Warm forming — the stress you do not build in
A kettle spends its life at 450 °C under hydrostatic load. Residual stress from manufacturing does not stay put under those conditions.
How you check this on any supplier, including us. Warm forming leaves evidence and cold forming leaves different evidence. Ask for the forming temperature record, ask what the pyrometer read, and look at the corner: warm-formed plate keeps an even surface through the bend, while heavily cold-worked plate shows its strain at the outer radius.
It is also visible in the numbers. Cold forming needs a bend radius of at least 3× plate thickness; warm forming gets to 2.5×. A supplier quoting you a tight corner radius on a thick wall is telling you something about their process whether they mean to or not.
Why cold bending is the wrong answer here
Cold forming a 50 mm plate to a 150 mm radius puts a large amount of locked-in strain into the corner — exactly where the kettle will later carry the highest hydrostatic load and the highest thermal gradient.
Heat it first and the same shape is achieved with a fraction of the residual stress. This is not a refinement; it is the difference between a corner that stays where you put it and one that moves in service.
| Parameter | Value | Source |
|---|---|---|
| Plate heating temperature | 850 – 900 °C | BG/T 1501-2015 §5.3.1 |
| Temperature monitoring | Infrared pyrometer | §5.3.1 |
| Bend radius, warm forming | ≥ 2.5 × plate thickness | §5.3.3 |
| Bend radius, cold forming | ≥ 3 × plate thickness | §5.3.3 |
| Maximum wall thinning | 5 % of plate thickness | §5.3.3 |
| Internal corner radius | 125 – 200 mm | §4.2.2 d) |
This is the same principle the most expensive kettle builders in Europe work to. It is not a secret and it is not proprietary — it is just slower and more expensive than cold bending, so not everyone does it.
What happens before and after the press
- Plate and die surfaces cleaned before pressingDebris pressed into a plate face becomes a defect that no later process removes.
- Scale and oxidation removed from bevels after formingBefore assembly, back to bright metal — otherwise it ends up in the weld.
- Edges trimmed or flame-straightened after formingAny bevel touched in the process is re-prepared.
- Assembly platform flatness ≤ 1 mm/m²A kettle assembled on an uneven platform is built out of shape from the start.
Where forming meets welding
Warm forming and electroslag welding are two halves of the same objective: put as little residual stress into the vessel as possible before it ever sees zinc. The corner radii those two processes produce are also what decides which geometry suits your bath depth.
Source: BG/T 1501-2015 — §4.2.2, §5.3, §5.4.