How to calculate welding heat input
Heat input is the energy that enters the workpiece per millimetre of weld. It controls cooling rate, heat-affected-zone toughness and distortion, and most WPSs limit it. It is calculated from arc voltage, current and travel speed.
The formula
Arc energy E = U × I × 60 / (v × 1000) [kJ/mm, U in V, I in A, v in mm/min] Heat input Q = k × E
k is the thermal efficiency of the process from EN 1011-1 (and ISO/TR 17671-1): 1.0 for submerged arc (121), 0.8 for manual metal arc (111), MIG/MAG (131/135) and flux-cored wire (136/138), 0.6 for TIG (141) and plasma (15).
Worked examples
| Process | k | U (V) | I (A) | v (mm/min) | E (kJ/mm) | Q (kJ/mm) |
|---|---|---|---|---|---|---|
| 135 MAG | 0.8 | 24 | 200 | 350 | 0.82 | 0.66 |
| 135 MAG | 0.8 | 28 | 260 | 300 | 1.46 | 1.16 |
| 136 FCAW | 0.8 | 26 | 230 | 280 | 1.28 | 1.03 |
| 111 MMA | 0.8 | 23 | 140 | 150 | 1.29 | 1.03 |
| 141 TIG | 0.6 | 12 | 120 | 100 | 0.86 | 0.52 |
| 121 SAW | 1.0 | 30 | 550 | 500 | 1.98 | 1.98 |
EN vs ASME
ASME Section IX (QW-409.1) does not apply an efficiency factor: heat input there equals arc energy, usually in J/in. Multiply kJ/mm by 25,400 to get J/in. When comparing with a PQR, use the same definition on both sides.
Things that go wrong
- Travel speed is the speed along the weld axis, not the weaving speed. Weaving lowers travel speed and raises heat input.
- With pulsed or waveform-controlled processes, voltage × current understates the power; use the instantaneous energy or power shown by the machine (ISO/TR 18491, ASME QW-409.1 c).
- Measure voltage close to the arc; cable losses make the machine display read high.
The welding heat input calculator applies the k factor for the chosen process; the heat input check and t8/5 cooling time checks every run against the allowed window and calculates the t8/5 cooling time.
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