Comparison of spiral steel pipe, straight seam steel pipe, and seamless steel pipe

The production process of straight seam welded pipe is relatively simple. The main production processes are high-frequency welding straight seam steel pipe and submerged arc welding straight seam steel pipe. The production efficiency of straight seam pipe is high, the cost is low, and the development is relatively fast. The strength of spiral welded pipe is generally higher than that of straight seam welded pipe. The main production process is submerged arc welding. Spiral steel pipe can produce welded pipes with different diameters with the same width of billet, and can also produce welded pipes with larger diameters with narrower billets. However, compared with straight seam pipes of the same length, the weld length increases by 30~100%, and the production speed is lower. Therefore, smaller-diameter welded pipes are mostly straight seam welded, and large-diameter welded pipes are mostly spiral welded. When producing larger diameter straight seam steel pipes in the industry, T-welding technology is used, that is, short sections of straight seam steel pipes are butt-jointed to meet the length required by the project. The probability of defects in T-welded straight seam steel pipes is also greatly increased, and the welding residual stress at the T-weld is large. The weld metal is often in a three-dimensional stress state, which increases the possibility of cracks.

The first is, the welding process.

In terms of the welding process, the welding method of spiral welded pipe is consistent with that of straight seam steel pipe, but straight seam welded pipe will inevitably have many T-welds, so the probability of welding defects is greatly increased, and the welding residual stress at the T-weld is large, and the weld metal is often in a triaxial stress state, which increases the possibility of cracks.

Moreover, according to the process regulations of submerged arc welding, each weld should have an arc initiation point and an arc extinguishing point, but each straight seam welded pipe cannot meet this condition when welding the circumferential seam, so there may be more welding defects at the arc extinguishing point.

Second, strength characteristics.

When the steel pipe is subjected to internal pressure, two main stresses are usually generated on the pipe wall, namely radial stress δ and axial stress δ. The composite stress δ at the weld, 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 composite stress at the spiral weld is the main stress of the straight welded pipe. Under the same working pressure, the wall thickness of the spiral welded pipe of the same diameter can be reduced to that of the straight welded pipe.

According to the above characteristics, it can be known that:

When a spiral welded pipe explodes, the burst hole generally does not originate from the spiral weld due to the relatively small positive stress and synthetic stress on the weld, and its safety is higher than that of the straight welded pipe.

When there are defects parallel to the spiral weld near it, the spiral weld is less stressed, so the risk of its expansion is not as great as that of the straight weld.

Since radial stress is the maximum stress existing on the steel pipe, the weld bears the maximum load when it is in the direction of vertical stress. That is, the straight seam bears the largest load, the circumferential weld bears the smallest load, and the spiral seam is between the two.

Third, static pressure burst strength.

The relevant comparative tests have verified that the yield pressure and burst pressure of the spiral welded pipe and the straight welded pipe are basically consistent with the measured and theoretical values, and the deviation is close. However, both the yield pressure and the burst pressure of the spiral welded pipe are lower than those of the straight welded pipe. The burst test also shows that the circumferential deformation rate of the burst hole of the spiral welded pipe is significantly greater than that of the straight welded pipe. This proves that the plastic deformation capacity of spiral welded pipe is better than that of straight seam welded pipe, and the burst hole is generally limited to one pitch, which is due to the strong restraint effect of spiral weld on the expansion of the crack.

Fourth, toughness and fatigue strength.

The development trend of pipelines is large diameter and high strength. With the increase of steel pipe diameter and the improvement of steel grade used, the tendency of stable expansion of tough fracture tip is greater. According to the test of relevant research institutions in the United States, although spiral welded pipe and straight seam welded pipe are of the same grade, spiral welded pipe has higher impact toughness. Due to the change in transmission volume, the steel pipe is subjected to random alternating loads in the actual operation process of the transmission pipeline. Understanding the low cycle fatigue strength of steel pipe is of great significance to judging the service life of the pipeline.

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