Quality control of welded joints

Welding is very popular in thosecases, when it is necessary to connect together the metallic elements of any design, and the resulting connection must be one-piece. There are a lot of varieties of this technological process, but all are conventionally 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 whatever method is chosen, quality control of welded joints must be carried out necessarily. This will guarantee quality and durability of the finished product

Quality control of welded joints alwaysis performed in full compliance with regulatory enactments, to which, depending on the design purpose, GOST or SNIP are used, which regulate the order and requirements for the sequence of performance of this type of work. As a rule, it consists of two stages:

• Prevention of the formation of possible defects.At this stage, the main metal, all types of welding materials, compliance with the technology of preparation of parts for conducting welding operations are controlled. Most often, the method of visual control is applied here. The equipment and qualification of the welder are also checked. Especially in cases where the manufacture of critical structures is carried out.
• Defect detection. Here, most often, a nondestructive testing method is used, which makes it possible to identify all possible shortcomings in the weld seam without violating its integrity.

The last stage will be considered in more detail.Obviously, quality control of welded joints is performed in order to detect external, internal and through defects. To begin with, by external examination, the following are identified:

• Unevenness 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 is fully matchedrequirements to it, that is, smoothly passes into the base metal, has a fine-scaly or smooth surface without characteristic defects, the quality control of welded joints by external inspection of the finished product does not end. The main danger can be hidden inside the seam. Such defects can not be visually seen, but they can be the main cause of unexpected damage to the welded structure.

Internal defects of joints can be identified, 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 the crack or pores, are reflected and create a certain image on the screen of the device. In the second case, a substance is applied which penetrates the opposite side of the product when the seam is not perfect, and is manifested with the help of a second substance. In this way, it is possible to detect cracks even of a very small size.

Also the quality control of welded joints canare carried out by checking the quality of test samples made from the same materials as the finished products. In this case, in the laboratory conditions, the mechanical properties of the welded weld metal are tested, which must satisfy certain criteria. In particular, in addition to the ultimate strength, hardness and elongation, the bending angle and the toughness are determined. Also, thin sections are prepared, which allow to determine the quality of root penetration sufficiently accurately and to reveal internal defects of the joint.