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How to carry out ultrasonic surface wave flaw detection for bearing rings

2021-07-13 08:10:51
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1. Technical analysis when ultrasonic surface wave inspection is applied to bearing inspection:


In the actual production of bearing parts, mass returns are often caused by quality problems. Although the proportion of crack defects is not too large, there is no ideal way to pick out the surface cracks of the bearing.

  

Commonly used magnetic particle inspection and penetrant inspection methods must be applied after the bearing is disassembled into parts.


Although the above two methods can detect crack defects, they have great disadvantages for finished bearings, and both have some insurmountable shortcomings.

  

For example, for the penetrant testing method, because the grease on the bearing surface penetrates into the cracks of the surface openings, the penetrating liquid cannot penetrate into the cracks, resulting in product missed inspections during the inspection process, and the efficiency of penetrating flaw detection is extremely low, and there are disadvantages that it is not easy to clean;


Although the magnetic particle inspection method has a high detection efficiency, but according to the scope of application of magnetic particle inspection, the bearing must be disassembled and then cleaned. In this process, it is easy to cause the bearing parts (such as cage, seal ring, steel ball) to bump and The phosphate film on the surface of the bearing ring is damaged, which affects the appearance quality and performance of the bearing.

  

Therefore, it is imperative to find and select a non-destructive testing method that analyzes and judges the surface defects of parts without disassembling the bearing. This article carries out related experimental studies based on the principle and application of surface ultrasonic flaw detection, which shows that surface ultrasonic waves It has broad application prospects in the detection of defects in finished bearings.

2. Analysis of the principle of ultrasonic flaw detection and the possibility of its application


2.1 Principle of Ultrasonic Surface Wave Inspection
  

Ultrasonic flaw detection mainly has three kinds of waveforms: longitudinal wave, transverse wave and surface wave. The shorter wavelength of surface wave has higher detection sensitivity.  

Ultrasonic surface waves mainly propagate along the surface of the medium. It can propagate directionally in a uniform medium with little energy attenuation. It can travel a long distance. The energy is mainly concentrated at a wavelength from the surface depth. If it encounters a small Defects, such as pores, cracks, etc., will be reflected on the interface between the metal and the air, and this reflected wave can be displayed on the screen to obtain the defect wave signal. This is a reliable method for detecting surface defects.

  
2.2 Factors affecting ultrasonic surface wave flaw detection
  

(1) The influence of oil quality, there should be no oil stains in front of the probe during surface ultrasonic flaw detection. If oil is applied to the surface where the reflected wave propagates, the propagated wave is almost completely attenuated, such as touching the surface of the workpiece with a finger.


This is because the wave propagation effect is completely different when one side of the test piece is air and when there is liquid, and it can be understood that the vertical component of the ultrasonic surface wave causes attenuation in the liquid layer.

  

Therefore, it is possible to judge whether the surface is a reflection of cracks or edges by pressing the wave propagation surface by hand.

  

(2) Edge reflection. For example, when the ultrasonic surface wave encounters the edge of the workpiece during propagation, a reflected wave will be generated, and some energy will continue to propagate beyond the edge.


When the reflected edge of the ultrasonic surface wave is at a right angle, the reflectivity is high, and the wave reflected by the edge can be used as a reference point to detect defects in the circumferential direction.

  

(3) Edge reflection with curvature. If the edge is chamfered, then a reflected wave will be generated at the edge.


For example, the radius of the curved surface of the workpiece is r and as the radius of curvature increases, the reflected echo decreases, that is, more energy can pass through the arc surface.


The ultrasonic surface wave detection of finished bearings is based on this propagation principle, and the detection of longitudinal defects can be done with the edge reflection as the reference end point.

  
2.3 Ultrasonic surface wave probe selection
  

Requirements for the ultrasonic surface wave probe: The ultrasonic surface wave probe uses a wedge similar to the oblique probe. When the shear wave refraction angle is 90°, the ultrasonic surface wave is generated.

