Hot-Dip Galvanized Steel - Avoiding Distortion During Processing

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When steel fabrications or sections are immersed in molten zinc, their temperature increases to that of the molten zinc, which is usually 455 °C. The rate at which the steel attains this temperature across its total surface is dependent on:

• The total mass of the item
• The thickness of the separate sections
• The dimensions of the item

There is no variation in the metallurgical microstructure of the steel at galvanizing temperatures, and the galvanizing process is not sufficiently hot to cause any heat-treating effects on the mechanical properties of the steel following galvanization.

By contrast, the steel’s yield strength drops by about 50% at galvanizing temperatures. If the nearby steel is not at the same temperature and any stresses are present, the weaker area will be made to move by the stronger area. The designer, the fabricator, and the galvanizer are accountable and have to co-operate to make sure that distortion hazards are curtailed or eliminated.

Basic Design Rules

1. Plan to use sections of even thickness all through the fabrication.
2. Ensure welding and assembly methods lessen stresses in parts that make up the item.
3. Ensure that venting and draining are sufficient. This will enable the item to be immersed in and taken out from the molten zinc as rapidly as possible.
4. The item’s structural design has to be adequate to support its own weight at 50% of the steel’s stated yield strength. Temporary bracing must be considered if potential to yield is present.
5. Avoid using large areas of thin flat plate measuring below 8 mm.
6. Guillotine cut plate is desirable than oxy cut plate.

Case Study

The distortion pattern in a 3-mm floorplate is evidently the result of the welding method applied. Attaching the channels with fasteners following the galvanization process would considerably decrease the risk of distortion.