Manufacturing difference between SMLS and welded pipe

Seamless Steel Pipe (SMLS):
SMLS pipes are produced by drawing a solid billet over a piercing rod to create a hollow shell without any welding or seams. This manufacturing method enhances the pipe’s ability to withstand higher pressures, making it ideal for applications that require strength. SMLS pipes can also be bent, flanged, cut, threaded, and grooved. Coating options for SMLS include black/red lacquer, varnish painting, and hot-dip galvanization.

Welded Steel Pipe:
Welded pipes are formed from flat steel plates, known as skelp, that are shaped, bent, and welded into a tubular product. Due to their lower production costs and the availability of raw materials, welded pipes are widely produced and used across various industries. Examples of ASTM or API 5L welded steel pipes include SSAW, ERW, and LSAW pipes.

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Manufacturing Differences Between SMLS And Welded Pipe:

- Concentricity:
The process of creating SMLS pipes involves punching a hole in a stainless steel blank at high temperatures (around 2200°F) and then drawing it into a hollow shape. This results in uneven wall thickness and higher eccentricity. As a result, ASTM standards allow a greater wall thickness variance for SMLS pipes compared to welded pipes. In contrast, welded pipes, made from cold-rolled sheets, exhibit more consistent wall thickness and better concentricity.

- Welding Performance:
SMLS and welded pipes have differences in their chemical compositions. The steel used in SMLS pipes meets only the basic requirements of ASTM standards, while welded pipes contain specific elements like silicon, sulfur, manganese, and oxygen that enhance welding performance. These elements facilitate better heat transfer and promote complete penetration of the weld, making welded pipes easier to weld. SMLS pipes, lacking these elements, tend to have unstable welding characteristics and may be difficult to weld or be impermeable at the weld site.

- Grain Size:
The metal grain size in both SMLS and welded pipes is influenced by heat treatment. Annealed SMLS and welded pipes typically have similar grain sizes. However, welded pipes can exhibit a finer grain structure at the weld site, contributing to stronger welds compared to the surrounding material.

- Strength:
The strength of both SMLS and welded pipes largely depends on their alloy composition. When made from the same alloy and heat treatment, both types of pipes offer similar strength. However, the strength at the welded joint in welded pipes can be superior due to fewer impurities and a slightly higher nitrogen content in the weld. That said, welded pipes generally have lower pressure ratings (85% of the allowable pressure) compared to SMLS pipes, though advancements in welding technology, such as ultrasonic testing, have improved the performance of welded pipes.

- Corrosion Resistance:
Both SMLS and welded pipes have similar corrosion resistance when made from the same alloy. However, insufficient heating at the weld during the manufacturing process can lead to accelerated corrosion in welded pipes, particularly in acid chloride environments. ASTM standards require additional tests to ensure that the weld has equivalent or superior corrosion resistance compared to the base metal.

- Bending and Extension:
Welded pipes undergo bending tests to ensure their flexibility and strength. The ASTM standard requires welded pipes to pass a test where they are bent 45° to 90°, then rotated to make a 180° bend. If any tearing or separation occurs under magnification, the weld quality is considered inadequate. The minimum bending radius is typically 2D, controlled by the alloy composition of the pipe.

- Ideal Welding Conditions:
For optimal welding, the weld joint should be in a neutral or compressed state, and the pipe should undergo annealing to reduce hardness and improve bending performance.

Other Differences Between Smls And Welded Pipe