DETECTION OF DEFECTS IN SMOOTH-ROLLED PRODUCTS OF LIGHT-SECTION ROLLING MILLS

V. K. ZHUKOV, A. M. PANASYUCHENKO, B. A. DOBNER

(Presented by the joint scientific-technical seminar of the Department of Information and Measuring Technology and the Sector of Electromagnetic Defectoscopy of the Scientific Research Institute of Electronic Introscopy)

For modern industrial production, the problem of sharply improving the quality of all products and increasing the reliability of the machines and apparatus produced is acquiring exceptional importance. The use in industry of new, highly productive technological processes with large specific loads on the parts of machines and installations, and the rapid development of new branches of technology, including the exploration of outer space and nuclear energy, have required the widespread use of exceptionally high-quality materials and parts with guaranteed reliability. However, most existing technological processes in metallurgy are still such that various dangerous defects may arise in the materials and parts obtained, in the form of discontinuities (cracks, cavities, various nonmetallic inclusions, laminations in rolled stock, lack of fusion), as well as deviations in geometric dimensions. In this connection, quality control of rolled products from light-section mills is an important task.

The technological line of a light-section rolling mill is a complex system. Initially, a billet with approximate dimensions of \(80 \times 80 \times 10500\) is fed into a furnace, where it is heated to a temperature of \(1000^\circ\)C, after which it is passed through rolling stands, the maximum number of which may reach seventeen. During reduction, both the geometric dimensions and the structure of the metal change. From the last stand, the rolled product is delivered by roller table to a cooling bed, cools, is cut into bundles of the required length, tied into coils, and sent to the finished-product warehouse.

Such technology makes it possible to carry out quality control of the products only in the section between the last stand and the cooling bed, for the following reasons. The design of the cooling bed and of all subsequent units does not allow the product to be separated into individual pieces, since they move in batches of 10–20 pieces. Carrying out control before rolling, i.e., inspecting the billet, means not solving the assigned task completely, since during rolling some defects are welded up and disappear, while at the same time new ones may form.

Inspection of the rolled product after the last stand is associated with considerable difficulties because of the high temperature of the product (piece), which reaches \(1000^\circ\)C, the considerable speed—up to 20 m/s—and axial displacements, which can never be completely eliminated. All this sharply narrows

the range of inspection methods that can be applied in the given case. All contact methods, as well as the ultrasonic method—which is, in essence, a contact method—are inapplicable. Because of the high speeds, radiation methods would require excessively powerful radiation sources, which is undesirable. The most acceptable method in this case is the eddy-current method, and the most convenient sensor is a through-type sensor.

Because of the severe conditions in the inspection zone, special requirements are imposed on the sensor. First, it must have high mechanical strength, since when the leading end of a bar, possessing a large mass and high speed, enters the sensor, the latter is subjected to strong mechanical impacts. Therefore the sensor housing must be made of metal, preferably nonmagnetic stainless steel, and provided with a funnel-shaped entry. In order for the electromagnetic field to penetrate inside the sensor, the housing must be slit along a generatrix to eliminate a short-circuited turn. Because of the high temperature of the bar, even in the absence of contact between it and the sensor housing, the latter is subjected to considerable heating due to heat radiation, which can lead to disruption of the normal operation of the entire device as a whole. Therefore it is necessary to cool the sensor housing. Both air and water cooling systems were developed and tested. Figure 1 shows a sensor with an air-cooling system. The sensor is assembled from separate sections, the frames of which are made of copper and have a slit along a generatrix. The entry 3 is made of stainless steel. The sections are connected to one another and to the entry by means of tie bolts. Compressed air is supplied from below and, blowing over the frames, cools them. This cooling system proved, during testing, to be sufficiently effective; however, the drawback of the design as a whole was its low mechanical strength and the possibility that the leading end of the bar could become jammed in the sensor.

