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Welding heat input is the amount of arc energy delivered to a joint for a given length of weld. It affects penetration, bead size, cooling rate, distortion, and the properties of heat-affected steel. The calculation is simple, but the result is only useful when you use the correct travel speed, units, and arc-efficiency factor.

The basic heat-input formula

For welding speed measured in inches per minute, use:

Heat input (kJ/in) = (Voltage × Amperage × 60 × efficiency) ÷ (Travel speed × 1,000)

For metric work, with travel speed in millimeters per minute, use:

Heat input (kJ/mm) = (Voltage × Amperage × 60 × efficiency) ÷ (Travel speed × 1,000)

Voltage is arc voltage, amperage is welding current, and travel speed is the actual speed along the joint. The efficiency factor estimates how much electrical arc energy enters the workpiece rather than being lost in the arc, spatter, radiation, or electrode heating.

If your welding procedure specification (WPS) gives a required heat-input range, follow its calculation method. Some shops report “arc energy,” which omits efficiency, while others report “effective heat input,” which includes it. Do not compare the two numbers as though they were the same.

Worked MIG welding example

Suppose you are welding carbon steel with MIG at 22 volts and 180 amps. Your measured travel speed is 8 inches per minute, and you use an estimated MIG efficiency of 0.8:

(22 × 180 × 60 × 0.8) ÷ (8 × 1,000) = 23.8 kJ/in

Without the efficiency correction, the arc-energy figure is 29.7 kJ/in. Both numbers can be valid if clearly labeled, but the WPS or engineer must specify which one controls.

For a metric calculation at 200 mm/min using the same settings:

(22 × 180 × 60 × 0.8) ÷ (200 × 1,000) = 0.095 kJ/mm

That is equivalent to 95 J/mm. Since 1 inch equals 25.4 mm, 23.8 kJ/in converts to about 0.94 kJ/mm, so be careful: the example speeds are different. Heat input depends heavily on travel speed, not just machine settings.

Typical efficiency factors

These are practical estimates, not universal constants. Transfer mode, polarity, shielding gas, electrode type, and operator technique can change the actual value.

For a critical steel structure, use the factor specified by the welding engineer or code. A rough factor is useful for planning and troubleshooting, but it is not a substitute for procedure qualification.

Measure real travel speed

Do not calculate with the speed shown on a wire feeder or a guess based on bead appearance. Mark a 12-inch or 300-mm section of test plate, weld it at the intended settings, and time the arc while it crosses the marks. Travel speed equals distance divided by arc time.

For example, if the arc takes 90 seconds to travel 12 inches, the speed is 12 ÷ 1.5 minutes, or 8 inches per minute. If you stop to reposition, exclude that time. If the weld has starts and stops, calculate each segment separately or use total arc-on time and total deposited length.

Voltage and amperage should also come from the actual weld. A MIG machine may display set voltage while the arc voltage changes with stickout and contact-tip condition. A TIG or stick machine may show current accurately but require a separate meter for voltage. A welding data logger is useful for procedure qualification; for basic shop work, stable machine readings and a timed test are often adequate.

Why heat input matters on steel

Too much heat input can produce excessive distortion, wide heat-affected zones, grain growth, reduced strength, and slower cooling. On some high-strength or quenched-and-tempered steels, excessive heat can reduce hardness and toughness. It can also make thin sheet buckle or pull out of alignment.

Too little heat input can leave lack of fusion, cold lap, poor tie-in, and an undersized effective throat. Rapid cooling may increase hardness and cracking risk in susceptible steels, especially when the plate is thick, restrained, dirty, or high in carbon equivalent.

Heat input is not the same as heat accumulation. Preheat, interpass temperature, plate thickness, joint restraint, bead sequence, and ambient conditions also control cooling. A low calculated heat input can still overheat a joint if the interpass temperature is not controlled.

How to adjust heat input

To reduce heat input, increase travel speed, lower amperage, lower voltage where the arc remains stable, or use shorter weld segments with cooling time between passes. Increasing travel speed is usually the most direct adjustment, but moving too fast can cause undercut or incomplete fusion.

To increase heat input, slow down, raise amperage, or make a wider weave. A slower travel speed usually increases penetration and bead size, but it also increases distortion. Weaving does not automatically improve penetration; a narrow, controlled stringer bead is often safer and easier to qualify.

Changing processes can help. A MIG welder can deliver productive, consistent welds on clean carbon steel, while an AC/DC TIG welder offers precise control at a slower deposition rate. Stick is often the cheaper and more portable choice for outdoor repairs, where wind makes MIG shielding unreliable.

Checks before welding the production joint

Confirm the required heat-input range, preheat, interpass limit, amperage, voltage, polarity, filler metal, and travel-speed limits in the WPS. Clean mill scale, oil, paint, and moisture from the joint. Use a calibrated temperature stick or infrared thermometer to verify preheat and interpass temperature.

If distortion is the main problem, improve fit-up, tack sequence, clamping, and weld order rather than simply chasing a lower heat-input number. For code work or critical repairs, record actual settings, travel speed, preheat, and interpass temperature on the weld log. That record is more useful than a calculated number with uncertain inputs.