The Importance of Visual Testing (VT) in Steel Structure Welding Quality Inspection
The Importance of Visual Testing (VT) in Steel Structure Welding Quality Inspection – With the unique advantages that traditional civil structures do not have, steel structures have developed rapidly, involving more and more different types of buildings, such as bridges, high-rise buildings, stadium, etc. . Welding is an important technology of steel structure component connection. The quality of welding will directly affect the strength and safety of steel structure.
The welding quality of steel structures can be tested by different non-destructive testing methods, such as ultrasonic testing and radiographic testing to detect internal defects in welds, magnetic particle inspection, penetration inspection and visual testing are used to detect weld surface and near-surface defects. The importance of visual testing in welding quality inspection of steel structures is described.
In the weld inspection, visual testing is often ignored by the inspector, probably because the final acceptance, there is no requirement to submit visual testing report. Some people even think that the surface has been examined before the ultrasound test, in fact, this is a wrong point of view. The surface inspection before ultrasonic testing is only concerned with whether there are obstacles on the surface that affect the movement of the ultrasonic probe, not some welding defects and size requirements.
The regulations of GB 50661-2011 and GB 50205-2001 on visual testing are: first, observe with visual or magnifying glass, then use magnetic particle inspection or penetration inspection for surface inspection when crack-like defects are suspected, to help identify surface defects. And almost all the domestic steel structure weld inspection are not required to do visual testing, and do not require visual testing report. In recent years, with the foreign business contacts, found in the visual testing on the understanding, the domestic and foreign counterparts of the importance of the degree or difference.
The importance of visual testing:
1. The right international approach
The correct international practice is to require ultrasonic testing or X-ray testing before the visual testing, only when all qualified, and then carry out ultrasonic testing or X-ray testing.
The welding personnel and the testing personnel all need to hold the certificate to go on duty, and the welding personnel is in the quality control of the non-destructive testing personnel from the beginning of the work, from the record of receiving electrode, the record of oven temperature inspection, the storage of electrode at welding site to the verification of welder’s qualification, the inspection of welding groove angle, the inspection of root clearance and the inspection of electrode specifications before welding, in the welding process to detect the flow of welding shielding gas, welding current size, inter-layer temperature test, after each layer of welding should be carefully checked whether the slag clean before the next process. After welding to check whether the weld is sanded and polished, whether there is welding scar, and finally cooling to room temperature and then carry out other non-destructive testing.
During the whole process of nuclear island construction and equipment installation, the welding quality control was completed under the visual supervision of the inspection personnel, the importance of visual testing for each link is self-evident.
2. The significance of visual testing of macroscopic defects
According to the research, if the metal surface does not form fatigue cracks, then theoretically, these materials are infinite service life.
Normal machining holes, grooves or sudden changes in the section, but also the aviation parts or important components are prone to failure of the parts, these parts are the use of surface detection methods for quality inspection of the key parts.
Some seemingly insignificant problems on the welding surface, such as welding dislocation, deformation, undercut, welding surplus is too high, it is closely related to quality. Welding dislocation will lead to the thinning of the actual thickness of the welding workpiece, resulting in the weakening of the welding strength; The existence of weld undercut is equivalent to the existence of a crack opening at the edge of the weld, which will lead to the cracking and breakage of the weld over time, this is why all welds in nuclear power plant pipelines must be ground and polished.
Welding quality control of steel structure is a systems engineering. Visual monitoring and process tracking before welding and non-destructive testing after welding are very important.
Application requirements of visual testing in steel structure inspection:
1. Pay attention to visual testing
A large number of cracks were produced in the welding parts of the steel structure of a department store. After analysis, it was found that the main reason was that the anti-rust zinc coating at the welding groove could not be cleaned, and then the welding was carried out directly, leading to zinc entering the welding seam, the mechanical properties of weld metal are reduced. Inspection organizations and personnel must pay attention to visual testing and visual testing record in process control, which provides traceable function for quality control system, and facilitates the late spot inspection and accountability.
2. Training professionals
In recent years, China has set up a visual testing personnel training forensics project. From the view of the requirement of visual testing, the requirement of knowledge is wide, and the requirement of knowledge of metal material, metal processing, metal characteristic and metal welding is high.
3. Enhance operational training
Although some testing organizations have purchased common instruments for visual testing, such as tool rulers, but for a large number of tools used in shape, profile, arc, concave, convex, inner hole, outer hole, gap detection, etc. are not too much attention, detection agencies should actively invest human and material resources, systematically improve their detection and analysis capabilities.
4. Be familiar with testing standards
Each non-destructive testing method must be based on the relevant standards. visual testing personnel should be familiar with the domestic and foreign standards on visual testing, and understand the differences in the standards, which has a good role in promoting the exchange and progress between domestic and foreign peers.
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