From a welding process perspective, spiral welded pipes and straight seam steel pipes use the same welding methods. However, straight seam welded pipes inevitably have many T-shaped welds, significantly increasing the probability of welding defects. Furthermore, the residual stress at these T-shaped welds is high, and the weld metal is often under triaxial stress, increasing the likelihood of cracking.

Moreover, according to submerged arc welding specifications, each weld should have an arc initiation point and an arc extinguishing point. However, this condition cannot be met when welding the circumferential seam of a straight seam welded pipe, potentially leading to more welding defects at the arc extinguishing point.

When straight seam steel pipes are subjected to internal pressure, two main stresses are typically generated on the pipe wall: radial stress δ and axial stress δ. The combined stress at the weld is δ, where α is the helix angle of the spiral welded pipe.

The helix angle of the spiral welded pipe is generally 10 degrees; therefore, the combined stress at the spiral weld is a significant portion of the principal stresses of the straight seam steel pipe. Under the same working pressure, the wall thickness of spiral welded pipes of the same diameter can be reduced compared to straight seam welded pipes.

First, what are the diameter expansion techniques for straight seam welded pipes?
1. Initial rounding stage: The sector blocks open until all sector blocks contact the inner wall of the straight seam welded pipe. At this point, the radii of all points within the inner circle of the straight seam welded pipe within the step length range are almost uniform, and the straight seam welded pipe achieves initial rounding.
2. Nominal inner diameter stage: The sector blocks reduce their speed from the initial position until they reach the required position, which is the required inner circumference position of the finished straight seam welded pipe.
3. Springback compensation stage: The sector blocks further reduce their speed from the position in stage 2 until they reach the required position, which is the inner circumference position of the pipe before springback as required by the process design.
4. Pressure holding and stabilization stage: The sector blocks remain stationary at the inner circumference position of the pipe before springback for a period of time. This is the pressure holding and stabilization stage required by the equipment and diameter expansion process.
5. During the unloading and return phase, the sector-shaped blocks rapidly retract from their initial position on the inner circumference of the steel pipe before springback, until they reach the initial expanded diameter position. This is the minimum shrinkage diameter of the sector-shaped blocks required by the expansion process.

Second, what are the classifications of straight seam steel pipes?
1. Straight seam high-frequency welded pipe: Straight seam high-frequency welded pipe is a welded pipe continuously produced on a production line using steel strip (coil) as raw material and a high-frequency welding process. 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 pipe is 14-610mm, and the wall thickness is 1-23.8mm. Straight seam high-frequency welded pipe adopts a multi-stand continuous forming process, with high production efficiency (production speed 15-40m/min). The production line has complete sizing, straightening, and rounding equipment, resulting in steel pipes with good roundness, straightness, and welding performance.
2. Straight Seam Submerged Arc Welded Pipe: Straight seam submerged arc welded pipe is produced using single-sheet steel plates as raw material, through JCO or UO forming, submerged arc welding, or a combination of submerged arc welding and other welding processes. Common sizes include X70, X80, and X120. The diameter range of straight seam submerged arc welded pipe is 406-1422mm, and the wall thickness is 8-44.5mm.

For edge processing, milling machines are used. In addition to conventional JCO and UO technologies, some manufacturers use Progressive Forming (PFP) and Roll Forming (RBE) technologies. For welding, automatic pre-welding machines with argon or CO2 gas protection and dedicated multi-wire (4-wire and 5-wire) internal and external submerged arc welding equipment are used, along with square wave and dynamic wave power supply devices. For diameter expansion, mechanical expansion is used throughout the entire pipe length. For inspection, online flaw detection is performed on the plates, and automatic radiographic and hydrostatic testing is conducted on the welded steel pipes. After diameter expansion, secondary online or offline radiographic and hydrostatic testing is performed.

Straight seam steel pipe sandblasting removes rust by using a high-power motor to drive the blasting blades at high speed. Steel shot, steel grit, iron wire segments, minerals, and other abrasives are blasted onto the surface of the straight seam steel pipe under the powerful centrifugal force of the motor. This not only removes oxides, rust, and dirt, but also achieves the required uniform roughness under the intense impact and friction of the abrasives.

Sandblasting for straight seam steel pipes involves a high-power motor driving blasting blades at high speed. This propels abrasive materials such as steel shot, steel grit, wire segments, and minerals onto the pipe surface under the powerful centrifugal force of the motor. This process not only removes oxides, rust, and contaminants but also achieves the desired uniform roughness due to the intense impact and friction of the abrasive.

After sandblasting, the physical adhesion of the straight seam steel pipe surface is enhanced, and the mechanical adhesion between the anti-corrosion layer and the pipe surface is strengthened. Therefore, sandblasting is an ideal rust removal method for pipeline corrosion protection. Generally, shot blasting is mainly used for the internal surface treatment of straight seam steel pipes, while shot peening is mainly used for the external surface treatment.