Weld Heat Input Calculator

A spray-transfer MIG fillet. Lincoln Electric’s GMAW guide (C4.200) lists 0.045 in wire at 300 A, 29 V and 18 in/min for a 1/4 in fillet. Heat input = 29 × 300 × 60 ÷ 18 = 29,000 J/in = 29 kJ/in, or 29 ÷ 25.4 = 1.142 kJ/mm. The EN 1011-1 heat input is 0.8 × 29 = 23.2 kJ/in.

A pulsed weld from the machine’s energy reading. In Lincoln Electric’s example the power source shows 22.3 kJ at the end of a 3.6 in bead. Under QW-409.1(c) the heat input is 22.3 ÷ 3.6 = 6.19 kJ/in; no voltage, current or travel speed is needed.

The common mistake: mixing the ASME and EN numbers. A stick weld at 30 V, 120 A and 150 mm/min has 30 × 120 × 60 ÷ 150 = 1,440 J/mm, so 1.44 kJ/mm is its ASME/AWS heat input. The EN 1011-1 figure is 0.8 × 1.44 = 1.152 kJ/mm. Checking an ASME procedure limit against 1.152 lets the weld run 25% hotter than the limit allows, because 1.44 ÷ 1.152 = 1.25. Leaving out the 60 when the speed is per minute is the other slip: it gives 0.024 kJ/mm, sixty times too small.

Heat input matters because it sets how fast the weld and the heat-affected zone cool, and the cooling rate decides the microstructure and so the toughness and hardness. ASME Section IX, the welding and brazing qualification standard first published in 1941, deals with it in variable QW-409.1. Method (a) is the familiar volts × amps × 60 ÷ travel speed; method (b) works from the volume of weld metal deposited.

Pulsed and other waveform-controlled power sources broke method (a). Ordinary meters show average voltage and average current (or RMS values for AC), and the product of two averages is not the average power when both swing thousands of times a second. As Teresa Melfi of Lincoln Electric explained in the Welding Journal in June 2010, the 2010 edition of Section IX added method (c): take the energy, or the power and the arc time, from a meter that samples the waveform about 10,000 times a second, and divide by the bead length. For waveform-controlled procedures only methods (b) and (c) are allowed.

European practice draws a further line. The older British standard BS 5135 used the term arc energy; EN 1011-1 defines heat input as arc energy multiplied by a thermal efficiency factor k for the process, measured relative to submerged arc welding, which TWI tabulates as 1.0 for submerged arc, 0.8 for stick, MIG/MAG and flux-cored welding and 0.6 for TIG and plasma. American codes still call the arc energy the heat input, so always say which one a number is.

This calculator works the ASME IX QW-409.1 heat input from volts, amps and travel speed, or from an energy or power reading for pulsed and waveform-controlled welding, and gives the EN 1011-1 heat input alongside it. It can also turn a limit round into the slowest travel speed or the highest current or voltage that stays under it, and check a list of passes against that limit. The formulas follow ASME IX as quoted in Lincoln Electric’s Welding Journal article; the k factors are TWI’s.

The results are calculations, not qualification records: the limits that apply to your job come from the welding procedure specification and the code it was qualified to. Everything runs in your browser; nothing you enter is sent anywhere.

Related tools: Carbon Equivalent & Preheat Calculator, MIG Wire Feed Speed & Deposition Rate Calculator, and Welding Amperage Chart.