What is weld heat input and why does it matter?

Heat input is the arc energy delivered per unit length of weld, in kJ/in. It sets the cooling rate of the heat-affected zone, which controls grain size, hardness, and toughness. High input widens the HAZ and softens it; low input risks brittle martensite. At 24V, 200A, and 8 in/min the raw value is 36 kJ/in, before the process efficiency factor.

How do I calculate weld heat input step by step?

Multiply Volts by Amps by 60, then divide by travel speed in in/min times 1,000. For 24V, 200A, 8 in/min: (24 x 200 x 60) / (8 x 1,000) = 288,000 / 8,000 = 36 kJ/in. Then multiply by the efficiency factor. For GTAW at 0.60 you get 36 x 0.60 = 21.6 kJ/in effective heat input.

What is a typical heat input range for carbon steel welding?

Most carbon and low-alloy WPS windows run 15 to 50 kJ/in. Quenched-and-tempered steels are often held to 20 to 25 kJ/in max to protect HAZ toughness. Thin sheet may need under 15 kJ/in to limit distortion. Always weld inside the effective range your PQR qualified, not just the raw kJ/in number off the machine.

My welds have too much distortion, is heat input the cause?

Likely yes. Distortion scales with heat input, so a 36 kJ/in bead puts far more thermal expansion into the plate than a 20 kJ/in bead. Increase travel speed or reduce amperage to cut input, use smaller stringer beads instead of wide weaves, and control interpass temperature. Dropping from 8 to 12 in/min alone would take 36 kJ/in down to 24 kJ/in.

How do I convert kJ/in to kJ/mm for heat input?

Divide kJ/in by 25.4 to get kJ/mm. So 36 kJ/in equals 1.42 kJ/mm, and the effective 21.6 kJ/in equals 0.85 kJ/mm. Many European WPS documents specify limits in kJ/mm, so run the conversion before comparing. Also confirm both figures use the same efficiency basis, since raw and effective values differ by the process factor.

What is the difference between raw and effective heat input?

Raw heat input is the arc energy per inch straight from the formula, 36 kJ/in in the example. Effective heat input multiplies by the process efficiency factor to account for energy lost rather than transferred to the metal: GTAW 0.60, SMAW 0.80, MIG 0.85, SAW 1.0. So 36 x 0.60 gives 21.6 kJ/in effective. Codes like AWS D1.1 typically expect the effective value.

Engineering and Process

Weld Heat Input Formula

Weld heat input controls how much thermal energy enters the base metal per unit length. It affects grain structure, HAZ size, distortion, and mechanical properties. Use it when qualifying a WPS, troubleshooting distortion, or controlling interpass temperature.

Formula

Heat Input (kJ/in) = (Volts x Amps x 60) / (Travel Speed in/min x 1,000)

Variables

Volts: Arc voltage during welding
Amps: Welding current in amperes
Travel Speed: Speed at which the weld is deposited, in inches per minute

Understanding the Weld Heat Input Formula

Heat input tells you how many kilojoules of arc energy land in each inch of weld, which governs cooling rate in the HAZ. Slow cooling from high heat input coarsens grains and softens the HAZ; fast cooling from low heat input can form brittle martensite in hardenable steels. At 24V, 200A, and 8 in/min you get 36 kJ/in raw, so this number is what actually drives distortion, toughness, and hardness in the joint.

Pull the numbers straight from the machine: voltage and amperage from the meter or a data logger, travel speed by timing a measured length of bead. Keep amps in true amperes and travel in inches per minute so the units resolve to kJ/in. The single biggest gotcha is the efficiency factor. Multiply the 36 kJ/in by 0.60 for GTAW, 0.80 for SMAW, 0.85 for MIG, or 1.0 for SAW to get effective heat input, here 21.6 kJ/in.

Most carbon and low-alloy WPS windows fall between 15 and 50 kJ/in, with quench-and-tempered steels capped near 20 to 25 kJ/in to protect HAZ toughness. If you exceed the ceiling, speed up travel or drop amperage; if you fall below the floor and get cracking, add preheat or slow down. Log the effective value against your PQR range and adjust interpass temperature so the actual thermal cycle stays inside the qualified envelope.

Worked Example

Welding at 24V, 200A, travel speed 8 inches per minute. Process efficiency factor (GTAW) = 0.60.

Raw heat input = (24 x 200 x 60) / (8 x 1,000)
= 288,000 / 8,000
= 36 kJ/in
Effective heat input = 36 x 0.60 = 21.6 kJ/in

Result: 21.6 kJ/in effective heat input

Common Mistake

Ignoring the process efficiency factor. Different processes deliver heat to the weld at different efficiencies: SMAW ~0.80, MIG/MAG ~0.85, GTAW ~0.60, SAW ~1.0. Omitting the efficiency factor will overstate heat input for GTAW and understate it for SAW.

Frequently Asked Questions

What is weld heat input and why does it matter?: Heat input is the arc energy delivered per unit length of weld, in kJ/in. It sets the cooling rate of the heat-affected zone, which controls grain size, hardness, and toughness. High input widens the HAZ and softens it; low input risks brittle martensite. At 24V, 200A, and 8 in/min the raw value is 36 kJ/in, before the process efficiency factor.
How do I calculate weld heat input step by step?: Multiply Volts by Amps by 60, then divide by travel speed in in/min times 1,000. For 24V, 200A, 8 in/min: (24 x 200 x 60) / (8 x 1,000) = 288,000 / 8,000 = 36 kJ/in. Then multiply by the efficiency factor. For GTAW at 0.60 you get 36 x 0.60 = 21.6 kJ/in effective heat input.
What is a typical heat input range for carbon steel welding?: Most carbon and low-alloy WPS windows run 15 to 50 kJ/in. Quenched-and-tempered steels are often held to 20 to 25 kJ/in max to protect HAZ toughness. Thin sheet may need under 15 kJ/in to limit distortion. Always weld inside the effective range your PQR qualified, not just the raw kJ/in number off the machine.
My welds have too much distortion, is heat input the cause?: Likely yes. Distortion scales with heat input, so a 36 kJ/in bead puts far more thermal expansion into the plate than a 20 kJ/in bead. Increase travel speed or reduce amperage to cut input, use smaller stringer beads instead of wide weaves, and control interpass temperature. Dropping from 8 to 12 in/min alone would take 36 kJ/in down to 24 kJ/in.
How do I convert kJ/in to kJ/mm for heat input?: Divide kJ/in by 25.4 to get kJ/mm. So 36 kJ/in equals 1.42 kJ/mm, and the effective 21.6 kJ/in equals 0.85 kJ/mm. Many European WPS documents specify limits in kJ/mm, so run the conversion before comparing. Also confirm both figures use the same efficiency basis, since raw and effective values differ by the process factor.
What is the difference between raw and effective heat input?: Raw heat input is the arc energy per inch straight from the formula, 36 kJ/in in the example. Effective heat input multiplies by the process efficiency factor to account for energy lost rather than transferred to the metal: GTAW 0.60, SMAW 0.80, MIG 0.85, SAW 1.0. So 36 x 0.60 gives 21.6 kJ/in effective. Codes like AWS D1.1 typically expect the effective value.