STUDY OF A TWO-FREQUENCY METHOD FOR MONITORING THE HARDNESS OF SHAPED ROLLED PRODUCTS

B. A. DUTOV, I. G. LESHCHENKO

(Presented by the scientific and technical seminar of the Department of Information and Measurement Technology)

An instrument based on measuring the first harmonic of the secondary emf of a through-type transformer transducer monitors the hardness of turbine-blade blanks with sufficiently high accuracy [1]. However, the increased requirements imposed on instruments for nondestructive testing make it necessary to seek more advanced methods that allow the productivity of inspection and its accuracy to be increased.

Promising in this respect is the two-frequency method, which has high sensitivity to changes in the structural and mechanical properties of ferromagnetic products [2, 3]. In the present work, the possibility of applying the two-frequency method to monitoring the hardness of shaped rolled products is investigated.

The use of two frequencies for magnetizing products has its own special features. This is connected with the fact that, under such excitation, low-frequency harmonics \(n\Omega\) and high-frequency harmonics \(m\omega\) arise in the output voltage taken from the measuring windings of the transducers. In addition, the spectrum of the output signal is enriched by the combination frequencies \(n\Omega \pm m\omega\).

The amplitude-phase relationships of the frequencies entering the spectrum depend on many factors, including the electromagnetic characteristics of the product being tested.

The block diagram of the experimental setup for studying the relation between hardness and the secondary emf of a through-type transducer when magnetized by two alternating magnetic fields of different frequencies and amplitudes is shown in Fig. 1. A strong low-frequency field (50 Hz) is produced by a short through coil, inside which the measuring coil and the high-frequency magnetizing coils are placed. The sensor unit consists of a reference transducer \(D_1\) and a control transducer \(D_2\). The low-frequency and high-frequency magnetizing windings are connected in series aiding, and the measuring windings are connected in series opposition.

In the spectrum of the secondary emf of the transducers, the harmonics of the high-frequency field are of greatest interest; these are selected by a tunable selective amplifier SA. However, the influence of the combination frequencies is not completely eliminated, since the quality factor of ordinary selective elements, as a rule, does not exceed 200. This is why the amplitudes of the harmonics are modulated by the low-frequency components of the spectrum.

The output voltage from the selective amplifier is detected and fed for recording to the KSP-4 self-recording instrument.

To determine the optimal conditions for hardness monitoring, the intensity of the low-frequency field was varied from 10 to 500 A/m, and that of the high-frequency field from 0.5 to 2 A/m, while the frequency of the high-frequency field was varied from 2 to 20 kHz.

Fig. 1. Experimental setup

The test results showed that there is a fairly broad range of intensities of both fields for which the first, second, and third harmonics of the high-frequency field are correlated with hardness; moreover, as hardness increases, the first and second harmonics increase, while the third decreases (Fig. 2). For samples of profiles No. 173 and No. 416 (steel 1Kh13), the closest correlations with hardness are observed at a low-frequency field intensity \(H\Omega = 150 \pm 200\) A/m, a high-frequency field intensity \(H\omega = 1\) A/m, and a high-frequency field frequency of 10 kHz. As is seen from the dependences shown in Fig. 2, the error in measuring hardness in this case is no more than \(\pm 7\) units at an average hardness of 200 Vickers units.

Fig. 2

At the same time, the effect of mechanical stresses was investigated. The level of all the indicated harmonics decreases under mechanical stresses of up to 400 kg/cm². To detune from the influence of mechanical stresses that may arise during monitoring of elongated moving products, a two-channel instrument with measurement of, for example, the first and third harmonics of the high-frequency field can be recommended.

Thus, the investigations carried out showed that, with the two-frequency method, it is possible to significantly expand the field of application

and to improve the characteristics of the instrument in comparison with those presented in [1]. The choice of the field strength of the low-frequency field and of the frequency of the high-frequency field is not critical, and the inertia of the selective elements does not introduce any significant error at high monitoring speeds.

REFERENCES

1. B. A. Dutov, I. G. Leshchenko. An instrument for monitoring hardness by the harmonic composition of the secondary e.m.f. of a sensor. This collection.

2. A. M. Gruznov. Development and investigation of new circuits for monitoring ferromagnetic products by the eddy-current method. Candidate’s dissertation, Tomsk, 1967.

3. I. G. Leshchenko. Investigation and application in electromagnetic flaw detection of the magnetization of ferromagnetic products simultaneously by two parallel fields of different frequencies and amplitudes. The method of higher harmonics in eddy-current flaw detection (materials of the All-Union seminar). Krasnoyarsk, 1969.