Calculate welding heat input from voltage, current, travel speed and arc efficiency. Results are provided in kJ/mm, kJ/cm and kJ/in.
Welding heat input represents the amount of energy delivered to the workpiece for each unit length of weld. A common calculation starts with the electrical arc power and divides it by travel speed.
When arc efficiency is included, the estimated net heat input becomes:
When voltage is in volts, current is in amperes and travel speed is in mm/min, the result is expressed in kJ/mm.
Suppose a weld is made at 24 V and 180 A with a travel speed of 300 mm/min and an assumed arc efficiency of 80%.
This demonstrates why the efficiency assumption matters. The electrical energy calculation and the net heat transferred to the workpiece are not necessarily the same quantity.
| Parameter | Increase Parameter | Effect on Heat Input |
|---|---|---|
| Voltage | Increase voltage | Increases arc power and generally increases heat input per unit length. |
| Current | Increase current | Increases arc power and generally increases heat input per unit length. |
| Travel speed | Increase speed | Reduces energy delivered per unit length. |
| Travel speed | Decrease speed | Increases energy delivered per unit length. |
| Arc efficiency | Increase efficiency | Increases estimated net heat transferred to the workpiece. |
Arc efficiency is not a universal constant. It depends on the welding process and the conditions under which the weld is made. Published engineering references commonly use approximate process-dependent values, but the value required for a particular procedure may be defined by the applicable standard.
| Process | Common Engineering Approximation | Important Note |
|---|---|---|
| SMAW | Approximately 0.7–0.8 | Actual efficiency varies with electrode type and welding conditions. |
| GMAW | Approximately 0.8 | Transfer mode and operating conditions can affect effective heat transfer. |
| GTAW | Approximately 0.6–0.7 | Arc characteristics and torch conditions affect efficiency. |
| FCAW | Approximately 0.8 | Consumable and process conditions influence the effective value. |
| SAW | Approximately 0.9–0.95 | High heat transfer efficiency is commonly associated with submerged arc welding. |
Heat input influences the thermal cycle experienced by the weld and surrounding heat-affected zone. Changing heat input can change cooling rates, peak temperatures, bead shape and the amount of base metal affected by welding.
For carbon and low-alloy steels, heat input can influence HAZ hardness, toughness and microstructure. For stainless steels and other alloys, excessive or insufficient heat input can create different metallurgical concerns depending on the grade.
Heat input therefore should not be selected solely to maximize deposition rate. The welding procedure must balance current, voltage, travel speed, joint design, preheat and interpass temperature with the required mechanical and metallurgical properties.
Heat input is closely related to the thermal cycle but does not completely determine cooling rate. Material thickness, joint geometry, preheat temperature, interpass temperature, ambient conditions and heat flow through the workpiece also affect cooling.
For example, two welds with the same calculated heat input can experience different cooling behavior if one is made on a thin plate and the other on a heavy section.
This is why welding procedure qualification normally considers heat input together with the complete set of welding parameters and material conditions.
Some welding documentation uses the term arc energy for the electrical energy per unit length and applies no process-efficiency correction. Other calculations use net heat input, which applies an arc-efficiency factor.
These terms should not be treated as interchangeable when a code or project specification defines a specific calculation method.
| Output | Meaning |
|---|---|
| kJ/mm | Energy delivered per millimetre of weld. |
| kJ/cm | Energy delivered per centimetre of weld. 1 kJ/cm = 0.1 kJ/mm. |
| kJ/in | Energy delivered per inch of weld. 1 kJ/in = 0.03937 kJ/mm. |
| J/mm | Energy per millimetre expressed in joules. 1 kJ/mm = 1000 J/mm. |
What is welding heat input? Welding heat input is the energy delivered to the workpiece per unit length of weld. It is commonly reported in kJ/mm or kJ/in.
How do I calculate welding heat input? A common calculation uses voltage, current and travel speed. When an efficiency factor is applicable, the electrical arc energy is multiplied by the assumed or specified arc efficiency.
Does higher current increase heat input? Yes. With voltage and travel speed held constant, increasing current increases electrical arc power and therefore increases energy per unit length.
Does higher voltage increase heat input? Yes. With current and travel speed held constant, increasing voltage increases electrical power and therefore increases heat input per unit length.
Does faster travel speed reduce heat input? Yes. Faster travel speed spreads the available arc energy over a greater length of weld, reducing energy per unit length.
What is arc efficiency? Arc efficiency represents the portion of electrical arc energy that is effectively transferred to the workpiece. Its value varies with welding process and conditions.
What is the difference between arc energy and heat input? Arc energy commonly describes electrical energy per unit length, while heat input may apply an efficiency factor to estimate the energy transferred to the workpiece. The exact terminology depends on the applicable standard.
Does heat input affect weld strength? It can. Heat input changes the weld thermal cycle and can affect microstructure, HAZ properties, toughness and other characteristics depending on the material and procedure.
Can the same heat input produce different cooling rates? Yes. Thickness, joint geometry, preheat, interpass temperature and other heat-flow conditions can cause different cooling behavior even when calculated heat input is similar.
Can this calculator be used for WPS qualification? It can be used as a calculation reference, but the applicable welding code or standard must define the required heat-input calculation and qualified parameter limits.