Product Focus – INCONEL alloy 725
If you have a project that requires superior high strength alongside excellent corrosion resistance, you need to take a look at INCONEL alloy 725.
As a nickel alloy stockist, Corrotherm stocks a number of different Grades that offer exceptional corrosion resistance. One of the most popular alloys that we stock is INCONEL alloy 625. We have already featured this particular alloy in a previous product focus, which is known for its high strength and versatility. However, if you need something with even more strength, then have a look at INCONEL alloy 725 instead.
INCONEL alloy 725 has plenty in common with INCONEL alloy 625. Both alloys are perfect for use in severely corrosive environments, with similar corrosion resistance properties. They are both nickel-chromium alloys, which provide resistance in both reducing and oxidising situations. They both include a substantial amount of molybdenum, which gives improved resistance to pitting and crevice corrosion. The elements combine to make both the alloys resistant to hydrogen embrittlement and stress-corrosion cracking.
There are, however, key differences between these two alloys. INCONEL alloy 725 has 2.75-4.00% niobium and 1.00-1.70% titanium. It’s also age hardenable to approximately twice the strength of annealed INCONEL alloy 625. We look in more detail at the differences between INCONEL alloy 625 and INCONEL alloy 725 in this blog.
High-strength properties
The strength of INCONEL alloy 725 is developed by heat treatment, which means it remains both ductile and tough. It’s also very versatile, being applied to large and non-uniform sections that can’t be strengthened by cold working.
The alloy is strengthened by precipitation of gamma double-prime phase during an ageing heat treatment. In order to ensure the high strength is maintained, there are optimum temperatures to comply with. Before it’s aged, the alloy needs to be given a solution anneal at 1040°C (1900°F) and then air cooled. If the product is going to be used for sour gas applications, the recommended ageing treatment is 730°C (1350°F) before following with appropriate cooling methods.
As it’s a high-strength product, INCONEL alloy 725 can also be hot formed more easily; it’s more resistant than most against deformation during hot working. It can also be machined in both its annealed and aged conditions, making it a versatile alloy across a number of different applications.
For an even-higher strength version of this alloy, there is INCONEL alloy 725HS. This alloy variant still retains the corrosion resistance of its base alloy, yet is significantly stronger. In age-hardened condition, it’s around 2.3 times stronger than annealed INCONEL alloy 625. It also has around 20ksi (128 MPa) higher yield strength than INCONEL alloy 725. This extraordinary strength is achieved through grain refinement and optimising the heat treatment process.
Corrosion resistance
Due to the high levels of nickel, chromium and molybdenum in INCONEL alloy 725, the material is put to great use in a number of corrosive environments.
For example, in highly volatile sour gas conditions, it is shown to resist corrosion, pitting, hydrogen embrittlement and stress-corrosion cracking. It exhibits good resistance to stress-corrosion cracking in temperatures of up to around 230°C (450°F) in a severe sour environment. The alloy is also an excellent choice for marine use, as it has good resistance in both brine and seawater against general and localised corrosion.
Despite having a broadly similar level of corrosion resistance as INCONEL alloy 625, tests show that INCONEL alloy 725 is actually superior when it comes to resistance to crevice corrosion initiation in seawater.
Key applications of INCONEL alloy 725
The useful blend of outstanding corrosion resistance and extremely high strength, means this alloy has a number of potential applications. One application is within the sour gas industry.
The alloy is approved under NACE MR0175 for use in sour gas wells. NACE MR0175 is a document with recommendations around environmental threshold limits for use in the petroleum and natural gas industries. The alloy’s ability to resist the effects of hydrogen sulfide, chlorides and carbon dioxide make it a perfect material of choice for this industry. As such, the alloy is used for such components as hangars, landing nipples, side pocket mandrels and polished bore receptacles in sour gas service.
Thanks to its resistance to seawater, INCONEL alloy 725 has numerous applications within the marine industry. It’s used in naval and submarine systems, for example. It can also be used for high-strength fasteners. Before INCONEL alloy 725, MONEL alloy K-500 was widely used for similar marine applications and within the US Navy.
However, some problems around hydrogen embrittlement and accelerated corrosion have been linked to potential component failure. Likewise, INCONEL alloy 718 has also been used by marine engineers, but it has only moderate resistance to corrosion in these seawater conditions. INCONEL alloy 725 overcomes these issues and is now a more commonly used standard material of construction.
The aforementioned INCONEL alloy 725HS variant can be used for many of the same applications as its base alloy. However, it is particularly useful for components in ship construction and marine fabrication where the extra strength is required. Its inherent strength, along with its resistant to corrosion by seawater, makes it an optimal choice for fasteners, for example.
For detailed technical specifications of this alloy, see the website. If you have a project you think would benefit from INCONEL alloy 725, please contact a member of our sales team.
