Arc Energy vs Heat Input: AWS D1.1 Formula and WPS Documentation

Heat input is one of the most misunderstood parameters in WPS documentation. CWIs and QC managers routinely confuse "arc energy" with "heat input," leave the calculation off WPS forms entirely, or fail to recognize when a production parameter deviation triggers CVN requalification. This article covers the formula, the process factors, and exactly what AWS D1.1:2025 requires you to document.

The Arc Energy Formula

Arc energy is calculated from three measurable production parameters:

Arc Energy (J/mm) = (V × A × 60) / S

Where:

• V = arc voltage (volts)
• A = welding current (amperes)
• S = travel speed (mm/min)

If your travel speed is in inches per minute, convert: 1 in/min = 25.4 mm/min. For example, a pass at 24V, 180A, and 12 in/min (304.8 mm/min) gives:

(24 × 180 × 60) / 304.8 = 85.2 J/mm

This number is what gets recorded on production parameter logs and what your WPS should reference as the qualified range.

Arc Energy vs Heat Input: Why the Distinction Matters

"Heat input" technically equals arc energy multiplied by a thermal efficiency factor (η) specific to the welding process:

Heat Input (J/mm) = Arc Energy × η

These efficiency factors appear in references such as EN 1011-1 and AWS A3.0 commentary. AWS D1.1:2025 does not publish a table of efficiency factors within the standard text itself — it uses the arc energy formula directly when specifying calculation methods. When a project specification or owner document requires "heat input," verify whether they mean arc energy or the thermally-adjusted value, because the numbers differ meaningfully between processes.

The practical consequence: a SAW procedure qualified at 1,200 J/mm (arc energy) and an SMAW procedure qualified at 1,200 J/mm are not equivalent from a HAZ toughness standpoint. The SMAW weld deposited less actual thermal energy into the joint.

What AWS D1.1:2025 Actually Requires

Standard Procedure Qualification (Table 6.6)

For procedures qualified under the standard essential variables (AWS D1.1:2025 Table 6.6), heat input is not an essential variable. You can increase or decrease energy input significantly without triggering requalification, as long as other listed essential variables (amperage range, voltage range, travel speed range) stay within the qualified ranges. If you list specific ranges on the WPS, you must stay within them in production.

CVN-Supplementary Essential Variables (Table 6.8)

This is where heat input becomes critical. When a procedure requires CVN Charpy impact testing — typically for demand-critical welds, Seismic Category C or D connections per AWS D1.8, bridge welds per D1.5, or owner-specified toughness requirements — the rules change.

Per AWS D1.1:2025 Table 6.8, an increase in heat input beyond the maximum heat input used during PQR testing constitutes an essential variable change requiring requalification. Specifically, Table 6.8 calls out:

• Increase in heat input above the maximum heat input used in qualification
• Decrease in preheat or interpass temperature minimum below what was used in the PQR (Table 6.8 row 8 as revised in 2025 — note that this covers interpass maximum increase only; preheat was dropped from scope in the 2025 edition)

The 2025 edition also revised the minimum thickness floor for Table 6.8: it is now 1/2 in [12 mm] (reduced from 5/8 in [16 mm] in the 2020 edition), meaning CVN requirements now apply to thinner material.

Rule library based on AWS D1.1:2025; verify against your governing edition.

Recording Heat Input on a WPS

Even when heat input is not a code-required essential variable, recording it on the WPS has operational value:

1. Owner specs often require it. AISC-certified shops, DOT projects, and nuclear-adjacent fabricators frequently have supplemental requirements that go beyond D1.1 minimums.
2. Production monitoring. A heat input range on the WPS gives welders and inspectors a quantitative check that travel speed hasn't drifted out of bounds during a shift.
3. Audit defense. If a CWI or third-party auditor questions whether a production weld was made within PQR bounds, documented heat input ranges give you something concrete to point to.

