Preheat temperature calculator
Whether to preheat, and to what temperature — by method B of EN 1011-2, the method that actually accounts for hydrogen and heat input rather than reading a single number off a table.
What goes into it
Four inputs, and all four matter. CET — the carbon equivalent from the mill certificate, worked out by the carbon equivalent calculator. Combined thickness — the sum of the plate thicknesses meeting at the joint, because that is what draws the heat away. Hydrogen — from the consumable data sheet: a re-baked basic electrode gives about 5 ml/100 g, an ordinary rutile one about 15. Heat input — higher energy means slower cooling and less preheat.
Why it works
Cold cracking needs three things at once: a susceptible hard structure, dissolved hydrogen, and tensile stress. Preheat attacks the first two. It slows the cooling through the 800–500 °C range, so the heat-affected zone comes out softer, and it keeps the joint warm long enough for hydrogen to diffuse out instead of collecting at the fusion line.
Note the range the formula is valid in: CET 0.30–0.70 %, thickness 10–90 mm, hydrogen 1–20 ml/100 g, heat input 0.5–4.0 kJ/mm. Outside it the calculator warns you, and the answer needs a metallurgist rather than a formula.
The HD hydrogen scale
| Designation | HD, ml/100 g | Typical consumable |
|---|---|---|
| E — very low | < 3 | Basic electrodes after baking, vacuum pack |
| D — low | 3–5 | Basic electrodes, solid wire in a gas mixture |
| C — medium | 5–10 | Flux-cored wire, basic electrodes after storage |
| B — high | 10–15 | Rutile electrodes |
| A — very high | > 15 | Damp electrodes, cellulosic |
The scale comes from EN 1011-2. The HD value is on the consumable certificate; without one, reckon a step worse.
Frequently asked questions
Can I skip preheat if the plate is thin?
Often yes, and the calculation says so — combined thickness is one of its four inputs. But thin plate with a high carbon equivalent and damp electrodes still cracks. Preheat is decided by the combination, never by thickness alone.
How do I hold the interpass temperature?
Measure it with a contact thermometer or a temperature crayon 25 mm from the joint, immediately before striking the next run. If it has dropped below the preheat figure, warm it up again: the requirement applies to every run, not only to the first one.
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