Abstract
The susceptibility to hydrogen embrittlement (HE) of an ultra-high strength Corrosion Resistant Alloy (CRA) OCTG was evaluated. A high-strength nickel alloy OCTG with 140ksi (965MPa) grade has been newly developed for use in high temperature and high pressure corrosive wells. This new alloy has exhibited equivalent stress corrosion cracking resistance to UNS(1) N08535 with conventional 110ksi (758MPa) grade. The strength of the material was enhanced by alloying nitrogen and cold working. In this study, the galvanically induced hydrogen stress cracking (GHSC) susceptibility was evaluated in sour environments by galvanically coupled type constant-strain or sustained-stress tests with carbon steel. Another evaluation method using slow strain rate testing (SSRT) under cathodic charging condition was employed to determine the hydrogen embrittlement (HE) susceptibilities of the different alloys tested including the developed alloy. It is revealed that the resistance to HE of the developed alloy is superior to other CRAs such as type 304L and some precipitation hardened alloys. From the results obtained, it can be considered that the high stability of austenitic phase, proper cold working ratio and no significant precipitation in this alloy achieve the excellent resistance to HE.
Introduction
As explorations and developments for extreme High-Pressure High-Temperature (x-HPHT) hydrocarbon reservoirs with corrosive environments have been increasing, the demand for higher strength materials is increasing for x-HPHT corrosive deeper well applications. Corrosion resistant alloys (CRAs) have been used for exploration and production wells in high pressure and high temperature with significant amount of hydrogen sulfide, carbon dioxide and chloride ion.1,2,3 Oil country tubular goods (OCTG) for HPHT wells containing hydrogen sulfide requires the high strength of material (or wall thickness) and good corrosion resistance. 4,5,6 Hence development of high strength CRAs with good corrosion resistance are critical to the development of these HPHT wells.
The presence of hydrogen sulfide in various environments raises the hydrogen embrittlement (HE) susceptibility of high strength grades, even in the case of CRAs. For instance, when the material is coupled with carbon steel, galvanically induced hydrogen stress cracking (GHSC) could occur. The control of precipitation, inclusion and/or dislocation in materials should be taken into account to enhance the resistance to HE. 7,8,9,10,11,12
