Superficial Injury? Internal Injury? ——What Kind Of Damage Was Found By The Ultrasonic Flaw Detection?
Aug 07, 2023| 1. Common methods of rail ultrasonic flaw detection
The principle of ultrasonic testing is to generate reflections when ultrasonic waves propagate in rails and encounter defects. Since the energy reflected by ultrasonic waves is related to the difference in acoustic impedance of the medium on both sides of the interface and the orientation and size of the interface, the energy received by the probe can be Analyze the size and location of defects. Ultrasonic flaw detection is divided into shear wave method and longitudinal wave method. Generally speaking, the sensitivity of using the shear wave method is relatively high, but the defect location is relatively complicated, and the defect location using the longitudinal wave method is relatively simple. At present, the main purpose of line flaw detection is to detect transverse fatigue defects in rails, mainly using the shear wave method of inclined probe. Straight probe longitudinal wave method for detection. The defects discussed in this article were discovered by the shear wave method with a 70° angled probe.
2. Overview of damage found by ultrasonic flaw detection of lines
A certain railway unit found obvious damage waves when using a 70° inclined probe to conduct ultrasonic flaw detection on rails, as shown in Figure 1. After investigation, an opening defect was found on the rail tread, which caused the ultrasonic flaw detection alarm. Or is there some other defect inside causing the alarm?
3. Ultrasonic flaw detection in steel mills
At present, the rail production plant uses automatic ultrasonic flaw detection, and fixes more than a dozen probes on the corresponding positions of the full section of the rail, mainly straight probe flaw detection. The position of the rail head probe in the rail production plant is shown in Figure 3. Ultrasonic testing is carried out when the rail passes. The coverage area of the detection refers to the standard TB/T 2344.1-2020, and should not be less than 70%. From the analysis of its coverage, the damage wave generated by the defect detected by the tread probe is outside the alarm area of the gate and will not cause an alarm, while the probe on the side of the rail head may not be able to detect the defect, as shown in Figure 4. It should be noted that the rail production plant has artificial surface inspection before the ultrasonic flaw detection process. Surface defects that appear in the factory can be found through manual inspection. May be generated during use.
4. Ultrasonic flaw detection of lines
The principle of ultrasonic flaw detection using a 70° inclined probe is shown in Figure 5. Generally speaking, in order to increase the flaw detection coverage area of the rail head flaw detection, the 70° angle probe flaw detection needs to carry out primary wave detection without deflection angle and secondary wave detection with deflection angle. This defect is mainly discovered when deflection angle detection is used. The damage is near the gauge angle of the rail tread. When ultrasonic flaw detection is used for detection, the specific location of the defect cannot be confirmed only from the A-type display waveform (shown in Figure 1). It was misjudged as an internal nuclear damage defect, as shown in Figure 6.
5. Analytical means of ultrasonic flaw detection alarm caused by surface defects on the line
From the above results, it can be concluded that the alarm defects found by the preliminary analysis of line flaw detection are caused by surface defects. It can be verified by the following methods:
When the shear wave detection of the ultrasonic oblique probe finds that the alarm exceeds the threshold, fix the position of the probe, calculate whether the vertical depth of the defect is on the outer surface of the tested rail, and dip or squeeze the defect horizontally to locate the position by dipping in the coupling agent. If the defect wave is obvious change, it is preliminarily judged that the defect is caused by surface defects
If further confirmation is required, the surface defects of the rail should be ground clean. At the same time, the surface of the rail after grinding should be smooth and excessive. After grinding, a hand-held ultrasonic detector should be used to detect the defect position on the front and rear sides. No defect alarm wave indicates the position It is an abnormal ultrasonic alarm caused by surface defects. The alarm wave indicates that the surface defect may expand to the inside, and further analysis and confirmation is required.
In summary, when performing ultrasonic flaw detection on in-service rails, when damage reflected waves appear, it is necessary to carefully analyze the defect location to prevent misjudgment.