MEASURING THE HARDNESS OF PRESSURE-GAUGE SPRINGS BY ELECTROMAGNETIC METHODS

I. G. Leshchenko, V. D. Sartakov

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

Most methods of electromagnetic quality control of products made from ferromagnetic materials are based on the existence of unambiguous relationships between the electromagnetic parameters of a material and its mechanical or physical characteristics, such as composition, structure, depth of the hardened layer, hardness, and others.

Determination of the specific electrical conductivity \(\sigma\), magnetic permeability \(\mu\), and static coercive force \(H_c\) is associated with great difficulties because of the complex geometrical shape of pressure-gauge springs and the large variations in their geometrical dimensions.

As a result of various investigations of the electromagnetic characteristics of springs, it was found that, for practical use, it is expedient to employ a method for monitoring the hardness of springs by the components of the secondary emf of a feed-through transducer with the article being tested, and by the dynamic coercive force of a partial cycle. In the first case, a transducer was used with the number of turns of the magnetizing winding \(W_1 = 450\) and of the measuring winding \(W_2 = 1000\).

The dimensions of the transducer (length \(l = 20\) mm, internal diameter of the transducer \(d_{\mathrm{in}} = 16\) mm) were chosen to be sufficiently small so that it would be possible to monitor the electromagnetic characteristics of springs in their middle part without a noticeable influence of length.

The use of a surface-type transducer without a magnetic core is limited by the impossibility of creating a high magnetic-field strength \(H\), while the electromagnetic characteristics (coercive force, magnetic permeability) depend to a large extent on the strength of the magnetizing field [1, 2].

The layout of the experimental setup (Fig. 1) includes a regulated alternating-current source, a two-winding feed-through transducer into which the article being tested is placed, a compensating device based on a VTM-5 rotating transformer, and a phase-sensitive voltmeter V5-1, which makes it possible to measure the quadrature components of the circuit unbalance voltage.

To power the setup, a mains voltage of 220 V, 50 Hz was used.

The setup makes it possible to obtain a hodograph of the unbalance voltage of the measuring circuit as a function of frequency from \(f_{\min} = 20\) Hz to \(f_{\max} = 20\) kHz. In this case it is necessary to observe the condition \(RC\omega = 1\), which is due to the use of the VTM-5 as a phase shifter.

The use of a differential circuit in this case significantly increases the sensitivity of the experimental setup.

The compensating device makes it possible, with sufficient accuracy, to compensate the emf of the secondary winding of the sensor in magnitude and phase, which favorably distinguishes the measuring circuit of the setup from circuits using a compensating sensor.

A spring with average geometric dimensions and maximum hardness was selected as the reference specimen. For this spring, the circuit was balanced along the imaginary and real axes by changing the position of the rotor of the rotary transformer and the slider of rheostat \(R_3\). As the reference voltage for the phase-sensitive voltmeter, the voltage drop across resistance \(R_2\), coinciding in phase with the magnetizing current, is used.

Fig. 1

Since the sensor operates in a prescribed-current mode, the waveform of the emf in the secondary winding is distorted. If one takes into account that in the phase-sensitive voltmeter the attenuation of the higher harmonics is not less than 40 dB, then the higher harmonics do not noticeably affect the measurement results; the characteristics were determined from the fundamental harmonic of the emf \(E_2\). When the test article is placed in the sensor, the real and imaginary components of the circuit unbalance voltage are measured from the voltmeter indicators.

From the measurement data, complex planes of voltage unbalance were constructed, the analysis of which makes it possible to determine such a phase shift between the primary current of the sensor and the reference voltage that the circuit has maximum sensitivity to the measured parameter of the article.

Measurements of the increment of emf in the secondary winding of the sensor were carried out at magnetizing currents \((0.1;\ 0.2;\ 0.3;\ 0.5\ \text{A})\) for a whole series of pressure-gauge springs made of steel 50KhFA of the following standard sizes: MGn \(160 \times 160\), MGn \(160 \times 250\), MGn \(160 \times 400\), MGn \(160 \times 600\).

Figure 2 shows the dependence of the imaginary component of the unbalance voltage of the circuit on the hardness of pressure-gauge springs of standard size MGN \(160 \times 250\) at a magnetizing current of \(I = 0.2\ \mathrm{A}\). At other magnetizing currents, a larger scatter of the experimental points is observed. The scatter of the experimental points relative to the averaged characteristic can be explained by the different approaches to changing hardness by mechanical and electromagnetic methods.

With the electromagnetic method of inspection using alternating current, one should speak of the average electromagnetic parameters of the material, which is connected with the nonuniform distribution of magnetic flux over the cross section of the material, as well as with nonidentical parameters of the material over the cross section.

