Comparison of Spiral Steel Pipes, Straight Seam Steel Pipes, And Seamless Steel Pipes

The production process of straight seam welded steel pipes is relatively simple. The main production processes include high-frequency welded straight seam steel pipes and submerged arc welded straight seam steel pipes. Straight seam pipes have high production efficiency, low cost, and rapid development. The strength of spiral welded steel pipes is generally higher than that of straight seam welded steel pipes. The main production process is submerged arc welding. Spiral steel pipes can use blanks of the same width to produce welded steel pipes with different diameters, and narrower blanks can also be used to produce larger diameters. Welded steel pipe. However, compared with straight seam pipes of the same length, the weld length is increased by 30~100%, and the production speed is lower. Therefore, welded steel pipes with smaller diameters mostly use straight seam welding, while welded steel pipes with large diameters mostly use spiral welding. In the industry, T-welding technology is used when producing larger-diameter straight-seam steel pipes. That is, short sections of straight-seam steel pipes are butted together to a length that meets the needs of the project. The probability of T-weld straight-seam steel pipe defects is also greatly increased, and T-welding The welding residual stress at the seam is relatively large, and the weld metal is often in a three-dimensional stress state, increasing the possibility of cracks.

The first is, the welding process

In terms of the welding process, the welding methods of spiral welded steel pipes and straight seam steel pipes are the same, but straight seam welded steel pipes will inevitably have many T-shaped welds, so the probability of welding defects is also greatly increased, and the T-shaped welds are Welding residual 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 steel pipe cannot meet this condition when welding the circumferential seam, so there may be arc extinguishing points. There are more welding defects.

Second, strength characteristics

When a pipe is subjected to internal pressure, it usually produces 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 steel pipe weld. The helix angle of the spiral welded steel pipe weld is generally degrees, so the resultant stress at the spiral weld is the main stress of the straight seam welded steel pipe. Under the same working pressure, the wall thickness of spiral welded steel pipes of the same diameter can be smaller than that of straight seam welded steel pipes. Based on the above characteristics, it can be seen that when a spiral welded steel pipe explodes, since the normal stress and synthetic stress on the weld are relatively small, the blast hole generally does not originate from the spiral weld, and its safety is higher than that of straight seam welded steel pipes. When there are parallel defects near the spiral weld, the risk of expansion of the spiral weld is not as great as that of the straight weld because the force on the spiral weld is smaller. Since radial stress is the maximum stress existing on the steel pipe, the weld will bear the maximum load when it is in the direction of vertical stress. That is, the straight seam bears the greatest load, the circumferential weld bears the smallest load, and the spiral seam is between the two.

Third, static pressure bursting strength

Through relevant comparative tests, it has been verified that the measured values and theoretical values of the yield pressure and burst pressure of spiral welded steel pipes and straight seam welded steel pipes are consistent, and the deviations are close. However, whether it is yield pressure or burst pressure, spiral welded steel pipes are lower than straight seam welded steel pipes. The blasting test also shows that the circumferential deformation rate of the blast hole of the spiral welded steel pipe is significantly greater than that of the straight seam welded steel pipe. This confirms that the plastic deformation ability of spiral welded steel pipes is better than that of straight seam welded steel pipes, and the blast opening is generally limited to one pitch. This is because the spiral welds play a strong role in constraining the expansion of the crack.

Fourth, toughness and fatigue strength

The development trend of steel pipes is large diameter and high strength. As the diameter of the steel pipe increases and the grade of steel used increases, the tendency of the ductile fracture tip to expand steadily increases. According to tests conducted by relevant research institutions in the United States, although spiral welded steel pipes and straight seam welded steel pipes are at the same level, spiral welded steel pipes have higher impact toughness. Due to changes in the transmission volume of transportation pipelines, steel pipes are subject to random alternating loads during actual operation. Understanding the low cyclic fatigue strength of steel pipes is of great significance to judging the service life of pipelines.

According to the measurement results, the fatigue strength of spiral welded steel pipes is the same as that of seamless pipes and electric resistance welded steel pipes. The test data are distributed in the same area as seamless steel pipes and electric resistance steel pipes but are higher than ordinary submerged arc straight seam welded steel pipes.