When a product is hot dip galvanized it is submerged into molten zinc at 450°C. This temperature is at the lower end of the stress relieving range for heat treating of steels, which will relieve any inherent stresses in the steel. This could result in a dimensional change of a product, perceived as distortion if a change in shape of the product takes place.
Internal stresses to some degree are present in most steel structures and as a rule are not a problem when hot dip galvanized. Internal stresses are mainly in equilibrium with each other and would not result in distortion.
Some sources of internal stresses are:
- Residual stresses induced at the mill during rolling of structural section or plate
- Residual stresses created by bending or welding
- Lack of symmetry in sections such as channels or in built up sections
- Combination of thick and thin material in the same assembly
- Assemblies so large that they require double dipping to cover the entire surface of the product in the zinc kettle.
Explosions in the zinc kettle due to Insufficient venting holes can also contribute to distortion, especially when moisture has been trapped. (Refer to previous article)
Large overlapping surfaces which are seal welded can also contribute to distortion, should moisture be able to get in between the overlapped surfaces through a pin–hole or porosity in the weld.