Abstract
Fracture modes and carbides of Hastelloy XR (a modified version of conventional Hastelloy X) have been examined metallographically. Creep tests were carried out at 800, 900 and 1000°C in a simulated environment of high-temperatute helium-cooled reactor (HTGR). At 800 and 900°C, wedge-type cracks were observed when specimens were tested under higher stresses, while cavity-type cracks were formed under lower stresses. At 1000°C, localized recrystallization was observed in fractured zone. Stress exponent for rupture life was 5.7 at 800°C and 3.4 at 1000°C, while the value changed from 5.9 to 3.3 in those with longer rupture life at 900°C. Nucleation and growth of crack were strongly associated with grain boundary carbides under tensile stress. Carbides were classified into two types at 900 and 1000°C; Cr-rich M23C6 and Mo-rich M6C, respectively. At 800°C, another type of carbide was found also. The two types of carbide had no different effect on nucleation and growth of crack. Comparison of the results in helium environment with that in air indicated that cracks and carbides were similar to each other except for the regions fractured under heavily oxidized condition. M23C6 was stable up to 1050°C and co-existed with M6C. In conventional Hastelloy X, precipitate of M6C predominated over M23C6 at 900°C.
