SHORTLY AFTER DEPARTURE & LEVEL OFF, THE PLT REPORTED AN ENG MALFUNCTION & INITIATED A RETURN TO THE ARPT. ABOUT 2 MIN LATER, HE REPORTED AN ENG FIRE & DECLARED AN EMERGENCY. APRX 1 MIN THEREAFTER, THE RGT WING SEPARATED FROM THE ACFT & THE PLANE CRASHED. AN EXAM OF THE WRECKAGE REVEALED EXTENSIVE INFLT FIRE DAMAGE IN THE AREA OF SEPARATION. A 2.5 BY 3 INCH HOLE WAS FOUND IN THE RGT INBOARD EXHAUST TUBE, PN 0850712-6. IT WAS ORIENTED TOWARD THE ACCESSORY COMPARTMENT WITHNUMEROUS FUEL & OIL LINES WHICH WERE FIRE DAMAGED & CHARRED. ALSO, MUCH OF THE ENG SUPPORT STRUCTURE & WING SPAR WERE MELTED IN THAT AREA. AN EXAM OF THE EXHAUST TUBE REVEALED ITS WALLS WERE THIN, BRITTLE & DISCOLORED IN THE AREA OF THE RUPTURE & SEVERAL AREAS OF CRACKS & SMALL HOLES WERE FOUND. THE EXHAUST TUBE HAD 5862 HRS OF TOTAL SERVICE & 1762 HOURS SINCE IT WAS OVERHAULED. RECORDS SHOWED THAT AD 75-23-08 HAD BEEN COMPLIED WITH DURING THE LAST INSPECTION, 25 FLT HRS PRIOR TO THE ACCIDENT.