DEVELOPMENT OF CLAD BOILER TUBES EXTRUDED FROM BIMETALLIC CENTRIFUGALCASTINGS

Wrought tubes of T-ll steel, externally clad with SS310, have been pro duced by a new method (U.S. Patent 5,558,150), The alloys were united directly from the molten state by centrifugal casting. In the optimum process, temperatures were controlled to prevent meltback of the SS310 outer layer by the higher melting T-ll stream, Hollow extrusion bille ts were prepared from the heavy-walled cast bimetallic tubes and succe ssfully hot extruded (at a ratio of 13.4) to 84-mm (3.3 in.) OD x 64-m m (2.5-in.) ID tubes, and (at a ratio of 37.6) to 51-mm (2-in.) OD x 3 8-mm (1.5-in.) ID tubes, In all, 10 castings were produced, and 12 bil lets were extruded to tubes. For the most part, thicknesses of the cla dding and of the tube wall are rather uniform around the circumference and from end to end of the tubes. Hardness and tensile properties of annealed 51-mm (2-in.) tubes are uniform from end to end of a tube, an d between tubes, and readily conform to ASTM A 213; tubes satisfy the flattening and flaring requirements of ASTM A 450, The cladding is met allurgically bonded to the base metal, as revealed by metallography, a nd by two tests developed for this study: a bond shear strength test a nd a twist test, In the latter test, rings 3.1 mm (0.125 in.) in thick ness are slotted and severely twisted with a special tool. In tubes ma de by the optimum process, minute fissures that form adjacent to some of the pressure points during twist testing occupy just 3 % of the bon d-line length, Cost estimates for commercial production of 51-mm (2-in .) tubes via the centrifugal casting route suggest that such tubes sho uld be considerably less expensive than conventionally clad tubes (ext ruded from composite billets assembled from heavy-walled wrought tubes ).