Every weld puts heat into the base metal. That heat changes the metallurgy of the material around the weld, a region called the heat-affected zone (HAZ). How much heat goes in, and how fast, determines whether the HAZ is a narrow stripe of slightly altered metal or a wide band of weakened, brittle, or sensitized material that will fail in service. Heat input is the single number that captures this, and most welding codes specify maximum limits for good reason.
The concept is simple: heat input equals the energy delivered per unit length of weld. High heat input means more energy per inch, a wider HAZ, more grain growth, and more distortion. Low heat input means a narrow HAZ but risks incomplete fusion and lack of penetration if you go too low. This guide covers the calculation, explains what happens metallurgically, and gives you practical methods to control it on the shop floor.
The Formula: Amps, Volts, and Travel Speed
Heat input is calculated as: H = (60 × E × I) / (S × 1000), where H is heat input in kilojoules per inch (kJ/in), E is arc voltage in volts, I is welding current in amps, and S is travel speed in inches per minute. The numerator is power (volts times amps), and the denominator is how fast you move.
For example, a GMAW weld at 28 volts, 250 amps, and 12 inches per minute: H = (60 × 28 × 250) / (12 × 1000) = 35 kJ/in. The same parameters at 8 inches per minute: H = 52.5 kJ/in. Slowing down by 4 inches per minute increased heat input by 50 percent. Travel speed has a huge effect.
Some codes apply a process efficiency factor. GMAW is typically 0.8, SMAW is about 0.8, GTAW is about 0.6, and SAW is about 1.0. When specified, the formula becomes H = (60 × E × I × f) / (S × 1000). GTAW puts less heat into the workpiece per watt of arc power because more energy is lost to radiation from the exposed arc.
Measure your travel speed accurately. Most welders estimate by feel and are almost always slower than they think. Use a stopwatch and a marked piece of flat bar to calibrate your actual travel speed before assuming the WPS values match what you are doing.
H = (60 × V × A) / (S × 1000)
V = arc voltage (volts)
A = welding current (amps)
S = travel speed (in/min)
H = heat input (kJ/in)
Process factors: GMAW 0.8 | SMAW 0.8 | GTAW 0.6 | SAW 1.0
Weld Heat Input Calculator
Calculate weld heat input per AWS D1.1 and ASME Section IX. Enter amperage, voltage, and travel speed to get kJ/in and kJ/mm with process efficiency correction and risk tier classification by material type.
What Happens in the HAZ: Grain Growth and Embrittlement
The heat-affected zone is the region of base metal that got hot enough to change its microstructure but did not melt. In carbon steel, the HAZ closest to the fusion line reaches temperatures above 1,800°F, which causes the fine-grained structure to transform to austenite and then, on cooling, to a coarser grain structure. Coarser grains mean lower toughness.
Higher heat input means the HAZ stays hot longer, which allows more time for grain growth. A weld at 35 kJ/in might produce acceptable grain size. The same joint at 80 kJ/in might produce grain size that does not meet Charpy impact requirements. This is why structural codes set maximum heat input limits.
In quenched-and-tempered steels (like A514 or T-1), excessive heat input is even more damaging. These steels get their strength from a controlled heat treatment. The HAZ from a high-heat-input weld effectively re-tempers the base metal, reducing hardness and tensile strength below the minimum. Codes typically limit heat input to 45 to 65 kJ/in for these steels.
Preheat interacts with heat input. Preheat slows the cooling rate, which is beneficial for preventing hydrogen cracking but adds to the total thermal energy in the HAZ. When a code specifies both preheat and maximum heat input, you need to balance both.
Stainless Steel: Sensitization and Carbide Precipitation
Austenitic stainless steels (304, 316, 321, 347) have a specific heat input problem: sensitization. When the HAZ spends too long in the 800 to 1,500°F temperature range, chromium combines with carbon to form chromium carbides along the grain boundaries. This depletes chromium from the surrounding metal, dropping it below the 10.5% minimum needed for corrosion resistance.
High heat input keeps the HAZ in the sensitization range longer. Low heat input moves through that range quickly, minimizing sensitization. This is why stainless welding codes typically specify maximum interpass temperatures (often 300 to 350°F) and encourage low heat input techniques. GTAW with fast travel speed is the preferred process for critical stainless work.
The "L" grades (304L, 316L) have reduced carbon content (0.03% max vs 0.08%) specifically to resist sensitization. For critical corrosion service, use L-grade base metal, L-grade filler (308L, 316L), and still control heat input. Belt and suspenders.
If you suspect sensitization, the ASTM A262 oxalic acid etch test can reveal it. A polished cross-section etched in oxalic acid shows ditched grain boundaries if sensitization has occurred. The test takes about 30 minutes and costs very little.
Weld Heat Input Calculator
Calculate weld heat input per AWS D1.1 and ASME Section IX. Enter amperage, voltage, and travel speed to get kJ/in and kJ/mm with process efficiency correction and risk tier classification by material type.
Practical Control: What You Can Actually Adjust
You have three variables: voltage, amperage, and travel speed. Travel speed is the variable with the most room to adjust without compromising weld quality. Increasing travel speed by 20% reduces heat input by roughly 17%.
Stringer beads vs weave beads is another form of heat input control. A stringer bead moves straight along the joint. A weave bead oscillates side to side, slowing the effective travel speed and increasing heat input. For heat-input-limited applications, stringer beads are required.
Interpass temperature is the temperature of the weld zone before you start the next pass. Codes specify maximums (typically 300 to 500°F) to limit cumulative heat buildup. Use a contact pyrometer or temperature-indicating crayon to check. If it is too high, wait.
Wire feed speed on GMAW is directly proportional to amperage. If you need to drop heat input, reduce wire feed speed. On pulsed GMAW, the average current is lower than spray transfer at the same wire feed speed, which is why pulse is preferred for heat-input-sensitive applications.