Pre-galvanizing Considerations for Control of Distortion & Warping
When the decision is made to hot-dip galvanize, the design engineer should ensure that the pieces can be suitably fabricated for highest-quality galvanizing. Best practice suggests steel to be galvanized should be symmetrical and of similar thickness.
With proper consideration and understanding of how the hot-dip galvanizing process affects steel, asymmetrical designs or structures containing sections of unequal thickness can be successfully galvanized, as can fabrications where cold-working techniques (bending, hole-punching, rolling, shearing) are employed.
Steel being galvanized progresses through a temperature cycle upon immersion into and withdrawal from the galvanizing bath. Because parts are immersed at an angle, uneven heating occurs, creating a temperature profile along the part being galvanized. This temperature profile allows the steel’s internal stresses to be relieved at different times in the immersion cycle. These stresses may cause changes in shape and/or alignment (distortion and warping).
The following steps can be taken to minimize this risk:
- Use assembly parts that are of equal or near-equal thickness, especially at joints
- Use standard rolled shapes
- Use symmetrically rolled sections rather than angle or channel frames; I-beams also are preferred to angles or channels
- Galvanize a channel frame with a plate separately and bolt or weld post-galvanizing
- 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 them into position during joining
- Avoid designs that, because of size, require more than one dip into the molten zinc bath (“progressive galvanizing,” sometimes called “double-dipping”)
- Relieve welding- or cold-working-induced stresses by heat-treating the part at 1100 F (590 C) for one hour per inch (2.5 cm) of section thickness
Consult your galvanizer regarding the use of temporary bracing or reinforcing.
The guidelines for safeguarding against warping and distortion during hot-dip galvanizing of steel assemblies are outlined in ASTM A 384.