The Alloy UNS(1) N07718 is among the most used alloys in the oil and gas industry. Due to the presence of the alloying elements niobium, aluminum and titanium, this alloy is precipitation hardenable by the formation of the phases Gamma’ and Gamma". Although presenting excellent strength properties and good resistance in sour gas applications, this material is known to be susceptible to hydrogen embrittlement and most field failures are related to this limiting property.
The use of Alloy 718 (UNS N07718) for oil & gas applications is regulated by the API(2) 6ACRA1 standard and it is available in three different grades, the 120K, with minimum 120 ksi yield strength, the 140K, with minimum 140 ksi yield strength, and the 150K, with minimum 150 ksi yield strength. Previous studies showed that, due to the different hardening heat treatment parameters, each of the available grades presents a different precipitation behavior in terms of distribution and amount of precipitates, and the obtained microstructure is directly related to the resistance of the material to hydrogen embrittlement.
In this context, this study was carried out with the aim of verifying the previous results in heats with modified compositions based on Alloy UNS N07718. The modified compositions were designed to produce microstructures containing different fractions of Gamma’ and Gamma". The presence and fraction of each of the hardening phases were then correlated to the hydrogen embrittlement susceptibility of the material, assessed by means of slow strain rate tests under cathodic application.
A great number of alloys that can provide good corrosion resistance only achieve high strength properties when cold worked, nevertheless cold worked products are not suitable when thick sections or complex shapes are required. Carbon and low-alloy steels, although having good mechanical properties, do not present enough corrosion resistance. When the application requires thick sections or complex structures, allied with high corrosion resistance, an alloy must be preferentially heat treated to grant more uniform mechanical properties through the cross-section.2 In the oil and gas industry, Alloy UNS N07718 is found among the most used alloys since it can be precipitation hardened to different high strength conditions by the formation of the phases Gamma’ and Gamma", which are granted by the presence of the alloying elements niobium, aluminum and titanium.
