Galvanizing Warping Panel

[kaizer7]

Hello,

I've got a weldment thats about 10'x10'x1', that is made up of several rows of welded webs in the vertical and horizontal direction. It looks like a 3 dimensional grid. One side is 3/8" plate, and after galvanizing, in each "square" the plate has bubbled inwards.

Is there any way to eliminate this deformation? - Some stress relief before the galvanizing process ??

The opposite side only has flanges mating together, so it does not suffer from the same deformation.

 

[Rich2001]

Stress relieving is of limited use, most of your deformation is caused by thermal stresses due to the dipping process. Different material thicknesses results in a varying temperature distribution, causing plastic deformatation. On method that might help is to find a galvanizer that has a preheater, to heat the steel up to 100-200C prior to dipping. This reduces the thermal stresses. Preheating is common in Europe but I don't know of any Galvanizers in the North America that have a preheater.

Some galvanizers will claim that they hold the assemble over the dip tank to preheat, which would work if they held it there for sevarl hours/ days for it to heat up completely. Check the temperature at various location with an IR thermometer.

 

[moltenmetal]

Do you need the heavy zinc deposition that hot dip galvanizing provides? Can you do something else, like sandblast and coat with a zinc-rich primer, or perhaps electrogalvanize instead?

 

[kaizer7]

moltenmetal,

We usually paint our parts with a zinc-rich primer, epoxy, urethane top-coat, etc, but occasionally we are required to use hot-dip galvanizing.

 

[rnd2]

kaizer7
What you have here starts with the welding process.
The welders are careful enough to place only sufficient weld so at room temp the plate doesn't buckle. When the whole part is heated by molten zinc there is only mild stress relief. The welds, which retain differential elastic tension relative to the weldment are able to pull the heat-relaxed plate inwards (the welds are fillets, yes?
Things to try:
1) Heat the entire weldment to above 700 deg. C This will allow the welds and the weldment to stress equalize. At that temp press flat and hold. Allow to cool before galvanising.
2) Move up to 1/2" plate which will reduce but not eliminate the buckling.
3) Re-design by avoiding as much as possible fillet welds on only one side of the plate