Nickel-chromium alloys are probably the most widely used nickel-based alloys. They are characterised by their excellent corrosion resistance at both ambient and elevated temperatures – forming a protective film that prevents further oxidation and retention of their mechanical properties over a wide temperature range. In addition, they achieve excellent levels of strength and toughness through increasing levels of heat treatment (ageing).
Alloy 718 is best known by the Special Metals Corporation tradename Inconel® as the originator of this family of grades. Inconel 718 is perhaps the most widely known and specified grade within this family of alloys. It was introduced in the 1960s and initially used in the aerospace industry, but due to its ready availability in the market, it was taken up by the oil and gas industry. This was associated with wells moving offshore, drilling deeper with higher temperatures, pressures and corrosive contaminants. Other nickel-based alloys such as Hastelloy C276 and Inconel 625 provided sufficient corrosion resistance, but insufficient strength without extensive cold working. Martensitic stainless steels were limited by their potential for hydrogen embrittlement and stress corrosion cracking. Therefore, Inconel 718 quickly became a default choice for many pressure-containing and load-bearing components in aggressive environments.
As such, it is covered by a variety of specifications. In terms of applications in the oil and gas market, the main standards to consider are API 6A, API 6A CRA and NACE MR1075 – ISO 15156. For aerospace applications, standards such as AMS 5662 and AMS5663 tend to dominate.
1) API 6A (also known as ISO10423 – originally issued in 1994, and last revised in 2018).
API is the American Petroleum Institute. Formed in 1919, the trade association aims to promote safety across the natural gas and oil industry globally. One of its main activities is to bring together subject matter experts to develop and maintain standards that cover materials and operational practices. The title of this standard – ‘Petroleum and natural gas industries — Drilling and production equipment — Specification for valves, wellhead and Christmas tree equipment’ suggests the target applications for this high-performance alloy.
A wellhead is the collection of components that sit at the surface of an oil and gas well and acts as the interface for the drilling and production equipment. It must, therefore, contain the pressure of the oil and gas within the well, and be sufficiently strong to support the various items that are joined to it. When the well has been drilled, it is completed to provide an interface with the oil and gas reservoir and pipework. The surface pressure control is provided by a Christmas tree.
In the context of oil and gas, a Christmas tree is the collection of valves, spools, and fittings used to control the flow to or from the well, and so is the major part of a wellhead. It may also house additional functions, such as the ability to inject chemical, detect or monitor various conditions, flow rates and temperatures.
The specific components described in further detail within this standard are:
a) wellhead equipment – casing-head housings, casing-head spools, tubing-head spools, cross-over spools, multi-stage head housings and spools
b) connectors and fittings – cross-over connectors, tubing-head adapters, top connectors, tees and crosses, fluid-sampling devices, adapter and spacer spools;
c) casing and tubing hangers – mandrel hangers, slip hangers
d) valves and chokes – single valves, multiple valves, actuated valves, valves prepared for actuators, check valves, chokes, surface and underwater safety valves and actuators, back-pressure valves
e) loose connectors – weld neck connectors, blind connectors, threaded connectors, adapter and spacer connectors, bull plugs, valve-removal plugs
other-equipment – actuators, clamp hubs, pressure boundary penetrations, ring gaskets, running and testing tools, wear bushings
This standard covers equipment operating at varying conditions; pressures from 2,000psi to 20,000psi (13.8 – 138MPa) and temperature ranges from -60degC (-75degF) to 121degC (250degF).
This standard focuses more on the design criteria of the components than the material properties of the metals that are specified. It is therefore used in combination with API 6A CRA and NACE MR-0175.
2) API 6A CRA (first issued in 2015, but builds upon standards such as API 6A718 from 2004).
The title of this standard is ‘Age-hardened Nickel-based Alloys for Oil and Gas Drilling and Production Equipment’. As a supplement to API 6A, it covers a group of high-performance alloys – specifically age-hardened nickel alloys, which includes 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 the properties of the metals: production route, chemical composition, mechanical properties, and microstructure and test conditions.
3) NACE (MR0175 – ISO15156-3)
To give its full name, NACE MR 0175 (‘Metals for Sulphide Stress Cracking and Stress Corrosion Cracking Resistance in Sour Oilfield Environments’), specifies the types of corrosion-resistant materials that can be used in specific oilfield environments. One particular feature of this standard is the use of limits for material hardness, as it is the only practical material measurement that can be conducted in the field as a validation of material specification. Despite this restriction, hardness has a reasonable correlation with overall mechanical properties and is applied to both the parent metal and any weld features.
Given that the standard is focused purely on sulphide stress corrosion cracking, it does not address the issues of general corrosion or pitting corrosion. Similarly, it does not stipulate minimum mechanical properties, the required manufacturing route nor general testing standards, so does not provide a complete selection tool. However, it is a useful complement to other supplier standards such as API, and so is typically used in combination.
The hydrogen sulphide threshold limits have been established based upon a combination of real-world experience and laboratory testing. The standard has evolved over time, such that both the operating limits and the material requirements will vary quite considerably. The standard also provides guidance for the selection and specification of materials when the hydrogen sulphide thresholds are exceeded. This allows for ‘fit-for-purpose’ testing to be undertaken in order to qualify grades for a particular application.
NACE MR0175 is now published and maintained as ISO 15156-3.
4) AMS 5662 and AMS 5663.
AMS stands for ‘Aerospace Materials Specification’, and covers a range of standards maintained by SAE (Society of Automotive Engineers) in the United States, which despite their name cover applications in aerospace as well as automotive and commercial vehicles.
AMS 5662 is the base specification for Alloy 718 typically used in the aerospace market (‘Nickel Alloy, Corrosion and Heat Resistant, Bars, Forgings, and Rings’). It covers the product in the solution annealed condition only, and so is relatively low strength and low hardness (20-25 HRC) condition compared with the more common solution annealed and aged condition.
It is therefore much easier to machine than Alloy 718 in the aged condition, where the strength and hardness are increased significantly. For some components, it is desirable to machine, or at least proof machine, the metal in this condition before heat treatment to achieve the desired mechanical properties and then complete finish machining.
AMS 5662 is commonly supplied with a ‘capability test’ to demonstrate that, with appropriate heat treatment temperatures and times, it can meet the requirements of specification AMS 5663.
AMS 5663 takes the same product as AMS 5662, but controlled heat treatments known as ageing achieve significantly increased mechanical properties. The mechanical properties are not dissimilar to API 6A CRA.
Inconel is a trademark of the Special Metals Corporation group of companies.
Hastelloy is a trademark of Haynes International Inc.