Effect of Grain Size Variations on the Long-Time Stability of Alloy 718

Abstract

Uniform, fine grain size is generally preferred in forgings of Alloy 718 for optimum properties. However, due to limitations imposed by forging equipment, forging size and forging configuration, it is often impossible to produce this desired grain size; instead, some grain duplexing or grain coarsening occurs in certain areas throughout the forging. Stress-rupture tests were conducted before exposure, and after exposing specimens at 1250°F, 50 ksi or 1300°F, 50 ksi for 500 hrs. on material having (1) uniform fine grain size (ASTM 7 or finer), (2) duplex grain size, and (3) uniform coarse grain size (ASTM 5 or coarser). These grain sizes were obtained in forgings with a simple pansake configuration using varying forging techniques. The results of the rupture tests conducted before exposure showed an excellent correlation with grain size and Ni3Cb plate size. After exposure, rupture ductility increased and strength decreased due to overaging. In duplexed samples the controlling factor appears to be the amount and size of the coarse grain. The optimum grain size for the best balance of rupture properties appears to be ASTM 6–7.