The steel structure of the main workshop of thermal power plant bears the static load of 10 thousand tons to tens of thousands tons, the dynamic load caused by the start and stop of boilers, and sometimes even affected by natural disasters such as earthquakes. Therefore, the welding quality of steel structure is of great significance to the safe and reliable operation of the unit. Ultrasonic flaw detection is an effective and reliable means to detect its internal defects, which is of great practical significance to accurately evaluate the quality, reliability and even service life of steel structures.
Ultrasonic wave of 1 T-weld in steel structure
1. 1: T-shaped weld structure introduction: The T-shaped weld of the steel structure of the main workshop is welded by wing plate and web plate, and the weld can be divided into incomplete and penetration type. Incomplete welds are divided into non-groove and V-groove, and T-joint paper with penetration welds divided into K-groove is written on the steel structure of the main workshop on site.
1.2 Selection of basic detection conditions
1.2. 1 probe selection:
① Selection of the refraction angle of the probe: In order to ensure that the main sound velocity of the probe can scan the weld section of the steel structure of the main building, and the center of the main sound velocity is perpendicular to the dangerous defects, and there is enough flaw detection sensitivity. The refraction angle of the probe used for defect detection on the web is selected according to the thickness of the web. (See table 1)
② Selection of probe frequency: According to the small thickness of weld web of steel structure in the main building, a higher frequency, generally 2.5MHz, should be adopted.
③ Selection of probe wafer size: In order to ensure the efficiency of probe detection, the wafer size is generally 13× 13.
1.2.2 Selection of coupling agent: In the flaw detection of steel structure welds in the main workshop, the coupling agent with appropriate fluidity, sound transmission and viscosity, good adhesion, easy cleaning after detection and harmless to human body is selected, and chemical paste is generally selected.
1.2.3 Selection of detection surface: According to the actual situation on site, choose oblique scanning on the web, and the web must be qualified after grinding.
1.2.4 selection of instrument and test block: use PXUT—280B full-digital intelligent ultrasonic flaw detector, use CSK- 1A test block to check probe front, sound velocity and refraction angle, and use Rb-3 test block to make DAC curve.
1.2.5 grinding of inspection surface: use hand grinder to remove spatter, paint and scale on the surface of weld web. The grinding width is determined by the web thickness, and the grinding width is: P≥2TK+50.
1.2.6 sensitivity compensation: in on-site flaw detection, when the test block is used to adjust the sensitivity to detect the weld, in order to ensure that the specified size defects are found in the weld, the acoustic energy transmission loss difference between the test block and the weld is appropriately compensated, and the sensitivity compensation in actual detection is +2dB.
2 DAC curve making
2. 1 sound velocity measurement: select the sound wave mode as shear wave, the primary sound path input of the test block is 50mm, and the secondary sound path input is100 mm. After confirmation, move the probe in the CSK- 1A test block to make the highest echo of R50 appear at the wave entrance, and when it drops to 60%, stabilize the probe and. Then reduce the echo of R 100 to 60% and stabilize the probe. Press OK again, measure the horizontal distance from the front end of the probe to R50, L=4 1mm, and input the instrument. The instrument automatically calculates the sound speed: 324 1m/s, and the front end of the probe:10 mm.
2.2 Measurement of K value (measurement of refraction angle): input mirror depth: 40mm, mirror diameter: 3mm, probe K value: 2.5. After confirmation, move the probe in RB-3 test block so that the highest echo with a depth of 40mm appears at the entrance door. The instrument automatically calculates K value (measuring refraction angle): K = 2.57 (β = 68.7).
2.3 The maximum detection depth for making DAC curve input is 60mm, the diameter of the mirror is 3mm, and the length of the mirror is 40mm. After confirmation, move the probe in RB-3 test block, adjust the gain, and ensure that the highest echo of the hole with the depth of 10mm is 80%. Then move the probe to find the highest echo of the hole with the depth of 20mm, 30mm, 40mm and 60mm in turn. Input waste judgment offset -4dB, quantitative offset-10dB and length measurement offset-16dB in turn, and press OK to make DAC curve.
3 Field actual detection and analysis
Bring the debugged instrument to the site, open the channel selected by the instrument, adjust the gain and DAC gate, and the probe 1 will be tested in the front and back, left and right, left and right, corner and other ways on the A surface of the web. At the end of the weld to 400mm, a defect wave will be found in the first wave, which is above the scrap line. Measure the length with 6dB length measurement method and record the defect wave data, as follows:
3. 1 Horizontal distance from the leading edge of the probe to the wing plate defect: L 1=33mm.
3.2 Depth from defect to web surface A: H 1= 13mm.
3.3 Defect length: F=22mm
3.4 defect equivalent φ 3× 40+6DB
The probe 2 detects front and back, left and right, surroundings, corners, etc. On the b side of the net. At the end of the weld to 400mm, a defect wave was found by secondary wave, which was above the rejection line.
4 defect determination
According to the requirements of DL/T542-94 "Ultrasonic flaw detection method and quality classification of fillet weld of steel fusion welded T-joint", the defect wave is located in Zone III, with the indicated length of 22mm >: 15mm, and the weld grade is Grade IV, which is an out-of-standard defect, so it should be repaired. The repair results show that the defect is a dangerous defect without fusion.
5 concluding remarks
The above method is simple and easy to operate, suitable for sudden inspection on the construction site, and can effectively find dangerous defects in T-joints of steel structures, which is of great significance to the safe operation of power plants.
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