Steel Gate Inspection
Steel gates are widely used in water conservancy projects. They are mainly made of high-quality steel plates and welded. The surface is made of rubber water-stop and anti-corrosion methods. Water conservancy projects such as reservoirs, drainage and irrigation, river courses, environmental protection, sewage treatment, aquaculture, etc. The welding quality of the steel gate is directly related to the safety of people's life and property downstream of the gate, so the welding quality and welding detection method of the steel gate are very important.
Ultrasonic flaw detector, as one of the non-destructive testing methods, is to use ultrasonic instruments or equipment for testing on the basis of not destroying the processed surface. It can not only check the internal defects of the workpiece that cannot be checked by naked eyes, but also greatly improve the accuracy and reliability.
Ultrasound is a kind of mechanical wave with a very high frequency. The frequency ratio exceeds 20 kHz. Its energy is far greater than that of audible sound waves with the same amplitude, and it has a strong penetrating ability. The ultrasonic wave used for flaw detection has a frequency of 0.4-25 megahertz, and the most widely used is 1-5 megahertz. Because it can detect and locate various defects inside the workpiece quickly, conveniently, without damage, and accurately, and ultrasonic flaw detection has a large detection distance, small size and light weight of the flaw detection device, which is easy to carry to the site for flaw detection, fast detection speed, and flaw detection. It only consumes couplant and wear probe, and the total detection cost is low, so its application is more and more extensive.
There are mainly two methods of ultrasonic flaw detection: penetration method and reflection method. Penetration testing uses two probes, one for emitting ultrasonic waves and one for receiving ultrasonic waves. During detection, two probes are placed on both sides of the workpiece, and the internal quality of the workpiece is judged according to the energy change after the ultrasonic wave penetrates the workpiece.
The high-frequency pulse excitation signal generated by the high-frequency generator of the reflection method flaw detection acts on the probe, and the generated wave propagates to the inside of the workpiece. If there is a defect inside the workpiece, part of the wave is reflected back as a defect wave, and the rest of the emitted wave is used as a defect wave. The bottom wave will also reflect back. The position of the defect can be determined according to the position of the launch wave, defect wave and bottom wave relative to the scanning baseline; the size of the defect can be determined according to the amplitude of the defect wave; the nature of the defect can be analyzed according to the shape of the defect wave; if there is no defect inside the workpiece, only Launch wave and bottom wave.
In the process of flaw detection, it is first necessary to understand the technical requirements of the drawings for welding quality. The acceptance standard of steel structure is implemented according to GB50205-2001 "Code for Acceptance of Construction Quality of Steel Structure Engineering". The standard stipulates: for drawings that require the welding quality level of the weld seam to be first grade, when the assessment grade is grade II, the specification requires 100% ultrasonic flaw detection; for drawings that require the weld seam welding quality grade to be second grade, when the assessment grade is grade III, the specification stipulates It is required to do 20% ultrasonic flaw detection; for drawings that require the welding quality level of the weld seam to be level 3, no ultrasonic internal defect inspection is required.
It is worth noting here that ultrasonic flaw detection is used for full penetration welds, and its flaw detection ratio is calculated as a percentage of the length of each weld, and is not less than 200mm. For locally flawed welds
If an impermissible defect is found, the flaw detection length shall be increased at the extension of both ends of the defect. The increased length shall not be less than 10% of the weld length and shall not be less than 200mm. If there are still impermissible defects, the Welds are 100% inspected for flaw detection. In addition, you should also know the thickness of the base metal of the workpiece to be tested, the type of joint and the type of groove. Generally, the thickness of the base metal is between 8-16mm, and the groove types include I type, single V type, and X type. After figuring out the above things, the preparation work before the flaw detection can be carried out.
Before each flaw detection operation, standard test blocks (CSK-IA, CSK-ⅢA) must be used to calibrate the comprehensive performance of the instrument and calibrate the panel curve to ensure the accuracy of flaw detection results.
(1) Repairing of the detection surface: spatter, scale, pits and rust on the welding surface should be removed, and the finish is generally lower than ▽4. The trimming width of the flaw detection surface on both sides of the weld is generally greater than or equal to 2KT+50mm, (K: probe K value, T: workpiece thickness). Generally, a probe with a K value of 2.5 is selected according to the base material of the weldment. For example, if the base metal thickness of the workpiece to be tested is 10mm, then it should be ground 100mm on both sides of the weld.
(2) The choice of coupling agent should take into account viscosity, fluidity, adhesion, no corrosion to the surface of the workpiece, easy to clean, and economical. Based on the above factors, paste is selected as the coupling agent.
(3) Due to the thin thickness of the base metal, the detection direction is carried out on one side and both sides
(4) Since the plate thickness is less than 20mm, the horizontal positioning method is used to adjust the scanning speed of the instrument.
(5) Rough flaw detection and fine flaw detection are used in the flaw detection operation. In order to get a general understanding of the presence or absence of defects and their distribution, quantification, and positioning are precise flaw detection. Several scanning methods such as zigzag scanning, left and right scanning, front and rear scanning, corner scanning, and surrounding scanning are used to find various defects and determine the nature of defects.
(6) Record the detection results, and evaluate and analyze internal defects if found. The classification of internal defects of welded butt joints shall comply with the provisions of the current national standard GB/T 11345-2013 "Nondestructive Testing of Welds Ultrasonic Testing Technology, Testing Grades and Evaluation" to judge whether the welding is qualified. If any defect exceeding the standard is found, a rectification notice shall be issued to the workshop, ordering it to undergo re-inspection after rectification until it is qualified. Common defects in general welds are: pores, slag inclusions, incomplete penetration, incomplete fusion and cracks. So far, there is no mature method to accurately judge the nature of the defect. It is only based on the shape of the defect wave obtained on the fluorescent screen and the change in the height of the reflected wave combined with the position of the defect and the welding process to comprehensively evaluate the defect.