Temperature gradient control of frequency conversion heating for a thick-walled pipe based on energy transfer

Abstract

Characterized by energy concentration, electromagnetic heating technology has a significant advantage in achieving local rapid heating of a workpiece. However, electromagnetic heating may create a great temperature gradient in the wall thickness direction for a workpiece with a thick wall that requires penetrating heating. To improve the evenness of the temperature between the internal and external surfaces, a frequency conversion heating method is proposed. An electric–magnetic–thermal coupling model for frequency conversion heating of welded pipes is established and verified by temperature measurement experiments, thereby revealing the distribution characteristics of the induced current, temperature, and magnetic flux intensity. On this basis, a new concept—the energy surface—is defined. An induction heating schedule suitable for thick-walled pipes is formulated with the energy surface transfer characteristics. The electromagnetic distribution resembles the topology of an underwater lake. There is a "demagnetization region" with a magnetic flux intensity of < 0.321 T. The field width varies with the frequency. The temperature difference along the wall thickness of the welded pipe reaches its maximum after the critical demagnetization surface manifests. By controlling the frequency conversion heating, the temperature difference can be decreased to 88.82%, thus improving the induction heating quality of the welded pipe. This study provides a measurable theoretical reference basis for investigating the change in the internal heat source caused by the change of electromagnetic characteristics to accurately control the temperature gradient.