Visual and dimensional control is performed at the welding stage to assess the quality of a weld. The welder checks the joint for lacks of fusion, undercuts and leg accuracy. The method is widely used for diagnostics in large metal structures as well.
Fault search is performed on the external side of a weld joint. Special devices have been designed for finding even minor defects hidden from the naked eye. Thus, visual and dimensional control inspectors use viewing, telescopic and floor-mounted magnifiers, lenses, microscopes and endoscopes.
Moreover, an NDT inspector's toolkit includes devices designed for use in the shop or in the field. Devices of the first type are designed for full rest conditions, +5 to +20˚ С temperatures, normal atmospheric pressure and moderate humidity. Field devices can endure temperatures of −55 to +60˚ С, vibrations, shaking and precipitation.
Visual and dimensional control is applicable to very different objects. This includes high temperature and dangerous radiation conditions, chemically active media, etc. The NDT process after this method is strictly defined by regulatory documents. A defect is assigned a certain category based on its characteristics. The values of metrological indications and requirements for instruments should also comply with official standards.
Overall, visual control requires a minimum of instruments. The amount of object-related information collected is determined by the qualification of the NDT inspector. Nevertheless, the protocol prepared by the inspector is regarded as quite an objective report.
With regard to weld joints, visual and dimensional control is intended for checking a weld for corrosion and faults. Apart from it, it is important to evaluate their sizes and the distortion level, that is the percentage of deviation from the norm.
The sooner inspection is carried out and the weld destruction fact is identified, the lower the chance of fault emergence.
Compared to the other technologies, visual and dimensional weld control has rather many advantages:
· a simple and easy method;
· possibility to obtain a lot of information on the quality of a metal structure;
· low labor;
· inexpensive devices, low process costs;
· possibility of a repeated inspection and result verification.
Visual and dimensional control is a key stage of integrated diagnostics. For more credibility, we will mention a number of disadvantages of this method:
· Subjectivity. The results are highly dependent on the human factor. Due to this, the accuracy of the diagnostics is often questioned.
· Only large defects can be identified (0.1–0.2 mm and larger). Small faults can only be assumed.
· Only a visible part of a structure can be checked.
Nevertheless, visual and dimensional control is performed during the following process operations:
· cladding jobs: acceptance and handover stage;
· layer-by-layer evaluation of multi-layered weld joints;
· final inspection of areas where the welding arc contacted the surface of the base metal;
· assembly of welded structures;
· automatic part production and technical evaluation of materials used;
· reliability verification for expired welds.
Visual and dimensional control jobs are necessarily preceded by surface cleaning. Paint, scale, oil, metal drops and other contaminations that may obstruct diagnostics should be removed. The inspection area is not limited to the weld proper. It includes adjacent areas minimum 20 mm in all directions. The weld type, its concavity, convexity, height and width should be checked.