Because the propagation speed of ultrasonic surface waves is relatively small and the wavelength is relatively short, the sensitivity of flaw detection is relatively high.

  
2.4 Scan range adjustment
  

The scanning speed adjustment method in surface ultrasonic flaw detection is different from that of ordinary oblique probes. The incident point is calculated according to the front edge of the probe (that is, the tip of the probe).


When adjusting the scan ratio, aim the probe at the edge of the test block, and adjust the echo signal to a certain ratio according to the horizontal distance. Generally, move the probe back and forth to change the L value.  


For example, when L1=20mm and L2=40mm, use the depth knob and the horizontal knob to adjust the horizontal scale to positions 20 and 40 respectively to complete the 1:1 adjustment.


3. Application of Ultrasonic Surface Wave Inspection in Bearing Inspection


By mastering and analyzing the principles and influencing factors of ultrasonic surface waves, relevant confirmatory experiments are carried out.

  

The experimental results show that the use of ultrasonic surface wave flaw detection will not affect the use of the bearing when the finished bearing is not disassembled. It will reduce the difficulties in the quality inspection and quality review of the finished bearing. It is the production and use of the key part of the bearing. , Maintenance and maintenance provide safe and reliable practical testing methods.

  
The specification of the surface wave probe used in the experiment is:
  

Frequency 2.5MHz, 5MHz;

Chip size: 6×8 mm, 8×8 mm, 10×10mm; the front edge of the probe: ≤3mm;

Sensitivity margin: ≥40dB (the leading edge is 20mm from the edge).

  
The following figure 1 and figure 2 are the waveforms of the ultrasonic surface wave detection of the finished bearing and the ferrule after disassembly. From the waveform diagrams of Figures 1 and 2, it can be seen that the defect waveform of the bearing ring flaw detection of the finished bearing and the bearing ring after disassembly is Consistent.

Figure 3 is a photograph of the finished bearing confirmed to have defects during the ultrasonic surface wave inspection, and the outer diameter surface defects.


The existence of defects is also confirmed by the use of magnetic particle inspection methods. The defects in the finished bearing of the ultrasonic surface wave inspection in Figure 3 are subjected to magnetic particle inspection after the sleeve is removed. From the analysis of the magnetic particle inspection results, the magnetic marks are located at the edge of the outer diameter surface of the bearing outer ring. The trace is very small, as shown in Figure 4, where the magnetic trace is clearly visible.

  

Wipe off the magnetic marks and observe them carefully under a magnifying glass. The micro cracks in the original magnetic marks can be seen, and it is difficult to distinguish and confirm them by visual observation without external detection means.


Wipe the cracks on the outer diameter of the bearing outer ring with cold acid. It can be seen that there are gray-black high-temperature tempering burns on the outer diameter surface of the cracks, and the distribution of secondary quenching burns in a bit of white and bright color, as shown in Figure 5. After analysis, it is considered that the cracks are burned. It should be produced when grinding the outer diameter surface of the part.


In addition, by sampling the cracks and observing under a microscope after being corroded by cold acid, the cracks are perpendicular to the surface, and no material inclusions, oxide scales and other foreign matters are found in the cracks. Under the metallographic microscope, there are obvious burns on the outer diameter surface where the cracks are located. Phenomenon, as shown in Figure 6.


From this analysis, it is judged that the cracks on the outer diameter surface of the test part ferrule are due to the grinding cracks produced during the grinding of the outer diameter surface.


in conclusion:


Through experiments, the non-destructive testing technology of ultrasonic surface wave flaw detection can be successfully applied to the surface and near-surface defect detection of finished bearings, and it is applied to the disposal of product quality under special circumstances. Because of its successful application, it avoids the disassembly of the bearing. The destruction of product parts reduces the cost of disassembling the bearing, thereby ensuring the product quality of the finished bearing.

  

The successful application of this method provides a very practical and effective detection method for bearing manufacturers and the bearing industry.


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