Corrosion Fatigue Crack Propagation of Oil-Grade Alloy 718 in NaCl Solution

Abstract

Nickel-based alloy 718 was originally developed for use in aircraft gas turbine engines, where there was a need for low cost, high temperature superalloys. Today, alloy 718 has evolved into a special grade (oilgrade) for use in the oil and gas industry for deep sea oil drilling where its good corrosion resistance and customization of mechanical properties via heat treatments are highly sought out. In recent decades, oil-grade Alloy 718 has been directly used in bottom-hole assembly components such as measurement while drilling tools and drill collars, where it applies a combination of good corrosion resistance, high fatigue/corrosion fatigue strength, and non-magnetism. However, as the modern oil and gas industry drilled into high pressure, high temperature, ultra-deep wells, the demand for increased strength and toughness is very critical for newly designed Alloy 718. Thus, a two-step aging treatment, which is used to maximize the yield strength of aerospace-grade Alloy 718, has been adopted and modified for oilgrade alloy 718. However, the effect of aging treatments on corrosion fatigue resistance of oil-grade alloy 718 has not been extensively investigated yet. The aim of this study is to investigate the effect of three different aging treatments on corrosion fatigue crack growth (CFCG) of oil-grade Alloy 718 in NaCl solutions with various temperatures, concentrations, and waveforms. The results show that there is no obvious effect of 3.5 wt.% NaCl solution on the CFCG rates of oil-grade alloy 718 in all three different aged conditions. However, the CFCG rates of oil-grade alloy 718 in 21 wt.% NaCl solution are increased as compared with the ones in air and in 3.5 wt.% NaCl solution. Moreover, aging treatments lead to lower CFCG rates of oil-grade alloy 718 in all tested conditions. Fractographical examinations of the fractured surfaces showed that the fatigue cracks propagated in the transgranular mode for all conditions.