Preheat is one of the oldest controls in structural welding practice. Raise the base metal temperature before striking an arc, and you slow the cooling rate of the HAZ — which reduces hydrogen cracking susceptibility, lowers residual stress, and improves HAZ toughness. AWS D1.1:2025 encodes minimum preheat requirements based on base metal chemistry (carbon equivalent), thickness, and process.
But specifying a preheat temperature on the WPS and actually achieving it in the field are different problems. The gap between what the WPS says and what the welder actually does — and what the CWI can verify and record — is where preheat nonconformances live.
This article covers how preheat is measured, where it must be measured, which tools are acceptable, and what a defensible preheat record looks like.
Rule library based on AWS D1.1:2025; verify against your governing edition.
Why measurement location matters
A base metal heated with a propane torch does not heat uniformly. The surface directly in front of the flame is hotter than the base metal 6 inches away. The through-thickness temperature gradient depends on heating time, section thickness, and heat source.
AWS D1.1:2025 requires preheat measurement on the base metal surface at a minimum of 3 inches (75 mm) from each edge of the weld joint. This location is chosen deliberately:
- It is away from the direct flame impingement point, which would give an artificially high reading
- It is within the zone that will be influenced by the heat of welding
- It approximates the temperature of the base metal at the HAZ before welding starts
A measurement taken at 1 inch from the weld edge, or worse, inside the joint groove immediately after heating, will read higher than the 3-inch measurement. That higher number may look like the preheat is achieved when the through-thickness temperature at 3 inches — the condition AWS D1.1 is actually requiring — has not been reached.
On heavy sections (2 inches and thicker), through-thickness temperature equilibration is the critical issue. The surface temperature reaches the minimum preheat temperature well before the interior of the base metal does. AWS D1.1:2025 allows sufficient time for the heat to soak in — in practice, this means maintaining the heat source long enough that the surface temperature at the required measurement location has been stable for several minutes. A CWI who verifies temperature while the welder's torch is still pointed at the base metal is not verifying the soak-through condition.
Temperature measurement tools
Temperature-indicating crayons (Tempilstik and equivalents)
Tempilstik crayons are wax-based sticks manufactured to melt at a specific temperature within a rated tolerance. They are the most common field tool for preheat verification. Method:
- Remove the heat source from the joint area.
- Draw a mark with the crayon at the required measurement location (3 inches from the joint edge).
- If the mark melts, the base metal surface is at or above the crayon's rated temperature.
- If the mark does not melt or smears but doesn't fully liquefy, the temperature has not been reached.
Tempilstik crayons are a pass/fail indicator. They do not give a continuous temperature reading. A shop using crayons must stock the specific temperature corresponding to the required minimum preheat — and the crayon temperature should be one calibration step below the required minimum, not above, to confirm the minimum has been achieved.
Crayons are inexpensive and require no calibration. They do not inherently generate a paper record — a CWI must note the observation in the inspection record manually.
Contact (probe) thermometers
Contact thermometers with a metallic probe give a direct surface temperature reading. They are more precise than crayons, faster to read, and generate a numerical value that can be recorded directly. A CWI carrying a calibrated digital contact thermometer can take multiple readings across the joint area, document each one, and produce a record that shows spatial uniformity of the preheat.
The probe must make solid contact with the base metal surface. A probe pressed lightly against rough mill scale will read lower than the actual surface temperature. Some inspectors lightly grind or clean the measurement spot before contact measurement on heavily scaled surfaces.
Calibration of the thermometer is a quality-system requirement. A contact thermometer that has not been calibrated within the interval specified by the shop's quality plan — typically annually — does not produce a defensible record.
Infrared (IR) thermometers and pyrometers
Infrared thermometers measure surface temperature without contact by detecting emitted thermal radiation. They are fast and can measure temperature at locations that are difficult to probe. However, their accuracy depends heavily on the emissivity of the surface. Mill scale, bare steel, and oxidized steel have different emissivities. An IR thermometer calibrated for one surface condition will read incorrectly on another.
AWS D1.1:2025 does not prohibit IR thermometers, but CWIs using them must account for emissivity correction. Some IR tools allow emissivity adjustment; without that adjustment, the reading may be meaningfully off. Using an IR thermometer on bare machined steel without emissivity correction can underread surface temperature by 50–100°F or more at typical preheat temperatures.
