Interpass Temperature Troubleshooting

Interpass Problems Quick Answer

Use this when the calculator max is known but production still overheats, under-cools below preheat, or logs temperatures that never existed on the plate.

1. Infrared Says Cool — The Joint Is Still Over Max

What it looks like: Gun reads 280°F on austenitic work (max 350°F). Tempilstik at 350 melts immediately. Or contact probe reads 420°F on carbon steel while IR shows 360°F.

Likely causes: Low emissivity on bright stainless or aluminum — IR under-reads. Spot size larger than the hot zone so the gun averages cooler plate. Measuring through smoke, slag, or at a glancing angle. Battery/out-of-cal gun drift.

Fixes: Dull a small patch (flat black marker or approved surface prep) and re-shoot, or switch to contact pyrometer / correctly rated temperature crayon. Treat IR as a production sequencer, not the sole code record, unless the WPS accepts it with verified technique.

2. Temperature Logged — Measured in the Wrong Place

What it looks like: Log sheet shows numbers inside the window. Inspector watches you shoot the back of a thick flange 6" from the groove while the toe is still cherry.

Likely causes: Habit of measuring wherever is convenient. Only one face of thick members. Reading weld metal instead of base metal when the WPS wants adjacent base. Measuring before slag-off when heat is still redistributing.

Fixes: Follow the WPS location — typically within about 1 inch of the joint on base metal. On thick or double-sided joints, check both accessible faces. Wait the brief soak the procedure implies after cleaning, then measure before the next arc.

3. Passes Stack Faster Than the Joint Can Cool

What it looks like: First two passes fine. By pass 4–6 every start is over max within seconds of cleaning. Production wants continuous arc time.

Likely causes: High heat input weaves (see Heat Input Troubleshooting). No leapfrog/skip sequence. Hot shop ambient starting near the ceiling. Illegal water quench or aggressive air blast when the WPS wants air cool only — or the reverse: waiting forever because nobody planned sequence.

Fixes: Leapfrog segments so each start location has a cool window. Drop HI or switch to stringers when the ceiling is tight. Only force-cool if the procedure allows. Plan grinding and inspection into natural wait time.

4. Carbon-Steel Habit on Stainless / Duplex / Aluminum

What it looks like: Crew treats 400°F as universal. Austenitic work runs 380–420°F between passes “because D1.1 muscle memory.” Corrosion or phase-balance failures show up later.

Likely causes: Wrong material family in the head (and sometimes in the calculator). Ignoring duplex ~300°F and aluminum ~250°F ceilings. High HI plus high interpass on sensitization-sensitive alloys.

Fixes: Re-select material in the Interpass Temperature Calculator before the job. Post the max at the booth. On stainless, pair interpass discipline with travel/heat-input control — fluid puddles are not permission to sit hot.

5. Under Max but Below Preheat — Cold-Crack Path

What it looks like: Team celebrates “we never went over 400.” Overnight toe cracks on restrained high-CE steel. Between-pass IR shows 150°F on a joint that required 250°F preheat.

Likely causes: Treating max interpass as the only number. Long waits, overnight holds, or wind cooling without reheat. Confusing “cooler is safer” with hydrogen-crack physics.

Fixes: Enforce the floor with the Preheat Temperature Calculator and reheat before the next pass. The window is two-sided. See Preheat vs. Interpass Temperature when the crew mixes the words.