Metallurgical properties of materials

Straight seam submerged arc welded pipe is made of steel plate, while spiral welded pipe is made of hot rolled coil. Hot strip mill rolling process has a series of advantages, it has the metallurgical process ability to produce high quality pipeline steel. For example, a water cooling system installed on the output bench to speed cooling allows the use of low alloy composition to achieve special strength grades and low temperature toughness, thus improving the weldability of the steel. But this system is largely absent in steel mills. The alloy content (carbon equivalent) of coil plates is often lower than that of similar grade steel plates, which also improves the weldability of spiral welded tubes.
More importantly, because the rolling direction of spiral welded pipe is not perpendicular to the axis of steel pipe (its clamping depends on the spiral Angle of steel pipe), and the rolling direction of straight seam steel pipe is perpendicular to the axis of steel pipe, the anti-cracking performance of spiral welded pipe material is better than that of straight seam steel pipe.

Welding process

From the welding process, the spiral welded pipe and straight seam steel pipe welding method is consistent, but the straight seam welded pipe will inevitably have a lot of t-shaped weld, so the probability of weld defects is also greatly improved, and the t-shaped seam welding residual stress is larger, the stress state of the weld metal are often in a three-way increases the possibility of a crack.
Moreover, according to the technological provisions of submerged arc welding, every weld should have arc initiation and arc extinguishing, but each straight seam welded pipe cannot meet this condition when welding around the seam, so there may be more welding defects at the arc extinguishing.

Strength characteristics

When the pipe is subjected to internal pressure, there are usually two main stresses on the pipe wall, namely the radial stress δY and the axial stress δX. The resultant stress at the weld δ=δY(L / 4Sin2 α+cos2α)1/2, where α is the spiral Angle of the spiral welded pipe weld.
The helical Angle of spiral welded pipe weld is generally 50-75 degrees, so the synthetic stress of spiral welded pipe is 60-85% of the main stress of straight welded pipe. Under the same working pressure, the wall thickness of the spiral welded pipe with the same diameter can be reduced than that of the straight welded pipe.
According to the above characteristics, it can be seen that:
A. When the spiral welded pipe is blasted, because the normal stress and the synthetic stress on the weld are relatively small, the bursting port generally does not originate from the spiral weld, and its safety is higher than that of the straight welded pipe.
B. When there is a parallel defect near the spiral weld, the risk of expansion of the spiral weld is not as great as that of the straight weld due to its small stress.
C. Since the radial stress is the maximum stress existing on the steel pipe, the weld shall bear the maximum load in the direction of the vertical stress. That is, the straight seam bears the maximum load, the circumferential weld bears the minimum load, and the spiral seam is between them.

Static blasting strength

The experimental results show that the yield pressure of spiral welded pipe and straight welded pipe is basically consistent with the measured and theoretical values of burst pressure, and the deviation is close. However, both yield pressure and burst pressure of spiral welded pipe are lower than that of straight welded pipe. The blasting test also shows that the circumferential deformation rate of spiral welded pipe is obviously greater than that of straight welded pipe. It is proved that the plastic deformation capacity of spiral welded pipe is better than that of straight welded pipe, and the blasting hole is generally limited to one pitch, which is caused by the strong constraint of spiral welded seam on the expansion of crack.

Toughness and fatigue strength

The trend of pipeline development is large diameter and high strength. With the increase of steel pipe diameter and steel grade, the trend of steady extension of ductile fracture tip becomes larger. According to the test of the relevant research institutions in the United States, the spiral welded pipe and the straight seam welded pipe are the same grade, but the spiral welded pipe has higher impact toughness.
The pipe is subjected to random alternating load in actual operation due to the variation of transmission capacity. It is important to know the low cycle fatigue strength of steel pipe to judge the service life of pipeline.
According to the test results, the fatigue strength of spiral welded pipe is the same as that of seamless pipe and resistance welded pipe. The test data are distributed in the same area with seamless pipe and resistance welded pipe, and higher than the general submerged arc straight seam welded pipe.

Field weldability

Field weldability is mainly determined by the steel pipe material and port fit size tolerance.
Considering the requirements of steel pipe installation and construction, the continuity of steel pipe processing and production and the consistency of geometric dimensions are particularly important.
The production of spiral welded pipe is basically a stable continuous process under the same working condition: and the straight seam welding control process is divided, including the whole plate/press head/prewinding/spot welding/welding/finishing/group of equal multi-step process. This is the important feature that the production of spiral welded pipe is different from that of straight seam welded pipe.

Stable production conditions are very convenient for welding quality control and geometric size assurance. Compared with the straight welded pipe, the spiral welded pipe has very good orifice ellipticity and end face verticality, which ensures the alignment accuracy of steel pipe welding in the field.