Use of UNS N07718 Bolting in ASME Pressure Vessel Applications in Wet Sour Services

Abstract

UNS N07718 (commonly known as Grade 718) is an age-hardenable nickel-chromium alloy which primarily has applications in aircraft components and engine parts, cryogenic tankages, and for downhole and wellhead components in oil and gas. It also has been used primarily as bolting for ASME Section VIII vessels, exchangers and other high strength applications. ASME SB-637, UNS N07718(1) has the highest design allowable stresses of all the bolting materials in ASME Section II, Part D(2), Table 3. There is a second industry standard which covers UNS N07718 components, namely API 6ACRA(3), which was developed for Oil and Gas equipment. Since this standard is for very different applications compared to the ASME applications, the recommended heat treatments and required mechanical properties of the two standards vary considerably. Bolting to either standard can be specified to meet “NACE hardness limits” for wet sour services, but for the ASME materials, this requires strict controls of the various heat treatment steps as the “box” for meeting all the required properties is tight. The NACE standards and requirements for wet sour service are discussed in the paper, along with the specified heat treatments and mechanical properties required by the two industry standards. The paper will review the effect of the heat treatments on the expected microstructures and properties. By using testing results of N07718 materials specified to both the materials standards, the heat treatment and properties were optimized for a specific application that was specified to meet NACE hardness limits as well as ASME Code strength requirements. The paper also makes a recommendation to ASME and ASTM committees to consider incorporating an additional grade of N07718 bolting with lower yield and tensile strength requirements and subsequently slightly lower allowable stresses which will provide lower hardness levels, improved toughness and improved ductility, and make it easier to source a viable N07718 bolting that is endorsed by the Code for wet sour services, and improved for low temperature, cryogenic and other services.