An investigation of the thermal stability of a commercial Ni-Cr-Fe-Mo alloy (hastelloy alloy X)

Abstract

The changes in hardness and room temperature impact toughness of Hastelloy ∗ Alloy X∗Hastelloy is a registered trademark of Cabot Corporation. after aging at 1000, 1200, 1400 and 1600°F (538, 649, 760 and 871°) for times up to 10,000 h were investigated. The alloy exhibits age-hardening at 1200 and 1400°F (649 and 760°).’A slight hardness increase was observed at 1600°F (871°) followed by overaging after 4000 h. No age-hardening was observed at 1000°F (538°) up to 10,000 h. Aging at all temperatures resulted in a substantial drop in room temperature impact toughness. The microstructure after aging was characterized by optical metallography and X-ray diffraction, while fracture mode was characterized by scanning electron microscopy. The results suggest that the toughness degradation is primarily associated with carbide precipitation. M₆C’ type carbides are believed to be the major phase precipitated during aging at all temperatures, although σ and µphases were also detected after 10,000 h at 1400 and 1600°F (760 and 871°), respectively.