Interpass temperature sits at the intersection of metallurgy, procedure control, and production throughput — which is exactly why it is one of the most inconsistently managed parameters on fabrication floors. The concept is simple: the weld joint must be above a minimum temperature before each pass (preheat) and below a maximum temperature before each pass (maximum interpass). In practice, the minimum gets measured at the start of the job and then largely ignored; the maximum gets ignored entirely until something goes wrong.
For a CWI doing in-process inspection, interpass temperature is an active monitoring obligation, not a form field to check at final inspection.
Why maximum interpass temperature matters
Preheat is protective — keeping the joint warm slows the cooling rate, prevents hydrogen-assisted cracking, and gives dissolved hydrogen time to diffuse out of the heat-affected zone. Most welders understand preheat intuitively.
Maximum interpass temperature is less well understood. Exceeding it causes:
Grain coarsening in the HAZ. Each thermal cycle with excessive heat input or elevated interpass temperature grows the prior austenite grain size in the heat-affected zone. Larger grains produce lower toughness — exactly the opposite of what CVN testing tries to confirm.
Reduced tensile strength on quenched-and-tempered steels. Q&T grades (A514, A517, A709 HPS grades) get their strength from heat treatment. Welding with excessively high interpass temperatures can effectively re-temper the base metal adjacent to the weld, reducing yield strength in a zone that is difficult to detect visually or ultrasonically.
Phase transformation problems on stainless and duplex steels. For AWS D1.6 stainless applications, high interpass temperatures drive unwanted sigma-phase precipitation and can shift the ferrite-austenite balance outside the qualified range.
The WPS maximum interpass temperature is not a conservative guess — it is a tested condition. Exceeding it places the production weld outside the qualified envelope.
Setting the maximum on the WPS
For non-CVN structural applications in common structural steel (A36, A572 Grade 50, A992), AWS D1.1:2025 does not set a universal maximum interpass temperature — that value comes from the WPS itself, based on the qualification test or the prequalified conditions. A typical maximum for these materials in standard practice is 450–550°F (230–290°C), though project specifications and material producer certificates sometimes tighten that window.
For CVN-qualified WPSs, the maximum interpass temperature is directly tied to Table 6.8. Under the 2025 edition, an increase in maximum interpass temperature beyond what was used during qualification is a supplementary essential variable (Table 6.8, row 8) — it triggers requalification if you need to go higher. Note that AWS D1.1:2025 dropped preheat from Table 6.8's scope relative to the 2020 edition; only the interpass maximum now drives supplementary requalification.
The WPS must state both:
- Minimum preheat temperature (and how it was established — code table or carbon equivalent calculation)
- Maximum interpass temperature
Measurement methods
Temperature-indicating crayons (Tempilstiks)
Temperature-indicating crayons melt at their rated temperature. Mark the base metal 2–3 inches from the joint and observe whether the mark liquefies during or between passes. Advantages: inexpensive, no batteries, immediate visual confirmation. Limitations: accuracy is ±10–15°C, and the crayons only confirm whether the surface reached the rated temperature — they do not give a reading between marked temperatures.
For minimum preheat verification on common structural steel, tempilstiks are widely used and generally accepted. For maximum interpass temperature on CVN-critical or high-strength applications, they lack the resolution to reliably catch a joint at 425°F when the maximum is 450°F.
Contact thermometers (bimetallic or thermocouple-based)
A calibrated contact thermometer placed on the base metal 1–2 inches from the joint gives a direct reading. These are the most common field instrument. Calibration matters: a thermometer that has been dropped or abused may read 30–50°F low, understating actual interpass temperature and creating a false sense of compliance.
Check instrument calibration against a known reference at least annually, and before any critical qualification coupon.
Infrared pyrometers
Infrared pyrometers measure surface temperature without contact. They are fast and useful when the joint is in an awkward position. The limitation is emissivity variation: mill scale, oxidized surfaces, and bright mill surface all have different emissivity values, and an uncorrected reading on a shiny weld bead can understate actual temperature significantly. Set the emissivity value for the specific surface condition, or use a contact instrument to verify.
Thermocouples with data logging
For high-volume production or critical structural applications, some QC programs attach temporary thermocouples to the joint and log temperature continuously throughout the weld sequence. This provides a complete thermal history and is especially useful for multi-pass heavy section welds where the thermal gradient from pass to pass is the compliance record. Thermocouple attachment adds setup time but eliminates the need for manual checks between passes.
Measurement location and timing
Location: Measure within 3 inches (75 mm) of the weld joint on the base metal, not on the weld bead itself. Weld bead surface temperature can be significantly higher than the base metal and HAZ, which is where the relevant metallurgy is occurring. For wide groove welds, check both sides of the joint.
Timing: Measure immediately before starting the next pass — after the slag from the prior pass has been removed but before arc strike. A measurement taken 30 seconds before welding while the welder repositions and the pyrometer is stowed is not a valid interpass check.
Frequency: The WPS or quality plan should specify how frequently interpass temperature is measured. For a 20-pass heavy plate weld, measuring at pass 1 and pass 20 is not adequate. A reasonable minimum for critical work is every 5–6 passes, with additional checks any time welding is interrupted.
CWI field verification
A CWI verifying interpass temperature compliance in the field should:
- Read the WPS before arriving at the joint. Know both the minimum and maximum before you touch the thermometer.
- Check the instrument calibration sticker. A field thermometer with no calibration date or an expired sticker is not a compliant measurement tool.
- Observe timing, not just results. A welder who takes the measurement mid-pass or long after the arc goes out is not capturing the correct parameter.
- Document the reading. On the traveler or inspection record, note the pass number, measured temperature, and instrument used. A verbal "it was fine" is not a quality record.
- Stop work immediately if the maximum is exceeded. Allow the joint to cool, verify temperature again, and document the nonconformance if it occurred.
Interpass temperature and WPS document control
The practical challenge is that interpass temperature limits are sometimes added to a WPS as a round number without clear traceability to either the qualification test or the applicable code provision. When a WPS states "maximum interpass 550°F" and there is no CVN requirement and no Q&T material, that limit may be perfectly adequate — or it may have been copied from another WPS covering a different material without adjustment.
As part of a WPS library review, verify that the maximum interpass temperature stated on each procedure is consistent with the material specification, the applicable heat input sensitivity, and any project specification requirements. For any WPS tied to a PQR, confirm the PQR records the actual interpass temperature measured during qualification. That recorded value is your upper bound for production.
Software-generated WPSs that carry essential variable fields through from the PQR record to the production document reduce the risk of this type of mismatch — see the GMAW WPS generator for A572 Grade 50 for an example of how thermal parameters are structured in a generated procedure.
For a broader treatment of what essential variables control WPS qualification envelopes — including how interpass temperature interacts with Table 6.6 and Table 6.8 — see WPS essential variables vs. nonessential variables and CVN supplementary essential variables: AWS D1.1 Table 6.8.
Rule library based on AWS D1.1:2025; verify against your governing edition (the AHJ or contract may specify 2020 or an earlier edition).