Abstract
Inconel 718 is a high strength, heat resistant superalloy that is used extensively for components in hot sections of gas turbine engines. This paper presents an experimental study on the thermal stability of broached Inconel 718 in terms of microstructure and nano-hardness. The broaching process used in this study is similar to that used in gas turbine industries for machining fir-tree root fixings on turbine discs. Severe plastic deformation was found under the broached surface. The plastic deformation induces a work-hardened layer in the subsurface region with a thickness of ∼50 μm. Thermal exposure was conducted at two temperatures, 550 ∘C and 650 ∘C respectively, for 300 h. Recrystallization occurs in the surface layer during thermal exposure at 550 ∘C and α-Cr precipitates as a consequence of the growth of recrystallized δ phases. More recrystallized grains with a larger size form in the surface layer and the α-Cr not only precipitates in the surface layer, but also in the sub-surface region when the thermal exposure temperature goes up to 650 ∘C. The thermal exposure leads to an increase in nano-hardness both in the work-hardened layer and in the bulk material due to the coarsening of the main strengthening phase γ′′.
Highlights
The use of Inconel 718 as a disc material in gas turbine engines has increased in recent years because it has good microstructural stability and can maintain high yield and high tensile strength at elevated temperature
It is believed that the formation of this layer is attributed to a combination of severe plastic deformation, high temperature generated at the machined surface and rapid cooling by coolant [17]
The higher low angle grain boundary (LAGB) density in the subsurface region in comparison to that of the bulk material reveals an increasing plastic deformation level induced by the broaching
Summary
The use of Inconel 718 as a disc material in gas turbine engines has increased in recent years because it has good microstructural stability and can maintain high yield and high tensile strength at elevated temperature. The precipitation of α-Cr and sigma after long-time thermal exposure at high temperature can be another factor that affects the mechanical properties [3]. 718 after turning, and the drop from a higher surface hardness to a lower bulk material hardness is explained by the work hardening effect [11,12,13]. Inconel 718 is extensively used for components in hot sections of gas turbine engines. In the case of thermal mechanical fatigue, cracks typically initiate at the surface and the crack growth behavior markedly depends on microstructure, strength and environment [14]
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