Comparative analysis of SSAW, LSAW, ERW Steel Pipe

The production process of longitudinally welded pipe is relatively simple, and the main production processes are high-frequency resistance welded longitudinally welded pipe (ERW steel pipe)and submerged arc welded longitudinally welded pipe(LSAW steel pipe). Longitudinal welded pipe has high production efficiency, low cost and rapid development.

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 pipes can be used to produce welded pipes of the same width with different diameters, and narrow square billets can also be used to produce welded pipes with larger diameters. But compared with the straight seam pipe of the same length, the weld length is increased by 30-100%, and the production speed is lower.

Therefore, the small diameter welded pipes mostly adopt straight seam welding, and the large diameter welded pipes mostly adopt spiral welding. In the production of industrial large-diameter straight seam steel pipes, T-shaped welding seam technology is adopted, that is, to meet engineering needs, short-section straight seam steel pipes are butt welded, which greatly increases the probability of defects in T-shaped welded seam steel pipes. The welding residual stress at the shape of the weld is large, and the weld metal is often in a three-dimensional stress state, which increases the possibility of cracks.

Welding process

In the welding process, the welding method of spiral welded pipe and straight seam welded steel pipe is the same, but the straight seam welded pipe will inevitably have a large number of T-shaped welds, which greatly increases the possibility of welding defects. The stress is large, and the weld metal is often in a triaxial stress state, which increases the possibility of cracks.

In addition, according to the submerged arc welding process specification, each weld should have arc ignition and arc extinguishing positions, but each longitudinal welded pipe cannot meet this condition when welding circular welds, so there may be more arc extinguishing positions Welding defects.

Strength characteristics

When the pipeline is subjected to internal pressure, two principal stresses are usually generated on the pipe wall, namely radial stress and axial stress. The resultant stress δ at the weld, where α is the helix angle of the weld of the spiral welded pipe.

The spiral angle of the spiral welded pipe weld is generally a degree, so the comprehensive stress at the spiral weld is the principal stress of the longitudinal welded pipe. Under the same working pressure, compared with the straight seam welded pipe, the wall thickness of the spiral welded pipe of the same diameter can be reduced.

According to the above characteristics:

When a spiral welded pipe explodes, because the normal stress and composite stress of the weld are small, the blast hole generally does not originate from the spiral weld, and its safety is higher than that of a straight welded pipe.

When there are parallel defects near the spiral weld, the expansion risk of the spiral weld is not as high as that of the straight weld because the stress of the spiral weld is small.

Since the radial stress is the maximum stress on the steel pipe, the weld is subjected to the maximum load in the vertical stress direction. That is, the load of the straight joint is the largest, the load of the circumferential joint is the smallest, and the load of the spiral joint is between the two.

Static pressure blasting strength

Through comparative tests, it is verified that the yield pressure of spiral welded pipe and longitudinal welded pipe is in good agreement with the actual and theoretical values of burst pressure, and the deviation is close. But regardless of the yield pressure or the burst pressure, the spiral welded pipe is lower than the straight seam welded pipe. The blasting test also showed that the circumferential deformation rate of spiral welded pipe blasting was significantly higher than that of straight seam welded pipe. The results show that the plastic deformation ability of spiral welded pipe is better than that of straight seam welded pipe, and because spiral welded pipe strongly restricts crack propagation, its blast hole is generally limited to one pitch.

Toughness and fatigue strength

The development trend of pipelines is large diameter and high strength. With the increase of steel pipe diameter and steel grade, the ductile fracture tip has a greater tendency to produce stable expansion. According to the test of relevant research institutes in the United States, although spiral welded pipe and longitudinal welded pipe have the same grade, spiral welded pipe has higher impact toughness.

Due to changes in conveying capacity, steel pipes bear random alternating loads during actual operation. Understanding the low-cycle fatigue strength of steel pipes is of great significance for judging the service life of pipes.

The test results show that the fatigue strength of spiral welded pipe is the same as that of seamless pipe and resistance welded pipe. The test data is distributed in the same area of seamless steel pipe and resistance welded pipe, which is higher than that of general submerged arc longitudinal welded pipe.