How to calculate welding heat input
Heat input is the energy delivered to the joint per unit of weld length, and it drives weld metal structure, heat-affected zone toughness and distortion. The formula is volts times amps times 60 divided by travel speed, expressed here in kJ/in. Welding at 200 A, 24 V and 12 in/min gives about 24 kJ/in, and applying an arc efficiency of 0.8 gives a net heat input near 19 kJ/in.
Arc efficiency is the share of arc energy that actually reaches the base metal. Commonly used values are 0.8 for SMAW, GMAW and FCAW, 1.0 for submerged arc and 0.6 for GTAW. Use the value listed in the welding procedure specification when one is given.
Structural carbon steel is often welded in a range of roughly 20 to 64 kJ/in (0.8 to 2.5 kJ/mm), but high-strength, stainless and low-temperature steels frequently carry their own documented limits. Too much heat coarsens the grain structure and increases shrinkage; too little causes rapid cooling and cold cracking. Keep the parameters inside the qualified range in the WPS and have a welding engineer or Certified Welding Inspector confirm the final settings.
Frequently Asked Questions
Heat input equals volts times amps times 60, divided by travel speed. With travel speed in inches per minute and the result divided by 1,000 you get kJ/in: 200 A, 24 V and 12 ipm gives about 24 kJ/in (0.96 kJ/mm). Multiplying by the arc efficiency gives the net heat delivered to the base metal.
Values widely cited in AWS and ISO/TR 18491 references are 0.8 for SMAW, GMAW and FCAW, 1.0 for submerged arc and 0.6 for GTAW. If the welding procedure specification lists a value, use that one.
Excess heat input slows the cooling rate, coarsens the heat-affected zone grain structure and lowers toughness while increasing distortion. Too little heat input causes rapid cooling, hardening and a higher risk of cold cracking, so stay inside the limits set by the procedure.