Innovations in the metal sector do not lie solely in the creation of new alloys or in the improvement of the machines used to create them. There are also improvements in existing parts, small details that make a part more practical for a specific function, as in the case of seamless tube manufacturing.

If you want to know their history, how they are manufactured and when you should use this type of tube instead of another, keep reading, because we are going to cover these and other topics in today’s post. It is our small tribute to the small big changes.

What are seamless tubes?

A seamless tube is so named because it is made from a single piece of metal. Hence, it does not have any type of joint or ‘seam’ markings. They are manufactured from a metallic ingot (steel, copper, aluminum, brass, etc.) and can be hot-rolled or cold-rolled. Techniques such as punching or forming are often used with the same result: a tube without any welding.

This is the most suitable way to manufacture thick pipes, normally created to withstand corrosion and pressures in any direction. Cold drawing can also be applied. This results in a medium-low strength tube, but completely smooth inside. Both steel and copper pipes are used in a multitude of industries, including thermal, plumbing, gas transportation, petrochemical, power plants, etc.

The origin of seamless pipes

The history of seamless pipe manufacturing is no more than 100 years old. It is a relatively recent invention and we owe it to the Mannesmann brothers, who invented in 1885 a machine that drilled through the rolling of two rollers. This discovery led to the invention of the rolling cycle in 1891. In 1903, the Swiss Ritteriefel created an automatic tube rolling machine. Gradually, and with the discovery of various types of extension machines (continuous tube rolling mills or jacking pipe machines), a modern seamless steel tube industry would begin to form.

In 1930, an improvement in seamless pipe manufacturing was introduced through the use of a three-roll mill, an extruder and a cold-rolling pipe mill. 3 decades later, and thanks to advances made in the 1930s, stress relieving machines and continuous casting blanks were applied, which increased production efficiency and gave seamless tubes the option to compete with welded tubes. In the 1970s, seamless and seamless tubes were sold in practically the same quantities, becoming a staple item. Its worldwide production increased at a rate of more than 5% per year.

API 5L Line Pipe Seamless Steel Pipe

How is a seamless steel pipe manufactured?

Two processes are involved in the manufacture of a seamless tube: the cross-rolling method (Mennesmann method) and the extrusion method. The first is generated by perforating the tube with a diagonal roller and extending it with a rolling mill. This procedure is quick to perform, but the tube blank requires high processability requirements. This method is the most recommended for the production of carbon steel and low alloy steel tubes.

This one uses a drilling machine to bore a rough pipe or steel ingot. It then punches it into a steel tube using an extruder. This process is definitely less efficient and is usually used to produce high-strength alloy steel tubes.

Both ways of manufacturing seamless pipe must first heat the tube blank or ingot. The resulting steel pipe is called hot-rolled pipe. Hot-working pipes can sometimes be cold-worked, depending on the final requirements. For this, there are 2 methods: cold drawing (pulling the steel pipe through the extrusion die to make it longer and thinner) or cold rolling (hot rolling is applied to the cold working method). Cold working can improve the dimensional accuracy and finish of pipe processing and improve the mechanical properties of materials.

Stress reduction

That uniformity of a hot-rolled seamless steel pipe is achieved primarily through stress reduction. It consists of heating the entire welded pipe to 950 degrees Celsius or more and applying a stress reducer (a total of 24 times the stress reducer). The resulting pipe is completely different from ordinary high-frequency welded pipe. After heating the part in the furnace, the metallographic structure and mechanical properties of the weld bead and main body are completely consistent. In addition, it reduces the stress of multiple passes.

The result of this process is a pipe of a low-strength material but with a very smooth internal surface.

What is a seam pipe?

A seamed or welded tube is one that does have a ‘seam’ where 2 or more pieces are joined together to form the tube. Its manufacturing process begins with the bending of the sheet metal. Now it is time to weld the part to get the cylindrical shape. The seam can be longitudinal (the oldest process) or spiral (helical). To achieve a longitudinal seam, a sheet is used which is bent with rollers. Thus, the ends are joined with a weld, forming the tube. The spiral or helical seam is generated by threading the weld along the pipe, rather than in a straight line. Sometimes the seam is concealed with a hot treatment and subsequent polishing.

The production of this type of pipe is cheap and requires little material, so there is a great economic saving. The final product is very resistant and is used in many industrial sectors (aerospace, aviation, energy, automotive, light industry…).

Differences between seamless and welded pipes

Now that we know what a seamless tube and a welded tube are and how they are manufactured, let’s take a deeper look at what other differences we can find between the two:

Appearance

Seamless steel pipe uses a cylindrical steel ingot as raw material. The external defects of this billet cannot be removed hot, so it would have to wait until the manufacturing of the product is finished to be polished, which is a waste of time and extra energy. The welded tube is generated from a hot rolled (or hot drawn) coil. Its quality makes it much easier to handle and the surface obtained from this process is much smoother and with less roughness. Therefore, the surface of the welded pipe will always be better than that of the seamless pipe.

Molding process

The seamless tube can only be formed when the rolling process is completed. However, since the welded tube is drawn from a sheet or plate that has to be bent, all types of welding can be performed while the tube is being produced (bending, rolling, etc.).

Performance and use

Seamless tubing withstands higher pressure levels. Its resistance is greater than that of welded pipe. For this reason, it is very common to find this pipe in high pressure equipment or in thermal industries working with boilers.

The seam of the welding tube is its weak point, and often affects its final performance. This type of pipe can withstand up to 20% less pressure than seamless pipes. This feature is the main factor that customers consider when choosing a seamless pipe. In fact, most industrial installations use seamless pipes. They are capable of withstanding extreme thermal, chemical and mechanical loads and levels. Welded tubes, on the other hand, are used more in other industries, such as aerospace, automotive and electronics.

Size

Most manufacturers produce seamless pipes with an outer diameter of 508 millimeters. The average diameter is less than 406.4 millimeters due to manufacturing equipment limitations. If a customer requires seamless tubes of a larger size, special machinery must be used to hot expand the material. However, the quality of the expanded pipe is not the same as that of the original seamless steel pipe. Welded steel pipe, on the other hand, has no such limitations. Their sizes can range from 48.3 milimeters to 2540 milimeters.

Cost and prices

Generally, the cost of a seamless tube is higher than a welded pipe. The causes of this difference are raw material, manufacturing equipment and the procedures employed in the production of each of the tube types. However, and due to market fluctuations, welded pipe can have a higher cost.