From strip steel to steel pipes

How are steel pipes manufactured?

We find steel pipes all around us. Just think of the exhaust of a car, the central heating pipe, garden chairs, the frame of a tent, scaffolding and fencing. These are all things that are made with steel pipes. And we have not even mentioned all the pipes that cannot be seen, such as pipes under the ground through which gas, water and other media are transported. The only question is: how are these pipes manufactured?

The manufacture of steel pipes

Raw materials for making the steel pipe consist of rolls of strip steel that are brought in from steel factories. For the production pipes, the rolls are unrolled one by one and welded together at the ends. This creates one endless strip of strip steel. The weld is then reground to get a surface that is as smooth as possible, so that the final forming process is not disturbed.

Moulding sections rollers

At the moulding section of the production line, the strip steel is fed in via a stock loop. We work with a number of rolling mills with horizontal and vertical rollers. The flat strip steel is gradually transformed between these rollers into a round tube. They still have an open seam at this point, but this is closed later on the welding plate. The strip edges are heated here to the welding temperature of 1500°C by means of high-frequency electric currents.

Butt-welding

By pressing the sides firmly against each other with the help of pressure rollers and without additive material, a ‘butt weld’ is created. During butt welding, any contaminants in the material are pushed outwards, resulting in a good, clean weld.

Planing

The material pressed outwards forms a weld bead on both the inside and outside of the pipe, which is immediately planed off on the outside. In some cases, this is also done on the inside, for example for pipes intended for gas transport.

Calibration rollers

After the planing, the steel pipes pass by the calibration rollers. There, the pipes are brought to the correct diameter. The steel is also pulled from strip to pipe through the moulding and welding sections.

Quality control

After the calibration section, the weld seam is checked for any faults in the weld with a specially designed device. These faults are made visible on a screen. This device also checks the strength of the moving pipe. A section of pipe that does not comply with the inspection is automatically ejected from the production line. The electronic control device is called the ‘disco controller’.

The desired steel profile

It is not always about producing a perfectly round pipe. It is also possible to make pipes with a different cross-section. In the profiling installation, the round pipe that has been produced is then brought into the desired profile by roller sets. This may, for example, be a square or rectangular, a flat or elliptical-oval, tunnel or trapezoidal cross-section.

After welding, an endless round pipe or profile pipe is obtained. This can be cut to the desired length for the customer. A flying saw is used for this. This is a trailing saw that receives the ‘cut’ command when the equipment sends it. The lengths can vary from 5 to 12 meters. The cut pipes are mechanically removed from the line of the welding machine by the ejector, which is electronically coupled to the disco controller. The disco controller then selects the pipes for quality and ejects where necessary.

Steel in the annealing furnace

A part of the produced pipe still has to undergo an annealing process in order to compensate for the stresses. The exiting pipe therefore slides in 12-meter long pieces into a 60-meter annealing furnace. Here, the pipes are heated to a temperature of 960°C. In a pass-through annealing furnace, there are 35 round chambers, which are placed in sequence. In each chamber, there are 280 burners that are fired by natural gas, where each burner can burn about 7 m³ of gas per hour. An attempt is made to reduce this gas consumption by preheating the combustion air with the heat from the combustion gases already present. Normally, they would leave the chamber, but in many cases, they can be used. After the pass-flow annealing furnace, the pipe is brought to the desired diameter in a stretch-reducing roller. After cooling on an almost infinitely cool bed, cutting is done to trade lengths of 6 meters.

Inspection of steel pipes

At the ejector, a cold saw and a sample press are at the service of the sample maker. With these, the pipes from the inspection bin are tested for quality. After the inspection by the official inspection body (the TUV), the pipes can be delivered with very high quality requirements. We are talking here about pipes with a weld seam factor of 0.9 or 1. This means that the pipe has 90 to 100% of the strength of a seamless pipe. For this, one in ten pipes (and sometimes even each pipe) must be inspected.

Straightening profile pipes

The approved pipes obtained from the production must be made straight for sale. This is done in the straightening machine, which is suitable for all lengths. The straightening machine has six rollers that cold-straighten the pipes. This is done according to the same principle as with the profile straightening machine. For further finishing, the long pipes are transported to a finishing line. Various finishing processes follow there, which the long pipes must undergo in transverse transport.

In this process, the pipes first pass (again) through a cold saw and a sample press. The pipes can still be inspected if this has not yet been done at the ejector of the welding machine.
After the inspection, the pipes are flattened on the surfacing line. There, the pipes are stripped of burrs (which are formed due to sawing) and made straight. At the request of the customer, the pipes can then have one bell end created. When the pipe is belled, the end of the pipe is heated and made wider by means of a mandrel.

After the surfacing line and any belling, the pipes reach the test press. There, they are pressurized with a liquid under a pressure of up to 700 bar. In the test press, the inner shavings are also removed by rinsing.

The surfacing line, the test press and the rinse section can be moved to one side. This makes them suitable for finishing pipes from 7 to 14 meters.

The rejected pipes fall into a rejection bin. The approved pipes are first conveyed via a roller transport past a stamping machine and then to the inspection bin. The quality department can also do ultrasonic testing at the location of the stamping device.

The finishing of pipes up to a length of approx. 7 meters is done on other installations and, after straightening, also consists of surfacing and pressurizing up to a pressure of 100 bar. Moreover, it is possible to have the ends of the pipe fitted with threads. There are different ways to protect the pipes. A zinc layer may be applied, or pipes may be coated with a polyethylene layer.

The galvanization of steel pipes

Before galvanizing, the pipes are first pickled in an acid bath to remove the iron oxides that have formed. The iron oxides bond with hydrochloric acid to form FeCl2 (FeCl₂) . An inhibitor or pickling inhibitor in the hydrochloric acid ensures that no separation of hydrogen gas takes place. After the pickling bath, the pipes are rinsed with warm water and then dried in a drying furnace. After the drying furnace, the pipes enter a flux bath. There must be a layer of flux on the steel surface because the reaction of the zinc with the iron otherwise cannot proceed properly. The flux consists of an aqueous solution of double salt in water. Double salt is a mixed crystal of one part ZnCl2 and two parts NH4Cl2, hence the name double salt. The two salts can crystallize with each other in almost all ratios.

The galvanizing is done in liquid zinc with a temperature of around 450°C; that is to say, at a temperature that is approximately 30°C higher than the melting point of zinc. When a steel object is immersed in this liquid zinc, various reactions occur on the steel surface.

With dry galvanizing, the relatively cold steel object is immersed directly in liquid zinc. The melting point of the flux is about 230°C, and the melting flux salt exerts a weak pickling effect on the steel surface. This removes any residual iron oxides that may still be present. The flux releases from the surface and floats upwards; a clean surface is then left behind. The liquid zinc reacts spontaneously to the iron in the steel and thus forms zinc/iron alloy layers. During this reaction, bubbling at the zinc surface is observed, this is also called boiling off.
When this boil-off has ceased, the object has reached the temperature of the zinc. The object can now be removed from the zinc. The duration of heating, called immersion time, depends greatly on the heat content of the object to be galvanized.