Preheat gets most of the attention on a structural WPS — and rightly so, because failing to preheat cold steel is a classic driver of hydrogen-assisted cracking. But maximum interpass temperature is the other end of the thermal window, and exceeding it causes its own distinct set of problems. High interpass temperatures coarsen the grain structure in the heat-affected zone (HAZ), lower impact toughness, reduce yield strength in some alloys, and — on multi-pass welds — can drive the heat input so high that the weld metal itself loses the mechanical properties it exhibited in the procedure qualification record (PQR).

This article covers how AWS D1.1:2025 treats maximum interpass temperature on the WPS, how it becomes a requalification trigger when CVN testing is involved, and the practical CWI workflow for measuring and documenting it in the field.

What Interpass Temperature Is — and Why It Differs From Preheat

Preheat is the temperature to which the base metal is raised before the first arc is struck. Interpass temperature is the temperature of the previously deposited weld pass (or the adjacent base metal) measured just before the next pass begins. They share the same unit and the same measuring tools, but they represent opposite constraints:

  • Minimum preheat prevents the weld from cooling too fast, which would trap hydrogen in the HAZ and raise the risk of cold cracking.
  • Maximum interpass prevents the cumulative heat of multi-pass welding from driving the joint temperature too high, which coarsens austenite grain size, reduces notch toughness, and can degrade weld metal properties.

On a thick plate joint requiring 20+ passes, interpass temperature is the more demanding control — each pass adds heat, and the joint temperature climbs unless the welder pauses. On thin members with few passes, preheat is typically the binding constraint.

What AWS D1.1:2025 Requires on the WPS

AWS D1.1:2025 requires the WPS to include both the minimum preheat and interpass temperature and the maximum interpass temperature. The standard sets a general upper bound of 550°F [290°C] for most carbon and low-alloy structural steels (ASTM A36, A572, A992, A500, and similar). Your WPS may specify a lower maximum if:

  • PQR testing was conducted at a lower interpass temperature — in which case the qualified range is bounded by what was tested.
  • The base metal or application demands it — high-strength steels (A514, A709 HPS 70W) are often qualified with lower interpass maxima to protect HAZ toughness.
  • CVN impact testing was performed — see the supplementary essential variable discussion below.

In practice, most fabricators target 400°F [205°C] as the maximum for standard carbon steel structural work. This leaves margin below the D1.1 default limit and is easily measurable with standard temperature-indicating crayons.

Interpass Temperature as an Essential Variable Under Table 6.6 and Table 6.8

AWS D1.1:2025 Table 6.6 governs essential and nonessential variables for SMAW, SAW, GMAW, FCAW, and GTAW procedures. Maximum interpass temperature appears as a nonessential variable under Table 6.6 — meaning you must record it on the WPS, but changing it does not by itself void the PQR or require a new qualification test weld.

The analysis changes when CVN (Charpy V-notch) impact testing was included in the PQR. CVN testing in structural welding is specified when the contract requires supplementary notch-toughness compliance (e.g., demand-critical welds per AWS D1.8, fracture-critical members, cold-service structures). When CVN testing was performed, AWS D1.1:2025 Table 6.8 (Supplementary Essential Variables — CVN Testing) becomes applicable.

Under Table 6.8, an increase in maximum interpass temperature beyond the value used during PQR qualification is a supplementary essential variable change that requires requalification. The logic is straightforward: the CVN specimen was tested at mechanical properties produced by the qualified thermal cycle. Raising the interpass cap means more heat soak, coarser grain, and potentially lower toughness — the CVN data no longer validates the production weld condition.

This means the same WPS parameter that is non-essential in an ordinary fabrication context becomes a requalification trigger the moment you add CVN testing to the qualification package. This distinction catches fabricators and CWIs off guard regularly.

See the detailed article on CVN supplementary essential variables under AWS D1.1:2025 Table 6.8 for the full list of parameters affected by CVN testing.

How to Measure Interpass Temperature in the Field

Three tools are standard:

Contact pyrometer (thermocouple or Seebeck-effect probe): The most accurate method. The probe tip touches the base metal surface within 1 in [25 mm] of the weld groove, and the temperature reads in under 5 seconds. Digital pyrometers with logging capability are available; the logged data can be attached to the inspection report.

