Welding

Specificities of the welding of stainless steels

Stainless steels contain at least 10.5% of chromium, making them highly oxidizable in the molten state. If they are not protected from the air, welding will lead to a loss of chromium and the formation of oxides, resulting in a lack of soundness and decreased corrosion resistance in the weld. Protection must therfore be provided during the welding operation to restrict or eliminate oxidation  of the weld surface and the neighboring regions. This also facilitates surface finishing in the weld zone (pickling or polishing).

Resistance welding

> General information

In this process, an electric current is made to pass through the parts to be joined and welding is produced by the resultant resistive heating. Numerous resistance welding techniques exist, the most common ones being spot welding and seam welding. 

Continuous seam welding
Discontinuous seam welding

                                                         

> Advantages of the resistance welding 

The advantages of this type of weld are considerable: 

- Very slight structural modifications in the vicinity of the weld.
- “Forging” deformation during welding, particularly useful for the joining of ferritic steels.
- No or very slight surface oxidation, due to short heating times (spot welding and water cooling).
- Low surface relief at the weld spots, due to an appropriate choice of electrodes (tip shapes) and power input (avoidance of overheating).

> Welding parameters

Welding parameters need to be defined for each stainless type to be joined.
In practice, the force between electrodes or between electrode wheels must always be adjusted so as to avoid “projections”, and must not be excessive in order to prevent “sticking”. All adjustments must be verified by tearing apart a sample weldment. Maintaing the electrode pressure after the welding phase accentuates the forging effect.
In seam welding, depending on the requirements, it is possible to make either a discontinuous “tack” weld or a continuous, perfectly leakproof joint.
It is possible with both the spot and seam welding processes to join sheets of thicknesses up to 2.5 mm thick each.
 
Resistance spot welding