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.
| Process | k |
|---|---|
| MMA (111) | 0.8 |
| MIG/MAG (135) | 0.8 |
| TIG (141) | 0.6 |
| Submerged arc (121) | 1.0 |
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.
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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