What is the welding heat input formula?

Heat input equals voltage times current times 60, divided by 1000 times the travel speed in mm per minute, giving kilojoules per millimetre. Multiply by 25.4 to convert kJ/mm to kJ/in. The 60 converts amp-volts (watts) into energy per minute of arc time.

Why apply a thermal efficiency factor?

Not all arc energy reaches the workpiece; some radiates and convects away. ISO/TR 18491 gives typical efficiencies of about 1.0 for submerged arc, 0.8 for stick, MIG, and flux-cored, and 0.6 for TIG. The effective heat input is the arc energy multiplied by that factor.

Why does heat input matter for procedure qualification?

Heat input controls the cooling rate of the weld and heat-affected zone, which sets the final microstructure, hardness, and toughness. Codes such as AWS D1.1 limit the qualified range, so welders must stay between the minimum and maximum recorded on the procedure qualification record.

What happens if heat input is too high or too low?

Too high and the weld cools slowly, causing grain growth, a soft heat-affected zone, and reduced toughness. Too low and it cools quickly, which can harden hardenable steels and promote hydrogen cracking. The correct window depends on the alloy and thickness.

Does this work for SAW and TIG?

Yes. The formula is identical; only the efficiency factor changes. Submerged arc uses about 1.0 because the flux blanket traps almost all the energy, while TIG uses about 0.6 because more heat is lost from the open arc, so the same parameters give a lower effective heat input.

Welding Heat Input Calculator

Welding heat input is the amount of energy delivered to the joint per unit length of weld. It is the master variable behind cooling rate, and therefore behind the strength, hardness, and toughness of both the weld metal and the surrounding heat-affected zone. Welding codes qualify a procedure over a tested range of heat input, and this calculator lets you confirm where any set of parameters lands.

How it works

The standard arc-energy equation used in AWS D1.1 and D1.5 is:

HI [kJ/mm] = (V x I x 60) / (1000 x S)      S in mm/min
HI [kJ/in] = HI [kJ/mm] x 25.4

V is arc voltage, I is welding current, and S is travel speed. The factor 60 turns instantaneous power (volt-amps, i.e. watts) into energy per minute, and dividing by 1000 converts joules to kilojoules.

Raw arc energy overstates the heat that actually enters the steel, so it is scaled by a thermal (arc) efficiency η from ISO/TR 18491:

effective HI = arc energy x η
SAW  ~ 1.00   SMAW ~ 0.80   GMAW/FCAW ~ 0.80   GTAW ~ 0.60

Example and notes

A GMAW pass at 25 V and 200 A travelling at 300 mm/min gives arc energy of (25 x 200 x 60) / (1000 x 300) = 1.0 kJ/mm, and with η of 0.80 an effective heat input of 0.8 kJ/mm.

To raise heat input, lower travel speed or raise voltage and current. To lower it, speed up.
Stay inside the qualified band on your WPS. Many codes allow heat input to be controlled either by the energy calculation or by a measured bead size; this tool covers the energy method.
For pulsed processes, use average voltage and average current; instantaneous peaks overstate the result.