Forging and Heat Treatment Conditions that Produce Visible Grains in a γ–γ′ Nickel-Based Superalloy

Abstract

Experiments were undertaken to understand forging and heat treatment conditions that give rise to large, visible grains in a new polycrystalline nickel-based γ–γ′ superalloy after solution heat treatment above the gamma prime (γ′) solvus temperature (T_solvus). Such grains are undesirable as they reduce strength and low cycle fatigue performance. The information that is reported is required to design an isothermal forging practice to manufacture closed die forgings, intended for disk rotors that are used in aircraft engines. The alloy is a development composition, which contains about 51 pct γ′ and has been produced by powder metallurgy. Compression tests were conducted to specified upsets on right circular cylinder and double-cone test pieces. Segments of double cones were heat treated and examined to characterize grain size. Visible grains were found in areas of low forging strain (< 0.75), particularly in localized areas of higher strain rate, from slow heating rates through the γ′ T_solvus. It is proposed that they are produced by selective grain growth from activation of a limited number of recrystallization nuclei that have sufficient retained strain energy to exceed a critical value, which reduces with increasing heating time. Higher strains are understood to promote the formation of additional nucleation sites for recrystallization and a consistently finer grain size. Localized areas of low strain that receive higher strain rates generate greater strain hardening, which creates a higher number of nuclei for selective grain growth, compared to surrounding regions.