Heat input

Heat input is the quantity that connects the settings on a power source to the metallurgy of the finished joint. It fixes how fast the weld and the surrounding parent material cool, and cooling rate is what decides hardness, toughness and the structure of the heat-affected zone.

Two conventions share the name, and confusing them is the most common error in practice. Arc energy is the electrical energy per unit length, straight out of the formula. Heat input in the EN and ISO sense is that value multiplied by a thermal efficiency factor k that accounts for the share of the arc energy actually entering the joint — different for submerged arc, for the consumable-electrode gas-shielded processes, and for tungsten arc welding. ASME and AWS documents use the unfactored value and call it heat input. Two records can therefore quote heat input for the same weld and differ substantially, so the convention belongs next to the number.

The travel speed term carries most of the measurement error. Voltage and current are read off the power source; speed is usually estimated, and because heat input varies inversely with it, a speed that is wrong by a fifth moves the result by more than the width of many qualified bands. Weave makes this worse and invisible at the same time: a wider weave means slower advance along the joint at identical meter readings.

Where a procedure qualification carries impact or hardness testing, the qualified value is bounded on one side or both. A production weld outside those bounds is outside the qualification, whatever the finished bead looks like — which is the practical reason to compute it from measured values rather than from the figures on the sheet.