INCOLOY Alloy 825 – stabilised for maximum performance

INCOLOY alloy 825 is a material that has been designed to last, even under extreme operating conditions. This powerful nickel alloy is considered the forerunner of alloys containing molybdenum, the inclusion of which lends the material additional strength.

Due to its hardiness, it is widely used in a number of volatile environments, including in chemical processing, pollution control equipment and the oil and gas industry. It is noted for its excellent corrosion resistance, which is part of the reason it is adopted in so many different industries.

The other reason that this material is such a popular choice among design engineers is that it is worked in such a way that it is stabilised for maximum performance. Through the years the process has been optimised and perfected.

Treatment and processing

While INCOLOY alloy 825 was introduced to the market over 50 years ago, questions still abound about the proper methods for fabricating, welding and heat treating the alloy.

Despite the fact that the alloy is a conventional iron-nickel-chromium-molybdenum alloy, it requires special attention to detail during processing to ensure that the product develops the optimum metallurgical structure, mechanical properties and corrosion resistance for the given application.

For optimum properties, the alloy should enter service in a stabilised condition. Proper mechanical processing and heat treatment are required to produce the microstructure required for optimum corrosion resistance.

Nickel-chromium-molybdenum alloys such as INCOLOY alloy 825 are often solution-annealed at a high temperature prior to service. However, stabilised alloys offer optimum corrosion resistance after annealing at a lower temperature. This process is commonly referred to as ‘stabilisation anneal’. This is the standard condition of supply for INCOLOY alloy 825 products and is the key to successful fabrication of the alloy. However carefully the material is formed and welded, if the material has not been correctly stabilised it will not give maximum performance.

Choosing the right annealing temperature for the intended end use is key here. It helps to maintain the high corrosion resistance that is required, for example, in the chemical processing industry. During the final stages of working and annealing, the temperature should be limited to 1700-1800°F (930-980°C). The optimum temperature for stablisiation is considered to be 1725°F (940°C).

The stabilising anneal heat treatment used for the alloy results in the optimum performance when unstabilised alloy is exposed to intergranular grain boundary attack.

Mechanical properties

The point of all this processing and optimising in order to stabilise the alloy is to retain its impressive mechanical properties. These are what make it such an attractive material to use, especially considering that it can operate in temperatures from cryogenic to moderately high. There are limitations, of course, to consider to ensure optimum performance. For example, when exposed to temperatures of more than 1000°F (540°C), the alloy may undergo microstructural change (phase formation) that significantly impacts the ductility and impact strength. As such, this alloy is not usually selected as material of choice for service environments where temperatures will exceed this and where creep-rupture properties are a key factor in the design spec. However, given the right operating conditions to suit this material, INCOLOY alloy 825 has a lot to offer. It can be strengthened quite substantially by cold working. The ultimate tensile strength of the material has been recorded as 104,500 psi, and it exhibits good impact strength at room temperature while also retaining its strength in cryogenic conditions.

Industry applications

A stabilised and corrosion-resistant alloy such as INCOLOY alloy 825 lends itself to many end uses where its mechanical properties are highly desired.

In particular, in the chemical processing industry, material selection can be especially tricky. An alloy that is stabilised for maximum performance is essential to ensure that it can hold up to the intense pressure, corrosive environments and high temperatures that are commonplace in such difficult service conditions.

The chemical composition of the alloy is what makes it perfect for use in the processing industry. INCOLOY alloy 825 is stabilised by the carbon content being limited to 0.05% max. The alloy is further stabilised by its content of 0.6 to 1.2% titanium. These elements, plus the major elements of nickel, iron, copper and molybdenum, give the material very good corrosion resistance to many corrosive chemicals used routinely in industry. This includes sulfuric and phosphoric acids, and both oxidising and reducing substances.

To find out more about INCOLOY alloy 825 and whether it’s right for your next project, please Contact Us and speak to a member of our experienced Sales team.