1.4980 | X6NiCrTiMoVB25-15-2 | UNS S66286 | Alloy 660 | Alloy A286 | AN5

Alloy 660 is the internationally common designation for an austenitic, precipitation-hardenable iron-nickel-chromium alloy. This material also appears in data sheets and orders under the following designations: 1.4980 (material number according to DIN/EN), X6NiCrTiMoVB25-15-2 (chemical short designation), UNS S66286 (US nomenclature) as well as A-286 or A286 (historically established US name, very widespread). At Valbruna the material is listed under the internal designation AN5. Properties can be found, among others, in the following standards: AMS 5731 / 5732 / 5737 / 5525 (aerospace sector), ASTM A453 / A638 and DIN EN 10269.

Alloy 660 is an austenitic, non-magnetic material – similar to a high-alloy stainless steel. The decisive difference, however, is precipitation hardenability: through heat treatment (solution annealing followed by aging at around 720 °C) titanium and aluminum form γ′ precipitates. These increase the strength even at higher temperatures. Compared with the grades of the Alloy 800 family, Alloy 660 is preferred when, in addition to high-temperature strength, high static and dynamic strength – typical for fasteners and rotating components – is also required.

Reference values as mass fractions: iron (Fe) balance (typ. ≈ 53 %), nickel (Ni) 24.0–27.0 %, chromium (Cr) 13.5–16.0 %, molybdenum (Mo) 1.0–1.5 %, titanium (Ti) 1.9–2.35 %, aluminum (Al) up to 0.35 %, vanadium (V) 0.1–0.5 %, boron (B) 0.003–0.010 %, manganese (Mn) up to 2.0 %, silicon (Si) up to 1.0 %, carbon (C) up to 0.08 %, sulfur (S) and phosphorus (P) each limited to low levels. Notable are the Ti, Al and V contents: they are set so that stable precipitates form during aging up to high temperatures. The addition of small amounts of boron additionally improves creep strength by stabilizing the grain boundaries.

In the hardened condition (solution annealing + aging), 1.4980 as bar or wire material typically achieves a tensile strength of 900–1,100 MPa, a 0.2% proof stress from 650 MPa and an elongation at break of about 15 % – values significantly above those of classic austenitic stainless steels. The actual hallmark, however, is the high-temperature strength: these high values are largely retained up to approx. 700 °C without the microstructure softening or embrittling. For high-temperature applications with long service lives, the well-documented stress-rupture and creep strength is the decisive design parameter. Added to this is a very low magnetic permeability, which also makes Alloy 660 attractive for magnetically sensitive components.

The standard heat treatment consists of two stages: 1. Solution annealing at around 980 °C (typically 900–980 °C) followed by rapid cooling (water or oil). This dissolves the hardening elements in the matrix and establishes the ductile initial microstructure. 2. Aging at approx. 720 °C for about 16 hours with controlled cooling. During this step the γ′ phase precipitates and produces the high strength.

The alloy is a classic wherever high mechanical stress and high temperatures act at the same time – especially in rotating components and in fasteners. Common fields of application: high-strength screws, bolts and stud bolts in gas and steam turbines, jet engines, combustion engines, turbochargers and motorsport; turbine disks, shafts and compressor components in aerospace and industrial gas turbines; springs and spring washers for high-temperature applications; components in exhaust and flue gas systems – especially in the high-performance and racing sector; structural components in rocket and aerospace technology. For fasteners, Alloy 660 is in many specifications the only approved alloy for use above about 500 °C, when classic bolt materials such as 1.7225 (42CrMo4) or 21CrMoV5-7 lose their suitability.

With around 15 % chromium and 25 % nickel, Alloy 660 offers solid corrosion resistance – comparable to the better austenitic stainless steels and significantly better than typical heat-resistant ferritic steels. In oxidizing atmospheres a stable chromium oxide layer forms, which reliably protects up to approx. 700 °C. In aqueous, neutral and weakly acidic media the resistance is good. The material reaches its limits in strongly reducing acids and in chloride-containing hot water applications, where stress corrosion cracking can be an issue – here higher-alloyed nickel-based materials are preferable. Important: corrosion and strength requirements should be considered together, since the material is rarely chosen for its corrosion properties alone.

Machining: Alloy 660 is difficult to machine in the hardened condition – the heat treatment and machining sequence should therefore be planned together. Ideally, the main machining is carried out in the solution-annealed condition, with finishing only after aging. Recommendations: stable machines, sharp/wear-resistant carbide or ceramic tools, low cutting speeds, high feed rates, continuous cooling. Welding: the alloy can be processed with the fusion welding processes TIG and MIG/MAG; as filler material for welding wire and wire electrodes, a NiCr variant such as ERNiCr-3 / SNi6082 is usually used. Welding should be carried out where possible in the solution-annealed condition, followed by a solution and aging treatment of the entire component. The hot-cracking tendency must be taken into account when selecting welding parameters. Ordering: Valbruna supplies 1.4980 under the internal designation AN5 as bar (round, hexagonal, square), as wire rod and drawn wire as well as drawn profile. For aerospace- and turbine-relevant applications the relevant AMS standards (e.g. AMS 5731, 5732, 5737, 5525) can be covered on request; the specification requirement should always be explicitly stated in the inquiry, as well as the required heat treatment (solution-annealed or hardened) and – for fasteners – the required property class or specification.