Temperature-indicating crayons (Tempilstik): A wax crayon rated to a specific temperature — e.g., a 400°F [205°C] Tempilstik melts at that temperature. Mark the base metal before welding; if the crayon mark has melted, the surface has reached (or exceeded) that temperature. Crayons are fast, inexpensive, and need no calibration, but they only confirm "above" or "below" the rated temperature, not the exact reading.

Infrared (IR) pyrometer: Measures surface temperature without contact. Convenient for field use but requires knowing the emissivity of the steel surface (mill scale vs. bare metal vs. painted differ significantly). IR is useful for trend monitoring but should be confirmed periodically against a contact probe on structural steel.

AWS D1.1:2025 does not specify the measurement tool, but it does require calibrated instruments. Calibration records for contact pyrometers should be maintained and traceable to NIST standards.

Measuring location: Always measure within 1 in [25 mm] of the weld joint on the base metal (not on the weld bead itself — the bead surface is hotter than the surrounding metal, giving a non-representative reading). For wide flanges and thick plates, check both sides of the groove if heat distribution might be uneven.

Documentation: What the CWI Records

A diligent CWI documents interpass temperature as part of the production welding inspection record — the same log that records preheat, pass sequence, and welder ID. At minimum, the record should capture:

  • Joint ID and weld designation
  • Pass number at which temperature was measured (or the time interval between measurements)
  • Measured temperature in °F [°C]
  • Measuring instrument type (pyrometer serial number, or crayon rating)
  • Time of measurement
  • Inspector signature

When the maximum interpass temperature is exceeded, the exceedance becomes a nonconformance. Document:

  • The measured temperature and time
  • The decision to stop welding and allow cooling
  • The temperature at which welding resumed
  • The disposition (typically "accept as-is after review" or "remove and requalify affected area")

For nonconformance record formats, see the article on weld nonconformance report documentation under AWS D1.1.

Practical Controls on the Shop Floor

Fabricators control interpass temperature through a combination of procedural requirements and shop discipline:

Interpass cool-down pauses: On thick plate joints, the WPS or traveler card includes a mandatory pause (e.g., "check temperature between every 3 passes; do not start next pass until at or below 400°F [205°C]"). This keeps the welder from rushing through passes without cooling checks.

Time-in — not temperature out: A common mistake is measuring interpass temperature the moment after the arc is extinguished, while the weld bead is still radiating heat into the base metal. Wait 30–60 seconds after arc-out, then measure on the base metal surface at the 1 in [25 mm] location for a representative reading.

Shop ambient temperature and ventilation: In summer months, a shop running at 95°F [35°C] with limited air movement can push a multi-pass joint well above 400°F without the welder realizing it. A brief temperature check at regular intervals — tracked on the production traveler card — catches this before the joint is 6 passes deep.

Preheat-to-interpass handoff: In cold weather, preheat temperatures of 225°F [107°C] are common for low-hydrogen SMAW on A572. If the shop loses heat between shifts and the joint cools below the minimum, the welder must restore preheat before restarting — the same pyrometer used for interpass checks confirms preheat at the start of each shift. See the article on carbon equivalent and preheat requirements under AWS D1.1 for the preheat side of the equation.

Interpass Temperature and Heat Input Interaction

Interpass temperature and heat input work together to determine the total thermal energy delivered to a joint. A weld pass made at high amperage, slow travel speed, and high interpass temperature delivers far more heat energy to the HAZ than a rapid, cool-start pass. The combination matters more than either parameter alone.

AWS D1.1:2025 does not mandate heat input limits as essential variables for most prequalified or standard qualified procedures (with some exceptions for CVN-tested joints and certain high-strength steels). But the engineer of record may add heat input constraints to the contract documents — and when they do, the interpass temperature limit and heat input limit must be managed together on the WPS.

For more on recording heat input and arc energy on the WPS, see heat input control and documentation.

When to Include Interpass Temperature in the Audit Packet

If your project has a contract requirement for NDE or a third-party structural steel inspection program (per IBC Chapter 17 or AISC COSP), the inspection records — including interpass temperature logs — may be part of the required audit package. An organized set of joint-level inspection records, where each weld shows preheat confirmation, interpass temperature checks, NDE result, and acceptance sign-off, is the standard expected in a Level II or Level III AISC certification audit.

If you track these records in a purpose-built welding quality system that links WPS → PQR → production inspection record, see pricing for the audit-packet export that compiles everything in one downloadable ZIP.

Rule library based on AWS D1.1:2025; verify against your governing edition (the AHJ or contract may specify 2020 or earlier).