Welding heat input (Q or HI — Heat Input) is the amount of energy transferred per unit length to a weld bead. The unit is kJ/mm. It directly determines weld microstructure, mechanical properties, distortion and defect risk.
General rule: Low heat input → hard and brittle microstructure, cracking risk. High heat input → soft HAZ, distortion. Staying within the range specified in the WPS is mandatory.
Basic Calculation Formula
Q = (k × U × I × 60) / (v × 1000)
- Q: Heat input (kJ/mm)
- U: Voltage (V)
- I: Current (A)
- v: Travel speed (mm/min)
- k: Thermal efficiency factor
Thermal Efficiency Factors (k)
| Welding Process | ISO Number | k Value |
|---|---|---|
| SAW (Submerged Arc) | 121 | 1.0 |
| SMAW (Covered Electrode) | 111 | 0.8 |
| MAG / MIG | 135 / 131 | 0.8 |
| TIG | 141 | 0.6 |
| Plasma | 15 | 0.6 |
Calculation Example
MAG welding S355 plate: U = 28 V, I = 280 A, v = 350 mm/min
Q = (0.8 × 28 × 280 × 60) / (350 × 1000) = 1.075 kJ/mm
Recommended Ranges by Steel Grade
| Material | Min Q (kJ/mm) | Max Q (kJ/mm) |
|---|---|---|
| S235JR | 0.5 | 4.0 |
| S355J2+N | 0.5 | 3.5 |
| S460M / S500MC | 0.5 | 2.5 |
| S690QL | 0.5 | 1.8 |
| S960QL | 0.5 | 1.5 |
| HARDOX 400/450 | 0.5 | 1.5 |
| 304L / 316L Stainless | 0.5 | 1.5 |
Effects of Low Heat Input
- Rapid cooling → martensite formation (hard and brittle)
- HAZ hardness increases — 380 HV10 limit may be exceeded
- Cold cracking risk increases
- Risk of lack of penetration and lack of side wall fusion
Effects of High Heat Input
- HAZ grain coarsening — toughness decreases
- HAZ softening (QT and TMCP steels) — yield strength decreases
- Increased distortion and residual stress
- Hot cracking risk (stainless, aluminium)
Heat Input Management Strategies
To Reduce:
- Reduce current or voltage; increase travel speed; use pulse mode
To Increase:
- Increase current; reduce speed; use SAW or FCAW
Relationship: Cooling Time t8/5
- t8/5 < 5 s → Martensite, high hardness
- t8/5 = 10–20 s → Bainite, balanced properties
- t8/5 > 30 s → Ferrite-pearlite, lower strength, high toughness
Practical control: Modern digital welding machines (Fronius TPS/i, Lincoln PowerWave, Lorch S-RoboMIG) display real-time heat input. The operator can verify compliance with WPS limits during production.
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