kilojoule per millimeter

Welding heat input is the single number that most determines what a weld becomes metallurgically. The arc delivers energy at a rate set by voltage and current; the travel speed decides how much of that energy each millimetre of joint receives. Slow travel means a hot, wide, slow-cooling weld; fast travel means a narrow one that quenches itself against the surrounding cold plate.

The relation is $H = \eta VI / v$, where $\eta$ is the arc efficiency — around 0.8 for shielded metal arc, higher for submerged arc, lower for gas tungsten arc. Ignoring the efficiency term overstates the true heat input, and it is the term most often left out.

The consequence that matters is the cooling rate through the transformation range, usually characterised as the time to cool from 800 °C to 500 °C. Fall below the minimum for a given steel and its carbon equivalent, and the heat-affected zone hardens enough to crack, often days later once hydrogen has had time to diffuse.

North American procedures write it in kilojoules per inch and metric ones in kilojoules per millimetre, a factor of 25.4 apart. A procedure qualification carried between the two systems without converting is the kind of error that passes every visual inspection and fails in service.