Alloy 625 is a nickel-chromium-molybdenum alloy used for its high strength, toughness and excellent corrosion resistance. It derives its strength from the hardening effect of molybdenum and niobium on a nickel-chromium matrix. The grade was developed for high-temperature strength, yet its highly alloyed composition also provides remarkable general corrosion resistance, which is why it serves both heat-resisting and corrosion-resisting duties. It is designated UNS N06625 and Werkstoff 2.4856.
| Element | Content (%) |
|---|---|
| Nickel | 58.0 min |
| Chromium | 20.0-23.0 |
| Molybdenum | 8.0-10.0 |
| Niobium plus tantalum | 3.15-4.15 |
| Iron | 5.0 max |
| Cobalt | 1.0 max |
| Manganese | 0.50 max |
| Silicon | 0.50 max |
| Aluminium | 0.40 max |
| Titanium | 0.40 max |
| Carbon | 0.10 max |
Industries and Applications
Alloy 625 is used in a variety of industries including automotive, marine, aerospace, oil and gas, chemical processing and nuclear energy. Typical end-use applications include heat exchangers, bellows, expansion joints, exhaust systems, fasteners, quick-connect fittings and many other components that need strength together with resistance to corrosive environments. In marine and offshore service the alloy is selected for seawater-exposed piping, sheathing and subsea hardware, while process plants use it for equipment handling mixed acids and chloride-bearing streams.
Corrosion Resistance
Alloy 625 has good resistance to oxidation and scaling at elevated temperatures. At 1800 degF (982 degC) scaling resistance becomes an important factor in service, and the grade outperforms many other high-temperature alloys under cyclic heating and cooling conditions. The combination of alloying elements allows it to withstand a wide range of severe corrosive environments, and it is virtually unaffected by mild environments such as fresh water and seawater, neutral pH environments and alkaline media. The chromium content gives excellent resistance to oxidising environments, while the high molybdenum content makes the alloy very resistant to pitting and crevice corrosion in chloride-bearing solutions.
Fabrication and Heat Treating
Alloy 625 can be formed using a variety of cold and hot working processes. The alloy resists deformation at hot working temperatures, so higher loads are required to form the material, and hot forming should be carried out in the 1700 degF to 2150 degF (927 degC to 1177 degC) range. During cold working the material work hardens faster than conventional austenitic stainless steels, so intermediate annealing may be needed. Three heat treatment routes are available:
Solution anneal at 2000-2200 degF (1093-1204 degC), air quench or faster
Anneal at 1600-1900 degF (871-1038 degC), air quench or faster
Stress relief at 1100-1500 degF (593-816 degC), air quench
Solution annealed material, often called Grade 2, is typically used above 1500 degF for applications where creep resistance is important. Soft annealed material, Grade 1, is typically used at lower temperatures and offers the best combination of tensile and fracture properties. Because the grade hardens only through solid-solution and precipitation effects rather than a martensitic reaction, it is not normally hardened by quenching.
Specifications and Product Forms
| Product | Common specification |
|---|---|
| Plate, sheet and strip | ASTM B443 / ASME SB443 |
| Rod, bar and wire | ASTM B446 / ASME SB446 |
| Seamless and welded pipe and tube | ASTM B444 and B704 / B705, ASME SB444 and SB704 / SB705 |
| Forgings | ASTM B564 / ASME SB564 |
| Fittings | ASTM B366 / ASME SB366 |
| Sour service | NACE MR0175 / ISO 15156 |
Standard forms include plate, sheet, strip, round bar, flat bar, hexagon bar, seamless tube, welded tube, pipe, fittings, forgings, wire and finished fasteners.
Frequently Asked Questions
Q: What is Alloy 625 steel grade?
It is a nickel-chromium-molybdenum alloy with niobium, designated UNS N06625 and Werkstoff 2.4856, used for high strength and corrosion resistance.
Q: Is Alloy 625 a stainless steel?
No. It is a nickel-based alloy rather than a stainless steel, although it is grouped with the corrosion-resistant alloys supplied in similar product forms.
Q: What is the difference between Grade 1 and Grade 2?
Grade 1 is soft annealed and used at lower temperatures for the best tensile and fracture properties; Grade 2 is solution annealed and used above 1500 degF where creep resistance matters.
Q: Can Alloy 625 be welded?
Yes. The alloy is readily welded by the common arc processes using matching nickel-chromium-molybdenum filler metal.
Q: What temperature range is used for hot forming?
Hot forming is carried out between 1700 degF and 2150 degF, and the alloy requires higher forming loads than ordinary stainless steels.
Q: Where is Alloy 625 most often specified?
Heat exchangers, bellows, expansion joints, exhaust systems, fasteners and seawater-exposed components in marine, chemical, nuclear and oil and gas service.
