Heat input calculator
Heat input is what ties your settings to the metallurgy: cooling rate, weld structure and the need for preheat all follow from it. Procedure specifications limit it at both ends.
The formula
Q = k · (U · I · 60) / (v · 1000), where U is the arc voltage in
volts, I the current in amperes, v the travel speed in mm/min and k the thermal
efficiency of the process.
The efficiency figures are those of ISO/TR 18491 and EN 1011-1. Without the coefficient the result is called arc energy, not heat input — in documentation the two are not interchangeable, and quoting one where the other is required is a common way to fail a review.
Where the number is used
Three places, mainly. In the preheat calculation to EN 1011-2, where higher heat input lowers the required preheat. In a WPS, where the qualified range of heat input is one of the essential variables. And in any argument about toughness: for fine-grained structural steels the upper limit exists because the heat-affected zone loses impact energy long before the weld metal does.
Thermal efficiency k
| Process | ISO 4063 | k |
|---|---|---|
| Submerged arc welding with solid wire electrode SAW | 121 | 1.0 |
| Manual metal arc welding with covered electrode MMA · SMAW | 111 | 0.8 |
| MAG welding with solid wire in active gas MAG · GMAW | 135 | 0.8 |
| MAG welding with flux-cored wire FCAW | 136 | 0.8 |
| TIG welding with solid filler TIG · GTAW | 141 | 0.6 |
| Plasma arc welding | 15 | 0.6 |
What the number means
| Q, kJ/mm | What it means |
|---|---|
| < 0.5 | Fast cooling; risk of hard structures and cold cracks in the HAZ |
| 0.5–1.0 | Thin material, root runs, stainless |
| 1.0–2.5 | The working range for most manual processes on steel |
| > 2.5 | Coarse grain in the HAZ, loss of toughness, distortion |
Frequently asked questions
Why does heat input matter?
It sets how fast the weld cools. Too little energy means rapid cooling, a hard structure and a risk of cracking; too much means coarse grain and poor impact toughness. That is why procedure specifications limit it from both sides, not just from above.
How do I measure travel speed?
With a stopwatch. Time a known length of weld — 200 mm is convenient — and divide. Speed judged by eye is usually out by tens of per cent, which moves the result more than any of the other inputs.
What voltage do I enter if the machine does not display it?
For manual metal arc you can estimate it as U ≈ 20 + 0.04 · I. That is an approximation: if the number is going into a document, voltage is measured, not estimated.
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