The heat input formula helps welding professionals understand how much energy is delivered along a weld. This matters because welding energy affects cooling rates, weld properties, and the heat-affected zone.
By using voltage, current, and travel speed, you can calculate energy per unit length and check whether welding parameters meet the requirements of the approved welding procedure specification (WPS).
Check out: What is P91 Welding?
During arc welding, electrical energy heats and melts the metal. The amount of energy delivered along the weld depends on three factors: arc voltage, welding current, and travel speed.
For metric measurements, the formula is:
Arc energy (kJ/mm) = (Voltage × Current × 60) / (Travel speed × 1,000)
Voltage is measured in volts (V), current in amperes (A), and travel speed in millimeters per minute (mm/min). The numbers 60 and 1,000 convert the result into kilojoules per millimeter.
Arc energy and heat input are related but not always interchangeable. Some standards use the unadjusted result, while others require an efficiency factor to account for energy losses during welding. Always follow the calculation method specified by the applicable code or WPS.
Here’s how you can calculate welding heat input.
Suppose a MIG welding operation uses 25 volts, 200 amperes, and a travel speed of 300 mm/min.
First, multiply voltage by current and by 60:
25 × 200 × 60 = 300,000
Next, multiply the travel speed by 1,000:
300 × 1,000 = 300,000
Divide the first result by the second:
300,000 ÷ 300,000 = 1.00 kJ/mm
The calculated arc energy is 1.00 kJ/mm. If the applicable method requires an efficiency factor of 0.80, the adjusted heat input is 1.00 × 0.80 = 0.80 kJ/mm.
This example shows why you must confirm whether your procedure requires adjusted heat input or unadjusted arc energy.
Sometimes, you know the weld length and time taken rather than the travel speed. Suppose a welder completes a 300 mm weld in 120 seconds of arc-on time. Convert the time into minutes and divide the weld length by that time:
300 mm ÷ 2 minutes = 150 mm/min
If the welding parameters are 24 V and 180 A, the arc energy is:
(24 × 180 × 60) ÷ (150 × 1,000) = 1.728 kJ/mm
This method is useful when travel speed is calculated from recorded weld length and arc-on time. Do not use the total time spent on the job, as breaks and setup time can distort the result.
The same calculation can be used with imperial measurements. Enter travel speed in inches per minute (in/min), and the result will be in kilojoules per inch (kJ/in).
For example, with 25 V, 250 A, and a travel speed of 10 in/min:
(25 × 250 × 60) ÷ (10 × 1,000) = 37.5 kJ/in
To compare results in different units, use these conversions:
Always convert values before comparing them with WPS limits.
Efficiency factors account for the portion of arc energy considered to reach the workpiece in an adjusted calculation. Illustrative reference values used in some methods include:
| Welding process | Abbreviation | Factor |
| Shielded metal arc welding | SMAW | 0.8 |
| Gas metal arc welding | GMAW | 0.8 |
| Flux-cored arc welding | FCAW | 0.8 |
| Gas tungsten arc welding | GTAW | 0.6 |
| Submerged arc welding | SAW | 1.0 |
These factors are not universal. Confirm the applicable standard and WPS before using one.
Heat input influences cooling rate and can affect the heat-affected zone (HAZ), microstructure, hardness, and mechanical properties. The effects depend on the base material, thickness, joint design, and welding process. Excessive heat input can affect toughness in some materials, while insufficient heat input may cause rapid cooling and increase the risk of excessive hardness or cracking. Following the approved welding procedure helps control the thermal cycle.
Read more about: Welding Quality Inspection Guide and Checklist
Compare the calculated result with the limits specified by the applicable WPS or governing code. Confirm that the units, calculation method, and definition of heat input match the requirements. For pulsed or waveform-controlled welding, multiplying average voltage by average current may not accurately represent average arc power. Use the measurement method required by the applicable procedure.
Preheat and post-weld heat treatment (PWHT) are separate controls. Preheat warms the base metal before and during welding, while PWHT applies a controlled thermal cycle after welding when required by the relevant procedure or code.
Avoid these common errors:
Consistent measurements and calculation methods make welding records more reliable.
AxiomHT provides on-site preheat and post-weld heat treatment services for industrial projects. Contact the team to discuss your welding requirements and thermal processing needs.
