Forming

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Forming describes a method used in electric welding to protect the part of the weld seam facing away from the welding electrode from oxidation and scaling . The root protection is mainly used for high-alloy materials such as stainless steels or Ni-alloys and parts made of stainless steels used.

Typical materials that have to be formed include:

  • X10CrMoVNb9-1 (1.4903)
  • X10CrWMoVNb9-2 (1.4901)
  • X20CrMoV11-1 (1.4922)
  • X12CrCoWMoVNb12-2-2 (1.4915)
  • NiCr23Co12Mo (2.4663)

With these materials, the corrosion resistance is already significantly reduced by moderate oxidation, which is visible as a tarnish or tempering color .

If a nitrogen-containing protective gas for the roots is used in titanium-stabilized steels, the roots will turn yellow due to the formation of titanium nitride. These are not tarnishing colors in the above sense.

During the forming process, protective gases wash around the highly heated seam root and seam edge areas and thus displace the air atmosphere. Inert or hydrogen-containing gases are used for root protection. The addition of hydrogen to the root protective gas (forming gas) binds the residual oxygen and improves root formation.

The forming gases are classified in EN ISO 14175. (Example names)

  • I1, argon, suitable for all materials
  • N1, nitrogen, austenitic CrNi steels, duplex steels
  • N5, nitrogen / hydrogen, austenitic CrNi steels
  • R2, argon / hydrogen, austenitic CrNi steels, nickel and nickel-based materials
  • N2, argon / nitrogen, austenitic CrNi steels, duplex and superduplex steels

The choice of protective gas depends, among other things, on the following factors:

  • Type of material to be processed
  • Welding process
  • if necessary, material transfer during welding
  • Material thickness

Forming is mainly used for hollow bodies (pipes and containers) because the back of the seam is not accessible here and subsequent removal of the oxide layers is therefore not practical. Before the welding process, the container or the pipe is sealed and filled with forming gas . Flushing gas continues to be used during welding. This prevents atmospheric oxygen from reaching the back of the seam. The hydrogen content also has a reducing effect and therefore does not allow the formation of oxides.

So-called forming chambers are built to seal the pipes or other hollow bodies. These can, for example, consist of:

  • water-soluble forming paper
  • Plastic forming chambers
  • foam
  • water-soluble forming balls

consist

Individual evidence

  1. ↑ Dipl.-Ing./EWE Thomas Ammann: White Paper. Forming during welding. In: formieren.com. Linde AG, April 2012, accessed on February 5, 2019 .