Experimental study of cooling during the temper-free induction bending of thick-walled super-duplex pipework to tight bend radii

Temper-free (TF) induction bending has been proven to produce complex pipework in a cost-effective manner. Super-duplex is eminently suitable for induction bending as it retains only a fraction of its ambient yield stress when exposed to elevated temperatures. Previous research has shown that metallurgical problems, namely the precipitation of chromium nitride (Cr₂N) and α′, may be encountered when bending thick-walled super-duplex pipe to radii of less than 2.5 × nominal pipe diameter. This paper uses evidence obtained from thermocouple testing to demonstrate that the typical cooling strategy employed for TF induction is insufficiently rigorous to guarantee the complete cooling of thick-walled pipe bent to tight radii. It goes on to show that, at the intrados, a reservoir of heat remains which may be sufficient to sustain the pipe within key critical temperature ranges long enough to allow the formation of Cr₂N and α′, as well as γ₂.