What are the new applications of eddy current nondestructive testing technology in the steel industry

As one of the five conventional non-destructive testing methods, eddy current testing is widely used in the steel industry, including metal rod and wire flaw detection, structural parts fatigue crack detection, material composition and impurity content identification, heat treatment status identification, mixed material sorting, and thickness measurement of metal sheets. In recent years, with the deepening of the understanding of eddy current testing technology and the development of computers, instrumentation and digital signal processing technology, the application of eddy current non-destructive testing technology in the steel industry has made certain breakthroughs, and solutions or ideas have been found for some problems that were previously considered to be detection limits or "impossible". For example, some people have proposed online detection of surface defects of high-temperature continuous casting slabs above 1100℃, which increases the temperature of traditional eddy current detection objects by several hundred degrees, and a Swedish company has developed eddy current detection equipment for detecting 1000℃ high-temperature steel and other metal plates and billets. In addition, the application of eddy current testing has also extended to stainless steel capillaries, wires with a diameter of less than 1mm, and crystallizer liquid level detection.

Eddy current testing is a nondestructive testing method that uses the principle of electromagnetic induction to nondestructively evaluate certain properties of conductive materials and their workpieces, or discover defects by measuring the changes in induced eddy currents in the workpiece being tested. When a high-frequency alternating current flows through a coil, an alternating magnetic field is generated in it. If the magnetic field is close to the surface of a metal workpiece, an electric current, referred to as eddy current, can be induced in the workpiece. The size of the eddy current is related to the electrical conductivity, magnetic permeability, geometric dimensions and defect morphology of the metal material. The eddy current itself will also generate a magnetic field, the intensity of which depends on the size of the eddy current, and its direction is opposite to the magnetic field of the coil current. It is superimposed with the magnetic field of the coil to form the AC impedance of the coil. Changes in the eddy current magnetic field will cause changes in the impedance of the coil. Measuring the amplitude and phase of the impedance change can indirectly measure the abnormality or defect size of the workpiece surface and near-surface material.

1. The limitation of eddy current detection of high-temperature products lies mainly in the temperature that the probe can withstand. The detection temperature of traditional eddy current detection technology can reach 550°C under high temperature conditions. If a water-cooled probe is used for detection, the temperature can be further increased. Jia Huiming and others developed a high-temperature eddy current probe using special materials. With the help of a combination of air cooling and water cooling, the internal temperature of the sensor is always kept below 40°C, and it can withstand strong high-temperature radiation for a long time. The test shows that the high-temperature probe can detect surface defects with a depth of 1.5mm, a width of 0.3mm, and a length of 10mm for ingots above 1100°C online. This technology can effectively suppress the noise influence caused by vibration marks on the surface of the ingot, and with the help of computer signal processing technology, it can realize the positioning, quantitative analysis and printing records of surface defects of hot ingots, providing a technical basis for the online non-destructive detection of continuous casting ingots.

According to the information, a Swedish company has designed and manufactured a device based on eddy current technology that can detect surface defects of steel and other metal plates and blanks at around 1000°C. The device can ensure that the two almost perpendicular directions of the steel surface are scanned. Using an analyzer composed of a computer, the input signal is divided into three main categories: serious defects, harmless defects, and unrecognized defects, and the location of any defects can be found. The device can accurately measure the location of a 0.5mm deep notch on the surface of the blank.

2. For extremely small diameter tubes such as stainless steel capillaries, the electromagnetic eddy current detection method is feasible for offline or online nondestructive testing, but special probes must be configured to achieve satisfactory results. Due to the extremely small diameter of the capillary, the current technological level is not yet able to produce an internal penetration probe, nor can it use a point probe for detection. It can only be detected by an external penetration probe. The differential external penetration probe jointly developed by Xi'an Jiaotong University and Edson (Xiamen) Electronics Co., Ltd. calculates and optimizes the width and thickness of the coil, the span between the two coils, the gap between the probe and the capillary, the wire diameter, and other aspects, and is equipped with a special high-level external penetration special probe. At a detection frequency of 666kHz, it detects Φ1mm and Φ0.45mm stainless steel capillaries, and obtains good results.

3. There are generally two methods for online detection of steel wire: one is the rotary detection type, that is, the eddy current detector rotates around the steel wire at high speed. This method is mainly used to detect cracks, scratches and wire drawing scratches extending along the longitudinal direction of the steel wire. Relative to the movement of the steel wire, the trajectory of the detector is spiral. Using multiple detectors in parallel and rotating at high speed can achieve 100% inspection, but its sensitivity for surface flaw detection is limited. It is not easy to maintain a constant spacing between the detector and the steel wire. The sensitivity decreases when the gap increases. If the steel wire is eccentric, the gap will change. The use of a high-speed processor can automatically sense the gap and continuously compensate for it, so that the sensitivity of the system is improved. The other is the surrounding coil type. The steel wire passes through the annular coil, and the transducer effectively checks the distribution of eddy currents in a section and compares it with the previous section. It is suitable for detecting point defects and cracks in the circumferential direction. It has high sensitivity for transverse cracks, V-shaped cracks, inclusions, pits and folds. The detection speed is fast and the detection diameter range is large.

The drive current of the surround coil type is higher than that of the rotary detection type, and it has better depth penetration. The system has good stability and is not affected by temperature changes and other factors. When the magnetic material is below the Curie point of 800℃, the signal will be suppressed after magnetic saturation, but the magnetic saturation can be avoided by adjusting the magnetic field strength to improve sensitivity. At present, most of the surround coils are used, and the above two methods can also be used in combination. Eddy current technology has been well applied in wire drawing, oil tempering production lines, cold heading steel or spring steel wire production. The water-cooled surround coil detects wire rods with a temperature exceeding 1100℃, and its detection speed exceeds 500km/h.

4、结晶器液位的精确检测是连铸生产过程中实现液位自动控制的关键。涡流式钢水液位计具有反应速度快、测量精度高、不需特殊安全防护、安装维护方便等显著优点，实用化进展很快。宋东飞介绍了攀钢改造采用国内生产的RAM系列涡流型钢水液位控制仪的情况。该测量系统采用涡流式传感器测量钢水液位，由振荡器产生的50kHz高频信号供给传感器的初级线圈(激励线圈)，由于受钢水内涡流电流的影响，由初级线圈产生交变磁场随液位高度变化。在次级线圈(测量线圈)内将产生与通过线圈磁场的强度成正比例变化的电压VγV2，从而差动电压(V1-V2)随液位高度变化。通过对V1-V2进行放大、相位、频率、振幅分析及线性化，送给16位的高性能单片机80C196KC处理，即得于液位高度测量信号，经控制仪转换成4～20mA信号送到结晶器液位控制系统PLC。该控制仪测量范围为0～150mm，分辩力为0.1mm。运行表明，该液位控制系统性能稳定可靠，使用精度达±3mm，不但减少了铸坯表面裂纹，提高了产品质量，而且经济效益显著。