After takeoff the flight attendants seated at the rear of the aircraft reported a bang followed by abnormal noises from the area of the rear airstairs. Shortly afterwards the flight crew observed that the lower aft body overheat light had illuminated. The appropriate drills were carried out, and the warning light extinguished when all bleeds were closed. The aircraft returned for an immediate landing. Maintenance investigation found that a titanium duct flange had ruptured upstream of the right wing anti ice valve. Failure of the flange is a known problem. A special inspection for cracks in the weld zone had been introduced many years ago and, in 1990, after an evaluation of service history, the inspection period was increased from 1,000 cycles to 2,000 cycles. The failed flange was nearly at the 2,000 cycle threshold and had failed approximately 60 cycles and just 12 days before the inspection was scheduled. The operator is continuing checks to confirm the mode of failure of the flange, and is reassessing the compliance period.