What Is Alloy 2.4669 (nickel alloy UNS N06625)?
Alloy 2.4669 is the European material number for NiCr22Mo9Nb, a nickel-chromium-molybdenum-niobium superalloy known globally as Alloy 625 and by UNS designation N06625. It is a nickel-based alloy rather than a stainless steel, and it is produced in plate, pipe, tube, bar, and forging forms for service where ordinary austenitic grades cannot survive. The balanced composition creates a fully austenitic, solution-strengthened microstructure with high strength from cryogenic temperatures up to about 1200°F (649°C) in continuous service, combined with outstanding resistance to pitting, crevice corrosion, and stress corrosion cracking in chloride-rich environments.
Chemical Composition and Mechanical Properties
The nominal composition of Alloy 2.4669 is nickel approximately 58% minimum as balance, chromium 20.00-23.00%, molybdenum 8.00-10.00%, and niobium (columbium) 3.15-4.15%, with iron 5.0% maximum and small additions of carbon, manganese, silicon, phosphorus, and sulfur. Chromium forms a protective chromium oxide scale for oxidation resistance; molybdenum and niobium raise resistance to localized attack in reducing and chloride media; and niobium also stabilizes carbides and contributes solid-solution strengthening. The annealed product is essentially non-magnetic.
| Property (annealed) | Typical minimum / value |
| Yield strength | 60 ksi (415 MPa) |
| Tensile strength | 120 ksi (827 MPa) |
| Elongation | 30% |
| Hardness | About 200 HB |
| PREN (pitting resistance) | Above 50 |
The values shown reflect typical minimums defined by ASTM B443 for annealed plate; actual figures depend on product form, section size, and heat treatment. Compared with 6% molybdenum stainless steel such as UNS S31254 (minimum yield strength around 45 ksi or 310 MPa per ASTM A240, PREN above 43), Alloy 2.4669 offers higher strength, broader resistance to reducing acids such as sulfuric and hydrochloric, and a substantially higher maximum service temperature.
Standards and Specifications
Alloy 2.4669 is covered by a well-established set of international standards. Under ASTM, plate, sheet, and strip are supplied to ASTM B443; seamless pipe and tube to ASTM B444; seamless and welded pipe and tube for general corrosion service to ASTM B829; bar and wire to ASTM B446; and forgings to ASTM B564. The same products are available with ASME Code designations SB-443, SB-444, SB-446, and SB-564 for pressure equipment. In Europe the material is designated NiCr22Mo9Nb with material number 2.4669; the American equivalent is UNS N06625. These equivalent designations all refer to the same base composition.
The standard delivery condition for both plates and pipes is solution annealed. Material is heated to 1700-1850°F (927-1010°C), held long enough to take the hardening elements into solid solution, and then rapidly cooled, typically by water quenching. The resulting single-phase, homogeneous microstructure delivers maximum corrosion resistance, good ductility and toughness, and good formability. If a full re-solution treatment is ever required after severe cold work or as a stress-relief measure, annealing at about 2100°F (1149°C) followed by rapid quenching is used.
Applications of 2.4669 Plates and Pipes
Typical applications include chemical processing vessels, reactors, heat exchangers, and columns handling sulfuric, phosphoric, and hydrochloric acids as well as chloride media; marine and offshore components such as subsea manifolds, seawater piping, and structural parts exposed to seawater; flue gas desulfurization scrubbers, ducting, and dampers; aerospace and defense components including engine mounts and thrust reverser systems; and nuclear reactor internals where dimensional stability under radiation and elevated temperature is essential.
Welding and Fabrication Considerations
Alloy 2.4669 is among the more weldable nickel superalloys, but success depends on disciplined procedure. Use low heat input with stringer beads rather than wide weaving to minimize the time the weldment spends in the crack-sensitive temperature range, and design joints to limit restraint and avoid hot cracking. Select an overmatching nickel-chromium-molybdenum filler, typically ERNiCrMo-3, so the weld deposit retains the corrosion resistance and strength of the base metal. Control interpass temperature and ensure rapid cooling through the 1200-1600°F (649-871°C) range to prevent sensitization. Post-weld heat treatment is generally not required and can be detrimental; if stress relief is mandatory, use a full solution anneal followed by rapid quenching. Keep joint surfaces free of oil, grease, paint, and marking compounds, because the alloy is sensitive to contamination by lead, sulfur, and phosphorus.
Frequently Asked Questions
Is 2.4669 the same as nickel alloy UNS N06625? Yes. 2.4669 is the EN material number, NiCr22Mo9Nb is the EN designation, and UNS N06625 and Alloy 625 are the American designations for the same nickel-chromium-molybdenum-niobium base composition.
What heat treatment does Alloy 2.4669 require? The standard delivery condition is solution annealed: heat to 1700-1850°F (927-1010°C), hold, then quench rapidly. Post-weld heat treatment is generally not required; if performed, it must be a full re-solution anneal, otherwise it can harm corrosion resistance.
Can 2.4669 replace 6% molybdenum stainless steel such as UNS S31254? It depends on the service. Alloy 2.4669 is stronger, resists a broader range of acids including reducing acids, and can serve at higher temperatures (continuous service near 1200°F, 649°C), but it costs more. Where the environment is mild, 6Mo stainless steel is a more economical choice.
Which filler metal is used to weld 2.4669? ERNiCrMo-3 is the standard choice. This nickel-chromium-molybdenum-niobium filler matches or exceeds the base metal in corrosion resistance and strength, protecting the weld deposit from preferential attack.
What is the maximum service temperature of 2.4669? For continuous service the alloy is commonly rated up to about 1200°F (649°C), with higher temperatures possible for short periods. Designers should verify the exact limit against the governing code and the actual load conditions.
Is Alloy 2.4669 magnetic? In the solution-annealed condition the microstructure is fully austenitic and essentially non-magnetic. Cold work may cause slight magnetic response, as with other austenitic nickel alloys.
