Interpass Temperature — D1.1:2025 Min & Max Requirements

Minimum Interpass Temperature

The minimum interpass temperature requirement under D1.1:2025 mirrors the minimum preheat temperature. Before depositing each subsequent pass, the weld zone must be at or above the minimum preheat temperature established for that material, thickness, and process combination. If the joint cools below the minimum preheat between passes, it must be reheated before welding continues.

This requirement exists because the metallurgical reasons for preheat — preventing hydrogen-induced cracking, controlling cooling rate, and reducing thermal gradients — apply equally between passes as they do before the first pass. A joint that cools to ambient temperature between passes loses the protective benefit of preheat.

Maximum Interpass Temperature

D1.1:2025 does not specify a universal maximum interpass temperature for all applications. A maximum applies when the WPS specifies one, when Charpy V-notch (CVN) impact testing is required, or when the base metal manufacturer's data or the applicable material specification limits interpass temperature to protect mechanical properties.

For certain high-strength steels, D1.1:2025 Table 5.11 directly encodes a maximum preheat and interpass temperature. ASTM A913 Grade 80 (Category G) and A913 Grade 70 (Category F) require H8 or H4 low-hydrogen electrodes — the process restriction itself controls heat input risk. ASTM A709 Grade HPS70W carries a Table 5.11 footnote b maximum of 400°F [200°C] for thicknesses up to 1-1/2 in and 450°F [230°C] for greater thicknesses. For materials where D1.1 does not specify a maximum, the WPS or the material specification may impose one. Note: ASTM A913/A913M is produced by quenching and self-tempering (Q&ST), not conventional Q&T — D1.1 Clause 5.9 explicitly excludes Q&T steels from prequalified PWHT, but A913 itself is categorized separately in Table 5.11 rather than under general QT restrictions.

Measuring and Documenting Interpass Temperature

Interpass temperature is measured using temperature-indicating crayons (Tempilstik), contact pyrometers, or infrared thermometers. D1.1 does not prescribe a specific measurement location or distance — a commonly applied industry practice is to measure on the base metal near the joint edge, not on the weld bead itself, which can give a falsely elevated reading. The method used should be consistent with the WPS and verified for accuracy. Interpass temperature readings should be recorded in the weld inspection records when required by the contract or quality plan.

Practical Measurement Considerations

Measurement distance from the weld matters because the temperature gradient across the base metal can be significant, especially on thicker plates. Measuring directly on the weld bead surface reads the solidified weld metal temperature, not the base metal heat-affected zone temperature that governs cracking susceptibility. A distance of 1 to 3 inches [25 to 75 mm] from the toe of the weld on the base metal surface is widely used in industry practice (D1.1 does not prescribe a specific measurement distance) to obtain a reading representative of the joint temperature.

Temperature-indicating crayons provide a go/no-go check at a specific temperature — the crayon melts when the surface reaches its rated temperature. They are inexpensive and require no calibration, but each crayon covers only a single temperature point. For interpass verification, a welder typically uses a crayon rated at the minimum preheat temperature: if it melts, the joint is at or above minimum interpass temperature. When a maximum interpass temperature applies, a second crayon rated at the maximum provides an upper-bound check.

Contact pyrometers and infrared thermometers give a numerical reading across a continuous range. Contact pyrometers require clean metal-to-metal contact and a brief settling time. Infrared thermometers read surface temperature without contact but are sensitive to emissivity — per instrument manufacturer guidance, shiny or freshly ground metal surfaces may give readings 50 to 100°F lower than actual temperature. Oxide scale, mill scale, and weld spatter also affect accuracy. Calibrating against a known reference at the expected temperature range before production welding is standard practice under most quality programs.

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