Fig. 1. Sensor with an air-cooling system: 1) magnetizing windings; 2) measuring windings; 3) stainless-steel entry; 4) product being inspected

The design shown in Fig. 2 is free of such shortcomings. Sensor 1 and sleeve 2 are made of stainless steel and have a longitudinal slit along a generatrix. Cooling water is supplied together with the product being inspected into the sensor entry; to prevent water from reaching the windings, the gap formed in the sensor after it was slit...

the gap is filled with epoxy resin. This type of cooling is the best, and the design is sufficiently strong. But such a cooling system has the drawback that, at high sensitivity of the circuit, turbulence of the water is detected by the instrument and recorded as defects, since water has a quite definite conductivity. In order to obtain a laminar flow of water, it was necessary to use the design

Fig. 2. Sensor with water cooling: 1) sensor body; 2) sleeve; 3) article being inspected

shown in Fig. 3. The sensor 1 and the protective sleeve 2 are made of stainless steel; the coil frame 4 is made of fluoroplastic 4. For rigid fastening of the coils, washers 3 are used, consisting of two half-rings, which are tightened on the sensor by means of bolts. Water enters the sleeve from below, owing to which, with

Fig. 3. Sensor with water cooling, with water fed into the body: 1. Sensor body; 2. Sleeve; 3. Washer; 4. Coil frame; 5. Insulating layer of epoxy resin

sufficiently effective cooling, there is no turbulence of the liquid. To prevent water from getting onto the winding lead, the latter is potted with epoxy resin 5. This design combines two

differential sensors, i.e., each coil has a magnetizing and a measuring winding.

The sensors were tested together with a flaw detector, the block diagram of which is shown in Fig. 4. The master oscillator \(G\) feeds the four magnetizing windings of the sensor. From the secondary windings of the first two coils, which are connected in opposition, the signal is fed to rectifier \(B'\). After suppression of the high-frequency components by the filter \(\Phi_{\mathrm{hf}}\), the signal is fed to the envelope amplifier \(UO\) and the low-frequency filter \(\Phi_{\mathrm{lf}}\), which suppresses the signal from axial displacements in the sensor and 50 Hz pickup. The output devices \(BU'\) are an amplifier combined with a one-shot multivibrator and are actuated at a certain signal amplitude. \(BU_+\) is actuated when a positive signal appears at the input of the circuit, and \(BU_-\) when a negative one appears. The signal then goes to the “I” circuits. From the other two secondary windings the signal goes to a parallel channel, which is completely identical to that described above, except for the final stages. In this case the signal goes from the output devices to the “NOT” circuits and to the second inputs of the “I” circuit. From the outputs of the “I” circuit the signal reaches the recording device \(RU\).

Fig. 4. Block diagram of the flaw detector

Such a complication of the circuit through the introduction of a parallel channel is justified, since the noise immunity of the entire device as a whole is increased substantially; namely, all interference arising in the electronic circuit itself passes simultaneously through both channels, but thanks to the logic circuits the signal does not reach the recording device. The signal from a defect passes through the channels with a certain time shift, because the sensor coils are spaced at a certain distance and the defect appears under them at different times. Such pulses, shifted in time in a definite way, pass to the recording device, and the instrument registers the presence of a defect.

The gain of each channel is one hundred thousand; therefore very stringent requirements are imposed on the shielding of the circuit, especially the input stages and the generator, on decoupling in the power-supply circuit, and on filtering both the carrier frequency and its harmonics, as well as the low frequencies.

As the recording device one may use a self-recording instrument, but in this case the problem of detecting defects cannot be considered completely solved, since the chart of the recorder, when the flaw detector is used for inspection in mass production, can provide information only on the presence or absence of defects in the rolled stock without indicating the location, i.e., to eliminate these defects—to cut them out—and thereby

it is impossible to improve the quality of rolled stock by the stands themselves. The best solution to the problem would be to create a marking device that would mark the location of the defect directly on the rolled stock. Mechanical markers, which would deform the rolled stock at the defect location, are not applicable in this case, since the dynamic loads arising from this would disrupt the normal operation of the mill. It was therefore necessary to proceed by applying a colored mark to the rolled stock. Major difficulties in solving this problem arose because of the high temperature of the rolled stock, reaching \(1000^\circ\mathrm{C}\), and the high rolling speeds. After prolonged investigations, a glass-based paint was created, having a color clearly distinguishable from that of the cold rolled stock, withstanding high temperature, and adhering well to the metal. The paint is applied to the rolled stock by a special sprayer.

On the basis of the investigations carried out, an experimental model of the defectoscope was developed; its tests were conducted at the West Siberian Metallurgical Plant. In the course of the tests it was established that the design of the sensor, the measuring circuit of the defectoscope, and the method of marking the defective section generally satisfy the requirements imposed on them and can serve as the basis for the development of an industrial version of the defectoscope.