Will it fit the kettle?
The first design decision is whether the fabrication fits the kettle in one dip. Check it against the kettle’s working dimensions, not its nominal ones, before the steel leaves the shop, and see what tilting, progressive dipping or a redesign would mean.
“Tight” takes the larger clearances (length and width less 200 mm, depth less 300 mm); “generous” the smaller ones (100, 100 and 200 mm). The piece’s length is checked hot, about 5 mm longer per metre at 450 °C.
Working-dimension reductions, thermal growth and the overlap band from the HDG book, Chapter 9. Distortion factors per ISO 14713-2 clause 4.4. Confirm every borderline fit with the galvanizer.
Working dimensions, not nominal ones
A kettle’s nominal dimensions are the inside of the steel vessel. The working dimensions are smaller, because the piece must clear the walls during immersion and withdrawal and stay clear of the surface while it is tilted. As a rule of thumb, working length and width are 100–200 mm less than nominal and working depth 200–300 mm less. Ask the galvanizer for the working dimensions and design to those.
Where a piece will not fit, the standard answer is modular fabrication: sub-assemblies sized for a single dip, bolted after galvanizing. It also simplifies transport and lets each module be vented and lifted properly. Bolting is preferred to field welding, which burns the coating and needs a repair.
Progressive (double-end) dipping is the option of last resort: one end is dipped, the piece is reversed, and the other end is dipped. Steel grows about 5 mm per metre on its way to 450 °C, and with half the piece hot and half cold the strain can distort it permanently. The overlap band, typically 200–400 mm wide, gets two thermal cycles and two zinc deposits and always shows. It is an accepted feature of the process, not a defect, but the customer has to be told before the order, in writing.
ISO 14713-2 adds the design side: symmetrical sections are preferred, large variations in thickness such as thin sheet welded to thick angles should be avoided, and residual stress relieved at galvanizing temperature is one of the main causes of unexpected distortion. ASTM A384 is the ASTM practice on safeguarding against warpage and distortion.
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Questions
What are the working dimensions of a galvanizing kettle?
The space a piece can actually use. As a rule of thumb, working length and working width are 100–200 mm less than the kettle’s nominal inside dimensions, and working depth is 200–300 mm less, depending on freeboard and the geometry of the work. Ask the galvanizer for the working dimensions, not the nominal ones.
What is progressive or double-end dipping?
Galvanizing a piece too long for the kettle in two stages: one end is dipped, the piece is reversed, and the other end is dipped. It is the option of last resort. The overlap band, typically 200–400 mm wide, receives two thermal cycles and two zinc deposits and is always visible.
Why does progressive dipping cause distortion?
Steel expands roughly 5 mm per metre as it heats from ambient to 450 °C. During progressive dipping one part of the assembly is at 450 °C while the rest is at ambient, and the differential strain can exceed the steel’s reduced yield strength at temperature, leaving distortion that does not recover on cooling.
What should I do if the piece does not fit?
Design it in modules: sub-assemblies sized for a single dip, bolted together after galvanizing. Bolting is preferred to field welding, which burns the coating and has to be repaired. If progressive dipping cannot be avoided, keep the piece symmetrical, vent and drain it for both orientations, and mark the overlap band on the drawing where it will not show.