Heat treatment effect on corrosion resistance and mechanical properties of EP718 precipitation-hardened nickel-based alloy

Abstract

The paper investigates the effect of heat treatment modes on the corrosion resistance and strength properties of the EP718 precipitation-hardened nickel-based alloy originally developed for the aircraft industry and currently used in the oil and gas industry. The effect of the annealing temperature (980–1130 °C), holding time (1–2 h) and the time of intermediate and final aging (4–20 h) at 780 °C and 650 °C was studied. It was found that EP718 alloy strength and corrosion properties are determined by the hardening temperature. Highest strength properties are achieved at a hardening temperature of 980 °C (yield strength σy = 950 MPa) due to a higher grain score equal to 3.5 and the presence of inclusions of different size. However, in this case corrosion rate reaches V = 5.88 g/(m2·h). The temperature of 1130 °C ensures the best corrosion performance (V = 2.04 g/(m2·h)) due to the dissolution of undesirable phases (volume fraction of non-metallic inclusions is 0.47 %), but strength performance is reduced (σy = 756 MPa) in this case as a result of the lower grain score – 2.7. Aging mode consisting of an intermediate aging stage with holding at t = 780 °C for 5 h and a final stage at 650 °C for 16 h with air cooling ensures maximum hardening, which is expressed in an increase in hardness to 37.5–38.5 HRC. Electrochemical studies demonstrated that an increase in the aging time leads to a decrease in the stability of the passive state.