Effect of service exposure on fatigue crack propagation of Inconel 718 turbine disc material at elevated temperatures

• The specimens were prepared from the Inconel 718 turbine disc used for 0 to 4229 h.
• FCP rates were measured at 738 and 823 K.
• The ΔKth values decreased with increasing service time.
• The FCP behavior showed a strong correlation with the grain size of used turbine disc.

In this study, the fatigue crack propagation behavior of Inconel 718 turbine disc with different service times from 0 to 4229 h was investigated at 738 and 823 K. No notable change in microstructural features, other than the increase in grain size, was observed with increasing service time. With increasing service time from 0 to 4229 h, the fatigue crack propagation rates tended to increase, while the ΔKth value decreased, in low ΔK regime and lower Paris' regime at both testing temperatures. The fractographic observation using a scanning electron microscope suggested that the elevated temperature fatigue crack propagation mechanism of Inconel 718 changed from crystallographic cleavage mechanism to striation mechanism in the low ΔK regime, depending on the grain size. The fatigue crack propagation mechanism is proposed for the crack propagating through small and large grains in the low ΔK regime, and the fatigue crack propagation behavior of Inconel 718 with different service times at elevated temperatures is discussed.