Hot-Dip Galvanized Stamped and Deep-Drawn Steel Parts: What to Specify
Where this question comes from
In October 2026 a European outdoor-furniture company sent us 2D drawings for two formed steel parts, a cap and a hat-shaped part, at about 6,000 pieces per year each. Their RFQ asked for hot-dip galvanizing after all forming, drilling, threading and welding, the zinc standard and thickness, and whether the cap could be deep drawn without welding. Those are the right questions for any galvanized pressed part, so the answers are collected here.
Process order for a galvanized pressed part
- Blanking and forming: single-stage or progressive stamping for flat and bent parts, deep drawing for caps, cups and shells.
- Trimming, piercing and drilling.
- Welding of any brackets, nuts or tubes, with slag and anti-spatter residue removed.
- Threading where the thread is cut before coating (see the threads section below).
- Hot-dip galvanizing as a batch process: degreasing, pickling, fluxing, then dipping in molten zinc at about 450 °C.
- Post-galvanizing work: re-tapping internal threads, removing zinc drips from functional faces, inspection of coating thickness.
Galvanizing after fabrication coats the cut edges, hole walls and welds that pre-galvanized sheet would leave bare. Hot-dip galvanizing is done in specialist plants; we arrange it for steel parts through galvanizing plants in our production network across Zhejiang, Jiangsu, Shandong, Guangdong and Fujian, and confirm the route and plant in the quotation.
Zinc thickness under ISO 1461
ISO 1461 sets the minimum coating by steel thickness. For parts that are not centrifuged:
| Steel thickness | Minimum local coating | Minimum mean coating |
|---|---|---|
| Below 1.5 mm | 35 µm | 45 µm |
| 1.5 mm to 3 mm | 45 µm | 55 µm |
| Over 3 mm to 6 mm | 55 µm | 70 µm |
| Over 6 mm | 70 µm | 85 µm |
Small threaded parts that are spun in a centrifuge after dipping have lower minimums. These are minimums: the real thickness is often higher, and it is checked with a magnetic coating gauge on sample parts from each batch. If you need a thicker coating than the standard minimum, write the required value on the drawing.
Steel grade: why chemistry matters
Silicon and phosphorus in the steel change how fast zinc and iron react. Some silicon levels give a thick, dull grey and more brittle coating instead of a bright, even one. For deep-drawn parts the usual choice is a cold-rolled deep-drawing grade (DC01 to DC04, or SPCC/SPCD), which is low in silicon and galvanizes evenly. Hot-rolled structural steel such as Q235 or S235 is common for thicker pressed parts; ask for the mill certificate so the galvanizer knows the silicon content before dipping.
Threads and holes
- Internal threads are normally tapped or re-tapped after galvanizing, because zinc fills the thread. The re-tapped thread is protected by the zinc on the mating bolt.
- External threads are centrifuged or brushed after dipping, or the mating nut is tapped oversize.
- Close-fitting holes lose roughly twice the coating thickness in diameter; drill them oversize or ream after galvanizing, and mark which holes are functional.
- Welded nuts are welded before galvanizing and re-tapped afterwards.
Venting, drainage and distortion
Molten zinc and cleaning liquids must flow in and out of every cavity. Open caps and hat sections drain well, but an inward-turned flange, a pocket formed by a welded plate or an overlapping joint can trap air, flux or zinc. Add vent and drain holes at the high and low points of the part as it hangs in the bath; the galvanizer can advise on size and position.
Dipping at about 450 °C can release stresses from forming and welding. Large flat areas of thin sheet are the most likely to warp; drawn shapes, flanges and ribs are much stiffer. If flatness matters, give a flatness tolerance and ask for it to be reviewed before tooling.
Deep drawing a seamless cap instead of welding it
A one-piece drawn cap avoids a weld seam, which also galvanizes more cleanly. Whether it needs one draw or several depends on the ratio between blank diameter and cap diameter, the height, the corner and flange radii and the steel grade. As a rough guide, a first draw reaches a blank-to-punch diameter ratio of about 1.8–2.0 in deep-drawing steel; each redraw then reduces the diameter by a smaller step. Shallow caps are usually one draw plus trimming; tall caps take one or more redraws. Hengya draws parts up to 400 mm deep and holds critical dimensions to ±0.01 mm; the draw sequence for your part is shown in the DFM review.
Tooling and annual volume
At several thousand pieces per year, a dedicated die for each part is normally worth it, and progressive or transfer tooling may combine forming and piercing. Tooling usually takes 15–25 days after drawing approval, and the die remains the customer's property. Unit prices are quoted for your batch sizes (for example 1,000 / 3,000 / 6,000 pcs); the main cost drivers are steel thickness and blank size, number of forming stages, welding and threading operations, and galvanizing, which is usually charged by weight.
What to send for a quote
- 2D drawings with all dimensions, tolerances, material and thickness; a STEP file if you have one (not required to start)
- Galvanizing standard (ISO 1461 or ASTM A123) and any required coating thickness
- Which threads, holes and faces must fit after galvanizing
- Batch quantity and annual quantity, and delivery country
We reply with a quote and DFM review within 24 hours. An NDA can be signed before we review your drawings.
Frequently asked questions
Can stamped or deep-drawn steel parts be hot-dip galvanized after forming?
Yes. The usual order is forming, piercing and drilling, welding and threading first, then batch hot-dip galvanizing as the last step, so that every cut edge, hole and weld is coated.
How thick is the zinc coating on galvanized sheet-metal parts?
Under ISO 1461 the minimum mean coating for non-centrifuged steel is 45 µm below 1.5 mm steel thickness, 55 µm for 1.5–3 mm, 70 µm for over 3 to 6 mm and 85 µm above 6 mm. Steel chemistry, especially silicon and phosphorus, can make the coating much thicker than the minimum.
What happens to threads and holes when a part is hot-dip galvanized?
Zinc builds up in threads and reduces hole diameters, so internal threads are normally tapped or re-tapped after galvanizing, external threads are centrifuged or cleaned, and close-fitting holes are drilled oversize or reamed afterwards. Mark these features on the drawing.
Can a steel cap be deep drawn in one piece instead of welded?
Often yes. Whether it takes one draw or several depends on the ratio of blank diameter to cap diameter, the height, the corner radii and the steel grade. As a rough guide a first draw reaches a blank-to-punch ratio of about 1.8–2.0 in deep-drawing steel, and taller caps need one or more redraws. The number of stages is confirmed in the DFM review.
Can you quote from 2D drawings only?
Yes, if the 2D drawings are fully dimensioned with material, thickness, tolerances and finish. A STEP file helps the engineers check draw stages and blank size faster, but it is not required to start a quotation.
Send your drawing for a quote
Send 2D drawings (and STEP if available) with material, thickness, galvanizing standard, quantities and delivery country. We reply with a quote and a DFM review within 24 hours.