AWS D1.1 Preheat Temperature Measurement Methods

Why Preheat Measurement Matters

Preheat is one of the most effective — and most consistently abused — defect-prevention steps in structural welding. Heat the base metal to the required minimum before striking the arc and you drastically reduce the risk of hydrogen-assisted cold cracking (HACC), also called underbead cracking or delayed cracking. Skip it, or hit the number on the surface while the core of the joint is still cold, and you may not see the damage for hours or days after welding.

AWS D1.1:2025 Table 3.3 sets minimum preheat and interpass temperatures by steel group and base metal thickness. For prequalified WPS, compliance is mandatory. For qualified WPS, the WPS must specify the minimum preheat established during the PQR — and the coupon was welded with that preheat applied.

Knowing the required temperature and actually achieving it in the field are two separate problems. This article covers the three field-practical measurement methods, their real-world limitations, and what a CWI looks for during production inspection.

Three Acceptable Measurement Methods

Contact (Digital) Pyrometer

A calibrated contact thermometer — a probe pressed directly against the base metal surface — is the most accurate tool available on the shop floor or in the field. Readings are fast, unambiguous, and can be logged by the instrument for traceability. A thermocouple-tip pyrometer rated for 0–600°F [–18–315°C] covers the full preheat range encountered in AWS D1.1 work.

Contact probes wear out and drift. They need periodic calibration verification against a traceable reference, and the calibration date label should be visible on the instrument or in the shop's calibration file. For any joint where the CWI holds a sign-off, a calibrated pyrometer is the standard of care.

Best for: thick-section steel (≥1½ in [38 mm]), alloy-group D and E base metals, Q&T steels, and any situation where the inspection record must document the actual measured temperature.

Temperature-Indicating Crayons (Tempilstiks)

Temperature-indicating crayons — sold under the Tempilstik trade name and by other suppliers — are wax-based sticks formulated to melt at a specific calibrated temperature. Mark the base metal near the weld joint before applying heat; when the crayon mark transitions from a dry streak to a wet, glossy line, the base metal has reached that temperature.

Tempilstiks are inexpensive, fast, and require no calibration. Their limitation is that they give a single-point go/no-go answer. To bracket a required range — for example, confirm that the base metal is above 225°F [110°C] minimum but below 400°F [205°C] interpass maximum — you need at least two crayons. A common production practice is to apply both a lower-limit crayon and an upper-limit crayon to the plate before heating; when the lower melts and the upper has not, you are in the window.

Tempilstiks do not produce a printable data record unless the CWI or welder photographs them or logs them in a hold-point checklist. For audited programs, pair Tempilstik use with a witness entry in the inspection log.

Best for: routine production runs where a CWI or QC manager witnesses preheat live and documents it contemporaneously, and for single-pass fillet welds on Group A and B base metals where temperature windows are wide.

Infrared Non-Contact Thermometer

IR thermometers read the thermal radiation emitted from the surface and convert it to a temperature display instantly, with no contact required. They are useful for checking large heat-soaked areas quickly and for spotting hot or cold zones across a joint without touching the plate.

Their major limitation in a fab shop is emissivity error. IR thermometers assume a fixed emissivity for the target surface. Mill scale, red rust, freshly ground steel, and painted steel all have different emissivities, and shiny bare steel in particular can return a reading 50–100°F below actual — falsely suggesting the preheat requirement is not met, or worse, falsely confirming it when it isn't. To use an IR thermometer reliably on structural steel, either paint a small patch with flat black spray paint as a reference zone, or dial the emissivity setting to approximately 0.95 for oxidized mill-scale surface.

Never rely solely on an IR thermometer for preheat sign-off on reflective freshly ground or machined steel without cross-checking against a contact probe.

Best for: initial screening across large plates, checking interpass temperatures on the weld face where contact is impractical, and identifying cold spots before applying supplemental heating.

Where to Measure

AWS D1.1 requires preheat measurement within 3 in [75 mm] of the weld joint in all directions, before welding begins. The base metal must have soaked long enough to reach temperature uniformly through the thickness — surface temperature alone does not guarantee through-thickness preheat.

On thick material (≥2 in [50 mm]), a heater applied to one face may show the required temperature on the heated side while the far face remains near ambient. For thick sections, experienced welding supervisors heat from both sides and verify with probes on both faces. For extremely thick joints where through-thickness temperature is critical, a more formal heat-soak procedure should be documented in the WPS or in a companion heat-treatment procedure.

Interpass Temperature: The Upper Limit You Cannot Ignore

Minimum preheat captures most of the production attention, but AWS D1.1:2025 also sets maximum interpass temperatures — particularly for quenched-and-tempered steels such as A514, A517, and A709 Grade HPS 100W. Overheating between passes on these materials over-tempers the heat-affected zone and reduces toughness, creating the exact failure mode preheat was supposed to prevent.

The WPS must state both the minimum preheat and the maximum interpass temperature. In production, the CWI or welder checks the interpass temperature before every pass on CVN-qualifying joints, not just the first. Interpass temperature is measured on the base metal or weld metal immediately before striking the arc.

Shops that run continuous, unbroken production cycles sometimes lose track of interpass temperature. A practical fix: keep a calibrated contact pyrometer on the fixture and log it with every pass start on Q&T or CVN joints. Five seconds with a pyrometer is not production overhead; it is the documentation that protects the shop if a question arises later.

What a CWI Checks

During a preheat inspection hold point, a CWI typically verifies:

• Hold-point compliance. Was preheat confirmed before the first pass and recorded in the inspection log or WPS traveler?
• Instrument traceability. Is the pyrometer calibrated? (A calibration label with date on the instrument, or a calibration certificate in the QC file.) Tempilstiks are go/no-go and do not require individual calibration certs, but the CWI may confirm the crayon's rated temperature matches the WPS requirement.
• Measurement location. Was the check made within 3 in [75 mm] of the joint — not on the web six inches away?
• Environmental conditions. Rain, drafts above 5 mph, or sub-freezing ambient all affect heat retention. AWS D1.1 addresses environmental welding limits; the inspector verifies conditions are acceptable or that preheat has been elevated to compensate.

For more on how preheat integrates with your WPS parameters, see arc energy and heat input calculations and why essential versus non-essential variables matter. When determining which base metal group your steel falls under, understanding carbon equivalent and preheat explains how D1.1 uses carbon equivalent as a group-assignment input.

If you are building or auditing your WPS library, wpswelding.com/pricing has per-seat plans that include preheat rules, qualification tracking, and a CWI-ready audit log.

Rule library based on AWS D1.1:2025; verify against your governing edition — the AHJ or contract may specify 2020 or earlier.