The effect of age hardening on creep crack growth in alloy 800

The creep crack growth properties of two versions of Alloy 800 have been compared at 500 and 6000C in the Grade II condition in tests up to 104 hours duration. The two alloys were a low carbon alloy containing (wt.%) 0.024 C, 0.5 Ti, 0.3 Al (Alloy B) and a higher carbon alloy containing (wt.%) 0.038 C, 0.2-0.3 Ti, 0.2-3.0 Al (Alloy J). At 6000C, Alloy B attained maximum age hardening in 102 hours, whereas J did not harden significantly in time up to 104 hours. Both alloys age hardened at 5500C with Alloy J hardening more rapidly than B. The creep displacements and displacement rates in small compact tension specimens at 6000C were smaller than Alloy B than in Alloy J. However, failure times were shorter in Alloy B because of lower displacements for crack initiation and propagation and higher crack growth rates. At 5500C the displacements for initiation and propagation in Alloy J were smaller than at 6000C. A consequence of this embrittlement was that the crack growth rates were similar at 550 and 6000C, i.e. were temperature independent. In Alloy B, the slowness of ageing resulted in the fracture being less brittle than at 6000C, and as a consequence the crack growth rates in B were similar to those in J. (Auth.)