Analysis of quality defects of welded pipe welding section

Posted by BAOWI STEEL MANUFACTURING CO.,LTD on July 3rd, 2024

The weld is an important indicator of the quality and life of the weld. Therefore, the analysis and treatment of weld section quality defects should be carried out around the weld. The main defects of the weld include weld cracks, cracks, incomplete penetration, overburning and perforation, weld dislocation, etc.
Weld cracking: including natural cracking of the weld (including stress corrosion) and force (additional external force) cracking.

Natural cracks in welds Features: Welds

Natural cracks refer to cracks after the welded pipe leaves the extrusion roller. The weld does not crack due to additional external forces such as cooling water tanks, sizing machines, warehouses, etc. Such cracks are staggered like canine teeth and are gray. The more obvious feature of the luster of non-ferrous metals is that no external force is required and cracking occurs for no reason.

Causes: There are many reasons for the natural cracking of welds. The main reasons are insufficient extrusion pressure, too low or too high welding temperature, narrow and thin tube billets, too deep deburring, excessive lateral residual tensile stress caused by the weld face ︿ or reverse ﹀-shaped butt joints and high-strength tube rings, etc.

Measures to prevent weld cracking naturally include:

1. Strictly check the raw materials and remove the hard tube billet

2. According to different tube billets, choose a reliable welding process to avoid cold welding and overburning

3. Strengthen forming adjustment, reduce forming residual stress, and achieve parallel butt welding of welds

Weld stress cracking

Flattening weld cracking includes positive flattening and side flattening cracking. Flattening can be divided into D/3 flattening and full flattening. According to relevant Chinese standards, the weld will not crack after D/3 under positive pressure. Qualified, but from the actual use requirements of welded pipes, most of them require full flattening without cracking. The so-called full flattening means 100% pressing to the inner wall of the pipe, and the gap is 0. Full flattening can not only check the welding quality, but also check the plastic state of the tube billet.

Weld cracks

Weld cracks refer to the presence of tiny hairline cracks in the weld. Most of these cracks occur on the surface part of the weld. Some are obvious at a glance, while others need to be carefully identified and even magnified.

The reasons for the cracks are nothing more than the following:

Backflow and slag inclusion caused by high welding temperature, thinner and narrower tube blanks, and low extrusion pressure. Some oxides will not be squeezed out of the weld. Non-metallic inclusions are formed after cooling. Edge microcracks, etc., V-shaped butt joints of the weld, loose surface fusion structure, cold shrinkage stress tears the loose structure, chemical composition segregation at the edge of the tube blank, thick oxide layer, resulting in high melting point non-metallic inclusions, most of which are cracked welded pipes can pass the water pressure test, but it is difficult to pass the non-destructive testing and side flattening test. Generally, measures such as increasing the opening angle, increasing the extrusion pressure, increasing the welding temperature, and improving the edge butt joint state are adopted.

Weld misalignment

Weld misalignment refers to the weld formed by two pairs of welding surfaces that are not welded on the same plane. Weld misalignment can be divided into three types: inclined weld misalignment, occasional weld misalignment and periodic weld misalignment, but they have a common defect feature, that is, after the normal removal of the external burr, the external burr still remains on one side of the weld. There are many reasons for the strength of the weld and the misalignment of the weld, including:

1. The first and second flat rollers are asymmetrical when pressed on both sides

2. The extrusion roller, closed hole roller or guide roller is asymmetrical, runout, asymmetrical concentricity, etc.

3. The forming flat roller bearing, vertical roller bearing, guide roller bearing, extrusion roller bearing, etc. are damaged but not found

4. The thickness, width, tolerance of the tube billet are narrow, S-bend, sickle bend, etc.

5. The tube billet runs unsteadily and swings from one side to the other

6. There are invisible bulges in the formed tube

7. The extrusion roller and the guide roller deviate seriously from the rolling center line

The causes of welding misalignment are relatively complex. It may be caused by a single reason or the result of a combination of multiple reasons. The specific search should be based on the principle of starting with the easy and then moving on to the difficult, starting from the visible and tangible causes, eliminating them one by one and taking corresponding measures.

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