Application of Ultrasonic Flaw Detection in Medium and Thick Plate Plant of Jigang

1. Overview

The online flaw detection of steel plates in Jigang Medium and Thick Plate Plant adopts ultrasonic flaw detection system. According to the standard regulations or user requirements, special purpose steel plates such as boiler and pressure vessel plates, bridges, buildings, pipeline plates, etc. need to be ultrasonically tested. With the increasing requirements for the intrinsic quality, performance and specifications of steel plate products, more and more users require 100% flaw detection of steel plates.

According to different usage methods, ultrasonic flaw detection can be divided into manual flaw detection and automatic flaw detection. Due to the advantages of low investment and flexible use, domestic medium and thick plate plants mostly adopt this method, but manual flaw detection also has many irreparable shortcomings. First, the flaw detection time is long and the production efficiency is low; second, the flaw detection labor intensity is high and easy to fatigue; third, the production site is occupied and the amount of steel plates is large; fourth, it is difficult to ensure that the ultrasonic beam covers 100% of the steel plate surface, which inevitably causes missed detection and false detection. Manual flaw detection is also difficult to adapt to the requirements of modern large-scale production due to its slow flaw detection speed. Automatic flaw detection has obvious advantages such as fast speed, high efficiency and small production space occupation. The online method basically does not occupy the production space and there is no problem of repeated handling of steel plates. The

SONOTROTM66 steel plate detector used by the factory is a non-destructive detection system for heavy steel plates produced by NDT Technology Company using ultrasonic technology. The online high-speed scanning speed of the steel plate is 2.0 meters per second, and the flaw detection ability can penetrate 6-60mm thick steel plates and cover 100% of the surface and the inside of the steel plate. The probe used is made of high-performance piezoelectric crystals and has two forms—single crystal probe and dual crystal probe. The high-frequency pulse waves generated by the electronic pulses pass through the water interference and enter the steel plate, and are reflected back from the defect point or wall surface. The larger the defect surface, the greater the reflected energy and produces a higher visible peak. This operation mode is called "pulse echo mode", and the scanning results are processed by the integrated electronic system. The types of steel plate materials that can be detected include: boiler plates, pressure vessel plates, bridge plates, ship plates, structural plates and steel plates used for other special structures.

2. Principle of ultrasonic flaw detection

The basic method of ultrasonic testing is based on the fact that the frequency of ultrasonic waves must be higher than 20KHz. For different frequencies and waveforms, the waveforms returned from the material are different. When ultrasonic waves enter the material, mechanical vibrations will be generated in the material. When ultrasonic waves propagate in the material being tested, the acoustic properties of the material and the changes in the internal organization will have a certain impact on the propagation of ultrasonic waves. The changes in material performance and structure can be understood by detecting the degree of influence and condition of ultrasonic waves.

The detection process is carried out by using a piezoelectric chip to emit ultrasonic waves into the material. This piezoelectric chip is called a probe or sensor. The probes used are divided into single crystal and double crystal probes, which are based on the principle of pulse reflection. The pulse reflection method is: when ultrasonic waves enter the material, when they encounter defects or interfaces, the sound waves will be reflected back to the probe. By observing the amount of energy reflected back in the waveform, the size of the defect can be known. Large defects return more energy than small defects. The test process is completed by two waveforms: longitudinal waves and transverse waves. Longitudinal waves are used to detect internal defects of steel plates. This wave is suitable for detecting delamination, slag inclusions and spherical cracks inside steel plates; transverse waves are used to detect longitudinal linear defects on the surface and inside of steel plates.

In order to make the ultrasonic wave enter the material and reflect from the material, the probe must be as close as possible to the material to be tested, and there must be a medium between the probe and the material to be tested. In the ultrasonic flaw detection of steel plates, water is often used as a medium, the so-called "coupling water". During the test, all probes are at the same level from the surface of the steel plate, and the sound coupling is completed by water.

3. System composition and function

The ultrasonic flaw detection device is a complete system, which consists of subsystems such as the detection system, data system, automatic control system, report output system, defect marking system, etc. In order to detect the steel plate, it is necessary to match the transport roller, clamping roller, side guide roller, straightening machine, etc.

The detector probe is installed in the probe machine base on the two servo drive racks under the test steel plate, and scans according to the width of the steel plate according to a certain coded channel. The narrowest steel plate requires one channel to be tested, while the widest requires three channels to be tested. Each time the test is performed, the probe performs the floating and tilting functions in the bracket. As a result, the probe surface is in contact with the plate as horizontally as possible, and the deviation value of the steel plate surface can also be compensated.

In order to achieve 100% flaw detection, certain speed detection and position detection elements are also required; the probe is the basic structure of the automatic ultrasonic testing system, arranged in an array, and it detects the steel plate from the bottom. The ultrasonic wave should cover 100% between the probes, and the entire probe covers 1/3 of the plate width. The probe must be as close as possible to the material being tested to facilitate the ultrasonic wave to enter the material and reflect from the material. Water is necessary for the transmission of ultrasonic waves. The detection probe is located under the steel plate and is mounted on a trolley that can move laterally. The servo drive can position the probe anywhere on the side of the steel plate and keep it synchronized with the edge of the steel plate, which is very important for 100% flaw detection.

The system is a fully automated control system using a Mitsubishi logic controller (PLC), which interlocks the main operator station with the field operation cabinet. The system collects data from the longitudinal and transverse probes and sends the data to the operator on the video monitor. Its functions include automatic diagnosis system, adjustable sensitivity, pulse frequency, DAC compensation and compensation range through computer, detection of steel plate passing speed and direction, automatic alarm and detection of bad coupling, detection of discontinuous defects exceeding calibration parameters, standard scanning, etc.

4. Brief description of the detection process

The position detection of the steel plate is completely realized by the action of four photoelectric switches. One or three round trips can be carried out on the transport roller as needed.

When the photoelectric switch PC-101 is activated, it indicates that the steel plate has begun to enter the test area. The steel plate slows down and starts the floating piston to track the waviness of the steel plate. When the steel plate has covered all the probe seats, the steel plate speed is accelerated to 2 meters/minute, and the ultrasonic flaw detection of the steel plate begins. After the steel plate is completely scanned, it slows down and continues to run until the PC-104 photoelectric switch is activated, and the steel plate exits the test area and stops.

Figure 1 Detailed view of the steel plate test area

During the inspection process, the system uses sensors and two industrial cameras to detect the edges of the steel plate. There are 16 high-temperature sensors on the longitudinal frame and 32 high-temperature sensors on the transverse frame. The camera is positioned on the probe frame to measure the width of the steel plate. In order to compensate for the distortion of the steel plate, all parts of the probe can move up and down or tilt to ensure that they are in full contact with the surface of the steel plate during the test. In order to protect the probe, a special anti-wear device made of high-efficiency anti-corrosion material is installed on the probe surface. The probe surface is not in direct contact with the steel plate below the surface, and an acoustic coupling device is used to achieve the purpose of inspection.

5. Conclusion

This ultrasonic detection system can cover 100% of the steel plate surface. The probe can accurately track the steel plate and couple well with the steel plate surface, reducing missed detection and false detection, and the detection defect size accuracy reaches a minimum of φ3mm; the supporting computer system can judge the steel plate defects according to various standards, and has good practical value for the analysis of the rolling quality of the steel plate. (end)