Heat input controls the thermal cycle at every point in the joint: the peak temperature reached, the width of the heat-affected zone, and above all the cooling rate through the 800 to 500 °C range that decides whether the HAZ transforms to bainite or to martensite. Low heat input means fast cooling, hard HAZ and cold-cracking risk. High heat input means slow cooling, coarse grain, low toughness and — in austenitic and duplex stainless steel — time spent in the sensitisation and sigma-forming ranges. Every welding procedure specification therefore carries a heat input or run-out length limit.

The measured quantity is arc energy: E = V·I·60 / (v·1000) in kJ/mm, with voltage in volts, current in amperes and travel speed in mm/min. That is the electrical energy delivered per unit length of weld. Not all of it enters the workpiece — some is lost to radiation from the arc, to spatter, to heating the electrode stub and to fumes. EN 1011-1 applies a thermal efficiency factor k to convert arc energy to heat input: 1.0 for submerged arc, 0.8 for manual metal arc, MIG/MAG and flux-cored, and 0.6 for TIG and plasma.

This is where the two conventions diverge, and it matters. ASME Section IX and AWS D1.1 define heat input as arc energy with no efficiency factor — k is effectively 1.0. EN 1011 and ISO/TR 18491 apply k. The same weld can therefore be recorded as 1.06 kJ/mm on an ASME procedure and 0.85 kJ/mm on a European one. Always state which convention a recorded value uses; comparing a European heat input to an ASME limit without correcting for k will under-report by 20 % or more.

For pulsed and waveform-controlled processes, instantaneous voltage times instantaneous current averaged over time is not the same as average voltage times average current. ISO/TR 18491 requires instantaneous power measurement for these processes; using meter averages can be substantially in error.

Worked example

Arc voltage	24 V
Welding current	220 A
Travel speed	300 mm/min
Process / convention	smaw
Thermal efficiency factor k	0.8
Travel speed	5 mm/s
Arc energy	1.056 kJ/mm
Heat input	0.845 kJ/mm

Arc energy = 24 x 220 x 60 / (300 x 1000) = 316800/300000 = 1.056 kJ/mm. SMAW efficiency k = 0.80 from EN 1011-1, so heat input = 0.80 x 1.056 = 0.8448 -> 0.845 kJ/mm. Travel speed 300 mm/min = 5.0 mm/s. Recorded on an ASME IX procedure the same run would be reported as 1.056 kJ/mm.