Preheat gets most of the attention in pre-weld inspection. Interpass temperature — the temperature of the weld zone and base metal between successive passes — gets far less, even though violating the maximum interpass limit can reduce mechanical properties and drive CVN requalification. For a CWI or QC manager running a multi-pass structural weld, understanding interpass temperature is as important as understanding preheat.

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

What interpass temperature is — and isn't

Interpass temperature is measured at or near the weld zone immediately before depositing the next pass. It is not the arc temperature, the weld pool temperature, or the temperature of steel several feet away from the joint.

The term "interpass" means between passes: after a pass is completed and before the next one starts. On a thick CJP groove weld with many passes, the inspector may need to measure interpass temperature multiple times throughout the weld sequence.

Interpass temperature has two distinct bounds:

  • Minimum interpass temperature = the same value as the minimum preheat. You must maintain the preheat temperature through the entire weld sequence, not just before the first pass. The plate cannot cool below the minimum preheat between passes on a complex joint.
  • Maximum interpass temperature = the upper limit stated in the WPS. Once reached, welding must stop until the joint cools to within the stated range before the next pass is deposited.

Both values belong on every WPS that covers multi-pass welds. An inspection hold point before each successive pass verifies both.

Why the upper limit matters

The mechanical properties of the heat-affected zone and weld metal are sensitive to thermal history. Depositing a pass on base metal or previously deposited weld metal that is significantly hotter than the maximum interpass limit causes several problems:

Grain coarsening in the HAZ. At temperatures well above 400°F [200°C], prolonged heating coarsens the austenite grain structure in the heat-affected zone. Coarser grains reduce notch toughness — a particular concern for fracture-critical and seismic members where CVN impact values are required.

Softening of quenched and tempered steels. High-strength Q&T steels like A514 are produced by heat treatment. Excessive interpass temperature re-tempers the microstructure of both the base metal HAZ and previously deposited passes, reducing yield strength below the specification minimum. This is why A514 WPSs typically specify maximum interpass temperatures well below 400°F [200°C] — some as low as 200°F [95°C] per the manufacturer's recommendation and project spec. See the related article on WPS requirements for high-strength A514 steel.

Reduced tensile properties. Even on mild steel, consistently running above the maximum interpass temperature can soften the weld deposit and reduce yield and tensile strength slightly. For typical A36 or A572 Grade 50 applications with E7018 or ER70S-6, the effect is usually small — but it becomes a qualification issue if the PQR was run with a lower maximum interpass temperature than production practice.

What value should the WPS specify?

AWS D1.1:2025 does not prescribe a universal maximum interpass temperature for all structural steels. The WPS is required to state the limit, and the inspector must enforce it.

Common practice for carbon structural steels (Group I, Group II):

  • 400°F [200°C] is the most widely used maximum interpass temperature for general carbon steel work. It is the limit stated in many prequalified WPS templates and is consistent with AWS D1.1 commentary guidance.
  • Filler metal manufacturers sometimes recommend lower values — especially for FCAW electrodes in certain classifications. The electrode technical data sheet should be consulted.
  • CVN applications — when CVN impact testing is required by the project spec or AISC 341 seismic requirements, the maximum interpass temperature qualified in the PQR must not be exceeded in production. For these applications, 400°F [200°C] or lower is typical, and the PQR documents the actual maximum used during coupon welding. See the related article on CVN impact testing and AWS D1.1:2025 Table 6.8 for the requalification implications.

For Q&T steels and non-standard base metals, always consult the base metal manufacturer's welding guidance and the project spec alongside the code.

Interpass temperature as an essential variable

Table 6.6 — baseline essential variables

Under Table 6.6 of AWS D1.1:2025, interpass temperature appears as an essential variable for the standard process list (SMAW, SAW, GMAW, FCAW, GTAW). Specifically, an increase in the maximum interpass temperature above the range qualified by the PQR triggers requalification. If a PQR was run at a maximum interpass temperature of 350°F, production welding at 400°F is a change from the PQR conditions and may require an amendment or new PQR.