For code-compliance verification, contact measurement or temperature-indicating crayons are more defensible and easier to explain to an AHJ or owner's representative than an IR reading with a complicated emissivity discussion.
Thermocouples
For production environments where continuous temperature monitoring is required — large-diameter pipe welds, heavy section connections, high-restraint joints requiring extended preheat and controlled post-weld cooling — thermocouples attached to the base metal give a continuous record. This is less common in structural fab than in pressure vessel or pipe fabrication, but it appears on critical projects with specific temperature control requirements.
The measurement sequence in practice
A defensible preheat sequence at the joint level:
- Before heating: Record the ambient temperature and any conditions affecting preheat (wind, rain, low ambient temperature). Cold-weather conditions may affect how quickly the base metal loses heat after heating stops. See cold weather welding requirements under AWS D1.1 for the ambient temperature provisions.
- Apply heat: Use the specified heat source (propane torch, induction heater, electric resistance blanket). Apply heat evenly, not localized to one spot. For thick sections, both sides of the joint should be heated simultaneously when possible.
- Remove heat source: Allow several seconds to a minute (depending on section thickness) for temperature equalization before measuring.
- Measure at required location: 3 inches from each side of the joint, on the base metal surface. Take measurements on both sides of the joint, and at both ends of the joint if it is long (longer than 2 feet).
- Record: Minimum temperature observed at any measurement point. If any point is below the minimum preheat, reapply heat and remeasure.
- Begin welding: Only after all measurement points confirm the minimum preheat has been achieved.
Interpass temperature: the other side of preheat
AWS D1.1:2025 specifies both a minimum preheat temperature and a maximum interpass temperature. The minimum preheat is a floor — the joint must not fall below it during the welding operation. The maximum interpass temperature is a ceiling — some base metals (particularly higher-strength, quenched-and-tempered grades) can be damaged by excessive interpass temperature, which effectively applies uncontrolled heat treatment to the HAZ.
Measuring interpass temperature is the same instrument and location. Before starting each pass, measure the temperature of the previously deposited weld and adjacent base metal. If the temperature has dropped below the minimum preheat, stop and reheat. If the temperature exceeds the maximum interpass, stop and wait for the joint to cool into the acceptable range.
For ASTM A913 high-performance steel, A709 Grade HPS 70W, and similar grades, the maximum interpass temperature is a material-protection requirement, not just a code formality. Exceeding the maximum interpass on these grades can degrade the HAZ toughness and yield strength that the material specification is relying on. See WPS for A913 high-performance steel for the interpass temperature considerations specific to that grade.
Documentation requirements
The WPS specifies the minimum preheat and maximum interpass temperature. The weld record — whether that is a formal daily welding log, a traveler, or a marked-up weld map — should show:
- Joint identifier (gridline reference, weld number, or weld map reference)
- Date and time of preheat verification
- Ambient temperature at time of welding
- Measured preheat temperature and measurement location
- CWI or welding inspector signature or initials
This is the paper trail an AHJ, owner's inspector, or third-party auditor will request when they ask how preheat was controlled. A weld record that shows "preheat OK" without a measured temperature, a tool, and a location is not a defensible record.
For shops running multiple welding stations on a large structural project, this documentation discipline across every joint adds up quickly. Welding quality management software that integrates the weld map, WPS reference, and inspection records in one place makes this traceable and auditable without separate spreadsheets. See weld map and WPS traceability in production for how this works in practice.
Preheat and carbon equivalent: the upstream calculation
The minimum preheat specified on the WPS comes from the base metal chemistry — specifically the carbon equivalent, which summarizes the combined effect of alloying elements on hardenability and hydrogen cracking susceptibility. See carbon equivalent and preheat under AWS D1.1 for the calculation method and how AWS D1.1:2025 translates CE into minimum preheat.
If your shop's WPS library needs preheat values reviewed for accuracy against the current 2025 edition, or if your existing WPSs were developed against an older edition, WPS requalification triggers checklist covers when a preheat change constitutes an essential variable that requires a new PQR.
For a complete digital approach to WPS development, preheat calculation, and weld quality tracking, see WPS software for structural fabricators.