Induction heating of thick-walled, variable cross-section pipe with an open/closeable inductor: strategies for temperature uniformity control

Quenching is a critical heat treatment process for thick-walled, variable-cross-section pipes. Traditional resistance-heating methods suffer from several drawbacks, including austenite grain coarsening in thin-wall segments, low heating efficiency, high energy consumption, and lengthy processing times. This study investigated induction heating as an alternative approach, to achieve rapid and uniform heating of such workpieces before hardening. An induction heating model was established, which integrated multi-zoned heating approach into an inductor with open/closeable structure, to explore the optimal heating parameters and dynamic power-control strategies. The results showed that using a closed-loop dynamic power control method based on the workpiece temperature feedback, adjusted the activated current (8000A–16000A, 300Hz) and idle current (70 % of activated magnitude, 300Hz) in real time, ensured the radial temperature distribution of the workpiece met production requirements within 40min.The incorporation of magnetic flux concentrator and heat treatment ring effectively minimized temperature differences around the step part. Applying higher power input addressed the low-temperature issues at the workpiece ends. Ultimately, the overall temperature variation of the workpiece was maintained within 860 °C ± 20 °C, with a uniform temperature zone proportion reaching 95 % (3800 mm/4000 mm) along the axial direction. The reliability of the analysis model was verified by an induction heating experiment. The findings indicate a practical pathway for improving ongoing heat-treatment workflows. • Open/closeable inductor and multi-zone heating method provided high heating quality for variable cross-section pipes induction heating. • Low frequency combine with dynamic power control strategy can minimize the radial temperature gradient. • Incorporation of magnetic flux concentrator and heat treatment ring significantly improved the temperature uniformity around step part.