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).
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).
API 6A CRA
This standard was first published in 2015, but is based on standards such as API 6A718 from 2004.
The standard is called "Age-hardenable nickel-base alloys for petroleum and natural gas drilling and production equipment". As a supplement to API 6A, this standard covers a group of high-performance alloys, especially age-hardenable nickel-base 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 the perspective of the material supplier, this standard covers the most relevant aspects of metal properties: production process, chemical composition, mechanical properties, microstructure and test conditions.
NACE
MR0175 – ISO15156-3
NACE MR 0175 (Metals Resistant to Sulfide Stress Cracking and Stress Corrosion Cracking in Sour Oilfield Environments) 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 vendor standards, such as API, and is therefore often used in conjunction with API.
The hydrogen sulfide threshold limits are developed based on a combination of real-world experience and laboratory testing. The standard has evolved over time, with operating limits and material requirements changing significantly. The standard also provides guidance for material selection and specification when hydrogen sulfide thresholds are exceeded. This allows for "fitness for purpose" testing to determine acceptable grades for a particular 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 aerospace, automotive and commercial vehicle applications.
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. The standard covers product in the solution annealed condition only, so its strength and hardness are relatively low (20-25 HRC) compared to the more common solution annealed and aged conditions.
As a result, 718 alloy is easier to machine than 718 alloy in the aged condition, where strength and hardness are significantly increased. For some parts, it is better 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 usually accompanied by a "capability test" to prove that the product can meet the requirements of the AMS 5663 specification at the appropriate heat treatment temperature and time.
AMS 5663 uses the same product as AMS 5662, but has significantly improved mechanical properties by a controlled heat treatment (i.e., aging). The mechanical properties are the same as API 6A CRA.