The minimum you should document on a WPS:

• Qualified amperage range (min–max)
• Qualified voltage range (min–max)
• Qualified travel speed range (min–max)
• Calculated heat input range at the extremes of those parameters

For CVN-required procedures, also document the maximum heat input from the PQR explicitly, because that is the threshold that triggers requalification on increase.

Calculating Heat Input Range for a WPS

The worst-case (maximum) heat input occurs at the combination of maximum voltage, maximum amperage, and minimum travel speed. The minimum heat input is at minimum voltage, minimum amperage, and maximum travel speed.

Example for a FCAW-G procedure:

• Qualified amperage: 180–220A
• Qualified voltage: 24–28V
• Qualified travel speed: 10–18 in/min (254–457 mm/min)

Max heat input: (28 × 220 × 60) / 254 = 1,457 J/mm Min heat input: (24 × 180 × 60) / 457 = 568 J/mm

For a CVN-qualified procedure, the maximum 1,457 J/mm becomes the requalification threshold. Any production pass exceeding that value in a CVN-required joint requires requalification before the procedure can be used above that level.

Heat Input and HAZ Toughness

The engineering reason heat input matters for CVN welds is straightforward: higher heat input produces a wider heat-affected zone and slower cooling rates. Slower cooling coarsens the HAZ grain structure, which reduces toughness. This is especially significant for:

• HSLA steels (A572 Gr. 65, A913, A992) where toughness is achieved through controlled rolling and may degrade under excessive heat input
• Quenched-and-tempered steels (A514, A517) where the QT condition can be annealed out of the HAZ by excessive heat input
• Demand-critical and fracture-critical connections where CVN requirements exist precisely because of the toughness risk

For standard A36 or A572 Gr. 50 fillet welds with no CVN requirement, heat input is a quality-of-life parameter rather than a code-essential one.

Practical Monitoring on the Shop Floor

The three parameters that feed the heat input calculation — voltage, amperage, travel speed — must all be monitored in production for CVN procedures. Travel speed is the easiest to mismanage:

• Automatic processes (SAW): travel speed is dial-controlled and easy to verify
• Semiautomatic (FCAW, GMAW): travel speed is welder-paced; timed chalk marks (1 in per second = 60 in/min) give a field check
• SMAW: electrode burn-off time and electrode length provide an indirect check

Production parameter logs should capture voltage, amperage, and travel speed for each CVN-required pass. This gives the CWI the data to back-calculate heat input during inspection and during audit. See also our article on production welding parameter logs for CWIs and heat input control documentation best practices.

Common Mistakes to Avoid

Mistake 1: Using wire feed speed instead of amperage. Wire feed speed is a process control input that correlates to current, but it is not the same thing. Always use measured amperage for heat input calculations, not WFS.

Mistake 2: Forgetting to convert travel speed units. The formula requires consistent units. If you collect travel speed in in/min, convert to mm/min before plugging in.

Mistake 3: Assuming heat input is symmetric. Increasing heat input past the PQR maximum is an essential variable for CVN procedures. Decreasing below the PQR minimum is not (Table 6.8). Don't restrict your welders unnecessarily on the low end.

Mistake 4: Using the same heat input formula for all processes without noting the efficiency factor. If an owner or engineer is comparing heat input across processes, make sure everyone is using the same convention (raw arc energy vs. thermally adjusted) to avoid a 15-20% discrepancy creating a false compliance or non-compliance call.

Software and WPS Documentation

Manual heat input calculation is error-prone when you have dozens of WPS with different processes and parameter ranges. A purpose-built WPS platform can auto-calculate min and max heat input from the qualified parameter ranges and flag when a CVN-required procedure's maximum is being approached in production. Learn more about how WPS software handles heat input tracking at wpswelding.com/pricing.

For related reading, see CVN supplementary essential variables under AWS D1.1:2025 Table 6.8 and interpass temperature limits for structural welding.