Heat input controls cooling rate, HAZ toughness, and distortion. It's the first number an engineer asks for on any critical weld. Plug in volts, amps, and travel speed to get kJ/mm in one step.
HI = (V × A × 60) ÷ (Travel mm/min × 1000)
Volts × Amps gives power in watts. Multiplying by 60 seconds converts it to joules per minute. Dividing by travel speed in mm/min gives joules per mm; dividing by 1000 converts to kJ/mm.
| Variable | Unit | Where From |
|---|---|---|
| Volts | V | Machine readout / meter |
| Amps | A | Machine readout / clamp meter |
| Travel Speed | mm/min | Measured along weld axis |
| Result | kJ/mm | 1 kJ/mm = 25.4 kJ/in |
Worked example: 25 V × 200 A × 60 ÷ (300 mm/min × 1000) = 300,000 ÷ 300,000 = 1.0 kJ/mm.
Heat input only means something when paired with the right temperature discipline. Carbon steel minimum preheat values below follow AWS D1.1.
| Thickness | Non-Low-Hydrogen | Low-Hydrogen (E7018) |
|---|---|---|
| Up to 3/4" | 50°F (10°C) | 50°F (10°C) |
| 3/4"–1-1/2" | 150°F (66°C) | 50°F (10°C) |
| 1-1/2"–2-1/2" | 225°F (107°C) | 150°F (66°C) |
| Over 2-1/2" | 300°F (149°C) | 225°F (107°C) |
Low-hydrogen processes (7018, solid wire with clean base metal) tolerate less preheat — that's part of why codes love them. Thick, cold, or restraint-heavy joints always get preheat.
Interpass is the temperature of the joint when the next pass starts. It must never drop below the minimum preheat — every pass re-applies the same discipline.
Measure with a contact pyrometer or temp stick at the weld joint, not the surface glow. When in doubt, cool with air (never quench) and let the joint breathe.
Fast cooling. On hardenable steels the HAZ turns to hard, crack-prone martensite. Fix: raise amps, slow travel, or add preheat.
0.8–1.5 kJ/mm is home for most GMAW and FCAW structural work. SAW and electroslag run 2–3+ kJ/mm by design.
Coarse HAZ grains, lost impact toughness, distortion, and — on quench-and-tempered steel — a soft, weak weld. Slow travel isn't always better.
Use your calculated requirements to get an equipment recommendation for your application.
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