Every CWI who has worked structural fabrication in winter has watched a welder wave a propane torch over a column flange and declare it hot enough. The question is not whether the steel got warm — the question is whether it reached the minimum preheat temperature uniformly through the section thickness, and whether you can prove it.
Propane and oxy-acetylene torches have been the standard structural preheat method for decades. Induction preheating — using electromagnetic coils to heat the steel from within — has become increasingly available in heavy structural shops. Both methods meet AWS D1.1:2025 requirements when applied correctly. The difference is in how easy it is to achieve, verify, and document the required temperature uniformity.
Why Preheat Temperature Uniformity Matters
The purpose of preheat in structural welding is to slow the cooling rate of the weld and heat-affected zone (HAZ), reducing the risk of hydrogen-induced cracking. Preheat works not just at the weld surface but through the section thickness — a cold core under a warm surface is not adequate preheat.
This is where torch heating frequently fails in practice. Torch heat is applied to the surface, and surface temperature is what a contact thermometer or temperature-indicating crayon reads. On heavy plate (over 1.5 inches), the temperature at the interior of the section may lag the surface by 50°F or more immediately after torch application. AWS D1.1:2025 requires measurement after the heat source is moved away and temperature has equalized — but in a production environment with schedule pressure, that wait rarely happens consistently.
Induction preheating operates differently. The alternating magnetic field induces eddy currents directly within the steel, generating heat internally rather than externally. The result is more uniform temperature through the section thickness and faster equalization time. The operator does not need to wait as long after removing the induction coil before taking a verification temperature reading.
Torch Preheat: Standard Method, Known Limitations
Propane torches (or oxy-acetylene for higher heat demand on heavy plate) remain the most common preheat method in structural shops and in the field because of their simplicity and low equipment cost. A single torch handles any joint configuration and requires no setup.
The limitations are real:
Non-uniform heat distribution: Torch operators naturally apply more heat to the near surface and near-edge zones. The center of a heavy plate and the zone 3 to 4 inches back from the weld preparation (where temperature should also be elevated to support the thermal gradient) are often undertreated.
Oxidation and scale: Excessive torch heating creates oxidation on the steel surface. Heavy scaling in the weld prep area requires additional cleaning before welding; it also makes temperature measurement by contact thermometer less accurate if the scale is not disrupted.
Operator dependency: Achieving code-required preheat with a torch depends heavily on the welder's technique, patience, and attention. On a multi-shift shop floor, preheat compliance varies.
Documentation lag: Verifying torch preheat temperature by contact thermocouple or temp-indicating crayon at multiple locations is the CWI's responsibility. On active production with multiple joints being preheated simultaneously, the CWI cannot be at every joint at exactly the right moment. This is the gap that causes preheat-related nonconformances during audits.
For the required minimum preheat temperatures based on material specification and thickness — and for the carbon equivalent method that determines them — see carbon equivalent and preheat requirements under AWS D1.1.
Induction Preheat: Advantages and Setup Costs
Induction preheating systems use a water-cooled copper coil placed around the joint. A high-frequency alternating current in the coil creates a magnetic field that induces eddy currents in the steel, heating the steel from the inside out. Modern portable induction units weigh 50–150 lbs and can apply controlled heat to structural members in the shop or at the field connection.
Advantages for structural welding:
Through-thickness uniformity: On heavy structural sections, induction provides noticeably more uniform temperature through the plate thickness compared to surface torch heating. Temperature equalization time after removing the coil is significantly shorter.
Controlled ramp rate: Induction units can be set to ramp to a target temperature at a controlled rate, reducing the risk of thermal shock on highly restrained or thick joints. The ramp rate is programmable rather than dependent on operator technique.
Reduced oxidation: Because induction heats from within and does not apply an open flame to the surface, oxidation of the weld prep surface is substantially reduced. Joint surfaces stay cleaner.
Recordable output: Many modern induction units log time-temperature data that can be exported as a PDF or CSV. This machine-generated record is more defensible in an audit than a handwritten CWI log.
Setup considerations:
Induction coils must be sized and configured for the joint geometry. A universal coil that wraps a wide flange section is different from one configured for a round pipe or plate edge. Setup time per joint is longer than positioning a torch, which affects production throughput on small or varied joints. The equipment cost is significantly higher than torch equipment: portable induction units run $10,000–$40,000 depending on output capacity.
What the CWI Documents Regardless of Heat Source
AWS D1.1:2025 requires that preheat temperature be measured and documented before welding begins. The documentation requirements are the same whether you used a torch or an induction coil:
- Minimum preheat temperature achieved, measured at the minimum distance from the weld preparation specified in the WPS (typically 3 inches for plate over 1.5 inches thick)
- Temperature measurement method: contact thermocouple, temperature-indicating crayon (Tempilstik), or infrared thermometer (with emissivity correction for steel)
- Measurement timing: after heat source is moved away, after equalization — not while actively heating
- Location: measurements should be taken on the opposite face from where heat was applied (for torch) to confirm through-thickness uniformity
The preheat and interpass temperature requirements must be stated in the WPS. The WPS does not need to name the heat source method unless the project specification or owner requires it, but temperature limits must be present. For guidance on documenting interpass temperatures during multi-pass welding, see interpass temperature control and documentation for structural welding.
Rule library based on AWS D1.1:2025; verify against your governing edition.
Temperature Measurement Methods: Accuracy Comparison
The measurement tool choice affects how reliably you verify compliance:
Temperature-indicating crayons (Tempilstik): Simple, inexpensive, widely used. Accurate to ±2% of nominal temperature. Apply the crayon 3 inches from the weld prep before heating; when the wax melts, the material has reached the crayon's rated temperature. The limitation: they only confirm that you have reached the minimum — they do not confirm you have not exceeded the maximum interpass temperature.
Contact thermocouples: More accurate over a wider range and can confirm both minimum preheat and maximum interpass. Require good surface contact; scale and coating interfere. Electronic pyrometers with Type K or J thermocouples are standard in most CWI field kits.
Infrared thermometers: Fast and non-contact. Require calibration of the emissivity setting for steel (typically 0.80–0.85 for mill-scale surface; higher for oxidized surface). Infrared readings are surface-only and can be misleading on a glossy or painted surface — not the preferred method for preheat verification, though acceptable when correctly calibrated.
Machine records from induction units: Where the induction controller logs thermocouple data from a fixed thermocouple attached to the steel near the joint, the machine record is the highest-quality documentation available. Request a printed or electronic export for the QC file.
Practical Recommendation for Shop and Field
For heavy structural shop fabrication on members with carbon equivalent at or above 0.45 (typical for A913, A572 Grade 50 over 1.5 inches, and A514 throughout), induction preheating offers a meaningful quality improvement and a more defensible documentation trail. The equipment investment pays for itself on projects with a high proportion of thick-plate CJP groove welds.
For lighter structural work, field connections, and mixed-section projects, torch preheat with rigorous CWI verification using contact thermocouples remains standard practice. The key is building the preheat verification step into the weld inspection hold point system so it does not get skipped under schedule pressure.
For more on preheat verification methods in the field, see CWI preheat verification methods and documentation.
Shops looking to track preheat compliance records, link temperature readings to weld IDs, and generate audit-ready QC packages will find wpswelding.com/pricing built specifically for the CWI and QC manager on structural steel projects.