UNS(1) N07718 is one of the most commonly used alloys in the oil and gas industry. The chemistry of the precipitation hardenable nickel-chromium-iron alloy is characterized by additions of niobium and molybdenum as well as certain amounts of aluminum and titanium, resulting in excellent corrosion resistance in oil and gas applications and high strength properties.
Based on the API(2) Standard 6ACRA1 the alloy is available in two grades (min. yield strength 120 ksi and 140 ksi). Due to a strong interest in the market to have a grade with higher strength, the development of a third, 150 ksi grade was started. It is well known that the heat treatment of an alloy can have significant effects on the microstructure, mechanical properties as well as corrosion resistance. To study the effect of the heat treatment and the resultant properties of the alloy an intensive testing program of all three grades (120 ksi, 140 ksi and 150 ksi) was launched. In addition to the microstructure and mechanical properties a variety of corrosion tests were performed. The different mechanisms of environmental cracking were considered by using several test methods. Sulfide stress cracking (SSC) and galvanically induced hydrogen stress cracking (GHSC) tests were performed according to NACE(3) TM0177-2016 Method A. 2
Stress corrosion cracking (SCC) resistance was investigated using C-ring tests according to NACE TM0177 Method C at Level VI/VII (NACE MR0175/ISO 15156-3, Table E.1)3 for 3 and 6 months. Slow strain rate (SSR) tests under cathodic polarization were performed to study the resistance to hydrogen embrittlement (HE).
In terms of resistance to corrosion and hydrogen embrittlement the 150 ksi grade of UNS N07718 was found to perform equally well or even better than the lower strength grades.
For the application of materials in the oil and gas exploration it is necessary to select suitable alloys very carefully. The environmental and operating conditions are very harsh and the material has to meet several different requirements at the same time.
High pressures and temperatures coupled with the presence of hydrogen sulfide and chlorides create challenging demands on the materials.4 NACE MR0175/ ISO(4) 15156-3 gives an overview of all approved materials in consideration of the environmental conditions.2, 5