In terms of the welding process, the welding methods of spiral welded pipes and straight seam steel pipes are the same, but straight seam welded pipes will inevitably have many T-shaped welds, so the probability of welding defects is also greatly increased, and the welding residues at the T-shaped welds are The stress is large, and the weld metal is often in a three-dimensional stress state, increasing the possibility of cracks.
Moreover, according to the process regulations of submerged arc welding, each weld should have an arc starting point and an arc extinguishing point. However, each straight seam welded pipe cannot meet this condition when welding the circumferential seam, so there may be an arc extinguishing point. More welding defects.
When steel pipes bear internal pressure, they usually produce two main stresses on the pipe wall, namely radial stress δ and axial stress δ. The resultant stress δ at the weld is, where α is the helix angle of the spiral welded pipe weld.
The helix angle of the spiral welded pipe weld is generally degrees, so the resultant stress at the spiral weld is the main stress of the straight seam welded pipe. Under the same working pressure, the wall thickness of spiral welded pipes of the same diameter can be reduced compared with straight seam welded pipes.
Diameter expansion technology of straight seam welded pipe:
1. In the preliminary rounding stage, the sector blocks are opened until all the sector blocks are in contact with the inner wall of the steel pipe. At this time, the radius of each point in the inner circular tube of the steel pipe within the step range is almost the same, and the steel pipe is initially rounded.
2. In the nominal inner diameter stage, the sector block starts to reduce its movement speed from the front position until it reaches the required position, which is the required inner circumferential position of the finished pipe.
3. In the rebound compensation stage, the sector block starts to further reduce the speed from the position in stage 2 until it reaches the required position. This position is the inner circumferential position of the steel pipe before rebound is required by the process design.
4. In the pressure-maintaining and stable stage, the sector-shaped block remains stationary in the inner circumferential position of the steel pipe for some time before springing back. This is the pressure-maintaining and stable stage required by the equipment and diameter expansion process.
5. In the unloading return stage, the sector block begins to rapidly retract from the inner circumferential position of the steel pipe before rebounding until it reaches the initial expansion position. This is the minimum contraction diameter of the sector block required by the expansion process.
Classification of straight seam steel pipes:
1. Straight seam high-frequency welded pipe: Straight seam high-frequency welded pipe is a welded pipe that uses steel strip (coiled plate) as raw material adopts a high-frequency welding process and is continuously produced on the production line. The material strength is generally below 450MPa, and the materials include J55, L450, X60, Q235, Q345, Q420, and Q460. The diameter range of straight seam welded pipes is 14-610mm, and the wall thickness is 1-23.8mm. The straight seam high-frequency welded pipe adopts a multi-rack continuous forming process with high production efficiency (production speed 15-40m/min). The production line has complete sizing, straightening, rounding, and other equipment. The steel pipe has excellent roundness, and straightness, and the Welding is better.
2. Longitudinal submerged arc welded pipe: Longitudinal submerged arc welded pipe is produced using a single steel plate as raw material, through JCO or UO forming, submerged arc welding, or a combination of submerged arc welding and other welding processes. Common ones include X70, X80, X120, etc. The diameter range of straight seam submerged arc welded pipes is 406-1422mm, and the wall thickness is 8-44.5mm.
In terms of edge welding processing, milling machine processing is used; in terms of forming, in addition to conventional JCO and UO technologies, some manufacturers use advanced progressive forming (PFP) technology and roll bending forming (RBE) technology; in terms of welding, using argon gas protection or CO2 gas protection automatic pre-welding machine and special multi-wire (4 wire and 5 wire) internal and external submerged arc welding equipment, and using square wave power supply and power wave power supply device; in terms of diameter expansion, The entire pipe length is mechanically expanded; in terms of inspection, online flaw detection is required on the plate, automatic radiographic automatic wave flaw detection and hydraulic pressure testing are performed on the steel pipe after welding and secondary online or offline radiographic wave flaw detection are performed after diameter expansion.
Sandblasting and rust removal of straight seam steel pipes use a high-power motor to drive the spray blades to rotate at high speed so that abrasives such as steel shot, steel sand, iron wire segments, and minerals are sprayed on the surface of the straight seam steel pipe under the powerful centrifugal force of the motor. Not only can Oxide, rust, and dirt, but the straight seam steel pipe can also achieve the required uniform roughness under the action of violent impact and friction of abrasives.
Sandblasting and rust removal of straight seam steel pipes use a high-power motor to drive the spray blades to rotate at high speed so that abrasives such as steel shot, steel sand, iron wire segments, and minerals are sprayed on the surface of the straight seam steel pipe under the powerful centrifugal force of the motor. Not only can Oxide, rust, and dirt, but the straight seam steel pipe can also achieve the required uniform roughness under the action of violent impact and friction of abrasives.
After spray rust removal, it can not only expand the physical adsorption on the steel pipe surface but also enhance the mechanical adhesion between the anti-corrosion layer and the steel pipe surface. Therefore, spray rust removal is an ideal rust removal method for pipeline anti-corrosion. Generally speaking, shot blasting is mainly used for the inner surface treatment of steel pipes, and shot blasting is mainly used for the outer surface treatment of straight seam steel pipes.
Post time: Oct-19-2023