The effect of forging history on the strength and microstructure of TDNiCr (Ni-20Cr-2ThO2)

Abstract

Forging variables were evaluated to determine their influence on the elevated temperature strength and microstructure of TDNiCr. Grain size was the principal microstrucrural feature related to elevated temperature strength and was controlled primarily by the thermomechanical variables of forging temperature and final annealing condition. Tests at 2000°F (1366 K) revealed a factor of eight increase in tensile strength as grain size increased from 1 to 150 μm, while stress-rupture strength improved by three to five times as grain size increased from 15 to 1500 μm. Forged material of grain size ≥ ∼150 μm displayed a level of elevated temperature strength comparable to that of optimized TDNiCr sheet. The presence of a preponderance of small twins and a strong preferred orientation may have also been factors contributing to the excellent high temperature strength of large grain forged material.