Moulding: For which materials is it important?

Forming and its significance for different materials

In welding, forming - i.e. protecting the weld seam from oxygen using special gases - is a crucial process for preventing corrosion and ensuring the quality of the weld seam. However, not all materials require the same forming conditions. In this article, we will look specifically at the materials for which forming is of great importance.

 

High-alloy steels & stainless steels: Forming requirements

High-alloy steels such as CrNi steels (chromium-nickel alloyed steels) and stainless steels are particularly sensitive to corrosion after heat treatment such as welding. These materials form a thin passive layer, which is crucial for their corrosion resistance. During the welding process, this layer can be destroyed by contact with oxygen.

Correct forming is essential to protect the weld seam from oxidation and to ensure the formation of the passive layer. The use of inert gases such as argon, nitrogen or special gas mixtures (e.g. argon-hydrogen mixture) displaces the oxygen in the weld joint and reduces the risk of tarnish colours and susceptibility to corrosion. The choice of the right forming gas depends on various factors, including the type of material, the welding process and the material thickness.

Forming plays a particularly important role for austenitic chromium-nickel steels and duplex steels. These materials are sensitive to oxidation, which can impair their corrosion resistance and strength. Optimum protection of the weld seam and the heat-affected zone is therefore essential. This can be ensured by a laminar supply of shielding gas to prevent turbulence and mixing with the atmosphere

 

Titanium and its special features during moulding

Titanium poses a particular challenge when moulding as it has a high affinity for oxygen and nitrogen. Even the smallest amounts of oxygen can lead to a significant deterioration in material properties, including discolouration or a reduction in mechanical strength. During welding, titanium must be flushed with an inert gas (e.g. argon) to prevent oxidation and discolouration, which can impair the quality of the weld seam.

 

Nickel-based alloys & corrosion-resistant materials

Nickel-based alloys, such as NiCr23Co12Mo (material number 2.4663), also require careful moulding. These materials are often resistant to high temperatures and corrosion. In order to maintain these properties during welding, the use of suitable forming gases is crucial. By excluding oxygen, the formation of undesirable oxides can be prevented and the strength of the weld seam ensured.

 

Examples of materials that require forming

There are specific materials for which forming is absolutely necessary to guarantee the welding quality. Here are a few examples:

  • 10CrMoVNb9-1 (1.4903)
  • 10CrWMoVNb9-2 (1.4901)
  • 20CrMoV11-1 (1.4922)
  • X12CrCoWMoVNb12-2-2 (1.4915)
  • NiCr23Co12Mo (2.4663)

     

Each of these materials is characterised by high corrosion resistance, which is maintained by the use of shielding gases during welding

 

Discolouration and its significance for different materials

Discolouration or tarnishing is often the first sign of oxidation in weld seams. In the case of high-alloy steels, stainless steels or nickel alloys, even slight discolouration can significantly impair corrosion resistance. In the case of titanium, a yellowish discolouration is often a sign of titanium nitride formation, but this is not necessarily an indication of oxidation. This discolouration can reduce the quality of the weld seam and should be prevented by correct forming.

 

Conclusion: Optimum forming for higher material quality

Forming is a crucial process for maintaining the corrosion resistance and mechanical properties of weld seams. Particularly with high-alloy steels, stainless steels, titanium and nickel-based alloys, the use of suitable forming gases and techniques is essential to ensure high-quality welded joints.

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