Why does process efficiency matter in heat input?

Not all arc energy reaches the workpiece. An efficiency factor (e.g., 0.80 for GMAW, 0.60 for GTAW) converts gross electrical input into a realistic net heat input.

Welding Heat Input Calculator

Enter voltage, amperage, travel speed, and process efficiency. Returns net heat input (kJ/in or kJ/mm), arc power, and gross heat input without efficiency. Use for WPS scoping and shop-floor checks.

GMAW / FCAW efficiency

0.80

SMAW efficiency

0.75

GTAW / PAW efficiency

0.60

SAW efficiency

1.00

Units Switches travel speed and output

Voltage (V) Arc voltage at the work lead

Amperage (A) Welding current

Travel speed (in/min) Measured arc-on travel speed

Process efficiency Energy transferred from arc to weld

Heat input (net)

24.00kJ/in

GMAW at 80% efficiency

Arc power

6,000W

Gross heat input (no efficiency)

30.00kJ/in

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Tip Measure travel speed by marking a weld start, welding for 30 seconds, and measuring bead length. Stopwatch and ruler beat assumed numbers from the procedure sheet every time.

Worked example

GMAW at 24 V, 250 A, 12 in/min travel speed, efficiency 0.80.

1. Arc power P = V × A = 24 × 250 = 6,000 W 2. Gross heat input HI_gross = (V × A × 60) / (1000 × v) HI_gross = (24 × 250 × 60) / (1000 × 12) HI_gross = 30.00 kJ/in 3. Net heat input (apply efficiency) HI_net = 30.00 × 0.80 HI_net = 24.00 kJ/in

Process efficiency reference

Process	Abbreviation	Efficiency factor	Typical range
Shielded Metal Arc Welding	SMAW	0.75	0.70–0.80
Gas Metal Arc Welding	GMAW	0.80	0.75–0.85
Flux Cored Arc Welding	FCAW	0.80	0.75–0.85
Gas Tungsten Arc Welding	GTAW	0.60	0.55–0.65
Submerged Arc Welding	SAW	1.00	0.95–1.00
Plasma Arc Welding	PAW	0.60	0.55–0.65

Common mistakes

Using nominal travel speed from the WPS. Real arc-on travel drifts up to 20% from the number on paper. Time and measure the actual bead.
Ignoring efficiency. A 30 kJ/in calculation for GTAW is really 18 kJ/in at the joint. That is the difference between "hot" and "cold" on a stainless procedure.
Mixing units. kJ/in and kJ/mm are not interchangeable — 25.4 mm per inch. Pick one and stay there.
Treating heat input as the only quality metric. Joint fit-up, arc length, oscillation, and preheat all matter too.

Warn AWS D1.1, ASME Section IX, and most code procedures qualify a heat input range. Welds produced outside that range are out of procedure — even if the visible bead looks fine. Record V, A, and travel speed for every coupon and every production weld when required.

FAQ

What is welding heat input?

The amount of arc energy delivered per unit length of weld. It is calculated from voltage, amperage, travel speed, and process efficiency. Units are kJ/in or kJ/mm.

Why does process efficiency matter?

A portion of arc energy is lost to radiation, spatter, and electrode heating before it reaches the base metal. The efficiency factor converts gross electrical energy into net joint energy so different processes can be compared fairly.

What is a typical heat input range?

No universal range. Thin-gauge manual welding may run a few kJ/in; high-deposition mechanized welding can be much higher. Your WPS and the material's code limits define what is acceptable.

How does heat input affect weld quality?

It changes cooling rate, penetration, HAZ size, and distortion. Low heat can cause lack of fusion; high heat can widen the HAZ and reduce toughness in alloy and stainless steels.

What is the difference between gross and net heat input?

Gross is V × A × 60 / (1000 × travel speed). Net multiplies by the process efficiency factor to estimate energy actually entering the weldment. Most code calculations use net heat input.

Sources

AWS D1.1 Structural Welding Code ASME BPVC Section IX — Welding Qualifications AWS A3.0 Standard Welding Terms & Definitions Lincoln Heat input and procedure qualification guides

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Log every pass before the CWI signs off

Welding & Sheet Metal Toolkit bands kJ/in against your WPS, stacks the pass log, and prints the day-sheet citing AWS D1.1 / ASME IX. Offline. Pay once.