Date of Graduation
Statler College of Engineering and Mineral Resources
Mechanical and Aerospace Engineering
Oil-grade nickel-base alloy 718 has been recently used in the oil and gas industry for downhole drilling components especially for the parts of the bottom hole assembly (BHA) due to its superior mechanical properties, good corrosion resistance, non-magnetic properties and ability to be heat treated to various strength levels. As the modern oil and gas industry explored into coastal and marine locations, deeper and deeper wells have been drilled with high pressure high temperature, making the demand for increased strength and good toughness super critical for alloys used in ultra-deep wells drilling. Moreover, the existence of high concentrations of chloride salts in the aqueous drilling fluids and high mechanical loading will lead to corrosion problems such as pitting corrosion and corrosion fatigue for alloys applied in BHA.
The goal of this research is to achieve ideal microstructure and to improve the strength of oil-grade alloy 718 through an optimized heat treatment and a novel mechanical surface treatment, while maintaining good pitting corrosion resistance and corrosion fatigue resistance of the alloy, to better serve ultra-deep well drilling.
In this study, a new two-step aging treatment is successfully applied to modify the microstructure and to increase the yield strength of oil-grade alloy 718. Based on the microstructure analysis, isolated platelet δ phases were precipitated at some of the grain boundaries in both one-step aged and two-step aged specimens. The one-step aged specimen shows a uniform distribution of fine spherical γ' and elongated γ" precipitates in the grains. On the contrary, the two-step aged sample shows much finer precipitates.
The effect of aging treatment on the pitting corrosion resistance and corrosion fatigue crack growth (CFCG) rates of oil-grade alloy 718 in different NaCl solutions is investigated and compared. The electrochemical measurements results show that the pitting corrosion resistance of oil-grade alloy 718, regardless of aging treatment, is nearly the same, indicated by the similar corrosion current density and pitting potential. The influencing factors including solution temperature and NaCl concentration on the pitting corrosion resistance of oil-grade alloy 718 are also studied, and the results indicate that the pitting corrosion resistance is decreased with increasing the solution temperature and NaCl concentration. The CFCG results show that there is no obvious effect of 3.5 wt.% NaCl solution on the CFCG rates of oil-grade alloy 718 samples with different aging treatments. Nevertheless, the CFCG rates of oil-grade alloy 718 in 21 wt.% NaCl solution are increased in comparison with the ones tested in laboratory air and in 3.5 wt.% NaCl solution. Furthermore, aging treatments lead to lower CFCG rates of oil-grade alloy 718 in all tested environments. However, no visible difference of CFCG rates in NaCl solution is observed between one-step aged and two-step aged specimens.
The effect of surface modifications induced by machine hammer peening (MHP) surface treatment on the pitting corrosion behavior of oil-grade alloy 718 in 3.5 wt.% NaCl solution is also investigated. Severe work hardening and high compressive residual stress are generated with surface smoothing and microstructure evolution in terms of the formation of nano-grains and nano-twins in the near surface region after MHP. The electrochemical tests results show that MHP has a beneficial influence on the corrosion resistance, indicated by a significant increase of the critical pitting potential (+134 mV) accompanied with lower corrosion current density and higher polarization resistance. The two-step aging treatment is applied to oil-grade alloy 718 that has been previously surface-treated by mill finishing (MF) and MHP. As a result, a Cr-enriched oxide layer is formed along with a nano-precipitates layer that consists of high precipitate fractions of γ'/γ" on the top surface. Surface hardness increases after aging but the compressive residual stress is almost relaxed. The synergistic effects of MHP and aging treatment on the pitting corrosion behavior are also studied. The results show that the corrosion resistance of the MHP specimens decreases after aging, although the corrosion resistance is still higher than the MF condition.
Chen, Ting, "Pitting corrosion and corrosion fatigue crack growth behavior of oil-grade alloy 718 in sodium chloride solution" (2014). Graduate Theses, Dissertations, and Problem Reports. 8165.