Table 6.8 — CVN supplementary essential variables

When CVN impact testing is required, Table 6.8 adds supplementary essential variables. Row 8 of Table 6.8 in the 2025 edition states that an increase in the maximum interpass temperature qualified on the PQR is a supplementary essential variable. This means that even a small upward change in maximum interpass temperature — if CVN is in scope — requires a new PQR with CVN specimens.

The 2025 edition also made a specific change to Table 6.8 Row 8: preheat was removed from the supplementary essential variable scope, leaving only the interpass maximum increase as the trigger. In AWS D1.1:2020, both a preheat decrease and an interpass temperature increase were listed. Confirming which edition governs your project matters for this row.

Measurement methods and limitations

Three common field methods for checking interpass temperature:

Temperature-indicating crayons (Tempilstik or equivalent). A crayon rated at a specific temperature melts when that temperature is reached. The CWI marks the base metal adjacent to the weld zone, waits for the previous pass to cool, and observes whether the crayon has melted. Practical and inexpensive, but only confirms a specific threshold point — it won't tell you the actual temperature or how far below the maximum you are.

Contact pyrometers (digital thermocouple probes). A calibrated thermocouple probe held against the base metal within 1 in [25 mm] of the weld gives a direct temperature readout. This is the preferred method when the WPS calls for documentation of actual measured temperatures. The probe tip must be clean, and calibration must be current.

Infrared thermometers. Non-contact IR guns are fast and useful for scanning, but their accuracy depends on the emissivity setting for the surface being measured. Mill scale, shiny weld metal, and bare steel all have different emissivity values. Never use an infrared thermometer as the sole measurement method on a critical joint without confirming against a contact pyrometer. Emissivity error can easily read 50–100°F low on bare weld metal.

For preheat measurement procedures under AWS D1.1, see the companion article on carbon equivalent and preheat requirements under AWS D1.1.

Documenting interpass temperature

The WPS must state the minimum and maximum interpass temperature. At minimum, the inspection record should note:

  • The WPS maximum interpass temperature value
  • That interpass temperature was checked (yes/no, or with actual readings for critical welds)
  • Any nonconformances — if a pass was deposited over-temperature, this should be documented along with the corrective action taken

On demand-critical, fracture-critical, or CVN-qualified welds, recording actual temperatures (not just pass/fail) is standard practice. Digital inspection apps that tie temperature records to specific pass numbers and joint IDs make this traceability straightforward. See WeldingWPS pricing for documentation features built around this workflow.

Inspection hold points

On multi-pass CJP groove welds, interpass temperature checks should be hold points — not witness points. The difference matters: a hold point stops the work until the inspector verifies the condition; a witness point allows work to proceed if the inspector doesn't show up in time.

For demand-critical welds in seismic applications and for A514 joints, treat interpass temperature verification as a hold point. For routine fillet welds on Group I steel with no CVN requirement, a witness point may be acceptable depending on the project QC plan.

Check with the owner's inspector (OI) and the inspector of record about which joints in the package require hold vs. witness for interpass temperature. This should be specified in the project inspection and testing plan (ITP), not left to field interpretation.

Common mistakes

Checking interpass temperature only on the first few passes then stopping. On a 20-pass CJP weld, the heat accumulation tends to rise as the weld builds. The joint may be well within limits for the first five passes and then creep above the maximum in the middle of the sequence when the surrounding base metal is saturated with heat. Check throughout the weld, not just at the start.

Confusing minimum and maximum. Inspectors sometimes focus on minimum preheat only and ignore the maximum interpass. Both are WPS requirements. A joint at 600°F interpass is as much a nonconformance as a joint that starts below minimum preheat.

No interpass temperature stated on the WPS. A WPS that lists preheat minimum but has a blank or "N/A" for maximum interpass temperature is incomplete. This is an immediate finding on most AISC certification audits.