Influence of Heat Treatment on the Corrosive and Strength Properties of the EP718 Dispersion-Hardening Nickel Alloy

We investigate the influence of the heat treatment regimes on corrosion resistance and strength properties of precipitation-hardening EP718 nickel-based alloy initially developed for the aviation industry conditions and currently used in the oil and gas industry. Here considered is the effect of the hardening temperature (980–1130°С) and the holding time (1–2 h), as well as of the intermediate and final aging stages duration (4–20 h) at the temperatures of 780 and 650°С. We state that the strength and the corrosion properties of the EP718 alloy are governed by the hardening temperature. At its value of 980°C, the highest strength characteristics are achieved (the yield point being σy = 950 MPa) due to the higher grain score equal to 3.5, and to the presence of the various size inclusions (their volume fraction being 0.61%); yet, in that case, the corrosion rate reaches V = 5.88 g/(m2 h). At the temperature of 1130°C, the best corrosion characteristics are observed (V = 2.04 g/(m2 h)) due to the detrimental phase dissolution (the volume fraction of non-metallic inclusions being 0.47%); yet, the strength properties decrease (σT = 756 MPa) because of the lower grain score – 2.7. The ageing regime consisting of the intermediate stage with the holding at t = 780°С for 5 h and the final stage at 650°С, τ = 16 h, with cooling in air, leads to the maximal hardening: to the hardness increase up to 37.5–38.5 HRC. By means of the electrochemical studies we show that the ageing duration increase leads to a decrease in the passive state stability.