Which specifications apply to Alloy 718?
Alloy 718 (Inconel 718, UNS N07718, 2.4668) has been widely used in many fields due to its high performance and availability in a variety of product forms.
Nickel-chromium alloys are probably the most widely used nickel-based alloys. They are characterized by excellent corrosion resistance at both room and elevated temperatures - they form a protective film that prevents further oxidation and maintains their mechanical properties over a wide temperature range. In addition, they can achieve excellent strength and toughness through increasing levels of heat treatment (aging).
History of Alloy 718
Alloy 718 is best known as Inconel, the trade name of Special Metals Corporation, the originator of the family of grades. Inconel 718 is perhaps the most well-known and most complete grade in the family of alloys. It was introduced in the 1960s and was originally used in the aerospace industry, but was later adopted by the oil and gas industry due to its abundant market supply. This is related to the migration of offshore wells, increased drilling depths, and higher temperatures, pressures, and corrosive contaminants.
Other nickel-based alloys, such as Hastelloy C276 and Inconel 625, while sufficiently corrosion resistant, are insufficiently strong without extensive cold working. Martensitic stainless steels are limited by their potential for hydrogen embrittlement and stress corrosion cracking. As a result, Inconel 718 has quickly become the default choice for many pressure- and load-bearing components in harsh environments.
As a result, 718 alloy covers a wide range of specifications. For applications in the oil and gas market, the main standards considered are API 6A, API 6A CRA, and NACE MR1075 – ISO 15156. For aerospace applications, standards such as AMS 5662 and AMS5663 tend to dominate.
API 6A
Also known as ISO10423 – originally published in 1994 and last revised in 2018.
API is the American Petroleum Institute (API). The industry association was founded in 1919 to promote safety in the global gas and oil industry. One of its main activities is to bring together experts in the field to develop and maintain standards covering materials and operating practices.
More about API 6A
The title of the standard – "Petroleum and Natural Gas Industry – Drilling and Production Equipment – Specification for Valves, Wellheads and Christmas Tree Equipment" – indicates the intended applications for this high-performance alloy.
A wellhead is a series of components located on the surface of an oil and gas well that serve as an interface to drilling and production equipment. Therefore, it must be able to withstand the pressure of the oil and gas in the well and have sufficient strength to support the various equipment connected to it. After drilling is completed, the wellhead completes the connection to the oil and gas reservoir and pipeline. Surface pressure control is provided by the Christmas tree.
In the oil and gas field, the Christmas tree refers to the collection of valves, spools and fittings used to control the flow in and out of the oil well, and is also the main part of the wellhead. It can also host other functions, such as injecting chemicals, detecting or monitoring various conditions, flow and temperature.
Specific components detailed in this standard include:
Wellhead equipment – casing head housings, casing head spools, tubing head spools, transition spools, multistage head housings and spools;
Connectors and accessories – transition connectors, tubing head adapters, top connectors, tees and crosses, fluid sampling devices, adapters and isolation spools;
Casing and tubing hangers – mandrel hangers, slip hangers
Valves and chokes – single valves, multiple valves, actuated valves, pre-actuated valves, check valves, chokes, surface and subsea safety valves and actuators, back pressure valves
Joints – weld neck joints, blind joints, threaded joints, adapter and gasket joints, large plugs, valve removal plugs
Other equipment – actuators, clamp hubs, pressure boundary penetration devices, annular gaskets, running and testing tools, wear bushings
This standard covers equipment operating under different conditions; pressure ranges from 2,000psi to 20,000psi (13.8 – 138MPa) and temperature ranges from -60°C (-75°F) to 121°C (250°F).
This standard focuses more on design criteria for components than on material properties of the metals being specified. As such, it is used in conjunction with API 6A CRA and NACE MR-0175.
API 6A CRA
This standard was first published in 2015, but is based on standards such as API 6A718 from 2004.
The name of the standard is "Age-hardenable nickel-base alloys for petroleum and natural gas drilling and production equipment." Complementing API 6A, it covers a group of high-performance alloys, particularly age-hardenable nickel alloys, including Alloy 718, Alloy 725 (Inconel 725, UNS N07725), and Alloy 925 (Inconel 925, UNS N09925), as well as Alloys 716, 935, and 945.
From a material supplier's perspective, it covers the most relevant aspects of metal properties: production processes, chemical composition, mechanical properties, microstructure, and test conditions.
NACE
MR0175 – ISO15156-3
NACE MR 0175 (Metals Resistant to Sulfide Stress Corrosion Cracking and Stress Corrosion Cracking in Sour Oilfield Environments), which stands for NACE MR 0175, specifies the types of corrosion resistant materials that can be used in specific oilfield environments.
A unique feature of this standard is that it specifies limits for material hardness, as it is the only material measurement that can be made in the field to verify material specifications. Despite this limitation, hardness has a reasonable correlation with overall mechanical properties and is applicable to both the parent material and any welds.
Given that the standard focuses only on sulfide stress corrosion cracking, it does not address general corrosion or pitting issues. Likewise, it does not specify minimum mechanical properties, required manufacturing processes, or general testing criteria, and therefore does not provide a complete selection tool. However, it is a useful complement to other supplier standards such as API and is therefore often used in conjunction with API.
The hydrogen sulfide threshold limits were developed based on a combination of practical experience and laboratory testing. The standard has evolved over time and the operating limits and material requirements have changed significantly. The standard also provides guidance for the selection and specification of materials when the hydrogen sulfide thresholds are exceeded. This allows for "fitness for purpose" testing to determine the acceptable grade for a specific application.
NACE MR0175 is now published and maintained as ISO 15156-3.
AMS 5662 and AMS 5663.
AMS stands for "Aerospace Material Specification" and covers a range of standards maintained by the Society of Automotive Engineers (SAE) that, despite the name, cover applications in the aerospace, automotive and commercial vehicle sectors.
AMS 5662 is the base specification for 718 alloy ("nickel alloy, corrosion and heat resistant, bars, forgings and rings") commonly used in the aerospace market. It is only available in the solution annealed condition, so its strength and hardness are relatively low (20-25 HRC) compared to the more common solution annealed and aged conditions.
As a result, the strength and hardness in the aged condition are significantly increased compared to 718 alloy in the aged condition, and it is easier to machine. For some parts, it is best to machine the metal in this condition before heat treating, or at least proof machine it to achieve the desired mechanical properties, and then complete the finishing process.
AMS 5662 is typically "capability tested" to demonstrate that it can meet the requirements of the AMS 5663 specification when subjected to the appropriate heat treatment temperature and time.
AMS 5663 uses the same products as AMS 5662, but significantly improves the mechanical properties through a controlled heat treatment called aging. The mechanical properties are comparable to API 6A CRA.
Learn more about 718 Alloy
Gnee Steel offers a full range of nickel-based corrosion resistant alloys (CRAs), including 718 Alloy. Our stock range covers bars from ½ inch to 10 inches in diameter, meeting API 6A, API 6A CRA and NACE MR-1075 standards.
