Welding heat input, explained

Heat input is the energy a weld deposits per unit length of joint — the single number that links your meter readings to what happens metallurgically in the heat-affected zone. The working formula:

HI = η × V × I × 60 / (v × 1000)  [kJ/mm, v in mm/min]

Voltage and current come off the machine (or better, a calibrated meter at the arc); travel speed is measured weld length over arc time; and η is the arc thermal efficiency — how much of the electrical energy actually enters the plate. The welding heat input calculator runs this in kJ/mm and kJ/in with the efficiency factor explicit, which matters more than it looks:

The convention trap

Two legitimate conventions coexist. The traditional US/ASME practice computes heat input from meter readings alone (η = 1 implicitly); ISO/EN 1011 — and the post-2019 ASME Section IX options — multiply by the process efficiency (≈1.0 SAW, ≈0.8 GMAW/FCAW/SMAW, ≈0.6 GTAW). The same GTAW pass can be reported 40% apart depending on convention. When a WPS or a customer spec states a heat-input window, confirm which convention it uses before qualifying or comparing — that is the most common heat-input paperwork error.

Worked example — a GMAW fill pass

24 V, 180 A, travel 300 mm/min (≈12 in/min), GMAW with η = 0.8:

HI = 0.8 × 24 × 180 × 60 / (300 × 1000) = 0.69 kJ/mm (≈ 17.6 kJ/in)

Mid-range for structural steel. Now the lever that surprises people: travel speed is in the denominator. Slow to 200 mm/min — one weave instead of a stringer — and heat input climbs to 1.04 kJ/mm, a 50% increase with identical meter readings. Welder technique moves heat input more than the machine settings do, which is why qualified procedures bound travel speed, not just amps and volts.

What the number controls

• Cooling rate and hardness. Low heat input on hardenable or thick steel cools fast enough to form martensite — hard, brittle, hydrogen-cracking-prone HAZ. Preheat and minimum heat input are the defenses; the carbon equivalent calculator screens which steels need them.
• Toughness. High heat input grows HAZ grains and degrades Charpy values — impact-tested work carries a maximum.
• Distortion. Energy per length is also shrinkage per length; controlling heat input is the first distortion-control tool.
• Q&T and HSLA steels. Quenched-and-tempered grades (A514, armor) cap heat input hard — exceed it and you locally undo the heat treatment.

Heat input pairs with the rest of the procedure arithmetic: the amperage calculator for setting current by electrode and thickness, the deposition rate calculator for how fast metal goes in, and the weld cost calculator for what the parameters do to the budget.

Frequently asked questions

What is the welding heat input formula?

Heat input = (efficiency × voltage × current × 60) / (travel speed × 1000), giving kJ/mm with speed in mm/min (or kJ/in with in/min). Some codes and older WPS forms omit the efficiency factor — always state which convention a number uses before comparing.

What is a typical heat input value?

Common structural-steel work runs roughly 0.5–2.5 kJ/mm (about 12–60 kJ/in). Thin sheet and high-strength steels sit at the low end; thick-section submerged-arc joints run higher. The governing WPS, not a generic range, sets the real limits.

Why does heat input matter?

It controls the weld cooling rate, which controls the heat-affected-zone microstructure. Too low on hardenable steel = fast cooling, martensite and hydrogen cracking risk. Too high = slow cooling, grain growth, lower toughness and more distortion. Charpy-tested and quenched-and-tempered work caps it both ways.

What are the arc efficiency factors?

Customary thermal-efficiency values: submerged arc ≈ 1.0, GMAW/FCAW and SMAW ≈ 0.8, GTAW ≈ 0.6 — the fraction of arc energy that actually enters the plate. ISO/EN heat input (and ASME Section IX post-2019 options) multiplies by it; the traditional US ratio-of-meter-readings convention does not.