Minimizing Distortion
Some fabricated assemblies may distort at the galvanizing temperature as a result of relieving stresses induced during steel manufacturing operations. A channel frame with a plate, for example, should be galvanized separately and bolted or welded later rather than welded together before galvanizing.
Guidelines for minimizing distortion and warpage are provided in ASTM A 384, Safeguarding Against Warpage and Distortion During Hot-Dip Galvanizing of Steel Assemblies, and CSA Specification G 164, Hot Dip Galvanizing of Irregularly Shaped Articles.
To minimize changes to shape and/or alignment, design engineers should observe the following recommendations:
- Where possible, use symmetrically rolled sections instead of angle or channel frames. I beams are preferred to angles or channels.
- Use parts in an assembly that are of equal or near equal thickness, especially at joints (see Figure 24).
- Bend members to the largest acceptable radii to minimize local stress concentration.
- Accurately pre-form members of an assembly so that it is not necessary to force, spring, or bend into position during joining.
Continuously weld joints using balanced welding techniques to reduce uneven thermal stresses. Pinholes from welding are very dangerous in items to be galvanized and must be avoided. Staggered welding technique to produce a continuous weld are acceptable. For staggered welding of 1/8-inch (4 mm) or lighter material, weld center should be closer than 4 inches (19 cm).
Avoid designs that require progressive-dip galvanizing. It is better to build assemblies and subassemblies in suitable modules so they can be immersed quickly and galvanized in a single dip. That way, the entire fabrication can expand and contract uniformly. Where progressive-dip galvanizing is required, consult your galvanizer.
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