Effect of Grain Size on High Temperature Creep Properties of Hastelloy X

Abstract

The effect of grain size, ranged from 37 to 1220 μm, on creep behavior of Hastelloy X was studied at temperatures of 950, 1 000 and 1 050°C. A steady state creep rate, ε_s, of the alloy decreased with the increase of the grain size, reached a minimum at the grain size of 100200 μm, L_M, and increased with the increase of that. From measurements of friction stress (σ_f) and observations of microstructures, it is concluded that the increase in ε_s under a given applied stress with the decrease of the grain size is attributable to the increase of an effective stress, σ_e(=σ_a-σ_f, whereey σ_a is the applied stress). On the other hand, recrystallization induced by a high stress concentration at grain boundary triple points in the coarse-grained alloys seems to accelerate the creep. By comparing the results in the Hastelloy X with those in carbon free 17Cr-14Ni steel, it is suggested that the grain size dependence of the ε_s of the Hastelloy X having finer grains than L_M is more remarkable than that of carbon free 17Cr-14Ni steel because of the decrease of fine precipitates within grains in the former alloy, while the dependence of the alloy having coarser grains than L_M is minor than that of carbon free 17Cr-14Ni steel because of preventing the stress concentration at the triple points by the grain boundary precipitates.