ABSTRACT
The influence of loading mode and specimen geometry on the measured hydrogen embrittlement susceptibility of UNS N07718 (alloy 718) has been assessed. Slow strain rate (SSR) experiments were performed with smooth specimens, while incremental step load (ISL) testing and rising displacement (RD) testing were performed on circular notch tensile (CNT) specimens with cathodic hydrogen charging. Several microstructures of alloy 718 with different strength levels and amounts of grain boundary d phase precipitation were evaluated. Fracture modes were also evaluated with scanning electron microscopy and correlated to the microstructures and testing methods. Hydrogen embrittlement susceptibilities and fracture modes were similar for each test method and were highly dependent on microstructure. It is interpreted that hydrogen-affected cracking of alloy 718 requires a certain level of plasticity and an adequate amount of absorbed hydrogen in both the SSR test and the accelerated fracture mechanics ISL and RD tests. These criteria are related to the measured threshold stress intensity factor for crack growth with cathodic polarization (Kth). The hydrogen embrittlement mechanism at the notch for a double-aged microstructure was independent of the loading mode (dynamic straining versus constant displacement holds). For alloy 718 microstructures where no d phase was present, the hydrogen embrittlement mechanism was similar for under-aged to peak-aged conditions. The presence of d phase in equivalent strength microstructures significantly increased hydrogen embrittlement susceptibility, and strength level did not correlate strongly to hydrogen sensitivity.
INTRODUCTION
UNS(1) N07718, hereafter referred to as alloy 718, is one of several nickel-base corrosion resistant alloys (CRAs) from which components for deep-sea wells in the oil and gas industry are produced. Packers, tubing hangers, fasteners, valves, and bolting components are all fabricated from CRAs because of their high-strength and resistance to general corrosion processes. Uptake of hydrogen in oil and gas environments can result in hydrogen embrittlement field failures of CRA components.1-3 In these cases, atomic hydrogen is adsorbed on the surface of the component and diffuses into the alloy, resulting in a decrease in plasticity and/or fracture strength.
