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Comprehensive flaw detection for welds

Time:2022-07-29 Click:804


Railway rail defects easily threaten the stability and safety of train operation, so it is necessary to do a good job in inspection, so as to accurately judge the quality situation and take treatment measures for defects. At present, ultrasonic technology has been widely used as a kind of non-destructive testing technology, which can complete the testing work in a more convenient way, and the obtained results are accurate, the operation time is short, and it is feasible.

The welding quality is directly related to the quality and safety of engineering construction. In order to meet the requirements of railway engineering construction, transportation and convenient use, steel structure welding is often adopted to become a long and large rail of 50m to 200mm. Therefore, it is necessary to specify the technical requirements of NDT, pay attention to relevant matters in the application process, and reasonably use the calibrated instruments.

Comprehensive flaw detection of weld:

1. Flaw detection for welded rail head

To accurately determine the detection range of the top surface of the track, the probe can be moved longitudinally for scanning. However, considering the limitation of small contact area of the probe, the method of longitudinal displacement of deflection angle should be adopted so as to fully scan the welded rail head. During the detection of weld rail head, the distance between probe and weld center should be 80mm. When the weld defect diameter is less than the ultrasonic beam width, the ultrasonic beam reflection phenomenon can be synchronously generated at the defect and the reinforcement, and corresponding to the fluorescent screen, the defect wave and the weld rib contour wave can be displayed at the same time; When the weld defect diameter is larger than the ultrasonic beam width, the display results will be different, and the fluorescent screen only displays the defect wave. The location of two angle probes is suitable for the two sides of rail head, i.e. the two probes detect the cross section by synchronously moving longitudinally.

In order to ensure the accuracy of the detection results, before the formal operation, the information such as the weld width and the probe sound beam width shall be collected, and the scanning frequency and the location of the incident point can be determined after calculation, so as to ensure the reliability of the results while carrying out the detection work efficiently. When there is no defect in the welding seam of rail head, the fluorescent screen has no echo display, the sound wave of probe A is reflected at the side of rail head, but probe B cannot receive the echo; If the rail head has flake defects, the ultrasonic wave of probe A will be reflected at the defect and can be received by probe B. If the weld defect is outside the scanning area of the probe, there will be no echo display on the fluorescent screen.

2. Flaw detection for welded rail web

(1) The straight probe is placed in the longitudinal middle area of the rail surface and moves the probe longitudinally. This method is used to detect flaws in a weld where the reflective surface is parallel to the detection surface.

(2) When there are defects in the rail weld, scattering will occur, which will make it difficult for the sound wave to form enough reflected energy at the rail bottom. If there is an inclined flaky defect, the detection results will show that the rail bottom wave will disappear.

(3) Serial-type reflection method is more suitable for vertical flaky defects on the rail surface. When two probes are placed on a detection surface, the two probes can move longitudinally synchronously. During this period, the probe distance can be appropriately adjusted to realize full-section scanning. Collected by the probe, the reflected echo of the rail is summarized, and the rail defect cannot be identified. Through the application of wavelet analysis method, the effect of simultaneous analysis of signal field of view and frequency domain can be achieved. Therefore, the method of wavelet analysis can be used to reconstruct the defect feature signal, and the spectrum analysis can be refined in combination with Hilbert demodulation, so that the rail defect information can be generated more comprehensively, and the defect location can be accurately judged according to the obtained information.

3. Flaw detection for welded rail base

The rail bottom can be divided into two main areas: two sides of the rail bottom and the connection part between the rail web and the rail bottom. According to the corresponding relationship between the rail bottom angle and the acoustic beam, the rail bottom angle can be further divided to form 6 detection areas. The probe is moved longitudinally to complete the detection work;

Scan the area 1-3 of rail bottom angle, take the weld center as the reference benchmark, the distance between the probe incidence point and it is 65 mm, and it can be found that the detection results at this time show the contour wave on the welding rib; When the distance between the two increases to 90 mm, the display changes, i.e. the lower profile wave is displayed. Scan the area 4-6 of the rail bottom angle, and when the spacing is 95mm, the contour wave on the welding rib is displayed. From the angle of two parameters of defect diameter and ultrasonic beam width, the analysis can be divided into two cases: when the former is less than the latter, the welding rib wave and defect wave will be displayed; When the former is bigger than the latter, only the defect wave will be displayed.