Comparison of spiral welded pipe and straight seam welded pipe technical characteristics

1、Metallurgical properties of materials

LSAW pipes are produced from steel plates, while spiral welded pipes are produced from hot rolled coils. Hot strip mill unit rolling process has a series of advantages, with the ability to obtain the metallurgical process to produce high-quality pipeline steel. For example, the output stands are equipped with a water cooling system for accelerated cooling, which permits the use of low alloy compositions to achieve special strength grades and low-temperature toughness, thus improving the weldability of the steel.

The alloy content (carbon equivalent) of the coil plate tends to be lower than that of similar grades of steel plate, which also improves the weldability of spiral welded pipe. As the spiral welded pipe coil rolling direction is not perpendicular to the direction of the axis of the steel pipe (its pinch solution depends on the spiral angle of the steel pipe), while the straight seam steel pipe steel rolling direction perpendicular to the direction of the axis of the steel pipe, and therefore, the spiral welded pipe material cracking resistance is better than the straight seam steel pipe.

2, welding process

From the welding process, spiral welded pipe and straight seam steel pipe welding method is the same, but straight seam welded pipe will inevitably have a lot of d-beam weld, so the chances of weld defects are greatly increased, and d-beam weld at the weld residual stress is greater, the weld metal tends to be in the state of three-way stress, which increases the likelihood of cracks.

Moreover, according to the provisions of the submerged arc welding process, each weld should have an arc initiation and arc quenching, but each straight seam welded pipe in the welded annulus, can not achieve the conditions, and thus in the arc quenching may have more welding defects.

3, strength characteristics

When the pipe is subjected to internal pressure, two main stresses are usually generated in the pipe wall, i.e., radial stress δY and axial stress δX. The synthetic stress at the weld seam δ = δY(l/4sin2α+cos2α)1/2, where α is the helix angle of the weld seam of the spiral welded pipe.

Spiral welded pipe weld helix angle is generally 50-75 degrees, so the spiral weld synthetic stress is 60-85% of the main stress of straight seam welded pipe. In the same working pressure, the same diameter spiral welded pipe than straight seam welded pipe wall thickness can be reduced.