Before testing on the installation, hardness was measured with a Vickers instrument at 7–9 points located close to one another.

It should be noted that careful preparation of the specimen surface is required; for this purpose the upper (most hardened) layer of metal is removed and the prepared flat surface is ground. Consequently, if the material has undergone nonuniform hardening, the hardness measured with the Vickers instrument is not equal to the average hardness of the material.

We also checked the possibility of inspecting spring hardness using the EMT-1 instrument (electromagnetic hardness tester), developed at the Department of Electrical Measuring Engineering of the Tomsk Polytechnic Institute. The instrument is intended for measuring the hardness of steel products by the dynamic coercive force and is a phase meter constructed on the principle of measuring time intervals [3, 4].

As the reference signal of the instrument, the output voltage of a bridge phase shifter supplied by industrial-frequency voltage is used. To obtain a signal proportional to the induction in the product, the emf of the secondary winding is fed to the input of an integrating device consisting of \(R\)–\(C\) elements. The integration error does not exceed 3%.

Fig. 2

Fig. 3

The EMT-1 has a pointer indicator, the scale of which is graduated in Vickers hardness units. With this instrument, springs made of steel 50KhFA of the standard sizes listed above and springs made of steel 30KhGSA of the following standard sizes were inspected: AM \(160 \times 10\), AM \(160 \times 16\), AM \(160 \times 25\).

For investigating the characteristics of pressure-gauge springs made of steel 50KhFA, a through-type two-winding transducer with a number of turns-

of the magnetizing and measuring windings \(W_1 = 2500\) and \(W_2 = 10000\), respectively, an inner diameter \(d_{\mathrm{in}} = 14\) mm, and a length \(l = 25\) mm; when testing springs made of 30KhGSA steel it had, respectively, \(W_1 = 980\) and \(W_2 = 6700\), \(d_{\mathrm{in}} = 20\) mm, \(l = 25\) mm.

To determine the maximum sensitivity of the instrument to hardness, testing was carried out at various magnetizing currents \((0.1;\ 0.15;\ 0.2;\ 0.25;\ 0.3;\ 0.35;\ 0.4;\ 0.45;\ 0.5\ \mathrm{A})\).

It was noted that the maximum sensitivity of the instrument to the hardness of springs made of 50KhFA steel is observed at magnetizing currents of \(0.1 \div 0.15\ \mathrm{A}\).

Figure 3 presents the dependence of the instrument readings \(\alpha\) on the hardness of MGn \(160 \times 400\) springs at a magnetizing current of \(0.1\ \mathrm{A}\).

At lower magnetizing currents, considerable instability appears in the operation of the instrument, which is explained by the presence, in the instrument circuit, of elements having a high sensitivity threshold.

If the magnetizing current is greater than \(0.2\ \mathrm{A}\), the sensitivity of the instrument to the hardness of springs made of 50KhFA steel practically tends to zero.

For springs made of 30KhGSA steel, the maximum sensitivity to hardness occurs at a current \(I = 0.15\ \mathrm{A}\).

Figure 4 presents the dependence of the readings of instrument \(\alpha\) on the hardness of AM. \(160 \times 25\) pressure-gauge springs, determined at a current \(I = 0.15\ \mathrm{A}\).

At currents greater than \(0.2\ \mathrm{A}\) and less than \(0.4\ \mathrm{A}\), no such unambiguous relationship is observed between spring hardness and the instrument readings.

However, if \(I > 0.4\ \mathrm{A}\), the relationship between hardness and the instrument readings appears in a different form than in the case of magnetization by a current of \(0.15\ \mathrm{A}\): hard parts correspond to higher instrument readings, soft ones to lower readings. This is confirmed by literature data [2], according to which the dependence of coercive force on field strength is clearly nonlinear in character.

Fig. 4

Conclusions

The proposed installation and instruments of the EMT-1 type may be used for rough sorting by hardness of pressure-gauge springs made of 50KhFA and 30KhGSA steels.

It has been shown that the strength of the magnetizing field must be selected experimentally in each particular case in order to obtain the maximum sensitivity of the method.

References

1. I. I. Kifer. Testing of Ferromagnetic Materials. Gosenergoizdat, Moscow–Leningrad, 1962.

2. Bozorth. Ferromagnetism. Foreign Literature Publishing House, Moscow, 1956.

3. I. M. Vishenchuk, A. F. Kotyuk, L. Ya. Mizyuk. Electromechanical and Electronic Phase Meters. Gosenergoizdat, Moscow–Leningrad, 1962.

4. G. A. Remez. Radio Measurements. Svyazizdat, Moscow, 1966.