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Spiral welded steel pipes and straight seam welded steel pipes have different technical characteristics and production processes, resulting in many differences in manufacturing, different functions and applications, and different values in use. We will make a simple comparison of the technical characteristics of spiral welded steel pipes and straight seam welded steel pipes:
Firstly, the welding processes differ.
In terms of welding processes, spiral welded steel pipes and straight seam welded steel pipes use the same welding method. However, straight seam welded steel pipes inevitably have many T-shaped welds, thus greatly increasing the probability of welding defects. Furthermore, the residual stress at the T-shaped welds is relatively high, and the weld metal is often in a triaxial stress state, increasing the possibility of cracking. Moreover, according to the process specifications for submerged arc welding, each weld should have an arc initiation point and an arc extinguishing point. However, when welding the circumferential seam of each straight seam welded steel pipe, this condition cannot be met, resulting in more welding defects at the arc extinguishing point. Therefore, spiral welded steel pipes have better quality assurance, ensuring that the produced products will not have defects such as cracks.
Strength Characteristics of Spiral Welded Steel Pipes and Straight Seam Welded Steel Pipes.
When a pipe is subjected to internal pressure, two main stresses are typically generated on the pipe wall: radial stress δY and axial stress δX. The combined stress at the weld is δ = δY(l/4sin²α + cos²α ¹/², where α is the helix angle of the spiral weld. The helix angle of the spiral weld is generally 50-75 degrees, and the combined stress at the spiral weld is 60-85% of the principal stress of the straight seam weld. Under the same working pressure, the wall thickness of a spiral welded steel pipe of the same diameter can be reduced compared to a straight seam welded steel pipe.
Metallurgical Properties of Spiral Welded Steel Pipes and Straight Seam Welded Steel Pipes.
Straight seam submerged arc welded steel pipes are produced from steel plates, while spiral welded steel pipes are produced from hot-rolled coils. The hot-rolled strip mill rolling process has a series of advantages and possesses the metallurgical capability to produce high-quality pipeline steel. For example, a water-cooling system is installed on the output rack to accelerate cooling, which allows for the use of low-alloy components to achieve specific strength grades and low-temperature toughness, thereby improving the weldability of the steel. However, this system is basically absent in steel plate production plants. The alloy content (carbon equivalent) of coiled steel is often lower than that of steel plates of similar grades, which also improves the weldability of spiral welded steel pipes.
Furthermore, because the rolling direction of spiral welded steel pipe coils is not perpendicular to the steel pipe axis (its clamping depends on the helix angle of the steel pipe), while the rolling direction of straight seam steel pipes is perpendicular to the steel pipe axis, the crack resistance of spiral welded steel pipe materials is superior to that of straight seam steel pipes.
A systematic comparison of spiral welded steel pipes and straight seam welded steel pipes is conducted in three aspects: welding process, metallurgical properties, and strength characteristics. The differences and main distinctions between the two are analyzed in detail, and the welding process and methods, strength properties, and metallurgical properties and processes of each type of pipe fitting are introduced in detail.
