Welded tubes imply a welding process for at least one step in the manufacturing process, typically, in forming a closed tube profile.

According to the method of preparation of a tube profile in regards to its welding and the position of a welded seam, the tubes are divided into:

  • LPD large diameter pipes - LDP.
  • Longitudinal: velded with multiple (both longitudinal and transverse) welded seams (the tube profile is formed and shaped by several steel plates in a certain way) - usually, LDP; with one longitudinal welded seam, the profile for welding is formed by sheet or a mill in a continuous cold rolling strip - the BaltInox company uses the latter method in the production of welded tubes. See the description of the production process concerning the production of tubes in mills of continuous welding.

Scope of application of welded stainless steel tubes

Welded stainless steel tubes are used where high corrosion resistance, an attractive appearance and a small cost of care are required.

Historically, the main customers of stainless steel tubes were the chemical and food industries, engineering, and later, after stainless steel ceased to be a "new" and prohibitively expensive material, the application of stainless steel tubes increased significantly, mainly due to the designs and products where the required pipe is not involved in the use of the TRANSPORT function of a tube, i.e. due to those areas of economic activity that use so-called ornamental tube.

Let’s consider the scope of the use of stainless steel tube by industry of economic activity.
1. Tubes for mechanical applications, so-called Ornamental tubes, are used where the ability to withstand mechanical stress, corrosion resistance, matching the geometric dimensions required by the standard, and the quality of the outer surface are required i.e. these are all the requirements for stainless steel tubes with the exception of mandatory use as a transport pipeline.
Accordingly, this type of tube is produced by any welding method suitable for the manufacture of welded stainless steel tubes that require carefully controlling the geometrical dimensions of the pipe according to EN ISO 1127.
For the tubes outside with a diameter of less than 168 mm, the D3/T3 class is used, and for pipe with a diameter greater than 168 mm, the D2/T3 class is used.
Ornamental tubes are made with round, square, rectangular or shaped profile sections with an untreated (mill finish), grinded or polished surface, depending on the requirements of the order.
Main applications of ornamental tubes:
A) Construction (handrails, interior and exterior parts of buildings, fences, columns, etc., weight-bearing structures).
B) Production of neutral equipment: manufacturing of kitchen equipment, furniture, storage structures.
C) Any other application where high corrosion resistance and a good product appearance are required.
Ornamental tubes are produced according to the following standards - DIN17455, ASTM A554, EN10296-2.
Tubes for fluid transport - industrial tubes - round tubes with an untreated (mill finish), grinded or polished surface - depending on the order specifications. The most widely used standard for this tube is EN10217-7, tubes for pressure, part 7, stainless steel tube. After welding, the tubes, depending on the application, may be subjected to further processing for improving the operational parameters of the pipe: the welded seam inside the tube may be normalized for its dimensions (partially) removed and / or by phase structure steel (annealled).
For types of heat treatment and its effect on the improvement of pipe performance – see heat treatment.

Major applications of these tubes:

A) Pipes for water supply (including all types of equipment for bathrooms), chemical and petrochemical industries; manufacturing standards: EN10217- 7 (replaced DIN 17457), ASTM A268, A269, A778. Typically, this is a tube without additional heat treatment (annealling) without inside bead remove.
B) Tubes for evaporators, heat exchangers and boilers; they are used in the processing equipment of chemical and petrochemical industries, pulp and paper production, food equipment (production of sugar and powdered milk), any type of evaporators, equipment for power plants, etc. Manufacturing standards - EN10217-7 and ASTM / ASME A249, the pipes are usually supplied in heat-treated (annealled) inside bead remove.
C) Tubes for tubular electric heaters, usually of small diameter, thin-walled pipes, from 6 to 20mm. Manufacturing standards - EN10217-7 and ASTM778. Typically, this is a pipe without additional heat treatment (annealling) without inside bead remove.

Dairy and pharmaceutical pipe: also known as sanitary tube. It is used in the manufacturing of the food processing equipment industry (dairy and beverage industry) and in the pipelines of food and pharmaceutical plants. It is also used wherever a smooth inner surface of the tube is necessary. Typically, this tube is without heat treatment (annealled) on demand, with inside bead remove. The surface, by default, may be subjected to orbital grinding, polishing - on demand. Production standards: EN 10217-7, DIN EN11850.
Main application of the tube - line tube for dairies, evaporators and other applications where dairy products come in contact with the inside tube surfaces.

These are welded stainless steel tubes of high accuracy and high purity classification (low roughness) of the inside surface of the pipe. This is necessary in order to comply with the requirements for a good inside surface of the tube to be met for customers and milk processing equipment standards as regulated by the tube and the equipment in general.

To achieve the above requirements, it is necessary to produce lactic electric-welded tubes made from cold-rolled stainless steel flat product and by carrying out a normalization welding seam (bringing it even with the surface level of the tube wall and the reduction of its roughness to the required standard).

Normalization is performed by drawing, rolling or by any other means such as temperature/cold pressure reduction during the pipe production.

Tube Production in Continuous Tube Welding Lines

The BaltInox company produces welded stainless tubes by the TIG method in continuous tube welding lines.

Welding line scheme (figure)

The welding line consists of the following technical and functional components:

1. A container where rolls of steel tape are placed; the tape goes from the container to an accumulator.
2. An accumulator designed for the accumulation of the necessary amount of tape to ensure the continuity of tape rolling in the rolling mill during the docking tape rolls (when one roll of tape ends, it is joined to another). The amount of tape placed to the accumulator is determined by the speed of tape rolling and the time necessary for joining the tape.
3. The joining of the tape ends is made by argon-arc welding in the tape joining device.
4. After passing through the guide rollers, the belt goes into the molding and finishing group of stands of the drive (motorized) and non-drive (idle) of the "Duo" type rolling stands – where each stand is provided with two rolling rolls. After passing through the aforementioned stands, the automatic welding of the edges of a formed tubular profile is performed.
5. Welding is performed by the AAW method, allowing it to achieve the same mechanical strength as of the pipe itself in the welding area and in the solid metal area of as well.
6. After welding, the unrolling of the internally welded seam is carried out (optional) by means of cold drawing.
7. At the same time, a continuous monitoring of the integrity of a welded seam is carried out in a non-destructive way by using the eddy current method.
8. Weld seams or marks outside the pipe are polished by means of a device for the removal of external burrs.
9. The pipe profile that has been prepared through this method goes to a group of gauge stands that are driven (motorized) or non-driven (idle) to rolling stands of the "Duo" type - each stand is provided with two rolling rolls. In each gauge group, cold deformation of the welded profile occurs - reducing the pipe diameter and wall thickness to the desired values - the process is generally similar to the rolling of seamless pipes on a rolling mill with two-roll stands.
10. After the gauge stands, the pipe passes through a set of four non-driven roller stands - round - heads designed to make the pipe straight.
11. Further on, the pipe surface is polished with an orbital grinding machine (optional).
12. Then the pipe is cut into pieces of the desired lengths in a pipe cutting device.
13. And finally enters the discharge roller table for the discharge of the pipes.
14. Sampling for the testing of mechanical properties and the tool control of the pipe geometry is performed.
15. Quality certificates are issued
16. The ready pipe is packed in bundles and stored in the warehouse – it is ready for shipment to the consumer.