Grades
Alloy 625
UNS N06625Alloy 625 is a nickel-chromium-molybdenum alloy with excellent corrosion resistance from sub-zero to 980°C and high strength.
Alloy 718
UNS N07718Alloy 718 is a precipitation-hardened nickel-chromium superalloy that keeps high strength and corrosion resistance to about 750°C.
Alloy 725
UNS N07725Alloy 725 is an age-hardened nickel-chromium alloy with roughly twice the strength of Alloy 625, approved for sour service.
Alloy 825
UNS N08825Alloy 825 is a nickel-iron-chromium alloy resisting reducing and oxidising acids and chloride stress-corrosion cracking.
Alloy 925
UNS N09925Alloy 925 is an age-hardened nickel-iron-chromium alloy combining very high strength with corrosion resistance in sour service.
Alloy K-500
UNS N05500Alloy K-500 is an age-hardened nickel-copper alloy combining very high strength with excellent seawater corrosion resistance.
How the grades compare
At a glance: UNS number, defining property, typical applications and the forms we stock for each nickel alloys grade.
| Grade | UNS | Key property | Typical applications | Forms stocked |
|---|---|---|---|---|
| Alloy 625 (Inconel® 625) | N06625 | Solid-solution strength with broad corrosion resistance to 800°C | Wellhead components, sheathing, propeller shafts, chemical plant | Bar |
| Alloy 718 (Inconel® 718) | N07718 | Age-hardened high strength, cryogenic to 650-750°C | Wellheads, gate valves, aerospace, power-gen fasteners | Bar |
| Alloy 725 (Inconel® 725) | N07725 | Age-hardened, ~2x the strength of annealed 625; NACE sour-service | Downhole equipment, hangers, marine fasteners | Bar |
| Alloy K500 (Monel® K500) | N05500 | Precipitation-hardened Ni-Cu, high strength, low magnetic permeability | Pump and propeller shafts, valve stems, fasteners | Bar |
| Alloy 825 (Incoloy® 825) | N08825 | Ni-Fe-Cr resisting both reducing and oxidising acids; SCC resistance | Chemical process, scrubbers, downhole | Bar, Tube |
| Alloy 925 (Incoloy® 925) | N09925 | Precipitation-hardened 825 chemistry for high-strength sour service | Downhole valves, hangers, packers, pump shafts | Bar |
Why specify nickel alloys?
Nickel alloys are chosen when standard grades cannot cope with the temperature, the environment, or the required strength. Across the range you get:
- Outstanding corrosion resistance across severe environments, including seawater, industrial acids and sour (H2S) service
- Strength retained from cryogenic temperatures up to 800°C and beyond, depending on grade
- Excellent resistance to stress corrosion cracking, including chloride and sulphide stress cracking
- Sour-service grades approved under NACE MR0175 / ISO 15156, notably Alloy 725 and Alloy 925
- Age-hardenable grades (718, 725, 925, K500) deliver very high strength while keeping good ductility and toughness
- Non-magnetic and spark-resistant options (625, 718), with low magnetic permeability from K500
Material tools
PREN calculator
PREN = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N. Guidance only; always confirm against the grade's datasheet before specifying.
Sour-service screen (NACE MR0175)
Sour-Service Screen
Work out whether a service is sour from the H2S partial pressure, and see each alloy's NACE MR0175 / ISO 15156 status and hardness limit. A first-pass screen for oil & gas material selection - the full environmental envelope for a grade is set in ISO 15156-3 Annex A.
Candidate grade
| Grade | Family | NACE status | Hardness limit |
|---|
First-pass screen only. This shows whether the service is sour (from H2S partial pressure) and each material group's NACE MR0175 hardness limit and general suitability. It does not reproduce the full environmental windows (maximum H2S partial pressure, temperature, chloride, in-situ pH per grade) - those are in ISO 15156-3 Annex A and must be checked there for the specific grade, product form and condition. Qualification also depends on heat treatment, cold work and welding.
Hardness converter
Hardness Converter
Convert between Brinell, Vickers, Rockwell C and Rockwell B, with the approximate tensile strength. Values follow the ASTM A370 conversion tables for non-austenitic steels; nothing is extrapolated beyond the tabulated range.
