\), which are fed to the oscilloscope modulator. Illumination pulses

are formed at the moment when the intensity of the magnetizing field passes through zero.

Broad prospects for the use of asymmetric magnetization also open up in quality control of products by harmonic analysis of the secondary e.m.f.

Fig. 3

LITERATURE

1. V. P. Dolgopolov, A. M. Gruznov, I. G. Leshchenko, V. D. Sartakov. Measurement of the hardness of steel products using an alternating-current coercimeter. Journal “Defektoskopiya,” No. 4, Nauka Publishing House, Sverdlovsk, 1967.