Comparative Study of Typical Electromagnetic Heating Methods in Welded Pipe: Induction Heating Versus Contact Heating

Typical electromagnetic heating techniques include induction heating (IH) and contact heating (CH). The study of these techniques in welding has always been a controversial topic owing to the lack of straightforward quantitative analysis of heat transfer characteristics. Therefore, to explore the comparative nature of heat transfer in CH and IH, high-frequency welding of ferromagnetic materials was studied. Our results revealed quantitative differences between the two heating methods in terms of heating rate and intercritical heat-affected zone (ICHAZ). Compared with IH, CH resulted in a higher active current accumulating at the V-shaped angle of a welded pipe and a greater heating rate for a thick-walled pipe, because of variable magnetic flux leakage in IH. Given identical electrical input, the area of the heat-affected zone (HAZ) in the case of CH was found to be <1/2 of that in the case of IH, which is the root cause of why the area of performance optimization in localized heat treatment of a contact welded pipe is much smaller than that in the case of induction welding. Our study also reveals that IH resulted in a smaller temperature gradient and more uniform temperature distribution than CH, which is the main reason why heating quality in induction welding is superior to that of contact welding. Such comparison between IH and CH in terms of welding quality and energy utilization helps reveal the intrinsic rules of high-frequency electromagnetic heating and provide scientific data and a rationale for promotion of electromagnetic heating in more applications.