Heat input affects weld quality, metallurgy, distortion, and residual stress. Understanding and controlling heat input is essential for meeting welding procedure specifications and avoiding defects. This guide covers the calculation and its practical implications.
The Heat Input Formula
Heat input is calculated from welding parameters:
H = (Voltage × Amperage × 60) / (Travel Speed × 1000)
Where H is heat input in kJ/mm, voltage in volts, amperage in amps, and travel speed in mm/min.
For kJ/inch:
H = (Voltage × Amperage × 60) / (Travel Speed in in/min × 1000)
Example Calculation
A weld made at:
- Voltage: 24V
- Amperage: 200A
- Travel speed: 250 mm/min
H = (24 × 200 × 60) / (250 × 1000)
H = 288,000 / 250,000 = 1.15 kJ/mm
Calculate Weld Heat Input
Enter voltage, amperage, and travel speed to get heat input in kJ/mm or kJ/in. Includes efficiency factors.
Process Efficiency Factors
Not all electrical energy becomes weld heat. Apply these factors for more accurate calculations:
| Process | Efficiency |
|---|---|
| SAW (Submerged Arc) | 0.90-0.95 |
| SMAW (Stick) | 0.75-0.85 |
| GMAW (MIG) | 0.75-0.85 |
| FCAW (Flux Core) | 0.75-0.85 |
| GTAW (TIG) | 0.60-0.70 |
Many specifications ignore efficiency, using the full calculated value for conservatism.
Why Heat Input Matters
Metallurgical Effects
- High heat input: Larger HAZ, slower cooling, softer HAZ in steel
- Low heat input: Faster cooling, risk of hard/brittle structures
- Affects grain size, hardness, and toughness
Distortion
- Higher heat input = more distortion
- Minimize heat input when distortion control is critical
- Balance with need for proper fusion and penetration
Qualification Limits
Welding Procedure Specifications (WPS) often specify:
- Maximum heat input for toughness-critical applications
- Minimum heat input to ensure proper fusion
- ±10-25% range from qualified parameters
Code Requirements
AWS D1.1 (Structural Steel)
Heat input primarily affects preheat and interpass temperature requirements. Check Table 3.2 for guidance based on carbon equivalent.
ASME Section IX
Heat input is an essential variable when impact testing is required. Exceeding qualified heat input requires requalification.
API 1104 (Pipelines)
Lower heat input limits often required for high-strength pipe steels to prevent softening in the HAZ.
Controlling Heat Input
To reduce heat input:
- Increase travel speed
- Reduce amperage (may need smaller electrode/wire)
- Use stringer beads instead of weave beads
- Multiple thin passes vs. fewer heavy passes
To increase heat input:
- Slow travel speed
- Increase amperage
- Weave technique
Practical Considerations
- Travel speed measurement: Can use timed bead length or encoders
- Voltage/amperage: Use true RMS meters; machine displays may be inaccurate
- Pulsed welding: Use average values or consult code requirements
- Documentation: Record parameters for each weld pass when required
Heat input control is a balance between productivity, metallurgy, and quality. Understanding the calculation helps you make informed adjustments while staying within specification limits.
