Quality Defect Analysis of Welding Section of Welded Pipe

Weld is an important sign of welded pipe quality and the life of welded pipe. Therefore, the analysis and treatment of welding quality defects should be carried out around the weld. The main defects of weld include weld crack, crack, incomplete penetration, overburnt and perforation, weld dislocation and so on.

Weld cracking: including weld natural cracking (including stress corrosion) and stress cracking (additional external force).

Natural cracking of weld

Features: weld natural cracking refers to the cracking of welded pipe after leaving the constraint of extrusion roller, and the cracking of weld in cooling water tank, sizing machine, warehouse and other places without additional external force. This kind of crack looks like dog teeth crisscross, showing gray and dark metallic luster. The more significant feature is that it does not need external force and cracks for no reason.

Causes: there are many reasons for natural cracking of weld, mainly due to insufficient extrusion pressure, too low or too high welding temperature, thin pipe blank, too deep burr removal, butt welding surface or reverse butt joint and high strength pipe ring, etc.

The measures to prevent the weld from natural cracking include:

1.Strictly check the raw materials and eliminate the excellent tube blank

2.According to the different tube blank, choose a safe welding process to avoid cold welding and over burning

3.strengthen the adjustment of forming, reduce the residual stress of forming, and realize parallel butt joint of weld joint

Stress cracking of weld

The cracking of flattening weld includes normal flattening and side flattening. Flattening can be divided into D / 3 flattening and full flattening It is considered qualified that the rear weld does not crack, but from the actual use demand of the welded pipe, most of them require full flattening without cracking. The so-called full flattening is that 100% of the welded pipe is pressed down 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 pipe blank.

Weld crack

Weld crack refers to the existence of small hairy cracks in the weld. Most of these cracks occur in the superficial part of the weld. Some of them can be seen at a glance, while others need to be carefully identified or even magnified.

The causes of cracks are as follows

The slag inclusion caused by high welding temperature leads to thin and narrow pipe blank and low extrusion pressure. Some oxides are not extruded from the weld, and non-metallic inclusions are formed after cooling. There are no through defects and micro cracks on the edge of the formed tube blank. The weld V-shaped butt joint, the shallow surface fusion fabric is loose, the cold shrinkage stress will crack the loose tissue, the chemical composition of the edge of the tube blank is segregation and oxidation The layer is thick, and the non-metallic inclusions with high melting point are produced. Most welded pipes with cracks can pass the hydraulic test, but it is difficult to pass the nondestructive test and the side pressure flattening test. Generally, the opening angle can be increased appropriately, the extrusion pressure can be increased, the welding temperature is increased, and the edge butt joint state can be improved.

Weld dislocation

Weld dislocation refers to the weld formed when two pairs of welding surfaces are not welded on the same plane. Weld dislocation can be divided into three types: tendentious weld dislocation, accidental weld dislocation and periodic weld dislocation. However, they have a common defect feature, that is, after normal deburring, one side of the weld still remains external burr, and weld dislocation not only causes weld dislocation The unsmooth surface not only affects the surface quality, but also reduces the welding area and weld strength

1.Asymmetric reduction of both sides of the first and second flat rolls

2.The extrusion roll, closed pass roll or guide roll are asymmetric, pulsating and not concentric

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

4.The thickness and Width Tolerances of tube blank are large, such as S-bend and sickle bend

5.The tube billet is unstable and swings from left to right

6.There is invisible bulge in the tube blank

7.Extrusion roll and guide roll deviate from rolling center line seriously

The causes of weld dislocation are complex, which may be caused by a single reason or a combination of multiple reasons. The specific search should be based on the principle of easy first and difficult later, starting from the visible and palpable reasons, and eliminate them one by one and take corresponding treatment measures.