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As one of the five conventional non-destructive testing methods, eddy current testing is widely used in the steel industry, including metal rods, wire flaw detection, fatigue crack detection of structural parts, identification of material composition and impurity content, identification of heat treatment status, mixing and sorting, Measuring the thickness of the metal sheet and many other aspects. In recent years, with the deep understanding of eddy current testing technology and the development of computer, instrumentation and digital signal processing technology, eddy current non-destructive testing technology has made certain breakthroughs in the application of steel industry, for some of the past is considered the detection limit or " The impossible problem, found a solution or idea. For example, some people have proposed on-line detection of surface defects of high temperature continuous casting slabs above 1100 °C, which has increased the temperature of traditional eddy current testing objects by several hundred Baidu, and a Swedish company has developed high temperature steel and other metal sheets for testing 1000 °C. Eddy current testing equipment for billets. In addition, the application of eddy current testing extends to stainless steel capillary tubes, wire materials with a diameter of less than 1 mm, and mold liquid level detection.
Principle of eddy current generation
Eddy current testing utilizes the principle of electromagnetic induction to non-destructively evaluate certain properties of conductive materials and their workpieces by measuring changes in induced eddy currents in the workpiece being inspected, or to find non-destructive testing methods for defects. When the coil flows through the high-frequency alternating current, an alternating magnetic field is generated therein. If the magnetic field is close to the surface of the metal workpiece, a current can be induced in the workpiece, which is referred to as eddy current. The size of the eddy current is related to the electrical conductivity, magnetic permeability, geometrical dimensions, and defect morphology of the metallic material. The eddy current itself also produces a magnetic field whose intensity depends on the size of the eddy current, which is opposite to the current field of the coil, which is superimposed with the magnetic field of the coil to form the AC impedance of the coil. The change of the eddy current magnetic field causes the change of the coil impedance, and the magnitude and phase of the impedance change can be measured to indirectly measure the abnormality or defect size of the workpiece surface and the near surface material.
Application overview
1 Detection of surface defects of 1000 °C high temperature continuous casting slab
The limitation of eddy current testing for high temperature products lies mainly in the temperature that the probe can withstand. The traditional eddy current testing technology can detect temperatures up to 550 ° C under high temperature conditions. If water cooled probes are used, the temperature can be increased. Jia Huiming and other high-temperature eddy current probes developed by special materials, combined with air-cooling and water-cooling, keep the internal temperature of the sensor below 40 °C, and can withstand strong high-temperature radiation for a long time. Tests have shown that the high temperature probe can detect surface defects with a depth of 1.5 mm, a width of 0.3 mm and a length of 10 mm on the slab at 1100 ° C or higher. The technology can effectively suppress the noise influence caused by the vibration marks on the surface of the slab, and realize the positioning, quantitative analysis and printing record of the surface defects of the hot slab by means of computer signal processing technology, in order to realize the online non-destructive testing of the continuous casting blank. The technical basis.
According to the data, a Swedish company designed and manufactured a device that can inspect the surface defects of steel and other metal sheets and billets at around 1000 °C according to eddy current technology. The device guarantees that both sides of the steel surface are scanned in almost vertical directions. Using an analyzer consisting of computers, the input signals are divided into three main categories: severe defects, harmless defects, and unrecognized, and the location of any defects can be found. The device is capable of accurately measuring the position of a score of 0.5 mm deep on the surface of the blank.
2 Stainless steel capillary detection
For extremely small diameters such as stainless steel capillary offline or online non-destructive testing, the electromagnetic eddy current testing method is feasible, but special probes must be configured to achieve satisfactory results. Due to the extremely small diameter of the capillary tube, the current state of the art has not been able to make an internal probe, and it is impossible to use a point probe for detection. It can only be detected by an external through probe. The differential external probe developed by Xi'an Jiaotong University and Edson (Xiamen) Electronics Co., Ltd. has the width and thickness of the coil, the span between the two coils, the gap between the probe and the capillary, and the wire diameter. After calculation and optimization, the special advanced external wearable special probe is equipped. When the detection frequency is 666kHz, the stainless steel capillary of Φ1mm and Φ0.45mm is tested, and good results are obtained.
3 wire online inspection
Wire-line inspection generally uses two methods: one is a rotary detection type, that is, the eddy current detector rotates around the wire at a high speed. This method is mainly used to detect cracks, scratches and wire scratches extending in the longitudinal direction of the wire. The trajectory of the detector is helical relative to the movement of the wire. Using multiple detectors in parallel with high speed rotation, 100% inspection is possible, but the sensitivity of surface inspection is limited. It is not easy to maintain a constant spacing between the detector and the wire, and the sensitivity is reduced when the gap is increased. If the wire is eccentric, the gap changes. The high-speed processor automatically senses the gap and continuously compensates for the sensitivity of the system. The other is a surround coil type. The wire passes through the toroidal coil. The transducer effectively checks the distribution of the eddy current in a section and compares it with the previous section. It is suitable for detecting point defects and circumferential cracks. For transverse cracks, V-shaped cracks, inclusions, Pit and folding have high sensitivity. The detection speed is fast and the detection diameter range is large.
The surrounding coil drive current is higher than the rotary detection type and has better depth penetration. The system is stable and is not affected by temperature changes and other factors. When the magnet material is below 800 °C at the Curie point, the signal is suppressed after magnetic saturation, but the magnetic saturation can be prevented by adjusting the magnetic field strength, and the sensitivity is improved. At present, most of them use a surround coil type, and the above two methods can also be used in combination. Eddy current technology has been well applied in the production of wire drawing, oil tempering production lines, cold rolled steel wire or spring steel wire. The water-cooled surrounding coil detects the wire rod with a temperature exceeding 1100 ° C, and the detection speed exceeds 500 km / h.
4 liquid level detection
Accurate detection of the crystallizer level is the key to automatic liquid level control during continuous casting production. The vortex molten steel level gauge has obvious advantages such as fast reaction speed, high measurement accuracy, no special safety protection, convenient installation and maintenance, and the application progress is very fast. Song Dongfei introduced the situation of Pangang's transformation using the domestically produced RAM series eddy current steel level controller. The measuring system uses a eddy current sensor to measure the molten steel level. The 50 kHz high frequency signal generated by the oscillator is supplied to the primary coil (excitation coil) of the sensor. Due to the eddy current in the molten steel, the alternating magnetic field is generated by the primary coil. Height changes. A voltage Vγ V2 that varies in proportion to the intensity of the magnetic field passing through the coil is generated in the secondary coil (measuring coil), so that the differential voltage (V1 - V2) varies with the liquid level. Through the amplification, phase, frequency, amplitude analysis and linearization of V1-V2, it is sent to the 16-bit high-performance single-chip 80C196KC for processing, which is obtained by measuring the liquid level height signal, and converting it into 4~20mA signal by the controller to send it to the crystal. Liquid level control system PLC. The controller has a measuring range of 0 to 150 mm and a resolution of 0.1 mm. The operation shows that the performance of the liquid level control system is stable and reliable, and the use accuracy is up to ±3mm, which not only reduces the surface crack of the slab, improves the product quality, but also has significant economic benefits.
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