Conversions are approximate and material-dependent. Based on ASTM A370 hardness conversion tables for non-austenitic steels (equivalent to ASTM E140). Brinell is not reported above about HRC 65, and each Rockwell scale is only valid over its own range - values outside it show as out of range, never extrapolated. Use as a guide, not a substitute for direct measurement on the actual material.
Applications
How do you machine nickel alloys?
Nickel alloys are tougher and more work-hardening than carbon or stainless steels, so they reward a rigid, deliberate approach. Use rigid set-ups and sharp, positive-rake tooling, keep the cutting edge continuously engaged to avoid work-hardening the surface, and run lower cutting speeds with a generous, constant feed and plenty of coolant.
Age-hardened grades such as Alloy 718, 725, 925 and K500 are harder and more abrasive to cut than the annealed grades like 625 and 825, and generally need reduced speeds and robust carbide tooling. Wherever possible, carry out heavy machining before final age-hardening and finish to size afterwards.
Technical resources
Common questions
What are nickel alloys?
Nickel-based alloys have nickel as the main balancing element. Copper is fully soluble in nickel, and iron and chromium over wide ranges, which allows many different alloys to be developed. They combine excellent temperature and corrosion resistance, so they are chosen for the most aggressive environments. The best-known families are marketed under trade names such as Inconel, Incoloy and Monel.
Do nickel alloys corrode?
Yes, nickel alloys can corrode in sufficiently aggressive environments, just as stainless steels do. Different grades offer different levels and types of corrosion resistance, and elements such as copper and molybdenum are added to target specific conditions.
Can nickel alloys be used at high and low temperatures?
Yes. Nickel alloys keep useful properties over a far wider temperature range than most stainless steels. At elevated temperatures they resist oxidation and hold strength - Alloy 718, for example, retains most of its room-temperature strength to around 650°C - and at cryogenic temperatures they keep their toughness rather than becoming brittle.
What is the difference between Inconel, Incoloy and Monel?
All three are trade names of Special Metals Corporation, now used generically across the industry. Inconel refers to nickel-chromium alloys (Alloys 625, 718, 725), Incoloy to nickel-iron-chromium alloys (Alloys 825, 925), and Monel to nickel-copper alloys (Alloys 400, K500).
What is the difference between Alloy 625 and Alloy 725?
The two are nearly identical in composition and give very similar corrosion resistance, but Alloy 725 is precipitation-strengthened: small additions form fine precipitates during an extended heat treatment that roughly double its yield strength.
What is precipitation hardening?
Precipitation hardening (also called ageing or age hardening) is a heat treatment that greatly increases an alloy's yield strength. After solution annealing, the alloy is aged so microscopic precipitates form throughout the metal, raising strength and hardness while retaining ductility.
History of nickel alloys
The nickel alloy story begins with Monel. In the early 1900s, the International Nickel Company refined a strong, corrosion-resistant nickel-copper alloy from the ores of Sudbury, Ontario, and named it after its chairman, Ambrose Monell. An age-hardenable version, Monel K-500, followed, adding aluminium and titanium for much higher strength while keeping the alloy's excellent seawater resistance.
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The search for high-temperature strength drove the next chapter. International Nickel introduced its Inconel nickel-chromium alloys in the 1930s, and by the 1940s nickel-chromium alloys had become essential to the first jet engines, where materials had to hold their strength in the heat of the turbine. The family grew: solid-solution Inconel 625, conceived in the late 1950s for high-pressure steam and later widely used in aerospace and seawater service, and age-hardenable Inconel 718, developed by Herbert Eiselstein around 1960, whose niobium hardening and good weldability made it one of the most widely used superalloys ever produced.
As industry moved offshore and downhole, the nickel-iron-chromium Incoloy grades followed. Incoloy 825 brought outstanding resistance to acids, and the age-hardenable Incoloy 925 added high strength for sour oil and gas service. Today these alloys remain the benchmark for the most punishing environments. Langley stocks the full nickel family, from Alloy 625, 718 and 725 to Alloy 825, 925 and Monel K-500, for aerospace, subsea and marine engineers worldwide.
From sub-zero to 800°C, in seawater, sour wells and acid plant, nickel alloys perform where standard grades fail.
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