Steel goes into the kettle at ambient and reaches 450 °C in minutes. Anything that heats unevenly, or is holding stress it has not been allowed to release, will move.
Two things cause distortion, and they compound. The first is differential heating: a thick part of an assembly reaches bath temperature well after a thin part, so one expands while the other has not yet started.
The second is residual stress. Cold rolling, shearing, punching and above all welding lock stress into a fabrication. At 450 °C the steel's yield strength falls sharply and that stress finally gets to relieve itself — by moving the piece.
Galvanizing does not create the stress. It reveals it. A member that distorts in the kettle was carrying that distortion the whole time.
| Feature | Why it moves | What helps |
|---|---|---|
| Large flat unstiffened plate | Nothing resists differential expansion across the area | Stiffeners, or a thicker plate, or accept and specify a straightening allowance |
| Assembly mixing thick and thin members | The thin parts reach temperature first and push | Balance section thicknesses where the design allows; discuss hanging with the galvanizer |
| Asymmetric or single-sided section | Heats and expands unevenly about its own axis | Symmetry in the design; a stiffening arrangement that resists the twist |
| Heavily welded fabrication | Welding locks in stress that releases at bath temperature | Balanced weld sequence, welding from the centre outward, avoiding over-welding |
| Long slender member | Sags under its own weight at temperature | Hanging point and orientation — ours to get right, but tell us if it is critical |
| Cold-worked or heavily punched material | Cold work is stored stress | Drill rather than punch where the design permits, or stress-relieve before dipping |
ASTM A385 covers most of this from the design side. The practical version: symmetry, stiffness and balanced welding, decided at the drawing stage.
Straightening galvanized steel is possible but not free. The coating is hardest at the alloy layers and will crack if the member is bent sharply, so straightening has to be gradual and the affected area then inspected and, if necessary, repaired to ASTM A780.
Which is why the right time to deal with this is at the enquiry. Send the drawing. We will tell you where we think a piece will move, how we intend to hang it, and whether a stiffener or a changed weld sequence would settle the question before any steel is cut. That review costs nothing.
Answered from our own line, not from the standard's cover page.
Two reasons that compound: differential heating, where thick and thin parts of an assembly reach 450 °C at different times, and the release of residual stress from cold work and welding, which the steel can finally relieve once its yield strength falls at bath temperature.
No. It reveals stress that was already locked in by rolling, shearing, punching and welding. The kettle is simply the first time the fabrication gets hot enough to let it go.
Mostly at the drawing board: symmetry, adequate stiffening, avoiding large unstiffened plates and assemblies that mix very thick and very thin members. Then at fabrication: balanced weld sequence, controlled heat input, drilling rather than punching where possible.
Gradually, yes — but the coating will crack if the member is bent sharply, and the affected area then needs inspection and repair to ASTM A780. It is far cheaper to prevent than to fix.
Yes, if you send the drawing with the enquiry. Because our kettle is in-house, the person who will hang the piece can look at it before it is fabricated. That review is free.
Tell us the standard, the coating requirement and the component, and we will confirm in writing what we can hold and how we will certify it. Galvanizing is on our own floor, so the answer comes from us and not from a sub-contractor.
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