Welding Heat Input.
Calculate heat input (kJ/mm) for SMAW, GMAW, GTAW, SAW and FCAW from voltage, current, and travel speed. With process efficiency factor — required for WPS qualification per ASME IX, AWS D1.1.
Per AWS D1.1 / ASME IX standard efficiency values
volts — arc voltage
amperes — welding current
mm/min — welding travel speed
mm — for cooling rate estimate
HEAT INPUT
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kJ/mm — gross heat input
Net heat input
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Arc power
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Travel speed (mm/s)
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HAZ classification
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Heat Input (gross) = (V × I × 60) / (S × 1000)
Net Heat Input = η × Gross HI
Units: V (volts), I (amps), S (mm/min) → kJ/mm
Net Heat Input = η × Gross HI
Units: V (volts), I (amps), S (mm/min) → kJ/mm
Typical heat input ranges
| Material | Recommended HI (kJ/mm) | Risk if too high | Risk if too low |
|---|---|---|---|
| Carbon steel (mild) | 0.8 – 2.5 | HAZ softening, distortion | Lack of fusion, hard HAZ |
| Low-alloy steel (Cr-Mo) | 1.0 – 2.0 | Toughness loss | Hydrogen cracking |
| Stainless 304/316 | 0.5 – 1.5 | Sensitisation, distortion | Lack of penetration |
| Duplex stainless | 0.5 – 2.5 | Sigma phase, intermetallics | Wrong ferrite/austenite ratio |
| Aluminium alloys | 0.4 – 1.2 | Burn-through, strength loss | Cold lap, porosity |
| HSLA / Q&T steels | 1.0 – 2.5 | HAZ softening (softens martensite) | Cracking, brittle HAZ |
Welding Heat Input
Heat input (kJ/mm) determines weld penetration, microstructure of the heat-affected zone (HAZ), distortion, and residual stress. WPS (Welding Procedure Specifications) always specify heat input limits.
Heat Input Formula
HI (kJ/mm) = (V × I × η × 60) / (S × 1000)
Where V = arc voltage, I = current (A), S = travel speed (mm/min), η = arc efficiency (~0.8 SMAW, ~0.85 GMAW, ~0.6 GTAW, ~1.0 SAW).
Worked Example
SMAW: 25V, 130A, 200 mm/min travel, η = 0.8.
- HI = (25 × 130 × 0.8 × 60) / (200 × 1000) = 0.78 kJ/mm
Typical Heat Input Ranges
- 0.3 – 0.6 kJ/mm: thin sheet metal, GTAW root passes
- 0.6 – 1.5 kJ/mm: standard structural welding
- 1.5 – 2.5 kJ/mm: thicker sections, multi-pass fill
- > 2.5 kJ/mm: high heat — risk of HAZ softening, coarse grain
Effects of Heat Input
- Too low: lack of fusion, cold cracking, poor penetration.
- Too high: excessive distortion, coarse grain in HAZ, reduced toughness.
- Multi-pass strategy: split high heat input into multiple smaller passes — refines microstructure between layers.
Materials That Care Most About Heat Input
- Quenched and tempered steels (Q&T): high HI softens HAZ permanently.
- HSLA steels: heat input window is narrow.
- Duplex stainless: too high or too low destroys phase balance.
- Aluminum: high conductivity needs adequate HI but distortion is severe.
Related Tools
For material hardness verification, see Hardness Conversion. For weld joint sizing, Material Weight. Document WPS/PQR data in AS9102 Form 3.