1000mm Large Diameter API 5LX70 Schedule 40 Spiral Welded Black Steel Pipe

What is Seamless pipe

Spiral welded pipe: The low carbon carbon structural steel or low alloy structural steel strip is rolled into a tube blank according to a certain helix Angle (called forming Angle), and then the pipe seam is welded together, which can produce large diameter steel pipe with a narrower strip.

The specification is expressed by the outer diameter * wall thickness, and the welded pipe should ensure that the water pressure test, the tensile strength of the weld and the cold bending performance should meet the requirements.

Our company's inspection process

Raw material inspection -- leveling inspection -- butt welding inspection -- forming inspection -- internal welding inspection -- external welding inspection -- cut pipe inspection -- Ultrasonic inspection -- groove inspection -- outline dimension inspection -- X-ray inspection -- water pressure test -- final inspection  

Since spiral welded pipe is often used in major projects such as oil exploration, our company has been implementing absolute safety standards in the quality testing of the product to provide customers with absolutely high quality products. In order to ensure product quality, we have developed a perfect quality plan, on-site work procedures and inspection and test plans. 

Our product features

The difference between spiral welded pipe and straight welded pipe

Technical comparison: A simple comparison of the technical characteristics of spiral welded pipe and straight welded pipe:

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. The rolling process of hot strip steel unit has a series of advantages, and it has the metallurgical process ability to produce high-quality pipeline steel. For example, a water cooling system is installed on the output bench 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 the steel. But this system is basically absent in steel plate production plants. The alloy content (carbon equivalent) of the coil is often lower than that of similar grades of steel, which also improves the weldability of spiral welded pipes.

It should be noted that because the rolling direction of the spiral welded pipe is not the direction of the vertical steel pipe axis (its clamping depends on the spiral Angle of the steel pipe), and the rolling direction of the steel plate of the straight seam steel pipe is perpendicular to the axis direction of the steel pipe, the cracking resistance of the spiral welded pipe material is better than that of the straight seam steel pipe.

Welding process

In terms of welding process, spiral welded pipe and straight seam steel pipe welding method is the same, but the straight seam welded pipe will inevitably have a lot of T-weld, so the probability of welding defects is greatly increased, and the welding residual stress at the T-weld is large, and the weld metal is often in a three-way stress state, increasing the possibility of cracking.

Moreover, according to the process regulations of submerged arc welding, each weld should have an arc starting point and an arc quenching point, but each straight seam welded pipe can not reach the condition when welding the ring joint, so there may be more welding defects at the arc quenching point.

Strength characteristic

When a pipe is subjected to internal pressure, two main stresses are usually generated on the wall of the pipe, namely radial stress δY and axial stress δX. The resultant stress δ= δy (l/4sin2α+cos2α)1/2 at the weld, where α is the helix Angle of the weld of the spiral welded pipe.

The spiral Angle of the spiral welded pipe is generally 50-75 degrees, so the synthetic stress at the spiral weld is 60-85% of the main stress of the straight welded pipe. Under the same working pressure, the wall thickness of spiral welded pipe with the same diameter can be reduced than that of straight welded pipe.

According to the above characteristics, we can know:

A. When the spiral welded pipe explodes, because the normal stress and synthetic stress of the weld are relatively small, the blasting opening generally does not originate at the spiral weld, and its safety is higher than that of the straight welded pipe.

B. When there are parallel defects near the spiral weld, the expansion risk of the spiral weld is not as great as that of the straight weld due to its small force.

C. Since the radial stress is the maximum stress existing on the steel pipe, the welding seam bears the maximum load when it is in the direction of the vertical stress. That is, the straight seam bears the largest load, the circumferential weld bears the smallest load, and the spiral seam is between the two.

Advantages of Spiral welded pipe

1. High strength: spiral welded pipe has high strength and can withstand greater pressure and weight, so it is suitable for making heavy support structures such as steel structure pillars.   

2. Corrosion resistance: After surface treatment such as hot dip galvanized, spiral welded pipe can effectively prevent rust and corrosion and extend its service life.  

3. Easy processing: Spiral welded pipe can be easily installed and manufactured by cutting, drilling, welding and other processing methods.  

 4. Low cost: Compared with other advanced steels, spiral welded pipes have a lower cost and are suitable for large-scale production and application.