ERW refers to high frequency electrical resistance welding straight seam welded steel pipe. ERW steel pipe is significantly different from spiral steel pipe in terms of the welding methods. Due to the welding process is carried out under high speed, the difficulty to ensure the quality of welding is much bigger compared with SSAW steel pipe.

According to the different welding method, straight seam high frequency (ERW) is divided; two forms of induction welding and contact welding, hot-rolled wide coil as raw material, through the process of pre-bent, continuous molding, welding, heat treatment, sizing, straightening, cutting, compared with the spiral, the weld is short, the dimensional accuracy is high, the thickness is uniform, the surface quality is good, and can bear high pressure, but the drawback is that it can only produce small diameter thin-walled tube, easy to produce gray spot, lack of fusion groove and shaped corrosion defects. Currently widely used in the city gas, crude oil and other oil products transportation.

While for SSAW steel pipe：forming by spiral bending with strip, and welding the inside seam and outside seam with automatic submerged arc.

It can be widely used for the following reasons in the production of large diameter steel pipe:

1) as long as change the forming Angle, you can use the same width strip in the production of various diameter steel pipe;

2) as it is a continuous forming, the length of steel tube is not restricted;

3) weld spiral evenly distributed throughout the circumference of the steel tube, so the size of the steel tube is of high precision, and the intensity is also higher;

4) equipment cost is cheap and easy to change size, suitable for small batch and many varieties production of steel pipe.

The length of ssaw steel pipe is larger than the straight seam steel pipe, such as for the

length of L, the weld length is L/cos (θ). And steel tube defects are mostly concentrated in the weld seam and heat affected zone, so large weld length means the defect appearing probability is large, and it is the main constraint of spiral weld pipe to be widely used for a long time, and turn to a problem argued for a long time-spiral steel pipe and straight seam steel pipe, especially compared with ERW steel tube, which one is superior.By now, the spiral pipe manufacturing technology has developed a lot, we should comprehensively and correctly evaluate and compare, rediscover the problem of too-long welding seam of ssaw steel pipe.

First of all, due to the defects and weld are in parallel, so for spiral pipe, the weld defects are lean-defects. When in use, the main stress direction of steel tube, namely the defect equivalent length in the axis direction of the steel tube is smaller than the ERW steel pipe;

Secondly, the pipeline steel are all based on rolling steel plate, the impact toughness have larger anisotropy, the CVN value along the rolling direction is three times higher than that of perpendicular to the rolling direction.And the main stress of ERW steel pipe is just perpendicular to the direction of pipe with the lowest impact resistant ability, and the spiral tube staggers the direction of the lowest pipe impact resistant ability, making the too-long welding seam from disadvantage into advantage.