RELATION BETWEEN FRACTURE MODE AND CARBID E OF HASTELLOY XR CREEP-TESTED IN HELIUM M23C6 M6C ENVIRONMENT/

Fracture modes and carbides of Hastelloy XR (a modified version of con ventional Hastelloy X) have been examined metallographically. Creep te sts were carried out at 800, 900 and 1000 degrees C in a simulated env ironment of high-temperature helium-cooled reactor (HTGR). At 800 and 900 degrees C, wedge-type cracks were observed when specimens Were tes ted under higher stresses, while cavity-type cracks were formed under lower stresses. At 1000 degrees C, localized recrystallization was obs erved in fractured zone. Stress exponent for rupture life was 5.7 at 8 00 degrees C and 3.4 at 1000 degrees C, whilethe value changed from 5. 9 to 3.3 in those with longer rupture life at 900 degrees C. Nucleatio n and growth of crack were strongly associated with grain boundary car bides under tensile stress, Carbides were classified into two types at 900 and 1000 degrees C; Cr-rich M23C6 and Mo-rich M6C, respectively. At 800 degrees C, another type of carbide was found also. The two type s of carbide had no different effect on nucleation and growth of crack . Comparison of the results in helium environment with that in air ind icated that cracks and carbides were similar to each other except for the regions fractured under heavily oxidized condition. M23C6 was stab le up to 1050 degrees C and co-existed with M6C. In conventional Haste lloy X,precipitate of M6C predominated over M23C6 at 900 degrees C.