What Makes an Alloy Nickel Based?
A nickel-based alloy is one in which nickel is the largest single element, normally 50 percent or more by weight. Chromium, molybdenum, iron, niobium, aluminium and titanium are then added to develop the strength, oxidation resistance and corrosion resistance demanded by a particular service. The nickel-rich matrix is austenitic and non-magnetic in the annealed condition, and it stays ductile over a very wide temperature range, from cryogenic duty up to furnace temperatures.
Because the matrix itself is tough and resistant to many chemical media, the alloying elements act as tuning tools. Chromium builds the protective oxide scale, molybdenum and niobium raise strength and resistance to pitting and crevice attack, and aluminium adds a second, very stable oxide for high-temperature duty. The three grades below share the same design philosophy but are tuned to different service conditions.
UNS N06625: Nickel-Chromium-Molybdenum
UNS N06625, widely known as alloy 625, is a niobium-stabilised nickel-chromium-molybdenum grade. Niobium plus tantalum forms a coherent second phase that gives the annealed material an unusually high yield strength for a solid-solution alloy.
Nickel: 58.0 percent minimum
Chromium: 20.0-23.0 percent
Molybdenum: 8.0-10.0 percent
Niobium plus tantalum: 3.15-4.15 percent
Iron: 5.0 percent maximum
Carbon: 0.10 percent maximum
Common product standards are ASTM B443 for plate, sheet and strip, ASTM B446 for rod and bar, ASTM B444 for seamless pipe and tube, ASTM B704 and ASTM B705 for welded tube and pipe, ASTM B366 for fittings and ASTM B564 for forgings. The grade is selected for seawater and chloride-bearing piping, chemical process equipment, flue gas desulphurisation ducting and scrubbers, sour gas components and weld overlay on carbon steel.
UNS N06600: High-Nickel Nickel-Chromium
UNS N06600, commonly called alloy 600, contains about 72 percent nickel with 14 to 17 percent chromium and 6 to 10 percent iron. It is not precipitation hardened, so strength comes from the nickel matrix, from work hardening and from grain size control.
Nickel: 72.0 percent minimum
Chromium: 14.0-17.0 percent
Iron: 6.0-10.0 percent
Carbon: 0.15 percent maximum
Manganese: 1.0 percent maximum
Silicon: 0.5 percent maximum
The very high nickel content delivers outstanding resistance to caustic alkalies and to high-purity water, including resistance to chloride stress corrosion cracking, while chromium provides resistance to oxidising media and to high-temperature oxidation. Typical standards are ASTM B168 for plate, sheet and strip, ASTM B166 for rod and bar, ASTM B167 for seamless pipe and tube, ASTM B516 and ASTM B517 for welded tube, ASTM B366 for fittings and ASTM B564 for forgings. Heat treating equipment, furnace components, nuclear and power plant parts, and instrumentation are the classic applications.
UNS N06601: Nickel-Chromium with Aluminium
UNS N06601, usually written as alloy 601, contains about 58 to 63 percent nickel, 21 to 25 percent chromium and roughly 1.0 to 1.7 percent aluminium. The combination of chromium and aluminium forms a dense, tightly adherent oxide scale that resists oxidation, carburisation and scaling at high temperature.
Nickel: 58.0-63.0 percent
Chromium: 21.0-25.0 percent
Aluminium: 1.0-1.7 percent
Carbon: 0.10 percent maximum
Iron: balance
Product forms are covered by ASTM B168 for plate, sheet and strip, ASTM B166 for rod and bar, ASTM B167 for seamless pipe and tube, ASTM B516 and ASTM B517 for welded tube, and ASTM B564 for forgings. Applications include furnace internals, radiant tubes, retorts, combustion components and other equipment exposed to strongly oxidising or carburising high-temperature atmospheres.
How to Select Between N06625, N06600 and N06601
Start from the dominant service condition rather than from a habit or an existing bill of materials. The table below summarises the decision.
| Grade | Primary strength | Choose it when |
|---|---|---|
| UNS N06625 | Strength plus chloride and acid resistance | Seawater, chlorides, mixed acids, flue gas, weld overlay |
| UNS N06600 | Caustic and high-purity water resistance | Caustic service, high-purity water, elevated temperature, nuclear and power plant parts |
| UNS N06601 | High-temperature oxidation and carburisation resistance | Furnace internals, radiant tubes, combustion equipment in strongly oxidising atmospheres |
Confirm the product form and the applicable specification before ordering, because each grade is covered by separate standards for plate, sheet, bar, pipe, tube and fittings, and the mechanical property requirements differ between them. State the UNS number on the enquiry so that similar trade names cannot be confused.
Frequently Asked Questions
Q: What is a nickel-based alloy?
An alloy in which nickel is the largest single element, usually 50 percent or more by weight, with other elements added to develop specific mechanical and corrosion properties.
Q: What is UNS N06625 used for?
Seawater and chloride service, chemical process and flue gas equipment, sour gas components and weld overlay, because it combines high strength with resistance to pitting, crevice corrosion and chloride stress corrosion cracking.
Q: What is special about UNS N06600?
Its very high nickel content of about 72 percent gives outstanding resistance to caustic alkalies and high-purity water, while the chromium addition provides oxidation resistance.
Q: What is UNS N06601 used for?
High-temperature furnace internals, radiant tubes, retorts and combustion components exposed to strongly oxidising or carburising atmospheres.
Q: How do I choose between N06625, N06600 and N06601?
Choose N06625 for chloride and mixed acid duty, N06600 for caustic and high-purity water service at elevated temperature, and N06601 for high-temperature oxidation and carburisation resistance.
