Welding is very popular in cases where it is necessary to connect together the metal elements of some design, and the resulting connection should be one-piece. There are a large number of varieties of this technological process, but all are conditionally divided into two large categories. In the first case, the melting of the base metal during the formation of the seam will occur, in the second - no. But no matter what method is chosen, quality control of welded joints must be performed. This will be a guarantee of the quality and durability of the finished product.
Quality control of welded joints is always carried out in full accordance with regulatory enactments, which, depending on the design purpose, include GOST or SNIP, which regulate the procedure and requirements for the sequence of this type of work. As a rule, it consists of two stages:
- Prevention of the formation of possible defects. At this stage, the base metal, all types of welding materials, the observance of the technology for preparing parts for welding work are controlled. Most often, the visual control method is used here. The equipment and qualifications of the welder are also checked. Especially in those cases when the manufacture of critical structures is carried out.
- Identification of defects. Here, the non-destructive testing method is most often used, which allows you to identify all kinds of flaws in the weld without violating its integrity.
We will consider the last stage in more detail. Obviously, the quality control of welds is carried out in order to identify external, internal and through defects. To begin with, an external examination reveals:
- irregularities formed in both planes;
- lack of penetration, in which there is no connection between the parts of the base metal, the base and the layer (or layers);
- incompleteness;
- slag inclusions;
- visible cracks;
- undercuts;
- large pores.
Even if the weld fully complies with the requirements imposed on it, that is, it smoothly passes into the base metal, has a finely scaly or smooth surface without characteristic defects, then the quality control of the welded joints does not end with an external examination of the finished product. The main danger can be hidden inside the seam. It is impossible to visually see such defects, but it is they that can become the main cause of unexpected damage to the welded structure.
Internal defects of the joints can be detected, for example, using ultrasonic or color flaw detection. In the first case, a special device is used, the principle of which is based on the emission of ultrasonic waves, which, reaching a crack or pore, are reflected and create a specific image on the screen of the device. In the second case, a substance is applied, which, when the seam is of poor quality, penetrates to the opposite side of the product and appears with the help of the second substance. In this way, cracks of even a very small size can be detected.
Also, quality control of welded joints can be carried out by checking the quality of test samples made of the same materials as the finished products. In this case, in laboratory conditions, the mechanical properties of the weld metal are tested, which must satisfy certain criteria. In particular, in addition to the tensile strength, hardness and elongation, the bending angle and toughness are determined . Also, thin sections are prepared that make it possible to accurately determine the quality of root penetration and identify internal weld defects.