What's the formula for heat input?

For arc welding: Heat Input (kJ/in) = (Voltage × Amperage × 60) / (Travel Speed in inches per minute × 1000). For metric: Heat Input (kJ/mm) = (Voltage × Amperage) / (Travel Speed in mm/min × 1000) × 60.

Why is heat input an essential variable?

Heat input controls cooling rate, which controls grain structure and toughness. High heat input promotes coarse grain and reduces toughness. Low heat input promotes hardening in the HAZ. CVN-required work is especially sensitive.

Does the WPS need to show a calculated heat input number?

Best practice yes — include the calculation or the qualified range. A WPS that just lists amp/volt/travel-speed without showing the resulting heat input range leaves the boundary check to the auditor.

How do production welders stay in range?

By following the amperage, voltage, and travel speed ranges on the WPS. Travel speed is the most-violated variable in production — welders push it to keep up with schedule. A heat-input monitor on the welding power supply solves this for high-end production lines.

Heat input control: how to calculate and document it on a WPS

Heat input is an essential variable in AWS D1.1 Tables 6.6 and 6.8. Here's the calculation, the documentation pattern, and the production tracking that keeps it in range.

Heat input is the single most-violated essential variable in production welding. It's listed in AWS D1.1:2025 Table 6.6 baseline essentials and tightened under Table 6.8 for CVN-required work. Knowing how to calculate it, document it on the WPS, and keep it inside the qualified range during production prevents most heat-input-related audit findings.

The formula

For arc welding:

$$\text{Heat Input (kJ/in)} = \frac{V \times I \times 60}{S \times 1000}$$

Where:

V = voltage in volts
I = amperage in amperes
S = travel speed in inches per minute (ipm)

For metric:

$$\text{Heat Input (kJ/mm)} = \frac{V \times I}{S \times 1000} \times 60$$

Where S is in mm/min.

For pulsed processes, AWS D1.1 uses an instantaneous-power-integration method — the average power over time, divided by travel speed. Power supplies that report measured-instantaneous power simplify this.

A worked example

SMAW E7018, 1/8 in electrode:

V = 22 V
I = 130 A
S = 7 ipm

Heat input = (22 × 130 × 60) / (7 × 1000) = 24.5 kJ/in

If the supporting PQR qualified heat input at 25 kJ/in ± 10%, this WPS sits inside the qualified band (22.5–27.5 kJ/in).

What to put on the WPS

Three blocks make heat-input control auditable:

Parameter ranges. Amperage 115–145 A, Voltage 21–23 V, Travel speed 6–8 ipm.
Resulting heat-input range. Calculate both ends: minimum heat input (low V, low I, high S) and maximum heat input (high V, high I, low S). Show both.
Qualified heat input range (from the supporting PQR). Show side-by-side with the WPS range so the safety margin is visible.

A WPS that shows "WPS heat input: 19.4 – 31.8 kJ/in. PQR qualified: 18.0 – 32.0 kJ/in." makes the boundary check obvious to anyone reading.

Production tracking

Three approaches, in order of robustness:

Welder discipline. The welder is responsible for staying within the amp/volt/travel-speed range on the WPS. Cheapest, least reliable.
In-process auditing. A floor inspector spot-checks parameters on randomly selected joints. Catches drift but doesn't prevent it.
Welding power-supply data logging. Modern inverter supplies record actual welding parameters per arc-on event. Some can plot heat input in real time and alarm on out-of-range conditions. Standard on high-end production lines.

For CVN-required work, in-process auditing at minimum is expected. For ordinary structural production, welder discipline plus periodic audit usually meets code.

When heat input pushes you out of range

Two common production scenarios that drift heat input outside the qualified range:

Travel speed too high. Welder rushing the schedule. Heat input drops, weld may underbead, HAZ hardness rises. Common on FCAW production.
Voltage drift on long-arc welding. As the lead consumes, arc length changes, voltage rises. Heat input creeps up. Common on SMAW with manual amperage control.

Both are detectable in real time with a power-supply data log. Both result in lack-of-fusion or low-toughness defects that show up on radiographic or CVN production tests if performed.

The 2025 edition difference

Under AWS D1.1:2025, the Table 6.6 heat-input row threshold is generally ±10% to ±20% depending on the process and the row's specific wording. For CVN-required work, Table 6.8 tightens this further. The 2025 supplementary rules removed preheat as a Table 6.8 trigger (Row 8 in the new numbering) but kept the interpass-maximum increase trigger — so CVN heat input still has tighter latitude than non-CVN work.

A rule engine that knows your supporting PQR and your code edition can calculate the allowed heat-input band and warn on every WPS revision that drifts.