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The Difference Between Spiral Steel Pipe and Straight Seam Steel Pipe


In recent years, there has been a large supply of spiral steel tubes in the market. Everyone knows the main uses and performance of spiral steel tubes, but most people still don't know the difference in performance between spiral steel tubes and straight seam steel tubes. Spiral steel pipe and straight seam steel pipe are both a kind of welded steel pipe. They are widely used in national production and construction. Spiral steel pipe and straight seam steel pipe have many differences due to different production processes. The following is made by Permanent Steel Manufacturing Co.,Ltd. The manufacturer specifically discusses the difference between spiral steel pipes and straight seam steel pipes.

1. Metallurgical properties of materials

ERW steel pipes are produced from steel plates, while spiral steel pipes are produced from hot rolled coils. The rolling process of the hot-rolled strip unit has a series of advantages, and has the metallurgical process capability to produce high-quality pipeline steel. For example, a water cooling system is installed on the output platform to accelerate cooling, which allows the use of low alloy components to achieve special strength levels and low temperature toughness, thereby improving the weldability of steel. But this system is basically not available in the steel plate production plant. The alloy content (carbon content) of the coiled plate is often lower than that of steel plates of similar grades, which also improves the weldability of the spiral welded pipe. What needs to be explained is that since the rolling direction of the coil of the spirally welded pipe is not perpendicular to the axis direction of the steel pipe (the cracking depends on the helix angle of the steel pipe), the rolling direction of the steel plate of the straight seam steel pipe is perpendicular to the axis direction of the steel pipe. The crack resistance of welded pipe materials is better than that of straight seam steel pipes.

2. Welding process

In terms of welding process, the welding method of spiral steel pipe and straight seam steel pipe is the same, but the straight seam steel pipe inevitably has many T-shaped welds, so the probability of welding defects is also greatly improved, and the welding residue at the T-shaped weld The stress is large, and the weld metal is often in a three-way stress state, increasing the possibility of cracks.

Spiral steel pipe is made of 16Mn steel plate welded, 16Mn is the old national brand name, now it is classified as low alloy high strength structural steel, the current brand, it is called Q345, but Q345 steel is a substitute for many types of steel such as 12MnV, 14MnNb, 18Nb, 16MnRE, and 16Mn, rather than just replacing 16Mn steel. In terms of chemical composition, 16Mn and Q345 are also different. More importantly, there is a large difference in the thickness group size of the two steels according to the difference in yield strength, and this will inevitably cause changes in the allowable stress of materials of certain thicknesses. Therefore, simply applying the allowable stress of 16Mn steel to Q345 steel is not suitable.

According to the technical regulations of submerged arc welding, each welding seam should have an arc starting point and an arc extinguishing point, but each straight seam steel pipe cannot meet this condition when welding a girth, so there may be more at the arc extinguishing point Welding defect.

3. Strength characteristics

When the pipe is under internal pressure, it usually produces two main stresses on the pipe wall, namely radial stress δY and axial stress δX. The synthetic stress at the weld δ=δY(1/4sin2a+cos2a)1/2, where a is the spiral angle of the weld of the spiral welded pipe. The spiral angle of the welded seam of the spiral welded pipe is generally 50-75 degrees, so the resultant stress at the spiral welded seam is 60-85% of the main stress of the straight welded pipe. Under the same working pressure, the wall thickness of the spiral welded pipe with the same pipe diameter can be reduced compared with the straight welded pipe.

According to the above characteristics:
A. When the spiral welded pipe is blasted, because the normal stress and the synthetic stress on the weld are relatively small, the blasting hole 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 defect parallel to the spiral weld, the spiral weld is less stressed, so the risk of expansion is not as great as that of the straight weld.
C. Since the radial stress is the maximum stress that exists on the steel pipe, the weld is subjected to the maximum load when it is in the direction of the vertical stress. That is, the load on the straight seam is the largest, the load on the circumferential weld is the smallest, and the spiral seam is between the two.

4. Withstand pressure

The steel thickness group size re-determines the allowable stress. The main component elements ratio of Q345 steel is basically the same as that of 16Mn steel, the difference is the addition of trace alloying elements of V, Ti and Nb. A small amount of V, Ti, Nb alloy elements can refine the grains, improve the toughness of the steel, and the overall mechanical properties of the steel are greatly improved. Because of this, the thickness of the steel plate can be made larger. Therefore, the comprehensive mechanical properties of Q345 steel should be better than 16Mn steel, especially its low temperature performance is not possessed by 16Mn steel. The allowable stress of Q345 steel is slightly higher than that of 16Mn steel. The carbon content is less than or equal to 0.2%. This kind of steel guarantees its mechanical strength at the factory, and does not require alloy composition.

The production process of the straight seam steel pipe is relatively simple. The main production processes are high-frequency welded straight seam steel pipe and submerged arc welded straight seam steel pipe. The straight seam pipe has high production efficiency, low cost and rapid development.

T-welding technology is used in the production of large-diameter straight-seam steel pipes in the industry, that is, short sections of straight-seam steel pipes are butt-connected to a length that meets the needs of the project. The probability of T-weld straight-seam steel pipe defects is also greatly improved, and T-welding The welding residual stress at the seam is large, and the weld metal is often in a three-way stress state, increasing the possibility of cracks.

The strength of spiral steel pipe is generally higher than that of straight seam steel pipe. The main production process is submerged arc welding. Spiral steel pipe can use the same width of blanks to produce welded pipes with different diameters, and narrower blanks can be used to produce welded pipes with larger diameters. But compared with the straight pipe of the same length, the length of the welding seam is increased by 30-100%, and the production speed is lower. Therefore, the weld pipes with smaller diameter are mostly welded by straight seam, and the weld pipes with larger diameter are mostly spiral welded.



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