Device for Monitoring Hardness by the Harmonic Composition of the Secondary EMF of a Sensor
Abstract Generated abstract
This paper describes an electromagnetic device for production control of hardness in complex-profile steel products, particularly turbine-blade blanks, using the harmonic composition of the secondary EMF of through-type sensors. The proposed instrument uses two short differential transformer sensors, selective amplification of the first harmonic, and synchronous detection to measure the quadrature component associated with average surface-layer hardness. Tests on steel products established optimum magnetizing field strengths of 350 A/m for larger sections and 1400 A/m for smaller sections, with maximum scatter from the calibration line not exceeding about 8 Vickers units. Additional experiments assessed errors from magnetizing-current variation, ambient temperature, edge effects, and transverse displacement, informing the design of sensor guides for production use.
Full Text
DEVICE FOR HARDNESS CONTROL BASED ON THE HARMONIC COMPOSITION OF THE SECONDARY EMF OF A SENSOR
B. A. DUTOV, I. G. LESHCHENKO
(Presented by the scientific and technical seminar of the Department of Information and Measurement Technology)
As is known, the hardness of a metal is, to a certain extent, a generalized parameter of a product that has undergone heat treatment intended to improve its strength characteristics.
For hardness control of long products of complex profile, for example blanks for turbine blades, it is necessary to use fast-acting and efficient methods, among which may be included the electromagnetic method of control in alternating fields.
Studies carried out on products made of steel 1Kh13 with heat treatment according to ChMTU-1-187-67 showed the possibility of controlling hardness by the amplitude and phase of the fundamental and higher harmonics of the secondary EMF of a through-type sensor when the magnetizing windings are supplied from a 50 Hz network [1].
On the basis of the indicated studies, for controlling the hardness of profiled turbine-blade blanks under production conditions, a device was developed with two short through-type sensors connected according to a differential circuit.
The control method is based on the dependence of the quadrature component of the first harmonic of the secondary EMF of the sensor on the average hardness of the surface layer of the product section.
The block diagram of the device is shown in Fig. 1. The through-type transformer sensors \(D_1\) and \(D_2\) are supplied with stabilized industrial-frequency voltage from the power-supply unit BP in the specified field-intensity mode. Sensor \(D_1\) is the reference sensor, and sensor \(D_2\) is the control sensor. The compensating device (KU) makes it possible
Fig. 1. Block diagram of the device
compensate the difference in the sensors’ emf when two reference blanks of the specified profile and average hardness are placed in them. The selective amplifier SA extracts the first harmonic (50 Hz), while the higher harmonics are suppressed by the low-frequency filter LFF. Further amplification is carried out by the power amplifier PA1, from whose output the signal is fed to the synchronous detector SD. The reference-voltage channel for the synchronous detector consists of the phase shifter PS, the phase-shifting circuit PSC, and the power amplifier PA2. The phase shifter PS makes it possible to vary the phase of the reference voltage smoothly within the range from 0 to 180°. The phase-shifting circuit PSC changes the phase by 90°. By means of switch B1, the input of power amplifier PA2 can be connected to the outputs of PS or PSC, and this makes it possible to measure the quadrature components of the first harmonic.
Tests of the instrument were carried out in order to select the optimum magnetization mode and to estimate the error due to changes in the external inspection conditions. The field strength of the sensor, on the magnitude of which the relationships between the output signal and hardness strongly depend, was varied during the investigation from 350 to 3500 A/m.
Fig. 2. Profile No. 173—$H = 350$ A/m; profile No. 416—$H = 1400$ A/m
As a result of these experiments, the optimum field strength was determined: for samples of large cross section it is 350 A/m, and for samples of smaller cross section it is 1400 A/m. The dependence of the instrument readings on the hardness of the products at the indicated optimum field strengths is shown in Fig. 2. It follows from the graph that the maximum scatter of points from the mean line does not exceed $\pm 8$ Vickers units.
The comparatively high methodological error in measuring hardness is associated with the influence of the edge effect, whose action increases, as follows from the experimental data, with increasing magnetizing current [2] and with increasing cross section of the samples.
A change in the magnetizing current of the sensors by $\pm 10\%$ increases the measurement error by $\pm 1.5$ Vickers units.
For hardness control under production conditions it is necessary to take into account the increase in error due to changes in the ambient temperature. Experiments showed that a temperature change from $+20$ to $+50^\circ$C increases the error by no more than 1 Vickers unit.
Studies of the influence of transverse displacement of the product in the sensor were used in designing guide inserts intended to stabilize the blades in the cavities of the sensors.
LITERATURE
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B. A. Dutov, A. T. Kovrizhkin, I. G. Leshchenko, et al. Electromagnetic methods and means for controlling the hardness of drawn steel products. Moscow, MDMTP, 1971.
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I. G. Leshchenko, N. M. Malyshenko. Study of the influence of end effects on the spectral composition of the emf of pass-through sensors. In: Proceedings of the 2nd All-Union Seminar “Nondestructive Testing of Ferromagnetic Materials and Products by the Higher-Harmonics Method,” Tomsk, 1971.