Seamless steel pipe is manufactured by the round steel billet, through hot rolling, cold rolling, or cold drawn processes, with no seam on the pipe body.

Welded steel pipe is manufactured by flat steel plate, with a welding seam on the pipe body. Including ERW pipe, LSAW pipe, and SSAW pipe.

Octal tells you the differences between steel seamless pipe and welded steel pipe fittings on raw material, seamless pipe manufacturing processes, performances, appearances, sizes, applications, cost prices, etc.

From the names, it is obvious that the basic seamless tube manufacturing method differs. A seamless tube is extruded and drawn from a billet, while a welded tube is formed by rolling and welding a strip. A welded tube is much less expensive than seamless tubes and is readily available in long continuous lengths. In spite of the fact that welded tubes have a lower working pressure than seamless tubes, working pressure is not the deciding factor when choosing a seamless tube over a welded steel tube. 

A seamless tube is specified because of the difference in potential impurities, which reduces corrosion resistance. As a result, the weld area exhibits different malleability and corrosion resistance, as well as greater dimensional variation. It is possible to reduce these anomalies by drawing welded steel tubes. The majority of tubing (seamless and welded) is drawn to produce final dimensional tolerances. An operation called drawing involves pulling a tube through a die. 

Tubes can be drawn in two ways: sunk drawn and plug drawn. There is a difference in the roughness of the ID surface. In a sunk-drawn tube, there is no internal support. The diameter of sink-drawn tubes can be reduced without affecting the wall thickness. 

During the “crunch” of the ID, the tube develops a “sunburst” cross-section and wrinkles circumferentially. It is especially noticeable around weld-on welded tubing. The inner surface of a plug or rod-drawn tube is much smoother when it is pulled through a die with internal support. The rod or plug drawing removes the dimensional indication on welded tubing by breaking up the weld bead. The final annealing further promotes homogeneity. 

A fully finished tube is one that has been drawn and annealed sufficiently to remove any dimensional indication of the weld area and break up the dendritic structure of the weld bead and expedite homogenization. In many cases, it is very difficult to distinguish between welded and seamless tubing

How is Seamless steel pipe made?

Through a perforated tube, API 5L and ASTM Seamless steel pipes are formed by hot rolling, cold rolling, or cold drawing steel ingots or solid tubes as raw materials. The hot rolled seamless steel pipe is formed through billet forging, perforation, rolling, shaping, and other seamless steel tubing processes. In general, thick-walled seamless pipes are made in this way for large-diameter pipes. Using cold-drawn forming technology, steel seamless pipes are cold-drawn. There is a relatively low material strength, and the appearance and internal control surfaces are smooth. This method is usually used to make steel seamless pipes with small diameters and thin walls. Stainless steel, carbon structural steel, and low alloy structural steel are used to roll seamless stainless steel pipes. Several industries use seamless stainless steel pipes: pipelines, thermal services, mechanical industries, chemicals, etc.

How is Welded steel pipe made?

LSAW, SSAW, and ERW steel pipes are included in API 5L or ASTM welded steel pipes. Steel strips or steel plates are used to make these pipes. The steel plate is bent first, then welded into circular, square, and other shapes. Steel pipes that are welded have the advantages of high production efficiency, low seamless cost, and material saving. In addition to aviation, aerospace, energy, electronics, automobiles, light industry, and other industries, it has been widely applied. Corrosion-resistant in acidic environments.

Why Should You Prefer To Buy Seamless Steel Pipes?

Seamless steel pipe is preferred over welded steel pipe because it can withstand extremely high pressure without cracking. Unlike seam or welded pipes, however, welded points are vulnerable to cracking under high pressure. It is also very easy to calculate the pressure of seamless pipes accurately. Moreover, seamless pipes are thinner and lighter than welded ones, and they also have a uniform shape since they are formed by the uniform extrusion of an alloy. Octal Steel API 5L and ASTM seamless steel tube are best quality seamless steel pipes you can buy.

Seamless vs welded steel pipe differences summary

There are some differences between API 5L / ASTM seamless steel pipe and ERW welded steel pipe as follows:

1. Appearance difference

Steel billet is used as a raw material for seamless steel pipes. A billet’s outer surface defects cannot be eliminated during the hot rolling process, they are only removed after the product is finished. The defect can only be partially eliminated by wall reduction.

Welded steel pipe is made from the hot rolled coil as raw material, so coil surface quality is similar to pipe surface quality. Hot rolled coils have a high-quality surface.

So Welded steel pipe surface quality is much better than seamless steel pipe.

2. Molding process differences

Seamless steel pipe can be formed one time in the rolling process.

A welded steel pipe is made by bending and welding steel strips or steel plates.

3. Performance and Usage

Seamless steel pipe has a higher strength and pressure capacity than ERW welded steel pipe. Because of this, it is widely used in high-pressure equipment, thermal equipment, and boilers.

The weld seam is generally the weak point of the welded steel pipe, and its quality affects its overall performance.

The working pressure of welded steel pipes is generally 20% lower than that of seamless steel pipes. Seamless steel pipe is popular because of its reliability. The reason why all industrial pipelines are constructed with seamless pipes is that they are subjected to extreme thermal, chemical, and mechanical loads. Pipes that are welded are preferred in aerospace, automobile, and electronics industries where budgets are low and work pressure is high.

4. Available Sizes differences

The maximum OD of our original seamless pipes is 20 inches, and the maximum diameter is 508 mm. Due to equipment limitations, it is usually smaller than 16 inches, 406.4 mm. In addition, if the client would like to purchase seamless steel pipes larger than the above sizes, hot expanding machining must be used. The quality of this type of hot expanded seamless steel pipe usually cannot compare to that of the original seamless steel pipe.

Unlike welded steel pipes, which have these limitations, welded steel pipes are available in sizes ranging from 1-1/2 inches 48.3mm to 100 inches 2540mm.

5. Cost and prices

Due to the raw material, seamless pipe manufacturing equipment, and seamless steel tubing processes, seamless steel pipes usually cost more than welded steel pipes. If you encounter this situation, don’t hesitate to purchase the seamless steel pipe for the same dimensions if the welded pipe fittings are more expensive.

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