Background

When steel sections or fabrications are immersed in molten zinc, their temperature is raised to that of the molten zinc which is typically 455°C. The rate at which the steel will reach this temperature across its entire surface will depend on:

· The thickness of the individual sections making

· The total mass of the item

· The dimension of the item

At galvanizing temperatures, there is no change to steel’s metallurgical microstructure and the galvanizing process is not hot enough to have any heat treating effects on the mechanical properties of the steel after galvanizing.

However, at galvanizing temperatures, the yield strength of steel is lowered by approximately 50%. If the adjacent steel is not at the same temperature and any stresses exist, the weaker area will be subject to movement by the stronger area. There is a responsibility on the designer, the fabricator and the galvaniser to co-operate in ensuring that distortion risks are minimised or eliminated.
Basic Design Rules

1. Design to use uniform thickness sections throughout the fabrication.

2. Ensure welding and assembly techniques minimize stresses in components making up the item.

3. Ensure that venting and draining are adequate. This will allow the item to be immersed in and withdrawn from the molten zinc as quickly as possible.

4. Ensure that the structural design of the item is sufficient to support its own weight at 50% of the steel’s specified yield strength. Consider temporary bracing if potential to yield exists.

5. Avoid using large areas of thin (under 8 mm) flat plate.

6. Guillotine cut plate is preferred to oxy cut plate.
Case Study

The pattern of distortion in this 3 mm floorplate is clearly the result of the welding technique used. Attaching the channels with fasteners after galvanizing would significantly reduce the risk of distorting.