ABSTRACT
To extend the current understanding of the influence of microstructural particularities in age- hardenable UNS N07718 (alloy 718) on its susceptibility to localised corrosion and environmentally assisted cracking, a large research program dealing with the effect of precipitates such as ?', ?'', d-phase, and carbonitrides was conducted. Electrochemical results have shown that 6-phase had no significant effect on the electrochemical potential necessary for pit initiation or on pit nucleation time. Carbides, on the other hand, have been determined as preferential sites for pit initiation on UNS N07718. Hydrogen permeation tests on UNS N07718 have shown that both ?'/?''- and d-phase delay hydrogen diffusion, with ?'/?'-phase having a more pronounced effect than d-phase. On the other hand, the effect of both d-phase and ?' /?' ' -phase on hydrogen trapping was negligible. Using in-situ tensile testing under electrochemical hydrogen charging, revealed that 6-phase increases the susceptibility of UNS N07718 to hydrogen embrittlement by initiating cracks at the matrix/precipitate interface. Several mechanisms including Hydrogen-Enhanced Localised Plasticity (HELP) and Hydrogen-Enhanced DEcohesion (HEDE) have been identified as contributing to the hydrogen embrittlement susceptibility of UNS N07718, depending upon microstructural particularities in the material. These results can be used for redesigning age- hardenable Nickel alloys with improved corrosion resistance.
INTRODUCTION
UNS N07718 (alloy 718) is the most commonly used Nickel (Ni) alloy in upstream oilfield technology. However, it is known that UNS N07718 can be susceptible to localised corrosion and environmentally assisted cracking under certain service conditions. This susceptibility is affected by its complex microstructure containing several intermetallic phases, nitrides, carbides and carbonitrides. In addition, the size and amount of these precipitates depend upon the ageing time and temperature selected for heat treatment.
The alloy UNS N07718 derives its mechanical strength (and hardness) from fine ?' and ?'' precipitates, which typically precipitate at intermediate temperatures between 550 and 800 °C. The finer the precipitate, the higher is the yield strength of the alloy. At temperatures above 800 °C, the orthorhombic 5-phase becomes the most stable phase. In addition, UNS N07718 contains carbides and/or carbonitrides. Table 1 shows a summary of phases typically present in UNS N07718 and their corresponding solvus temperature.
