Preheat is one of the most effective tools for preventing hydrogen-assisted cracking and controlling HAZ hardness. The WPS specifies the minimum required temperature; the CWI or QC manager is responsible for confirming the base metal actually reaches it before the arc starts. These are two distinct tasks, and confusing them is a source of chronic inspection failures.
This article covers the verification side: what tools work, where to measure, and what records a compliant shop keeps.
Rule library based on AWS D1.1:2025; verify against your governing edition (the AHJ or contract may specify 2020 or earlier).
Why verification is separate from the WPS
The WPS documents the required preheat and interpass temperature range. It specifies minimums derived from the PQR, the base metal group, and the carbon equivalent calculation. But the WPS is a paper document. It says nothing about the 38°F shop floor on a February morning or the welder who started the arc 90 seconds after flame heating a 2-inch plate.
Verification is the real-world confirmation that the requirement was met, at the time of welding, on the specific joint. Without it, the WPS is a compliance document that provides no actual protection.
Temperature-indicating crayons (Tempilstik)
Temperature-indicating crayons are the most common preheat verification tool in structural fab shops. Each crayon is rated to a specific temperature: when the mark melts, the surface has reached that temperature. They are inexpensive, portable, and require no calibration.
How to use them correctly:
- Mark the crayon on the base metal 3 inches (75 mm) from the edge of the weld groove, on both sides of the joint if both members must be preheated.
- Apply the mark before heating, or on a cool zone, to create a solid line. The transition from solid to liquid is visible and obvious.
- Do not mark directly in the flame or on a surface that will enter the weld HAZ.
- Keep crayons labeled. A shop with 250°F and 300°F crayons in the same bin is asking for errors.
Limitations: Crayons only tell you the surface reached the rated temperature at the moment of marking. They do not confirm through-thickness temperature equalization, which matters on thick plate. A flamed surface can show crayon melt while the interior is still cold. Allow adequate soak time — typically several minutes per inch of thickness — before accepting the reading.
Contact thermometers and digital pyrometers
Contact-type digital thermometers use a thermocouple tip that is pressed against the steel surface. They read out in real time and can be used to verify temperature at multiple points quickly. Good instruments are accurate to ±2°F after calibration.
Advantages over crayons:
- Quantitative reading — you can document "measured 312°F at 3 inches from groove, required 300°F minimum."
- Instant reading at any point without pre-marking.
- Useful for interpass temperature verification, not just preheat.
Maintenance requirement: Contact pyrometers require periodic calibration verification. A shop relying on uncalibrated instruments has a paper trail that can undermine an audit. Keep calibration stickers current and document in the QC program.
When measuring interpass temperature, the contact pyrometer is the standard tool. Crayons are impractical mid-weld because the rated crayon typically melts on a surface that's already too hot. Use the thermometer to confirm the interpass maximum is not being exceeded — a common finding during NDE investigations of multi-pass weld cracking.
Infrared (non-contact) thermometers
Infrared guns are convenient but require extra care with steel. The reading depends on the emissivity setting matching the actual surface condition. Bare, lightly rusted steel at 200–400°F has an emissivity near 0.7–0.8. Many IR guns default to 0.95, which causes them to under-read by 30–60°F.
If you use an IR gun, verify its emissivity setting before each job and document the setting in the QC record. Cross-check against a contact thermometer when establishing the procedure. IR guns work well for quick surveys on large areas of plate, but should not be the sole verification method unless the emissivity issue has been explicitly addressed.
Thermal cameras
Thermal imaging cameras are the highest-resolution option. They show temperature distribution across an entire joint area in a single frame, which makes them useful for confirming that an entire groove face — not just a single spot — is above the minimum. They are also good for finding cold spots near backing bars or through-thickness cold zones in thick sections.
For most structural fab shops, thermal cameras are an engineering-review or audit tool rather than a daily inspection tool. The cost is not justified unless the project involves heavy plate, restricted-access joints, or repeated preheat deficiencies that need root-cause analysis.
Where to measure — the 3-inch rule
AWS D1.1 specifies that preheat be measured at least 3 inches (75 mm) from the edge of the weld groove. The intent is that the entire heat-affected zone will be at or above the required temperature, not just the surface directly under the flame.
For structural T-joints and corner joints, measure on both the web and the flange if both must be preheated. For a butt joint in groove prep, measure on both sides of the groove. Do not measure on the groove face itself — that surface may have localized heat from the flame that doesn't represent the bulk temperature.
Equalization time: After flame heating with a rosebud tip, the surface heats faster than the interior. AWS D1.1 does not prescribe a specific equalization time, but a common rule of thumb is 2 minutes per inch of plate thickness before taking the acceptance measurement. Electric resistance blanket heating distributes temperature more evenly and typically needs less equalization time than flame heating.
What records a compliant shop keeps
The minimum record of preheat verification is a notation in the welder's daily inspection log or the CWI inspection report: the joint ID, the required temperature, the measured temperature, the tool used, and the time of measurement. Many shops tie this to the weld identification number from the weld map.
For code-required preheat on higher-strength or thicker materials, retain:
- The WPS reference — which preheat requirement applied.
- The measured temperature and location (distance from groove).
- The verification method (crayon/contact pyrometer/IR, with instrument ID if applicable).
- The CWI or QC technician's initials and date.
This record supports the audit-ready welding procedure library and is what the owner's inspector or third-party auditor will request during fabrication inspection.
Preheat maintenance during welding
Verifying the starting preheat is necessary but not sufficient. The base metal loses heat during welding interruptions — coffee breaks, electrode changes, repositioning. On thick plate with high required preheat (250°F or above), a 20-minute break can drop the joint well below minimum, especially in cold weather.
The CWI's in-process role includes spot-checking temperature during active welding, not just at the start. For low-hydrogen SMAW procedures on A36 or higher-strength steels, a single in-process check at the midpoint of a long pass is standard practice on critical joints.
Document these in-process readings separately from the initial preheat verification, with the timestamp. If a temperature drop is found and reheating is performed, record that sequence. Gaps in the record create audit findings; complete records prove compliance.
Connecting preheat to hydrogen control
Preheat is one leg of the hydrogen-cracking prevention triad: low-hydrogen filler, preheat/interpass maintenance, and adequate heat input. The WPS for hydrogen cracking prevention explains the full picture. What preheat verification adds is the field proof that the procedure was actually followed.
A WPS that specifies 200°F minimum preheat protects the structure only when fabrication records confirm 200°F was present at arc start. Without that record, the WPS is an unfounded assertion.
Tools to consider for your QC program
Running preheat verification manually on dozens of joints per shift introduces human error and creates gaps in the record. Software that links inspection records to joint IDs, weld maps, and WPS references reduces both. The welding procedure and inspection tracking tools at wpswelding.com tie preheat requirements from the WPS directly to the inspection record form, so the CWI checks the box against the actual requirement rather than relying on memory.
Temperature verification is a five-second task if the tools are ready and the form is in front of you. The discipline is in making it routine on every joint that requires it — not just the ones under direct observation.