Resistance preheating

Resistance preheating describes the heat source and delivery method, not the temperature the joint must reach. Electric current passes through resistive heating elements, commonly arranged as flexible pads against the workpiece. The hot elements transfer energy into the material, while the procedure’s sensors and controller govern the heating cycle. The required preheat temperature still comes from the WPS and applicable construction rules.

AWS publishes D10.22/D10.22M:2021, which is specifically scoped to local electric-resistance heating of welds in creep-strength-enhanced ferritic steel piping and tubing. That scope is important: it is evidence that the method is formally specified for that case, not permission to apply its requirements to every alloy or product. For temperature measurement, the job’s edition of ISO 13916 is the place to look up the relevant measurement method and location; it does not choose the preheat value.

Worked distinction: resistance and induction preheating

Consider a pipe joint wrapped with ceramic heating pads and insulation. Leads supply current to the pads, and thermocouples report the pipe temperature to the controller. The pads become hot because their elements resist current, then heat the pipe through contact. This is resistance preheating.

Now remove the pads and place an induction cable or coil around the same joint. The coil’s electromagnetic field induces heating in the workpiece instead of first heating a resistive pad. That is induction preheating. Both methods use electrical equipment and can serve the same WPS temperature requirement, but the energy-transfer route identifies the method. Recording only “electric preheat” leaves that distinction unresolved.

The method name also does not prove uniform temperature. Heater coverage, insulation, joint geometry and the workpiece’s heat-sink effect shape the thermal gradient. Measurements must demonstrate the specified state at the required locations before welding and, where the procedure requires it, the interpass temperature between runs.