Heat Input Is Not Just a Number — It Has a Qualified Ceiling
When a fab shop completes a procedure qualification record (PQR), the amperage, voltage, and travel speed documented on the test represent more than just "what we used that day." For tested welding procedures under AWS D1.1, these parameters define the heat input range within which the WPS is qualified. Using the WPS outside that range — specifically at higher heat inputs — can invalidate the qualification without a retest.
This matters most for procedures where Charpy V-notch (CVN) impact toughness is required. High heat input coarsens the heat-affected zone (HAZ) grain structure, which lowers toughness. A procedure qualified at a specific maximum heat input cannot simply be applied at a higher heat input and assumed to still meet the CVN acceptance criteria. The test was done at certain conditions; the material behavior in service depends on replicating those conditions.
Rule library based on AWS D1.1:2025. Verify against your governing edition — the AHJ or contract may specify 2020 or earlier.
The Formula and What Goes Into It
AWS D1.1:2025 defines heat input using the standard arc energy equation:
HI = (E × I × 60) / (TS × 1000) kJ/in
Where:
- E = arc voltage (volts)
- I = welding current (amps)
- TS = travel speed (in/min)
- The divisor 1000 converts to kJ
Or in metric:
HI = (E × I × 60) / (TS × 1000) kJ/mm
With TS in mm/min.
Every variable in this equation appears on the WPS as a production range. The PQR records the actual values used during the test coupon run. The relationship between the PQR values and the WPS production ranges is where the essential variable logic applies. For a detailed walkthrough of the formula and its units, see arc energy and heat input calculation under AWS D1.1.
Table 6.6: Where the Essential Variable Rules Live
AWS D1.1:2025 Table 6.6 governs essential variable changes for SMAW, SAW, GMAW, FCAW, and GTAW procedures (Table 6.7 covers ESW and EGW; Table 6.8 addresses CVN supplementary essential variables). The heat-input-related rows in Table 6.6 establish the qualified range:
Amperage: A change in amperage that exceeds the ±10% tolerance listed in the WPS is considered an essential variable change for most processes. The ±10% tolerance is a production allowance — it lets welders adjust within the listed range during production. A WPS that does not list a range (only a single value) still allows the implied ±10% tolerance, but the WPS revision is triggered when production parameters consistently run outside that envelope.
Voltage: Similarly, a ±7% production tolerance on voltage is standard. Voltage changes affect arc length and therefore bead geometry, heat input, and penetration pattern.
Travel speed: For CVN-applicable procedures, a decrease in travel speed (which increases heat input) beyond the qualified range is an essential variable change requiring requalification. The minimum travel speed from the PQR sets the lower bound of the qualified range. Operating below that speed increases heat input above what was tested, and the CVN results cannot be extrapolated to higher heat input conditions.
Heat input directly: AWS D1.1:2025 Table 6.8 (CVN supplementary essential variables) includes a separate line that expressly states an increase in heat input beyond the maximum qualified in the PQR is an essential variable change for CVN procedures. This is where the ceiling gets formally codified for impact-tested work.
What the PQR Must Document
For the qualified range to be usable, the PQR must record the actual production parameters for each pass run during the test, not nominal target values. Common PQR documentation failures for heat input:
Recording only the target, not the measured value. The PQR should show the actual ammeter reading and voltmeter reading during welding, not just the machine dial setting. Weld machines drift; a machine set to 200 A may deliver 185 A. The PQR is a record of what actually occurred, and that record defines the qualified range.
Failing to record travel speed by pass. On a manual or semi-automatic process, travel speed varies pass-to-pass. Each pass should have its own recorded travel speed. The slowest pass (highest heat input) typically becomes the qualified upper limit. If the PQR shows a single travel speed for all passes, that value applies to all; if different passes ran at different speeds, each is recorded and the minimum becomes the constraint.
Not calculating and recording the resulting heat input. The PQR should explicitly state the calculated heat input per pass, or the WPS reviewer has to reconstruct it. Best practice: calculate and record HI for every pass, flag the maximum, and carry that maximum forward to the WPS as the upper limit for production.
How the WPS Should Express the Qualified Range
The WPS is the production document that welders and supervisors consult on the floor. It should express the heat input constraints in a way that is checkable in production, not only traceable to the PQR.
