Interpass Temperature Guide — Max Temp by Material & Measurement

Why Interpass Limits Exist

After pass one, the joint is hotter than when you started. Pass two dumps more energy into metal that has not fully cooled. Without a maximum interpass temperature, production pressure turns into “weld as fast as the puddle allows” until the HAZ is over-tempered, grains coarsen, and impact toughness falls — even when amps look normal.

Interpass is also a minimum story on crack-sensitive steels: the joint should not drop below required preheat between passes. So the practical window is:

Preheat (floor) ≤ joint temperature before next pass ≤ Max interpass (ceiling)

The comparison article Preheat vs. Interpass Temperature explains when each applies. This guide goes deeper on the ceiling — over-temp risk, measurement between passes, and calculator logic by material.

What the Calculator Returns

The Interpass Temperature Calculator maps material family to a typical maximum (°F / °C) and shows the working range from your entered preheat up to that max. It warns when preheat already exceeds the max — a procedure conflict you must resolve before welding.

Optional CE override lets you document chemistry context; the max table is still driven primarily by material family. Carbon-steel CE from the preheat calculator may suggest a different interpass ballpark — when WPS and calculator disagree, the WPS wins.

Material-Specific Over-Temp Risks

• Carbon / low-alloy steel — excessive interpass → coarse HAZ grain, lower toughness, possible strength/HAZ property drift outside PQR expectations.
• Austenitic stainless — time at elevated temperature promotes chromium carbide precipitation (sensitization) and kills corrosion resistance the alloy was chosen for. Keep interpass down and travel disciplined.
• Duplex — phase balance is heat-sensitive; high interpass / high heat input can shift ferrite/austenite and hurt corrosion and toughness.
• Aluminum — heat-treatable alloys over-age; strength drops. Lower ceilings and patience between passes matter more than on mild steel.
• Chrome-moly — procedure-driven; preheat, interpass, and often PWHT are a package. Do not freestyle the ceiling.

Heat input per pass is the other half of thermal control — see the Heat Input Guide. High HI plus high interpass compounds grain growth; low HI with interpass below preheat reintroduces cold-crack risk on hardable steels.

How to Measure Between Passes

1. Finish the pass — slag off, brush as required.
2. Measure promptly at the location the WPS specifies (often adjacent to the weld toe, both sides when accessible).
3. If above max — wait, use forced cooling only if the procedure allows (many structural WPSs prefer air cool; quenching can create new metallurgy problems).
4. If below minimum (preheat) — reheat before the next pass.
5. Record on code jobs — inspectors expect evidence, not vibes.

Tools that work between passes

• Tempilstik — carry the max-interpass rating (e.g. 400°F). If it melts, you are at or above ceiling — wait. Pair with a preheat-rated stick for the floor.
• Contact pyrometer — quantitative reading for records; clean the spot.
• IR thermometer — fastest for production sequencing; verify emissivity and spot size. Shiny stainless lies.

Common Mistakes

• Stacking passes with no measurement until the joint is glowing.
• Measuring only on the torch side of thick members.
• Using the carbon-steel 400°F habit on austenitic stainless — too hot for sensitization control.
• Setting preheat above max interpass — impossible window; fix the procedure.
• Air-blast cooling when the WPS forbids it — or water quenching “to save time.”
• Assuming max interpass does not apply to two-pass fillets — short welds still overheat locally on thin legs.
• Ignoring ambient / shop heat — summer fab shops start closer to the ceiling.

Frequently Asked Questions

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