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.