FAQ

A. Steel tubes: production process and general information

1. How is a welded tube made?

A welded tube is made from a flat steel strip, usually slit from a  coil. This strip is pushed through a series of rollers, bending the strip into a round tube shape. Finally the ends of the strip are welded together, often by ERW technology (Electric Resistance Welding). This is a form of pressure welding which utilizes heat generated by electrical resistance of the material. No extra welding material is added. Welded pipes are cut to fix lengths, usually 6 or 12 m.

2. How is a seamless tube made?

Seamless tubes are made from a massive block of steel. First, this block is heated up to 1.200 °C and then pierced into a hollow by pushing it over a plug. Then, by a number of rolling stages, the pipe will be shaped into the desired diameter and wall thickness. This process is also called “stretch-reducing”. This process implies that the final length of seamless tubes is not as fixed as it is for welded tubes. We often speak of "single random lengths" and "double random lengths" (more info under 9. and 10.) Additional processes like straightening, end cutting and testing will complete the seamless tube. 

3. How is a square or rectangular hollow section made?

Square and rectangular hollow sections can be made in two different ways. Either by using a flat steel strip that is shaped into its square or rectangular form by a number of rolling processes, and finally welded together. There is no heating of the steel involved, therefore these hollow sections are referred to as “cold formed”.

The other way is by starting with a round tube, shaping it into a square or rectangular shape by rolling. This often requires heating of the tube (there are exceptions) so they are referred to as “hot formed”. The round tube that is used as pre-material often is seamless, but it can also be a welded tube.

4. What is the difference between hot and cold formed hollow sections?

The different production methods of hot and cold formed hollow sections cause different product properties. They cannot simply be exchanged. The main differences are:

• Cold formed hollow sections have a larger corner radius, causing lower static values. Also, more welding material is needed when welding them together.
• Due to the cold forming procedure, the stress values in the corner areas are higher compared to hot formed hollow sections. This means a lower buckling curve according to Eurocode should be applied.
• For the same application, when welding in the corners is required, you need a higher steel grade of cold formed hollow sections compared to hot formed hollow sections.

5. How to calculate the weight of a carbon steel tube?

There is a simple formula for calculating the theoretical weight per meter of a round carbon steel tube:

Weight = Outside diameter -/- wall thickness * wall thickness * 0,02466

For example, the weight of a tube with the dimension 168,3 x 7,1 mm is:

168,3 - 7,1 = 161,2  →  161,2 * 7,1 = 1.144,52  →  1.144,52 * 0,02466 = 28,22 kg/m

6.  What is a precision tube?

Precision tubes are made according to specific standards with stricter tolerances on dimensions compared to, for example, standards for construction tubes. The most common standard used for precision tubes is EN 10305. The staff of our company Van Leeuwen Precisie in Deventer are experts in this field. Contact them for more information.

7. Why are tube dimensions sometimes indicated in inches?

Diameters of tubes which are fabricated according to the American standards API and ASTM, are indicated in inches. These standards are often used in the process industry and pipeline business. On our website, you find a conversion table to easily convert inch dimensions to millimeters.

8. Why is the wall thickness of tubes indicated with schedules?

According to the American standards API and ASTM, the wall thickness is indicated with so called schedules. There is a range of schedule sizes, for example: STD (standard), XS (extra strong) and XXS. Other schedules are S10, S20, S30 up to S160. Their equivalent in millimeters depends on the outside diameter of the tube. Please check our pipe schedule table to see all possible diameter/wall thickness combinations and the conversion to mm sizes.

9. What is a single random length for tubes?

For seamless tubes, the length can be indicated as SRL, meaning single random length. This means a tube length between 4 and 7 meter.

10. What is a double random length for tubes?

For seamless tubes, the length can be indicated as DRL, meaning double random length. This means a tube length between 8 and 14 meter.

11. What does API mean for tubes?

API is an abbreviation for American Petroleum Institute. Here, the API standards were developed at the beginning of the 20th century. API (5L) is a standard specification for line pipe. Van Leeuwen has a wide range of API pipes on stock.

12. What is the ASTM standard for tubes?

ASTM is the abbreviation for American Society for Testing and Materials. This institute was founded in 1898 and has developed a large number of standards. Tubes according to ASTM standards are often used in the process industry and for offshore oil and gas applications worldwide. Van Leeuwen holds a large assortment of ASTM pipe material on stock.

13. Why do we have Euronorms for tubes?

Euronorms have been created to merge former national norms within Europe for pipe material, such as BS, DIN, NEN and NF norms. Using Euronorms means more standardization within Europe and easier trade between countries in the EU. The use of Euronorms depends on the application of the tubes. Please check our overview.

B: Steel Qualities for carbon steel tubes

14. How are steel qualities defined?

The description or definition of the steel quality depends on the standard that is used for the tube. Euronorms have different steel quality descriptions compared to API or ASTM standards.

15. How are steel qualities according to Euronorms defined?

Steel qualities for carbon steel are defined in the Euronorm EN 10027. Most EN norms for tubes, such as EN 10219, have a section to define steel qualities too, but they are based on the EN 10027.

Steel qualities according to Euronorms are usually identified by their application and their mechanical properties. Examples are: S235JRH, S355J2H or P355NH.

• The first letter represents the application the quality is suitable for: S for structural, P for pressure, E for engineering and L for line pipe.
• The following number represents the minimum yield strength in N/mm².
• Then, different indications per application follow. For steel qualities indicated with S, there is an indication of the impact test, indicated in Joules. The most common ones are JRH, J0H and J2H, meaning the impact test has been performed at room temperature, at zero degrees or at -20 degrees Celsius respectively.
• For steel qualities starting with P, for pressure applications, the following letters may indicate that the steel is normalized (N) and is suitable for high temperatures (H).

16. What does steel quality S235JRH mean?

This means the tube is suitable for structural applications (S), has a minimum yield of 235 N/mm² and the impact test has been performed at room temperature (JR). H stands for hollow section.

17. What does steel quality S355J2H mean?

This means the tube is suitable for structural applications (S), has a minimum yield of 355 N/mm² and the impact test has been performed at -20 °C (J2). H stands for hollow section.

18. What does steel quality P265GH mean?

This means the tube, fitting or flange is suitable for applications under pressure (P), it has a minimum yield strength of 265 N/mm², and is suitable for high temperatures (H). In this case, the letter G has no meaning.

19. What is the difference between carbon and stainless steel tubes?

Carbon steel is characterized by a high carbon content, usually up to 2.1% of its weight, making it stronger and harder than stainless steel. Because it has no corrosion protective properties, carbon steel will rust when exposed to oxygen (oxidation process). Stainless steel is protected against rust by its higher content of chromium, being 10.5% at least. Because of this, stainless steel looks better and is often used for decorative purposes. It is also used for applications where corrosion is absolutely undesirable, such as the food and pharma industry, water treatment and in the process industry. Contact our experts at our company Van Leeuwen Stainless for all your questions regarding stainless steel!