The amount of energy that is put into a weld during the arc welding process, the "heat input", is a critical parameter, that must be controlled to ensure consistent weld quality. There are several ways of calculating the energy put into a weld. The most common approach to calculating the heat for non-waveform controlled welding is to use the welding current, voltage and travel speed. An American system for this is given in ASME IX and various AWS standards, and a European system is given in EN ISO 1011-1 and PD ISO/TR 18491.

Both calculations call the energy put into a weld the "heat input", but the European system for calculating heat input differs from the American system by the additional parameter of "thermal efficiency/process efficiency/arc efficiency". Note, in the earlier standard, BS 5135, the heat input was referred to as "arc energy" and did not necessarily include the process efficiency. You may also hear the ASME IX heat input referred to as arc energy under the European system. Ensure all parties agree on the definition (and calculation!)

The two calculations are:

Heat input is typically given in kJ/mm, so it is necessary to convert the values to standard units, that is: Current (Amps), Voltage (Volts), Travel speed (mm/s) and Thermal efficiency (no units). Those units will give the value of heat input in units of J/mm, so dividing the value by 1000 will give it in units of kJ/mm.

Other possible aspects of the calculation are to use a travel speed in mm/min, which requires multiplying the result by 60 (already incorporated in calculator above) or inch/min, which again requires a 60x multiplication factor and will produce a heat input result of kJ/inch.

The thermal efficiency values for the different processes are given in the table below:

This method is suitable for calculating the heat input in simple DC welding, including dip-transfer MIG and manual metal arc welding. It may also be suitable for AC welding where there is an equal balance of the welding current in both directions. However, in "waveform controlled" welding, which uses rapidly changing outputs, phase shifts and synergic changes, it may be that the calculations above do not correctly represent the heat input. Waveform controlled welding includes all pulsed welding processes, including synergic.

One method that is used in these cases is to calculate average, or time weighted values for the different parameters, e.g. for pulsed welding you might use:

but this is a crude method. The alternative is to use a method of recording either "instantaneous power" or "instantaneous energy" of the welding arc. This is performed by a high sampling frequency measurement device, either as part of the welding power source or as an external piece of equipment. The sampling frequency should be 10x the frequency of the waveform. There are then calculations for the heat input, which are given below. These equations are present in ASME IX and PD ISO/TR 18491.

The instantaneous energy equation is:

Where energy is given in Joules, and so a bead length in millimetres or inches again gives a heat input in J/mm or J/inch.

And the instantaneous power equation is:

Where here the power is given in Joules/second or Watts and a bead length of millimetres or inches gives a heat input of Joules/mm or Joules/inch. Again a factor of 1/1000 is necessary (but not shown) to conversion to kJ/mm or kJ/inch.

The final option for control of heat input is a measurement of volume of weld metal deposited, either by measuring bead size (width * thickness) or by controlling run out length per unit length of electrode.

An excel spreadsheet is given below which can calculate the heat input using these methods. Two versions are supplied. One has 20 passes, one 50.

Heat Input Calculator

Heat Input Calculator (More rows)

Note, this page is based on simple calculations for a single set of welding parameters. Consideration is given to multi-pass weld heat input here:

Multi-pass weld heat input calculations

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