Microstructure and mechanical properties of carbon steel A210–superalloy Sanicro 28 bimetallic tubes

The viability by hot co-extrusion of a new bimetallic tube: carbon steel A-210-G°A1/iron based superalloy Sanicro 28 (UNS N08028) both by hot torsion tests and by diffusion bonding experiments using hipping has been verified. An excellent metallurgical bond was obtained after the industrial hot co-extrusion process. Both the interdiffusion of the elements across the interface, and the microstructure have been analysed by optical microscopy, SEM, TEM and EBSD (electron backscattered diffraction). On the Sanicro 28 side a profuse precipitation of Cr/Mo carbides was found in the region close to the interface. In the hipped specimens and in the heat-treated bimetallic tubes, as result of the nickel and chromium diffusion from the superalloy to the carbon steel, austenite and martensite sub-bands were observed parallel to the interface on the steel side. The optimum heat treatment performed on the bimetallic tubes consisted of an austenitising-solution treatment at 1100 °C, and a stabilisation annealing at 900 °C, followed by air cooling.