Heat input
Heat input from run-out length and arc time (MMA)
- where
- travel speed (mm/min)
- length of weld deposited (mm)
- arc time for the run (s)
- arc energy (kJ/mm)
- arc voltage (V)
- welding current (A)
- heat input (kJ/mm)
- thermal efficiency factor
Explain this equation
Works out heat input for manual metal arc welding from the length of weld one electrode lays and how long the arc was on. Useful when travel speed is hard to measure directly.
L is the run-out length and t the arc time in seconds, so 60 L / t is the travel speed in mm per minute. Volts times amps times time is the total energy in joules; dividing by the length gives joules per millimetre, and the 1000 gives kJ/mm. k is the thermal efficiency, 0.8 for MMA.
The same energy-per-length idea as the travel-speed version, worked from a measured run instead.
Sense check: 22 V, 140 A for 60 s laying 150 mm gives 150 mm/min, an arc energy of about 1.23 kJ/mm and a heat input of about 0.99 kJ/mm.
Watch out: time the arc only, not the gap between electrodes, and measure the length of weld actually deposited.
For stick welding: works out travel speed and heat input from how long one electrode burns and how much weld length it lays down.
Enter your values to see the result.
Good to know
Time the arc from strike to break and measure the length of the run that electrode produced. This is the practical way to check heat input for MMA, where travel speed is hard to time directly. The same equation works for any process if you time a run over a known length.
Common mistake
Using the full electrode cycle time including changing rods. Only the arc-on time counts.
Limits
Assumes steady volts and amps over the run. Use k = 0.8 for MMA per BS EN 1011-1.
Sources
- BS EN 1011-1:2009 Welding – Recommendations for welding of metallic materials – Part 1: General guidance for arc welding (arc energy, heat input and thermal efficiency factors k)
- TWI FAQ: What is the difference between heat input and arc energy? (HI = η × arc energy; η = 1.0 SAW, 0.8 MMA, FCAW and MIG/MAG, 0.6 TIG and plasma)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the welding procedure specification (WPS), BS EN ISO 15614/9606, BS EN 1090, BS EN 1993 and manufacturers’ data. Structural welds need qualified welders and engineered designs.