Arc energy and heat input
The amount of heat put into a weld affects its cooling rate, and with it the microstructure, hardness, toughness and distortion of the joint. Welding procedures therefore specify a range of heat input. Two related values are used:
Arc energy (kJ/mm) = voltage (V) × current (A) × 60 ÷ (1,000 × travel speed in mm/min)
Heat input = arc energy × thermal efficiency k
In US units, with the travel speed in inches per minute, the same formula gives kJ/in. ASME Section IX uses the arc energy directly, while the European standards (EN 1011-1 and ISO/TR 17671) apply the thermal efficiency of the process.
Thermal efficiency by process
| Process | Efficiency k |
|---|---|
| Submerged arc welding (SAW) | 1.0 |
| Shielded metal arc, stick (SMAW) | 0.8 |
| MIG/MAG (GMAW) | 0.8 |
| Flux-cored (FCAW) | 0.8 |
| TIG (GTAW) | 0.6 |
| Plasma | 0.6 |
Example
A MAG weld at 24 V and 200 A with a travel speed of 12 in/min has an arc energy of 24 × 200 × 60 ÷ (1,000 × 12) = 24 kJ/in, or about 0.94 kJ/mm. With an efficiency of 0.8, the heat input is about 19 kJ/in, or 0.76 kJ/mm.
Why it matters
- Too low: fast cooling can harden the heat-affected zone and cause hydrogen cracking in carbon and low-alloy steels, especially in thick sections.
- Too high: slow cooling can coarsen the grain and reduce toughness, and it increases distortion. Stainless steels and quenched-and-tempered steels are sensitive to excessive heat input.
Measuring correctly
- Measure the travel speed over a known length and time, from the start to the end of the arc.
- Use the actual voltage at the arc, not the setting on the machine, when the cables are long.
- For pulsed and waveform-controlled processes, the simple V × A product does not represent the real power; use the instantaneous energy or power displayed by the machine, as allowed by ASME IX and ISO/TR 18491.
- Weaving lowers the travel speed and raises heat input, so record weave width in the procedure.