Practical approach for the WPS:
- List amperage as a range (e.g., 180–220 A for E71T-1 in the 3F position), not a single value.
- List voltage as a range consistent with the ±7% production tolerance.
- List travel speed as a minimum value (e.g., "min. 8 in/min [200 mm/min]"), not a single value. This directly expresses the heat input ceiling in the variable that welders can control in real time.
- List the resulting maximum heat input explicitly in kJ/in. This gives supervisors and CWIs a checkable number when they observe a pass that looks slow.
The Annex M form (the standard WPS form referenced in AWS D1.1) has fields for amperage range, voltage range, and travel speed range. All three should be populated from the PQR data, not left blank or filled with generic ranges.
Production Monitoring: What the CWI Checks
A CWI performing in-process inspection on a CVN-critical weld verifies that production parameters fall within the WPS qualified range. Practical tools:
Calibrated clamp-on ammeters are the standard for amperage verification. Dial settings are not reliable; actual welding current at the arc is what matters. AWS D1.1 does not prescribe calibration intervals for welding equipment, but an auditor will expect some documented calibration or verification frequency. See welding equipment calibration records under AWS D1.1 for what documentation is typically required.
Timing of passes. For manual SMAW or FCAW, a CWI can time a pass and measure the bead length to verify approximate travel speed. This does not replace instrumentation, but it catches obvious outliers — a welder making very slow stringer passes who may be running heat input well above the qualified maximum.
Bead geometry check. A wide, flat bead at low travel speed is visible evidence of high heat input. If the bead profile is significantly wider than the WPS's prescribed bead width range, that is a flag for direct parameter verification.
When a Change Requires Requalification vs. WPS Revision
Not every parameter change triggers requalification. The distinction:
WPS revision only (no new PQR): Tightening or narrowing the parameter range within the already-qualified range. Adding a tighter maximum heat input on the WPS when the PQR already supports a lower maximum is conservative, not a requalification trigger.
Requalification required: Increasing the maximum heat input above the PQR value; decreasing travel speed below the PQR minimum on a CVN procedure. These extend beyond the tested range and require a new test coupon at the new conditions.
Neither required: Adjusting production parameters within the ±10% amperage or ±7% voltage tolerance windows already on the WPS. The WPS permits this range; the welder uses the range; no paperwork changes.
The WPS requalification triggers checklist provides a complete rundown of the Table 6.6 and Table 6.8 essential variable triggers — useful when reviewing a change order that affects welding parameters on a qualified procedure.
Practical Example: SAW on a CVN-Qualified Procedure
Submerged arc welding (SAW) uses high amperages and slow travel speeds that can produce very high heat inputs — 80–120 kJ/in is common on thick plate. When a CVN requirement applies (Table 6.8 is in force), the heat input ceiling from the PQR is binding.
A shop that qualifies a SAW WPS at 600 A / 30 V / 10 in/min (HI = 108 kJ/in) and then finds production efficiency improves by slowing to 8 in/min (HI = 135 kJ/in) has exceeded the qualified range for a CVN procedure. The higher heat input was not tested; the CVN data does not extend to it. The options are:
- Return to the qualified travel speed.
- Run a new PQR at the slower speed and verify CVN results.
- Verify with the EOR that CVN requirements do not apply to this WPS, removing the Table 6.8 constraint.
This kind of scenario is more common than it appears in shops that optimize for throughput without a formal change-management process tied to the welding procedure.
If you need to track parameter changes against the qualified range across a multi-WPS project and generate a defensible change log, the WPS generator includes the qualified range documentation tied directly to each PQR.
Key Takeaways
- Heat input (from amperage, voltage, and travel speed) defines the qualified production range on a tested WPS — specifically the upper ceiling.
- For CVN-applicable procedures, AWS D1.1:2025 Table 6.8 expressly makes a heat input increase above the PQR maximum an essential variable change requiring requalification.
- The PQR must record actual measured parameters per pass, not target values. The maximum heat input from the PQR becomes the WPS production ceiling.
- The WPS should express travel speed as a minimum (not a single value) so the heat input ceiling is directly enforceable in production.
- A ±10% amperage tolerance is a production allowance within the qualified range — it does not expand the qualified range beyond what the PQR supports.