Ni-base superalloys are candidates for operating in deep, sour-gas well environments. When strengthened by cold-working and aging they are susceptible to hydrogen-stress cracking. In this study cold-worked and cold-worked plus aged Hastelloy C-276 was cathodically charged in an H2S04 solution during time-to-failure and constant strain-rate tensile testing. The hydrogen content was measured, and scanning electron fractography was conducted for all fractured samples. The H2S04 charging process was found to provide a valid accelerated simulation of embrittlement by hydrogen in sour-gas environments. For all charging treatments, the aged material was more brittle and contained less hydrogen than the cold-worked material. All tensile test fracture surfaces exhibited an outer intergranular fracture zone surrounding a dimpled core. The outer zone was deeper for the aged material. The depth of the intergranular zone was much greater for both materials than predicted from bulk hydrogen diffusion considerations.
Deep sour-gas well production presents a difficult tubular material selection problem. Deep wells, approaching 10,000 m (30,000 ft.) in depth, contain hostile aqueous H2S - NaCl environments at bottom-hole pressures ranging from 140 to ZIO Mpa (20,000 to 30,000 psi) at temperatures of 200 to 300°C (350 to 600°F). High strength steels, with strengths greater than 560 MPa (80 ksi), cannot be used for this application because they are susceptible to severe hydrogen embrittlement, or sulfide stress cracking, in the hydrogen-generating sour-gas environment.
Single phase nickel-base alloys have presented an attractive alternative to high strength steels. A number of these alloys provide outstanding corrosion resistance and can be processed to the high strength levels of 1380 to 2070 MPa (200 to 300 ksi) required by design considerations and economics. Unfortunately, when the Ni-base alloys are cold-worked to the required strength levels) they also are susceptible to hydrogen embrittlement. Embrittlement is most likely to occur in alloys cathodically charged with hydrogen and stressed in tension, transverse to the direction of cold working. This situation can be anticipated for the Ni-base alloy tubing materials under deep-well conditions. High transverse tensile stresses result from high internal pressures in the tubing, and hydrogen-generation results when tubes are galvanically coupled to steel casing and wellhead components. Moreover, the embrittlement tendency is drastically increased when the alloys are aged at low temperatures (200-500°C, ~ 350-930°F). Aging either is performed intentially to moderately increase strength or it results from exposure to the deep-well environment.
Hastelloy Alloy C-276, one of the leading Ni-base alloy candidate materials for sour gas environments, exhibits this marked embrittlement in the cold-worked plus aged condition. Although embrittlement of Hastelloy C-276 is well documented, agreement has not been reached as to its mechanisms. It is particularly difficult to explain why aging leads to such drastic embrittlement when it causes only minor modifications in microstructure.
