Ultrasonic flaw detection probe sensor for generator rotor retaining ring

- Jun 11, 2019-

Ultrasonic flaw detection sensor

Dedicated test blocks Since the workpiece to be inspected is made of special materials, a dedicated reference test block that reflects the actual condition of the workpiece should be produced. The special test block is made of the same material and the same material as the retaining ring. The geometric shape is the arc shape of the actual workpiece, the thickness is 35mm, the arc height is 85mm (about half of the thickness of the workpiece), and the inner diameter is <1240mm.

Selection of probe parameters The higher the probe frequency f, the higher the test sensitivity and defect resolution, from which it is advantageous for flaw detection. However, as the frequency increases, the attenuation of the ultrasonic waves also increases, especially the scattering attenuation is particularly serious. When the grain size of the material is smaller than the wavelength λ (the actual flaw detection frequency is generally several megahertz, which satisfies this condition), the scattering attenuation coefficient is proportional to λ3f4<1>.

The workpiece to be inspected is austenitic stainless steel, and the influence of its grain size on scattering should be considered. If the frequency is too high, the ultrasonic wave may cause severe attenuation, reduce the signal-to-noise ratio, and have an impact on the inspection. A plurality of straight probes of different frequencies were used to measure the attenuation coefficient by using multiple reflection methods on the dedicated test block and the IIW test block, respectively, and the results are shown. The attenuation coefficient of the special test block is higher than that of the IIW test block, reflecting the inherent characteristics of coarse grain of austenitic stainless steel. However, when using a low-frequency probe, the attenuation coefficient of stainless steel is less than that of carbon steel, so the lower frequency should be used for flaw detection.

In addition, 2 and 4MHz 22mm × 20mm 45° probes are used for comparison. The difference of the No. 3 through-hole echo on the special test block is >20dB. The shear wave selection of 1~2MHz frequency probe has good effect. Longitudinal wave detection has a shorter sound path and a relatively small attenuation, so a relatively high frequency can be selected, but ≯3MHz is preferably <2>. Due to the test error, the test data cannot verify that the attenuation coefficient is proportional to f4, but it can be seen that the attenuation coefficient increases with f.

The wafer size is large in size, small in semi-diffusion angle, and concentrated in energy, which facilitates detection of long-distance defects. The guard ring inspection should pay special attention to the AB section defects on the inner surface of the workpiece. Since the workpiece is thick, it is possible to consider a probe of a larger size wafer; in addition, because of the large size of the workpiece, the use of a large wafer probe also contributes to improvement of work efficiency. However, when the size of the wafer is large, the length of the near-field region increases, which is unfavorable for flaw detection.

The length of the near-field region N is proportional to the wafer area Fs and the probe frequency f, so the selection of the wafer size should also consider the matching with the frequency to shorten the length of the near-field region, so that the inspection focus region, that is, the AB segment of the workpiece inner surface including the groove portion is separated from the vicinity. Field area. In fact, due to the large thickness of the workpiece itself, and the choice of lower frequencies, it is easier to meet this requirement. Outside the workpiece, the vertical distance from the Harbin generator rental to the groove is about 100mm.

Taking the <25mm3MHz straight probe as an example, the longitudinal sound velocity of general austenitic stainless steel is 5660m/s, and N=82mm<100mm is calculated according to the formula N=Fs/πλ. Fs is the wafer area. For shear wave detection, take the K1 probe as an example. When detecting the groove, the single sound path is >140mm. For example, Fs=500mm2, frequency 2MHz, take the general austenitic stainless steel transverse wave sound velocity 3120m/s, regardless of the near field N in the probe. The distribution, according to the formula N = Fsπλcoscoscosα, α --- longitudinal wave incident angle β --- transverse wave refraction angle can be calculated as N = 89mm <140mm.

The oblique probe K value shortens the sound path to reduce the effect of attenuation, and a probe with a smaller K value should be used. K1~1 is generally used in actual flaw detection. Transverse wave test (1) Time-base scanning is performed with depth adjustment of three <1.5 mm through holes in the test block. (2) Test sensitivity: The reflection sensitivity of the No. 1 through-hole reflection wave on the special test block is 80% of the screen height, and the compensation is 6dB when scanning.

The longitudinal wave test uses the <1.5mm flat-bottom hole reflection wave on the special test block to reach 80% of the screen height as the test sensitivity, and the compensation is 6dB during the scan. The test preparation is to prevent the couplant from entering the rotor and causing pollution. Before the test, the rotor near the retaining ring should be sealed. Area. The grommet is cleaned or smoothed with sandpaper without damaging the paint. In order to prevent damage to the electrical equipment insulation, oil-based coupling agents must be used.

Conclusion For the ultrasonic flaw detection of generator rotor retaining ring, special attention should be paid to it. 1 Due to the weak structure and the risk of stress corrosion, the AB segment should be paid attention to during the inspection. 2 Since the workpiece is austenitic stainless steel, consider selecting a low frequency probe to reduce the adverse effects of scattering attenuation. 3 In order to effectively detect the inner surface defects of the workpiece, a larger size wafer probe may be considered, but at the same time, an appropriate matching with the frequency should be considered to avoid the negative influence of the long field in the near field.

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