Effect of Heat Treatment on Fatigue Crack Propagation and Deformation Mode in Alloy 718at Elevated Temperatures.

摘要

Alloy 718 is a commercial, wrought nickel-base alloy widely utilized in moderate temperature structural applications and has been proposed for use in advanced nuclear systems. In such applications, Alloy 718 will experience the combined effects of cyclic and static loading and sustained elevated temperatures. However, when used in structures requiring welds, Alloy 718 exhibits reduced weld ductility and strain age cracking when given conventional post-weld duplex age heat treatments. A modified heat treatment has been previously proposed to obviate these problems and thereby improve structural reliability. The effect of conventional and modified heat treatments and combined cyclic and static loading (hold time) on crack propagation and deformation mode in two heats of Alloy 718 plate material was investigated at 427 and 593 deg C. The fatigue results show that, in general, the resistance of Alloy 718 to crack propagation at these temperatures was not adversely affected by the modified heat treatment and, under most conditions, was found to be improved in comparison with the conventional heat treatment. Scanning electron microscopy (SEM) examination of the fracture surfaces of the fatigue specimens revealed that the failure mode was dependent on test temperature and heat treatment. The material given the conventional heat treatment exhibited transgranular failure modes at both test temperatures. On the other hand, the modified heat treatment produced transgranular fractures at 427 deg C and intergranular or a mixed failure mode at 593 deg C.