How to Avoid Quality Defects of Welded Pipe in Sizing Section

The difficulty of adjusting the size of the welded pipe simply exceeding the upper or lower difference is far less difficult than the size fluctuation. The large fluctuation of the outer dimension of the welded pipe means that the dimension changes outside the longitudinal measurement direction of the welded pipe. It is close to or exceeds the tolerance range of the welded pipe, commonly known as "segment difference". , there are two phenomena: periodicity and randomness.

Surface defects
The surface defects of the sizing section of the welded pipe mainly include pits, bumps, indentations, scratches, oxide layer indentation, etc. The causes and solutions are similar to those of forming. For general pipes, these defects are allowed to be removed, but the removal depth must not be If the negative deviation is lower than the nominal wall thickness, otherwise it should be judged as scrap. As for the surface quality requirements of furniture pipes, automobile pipes, API pipes, etc., they should meet the requirements stipulated in the agreement between the two parties.
Here we will talk about the common cornering defects in (square and rectangular) special-shaped pipes, which are divided into single-sided cornering and double-sided cornering. If the feel is rough, it indicates that there is a corner defect. In most cases, only one or two corners are crushed.
The main reasons for the formation of roll corners are: roll misalignment, crushing the tube surface; too large roll gap, and the pass line is not long enough; excessive shaping allowance, the tube angle is squeezed by the roll gap angle; single-pass rolling pressure is too large, the tube The surface of the billet is squeezed by the edge of the pass; the roll deviates from the rolling center line, resulting in excessive rolling force at a certain corner, and the corner is crushed. For the corner defect, it is necessary to conduct a reverse investigation based on the mapping principle, and then take corresponding corrective measures .
straightness defect
The straightness defect here refers to the straightness of online straightening, including bending and twisting. During the production process of welded pipes, the straightness often changes. The essence of straightness is various stress balance problems in the pipe body, and the mutation factors affecting stress balance It is mainly the change of material, including width, thickness, hardness, etc., which is only explained by the change of hardness. After the stress of the straightening system is balanced, it shows that the existing hardness is confirmed by the system, and the welded pipe produced by the system is "naturally" straight, but once the hardness occurs Change, the system does not recognize the new hardness, the stress balance is disturbed, the welded pipe becomes bent, and if there is no manual intervention, the pipe will always bend until it is readjusted and a new stress balance system is established, which reminds the operator from one side that if the welded pipe is If the straightness changes suddenly in the normal production process, it is necessary to think of the situation in the production process. Similarly, when the equipment precision, cooling system, ambient temperature, operation adjustment, etc. The existing balance caused the welded pipe to bend. As far as the ambient temperature changes, I believe most adjustment workers have this experience: the unit that produced the previous day stopped overnight, and then the original team restarted and found that the produced welded pipe was bent. Well, this is actually an invisible factor of temperature, which involves a heat balance problem. The heat balance of the welded pipe unit means that after a period of operation, the temperature of equipment parts, bearings, hole patterns, coolant, etc., all rise to a certain height. This kind of stress reached a tacit agreement with the welded pipe at that time, and it was in a balanced state. After stopping overnight, the temperature of the machine parts, bearings, rolls and coolant were all in a cold state, and it was difficult to achieve stress balance with the hot welded pipe immediately, so the welded pipe was bent.
For welded pipe defects and the causes of defects, some are obvious, some are hidden, and some are manifested in sizing, but the root is in the forming or even the feeding part. Therefore, for the treatment of some welded pipe defects, it is necessary to have an overall concept. Use the concept of the system to guide the adjustment of the welded pipe and find out the cause of the defect.