Alloy 625, UNS N06625, 2.4856 - Nickel Alloy

Alloy 625, UNS N06625, Inconel 625, 2.4856, BS3075 NA21 according to UNS and ISO 14172

Alloy 625, UNS N06625, inconel 625 – description

Alloy 625 (UNS N06625) is a Ni-Cr-Mo alloy introduced in the 1960s. It contains a nominal 20% Cr, 9% Mo and 2.5% Fe and is stabilized with approximately 3.5% Nb.

It is a superalloy, meaning it can effectively be used as a corrosion-resistant, heat-resistant and creep-resistant material. It is resistant to severely corrosive environments, particularly to pitting, crevice corrosion and high-temperature oxidation, carburization and halogen attack. A popular choice for hostile high-temperature environments. It is very strong across a wide range of temperatures, from cryogenic up to 815°C.

Applications

Alloy 625 was developed for aircraft engines but have found wide use in other applications requiring high-temperature strength and corrosion resistance. Typically, it is used for aircraft engine and airframe components, chemical processing equipment (internal components of scrubbers, fans, ducting quenchers, absorber towers, dampers, stack gas reheaters, wet induced draft fans and fan housings, outlet ducting, and stack liners), ship and submarine parts, as well as nuclear reactors.

Inconel 625 - physical and mechanical properties

Physical properties at room temperature:

• Density: 8.44 g/cm³
• Thermal conductivity: 9.8 W/(m*K)
• Specific heat capacity: 410 J/(kg*K)
• Electrical resistivity: 1.29 μΩ*m
• Magnetic permeability: 1.006
• Solidus temperature: 1290 °C
• Liquidus temperature: 1350 °C
• Curie temperature: <-196 °C

Alloy 625's typical high-temperature tensile strength:

• In ambient temperature: 855 MPa
• 538°C: 745 MPa
• 649°C: 710 MPa
• 760°C: 505 MPa
• 871°C: 285 MPa

Incoloy 625's typical elevated-temperature yield strength:

• In ambient temperature: 490 MPa
• 538°C: 405 MPa
• 649°C: 420 MPa
• 760°C: 420 MPa
• 871°C: 375 MPa

Alloy 625's typical high-temperature elongation:

• In ambient temperature: 50 %
• 538°C: 50 %
• 649°C: 35 %
• 760°C: 42 %
• 871°C: 125 %

Typical 100 h stress-rupture strengths of solution treated Inconel 625:

• 649°C: 440 MPa
• 815°C: 130 MPa
• 871°C: 72 MPa
• 871°C: 34 MPa

Typical 1000 h stress-rupture strengths of solution treated Inconel 625:

• 649°C: 370 MPa
• 815°C: 93 MPa
• 871°C: 48 MPa
• 980°C: 20 MPa

Coefficient of thermal expansion and temperature (linear):

• 24-540°C: 14.0 μm/(m*K)
• 24-870°C: 15.8 μm/(m*K)

Thermal conductivity and temperature:

• In ambient temperature: 9.8 W/(m*K)
• 540°C: 17.5 W/(m*K)
• 870°C: 22.8 W/(m*K)

Charpy's impact energy of annealed Alloy 625 as-rolled plate:

• In ambient temperature: 66 J
• -80°C: 60 J
• -196°C: 47 J

Other room temperature mechanical properties:

• Young's modulus: 207 GPA
• Hardness: 190 HB

General corrosion behaviour

Alloy 625 is able to withstand a wide variety of corrosive environments. It is almost completely resistant to the atmosphere, fresh water, seawater, neutral salts and alkaline media. High nickel content means N06625 is resistant to chloride stress-corrosion cracking. Nickel and chromium provide resistance to oxidizing chemicals up to ~1050°C, and the combination of nickel and molybdenum gives resistance to reducing conditions. Molybdenum also makes it also highly resistant to pitting and crevice corrosion. The alloy is stabilized with niobium, which makes it resistant to sensitization and prevents intergranular corrosion.

Hydrochloric acid HCl - very good resistance to a wide range of HCl concentrations at ambient temperature.

• Acid concentration 5%; 66°C: 1.803 mm/year
• Acid concentration 10%; 66°C: 2.057 mm/year
• Acid concentration 15%; 66°C: 1.651 mm/year
• Acid concentration 20%; 66°C: 1.270 mm/year
• Acid concentration 25%; 66°C: 0.965 mm/year
• Acid concentration 30%; 66°C: 0.864 mm/year
• Acid concentration 37.1%; 66°C: 0.381 mm/year

In high-temperature hydrochloric acid HCl resistance tests alloy 625 is one of the very best performers, which is evident from small metal loss in the following tests:

• HCl 300 h; 400°C: 0.74 mg/cm²
• HCl 1000 h; 400°C: 1.1 mg/cm²
• HCl 100 h; 500°C: 2.42 mg/cm²
• HCl 300 h; 500°C: 3.78 mg/cm²
• HCl 1000 h; 500°C: 8.64 mg/cm²
• HCl 100 h; 600°C: 6.79 mg/cm²
• HCl 300 h; 600°C: 14.6 mg/cm²
• HCl 96 h; 700°C: 26.5 mg/cm²

Sulfuric acid H₂SO₄ corrosion rates:

• Acid concentration 15%; 80°C: 0.188 mm/year
• Acid concentration 50%; 80°C: 0.432 mm/year
• Acid concentration 60%; 80°C: 0.711 mm/year
• Acid concentration 70%; 80°C: 1.626 mm/year
• Acid concentration 80%; 80°C: 2.286 mm/year

24 h hydrogen fluoride HF corrosion rate tests with no control of aeration gave the following results:

• Acid concentration 2%; 70°C: 0.5 mm/year
• Acid concentration 5%; 70°C: 0.4 mm/year

Organic acids and other corrosive media corrosion rates:

• Acetic acid. 99%. boiling: <1 mm/rok
• Formic acid. 88%. boiling: 0.237 mm/rok
• Ferric chloride. 10%. boiling: 0.22 mm/rok
• Sodium hydroxide, 50%, boiling: 0.13 mm/year.

Nitric acids - In boiling 65% nitric acid, Inconel 625 typically corrodes at a rate of 0.76 mm per year.

Phosphoric acid H₃PO₄ - corrosion rates vary depending if the sample was suspended or lying on the bottom of the beaker:

• Boiling acid 20%: ~0 mm/rok
• Boiling acid 40%: ~0.3 mm/rok
• Boiling acid 60%: 0.3-1.3 mm/rok
• Boiling acid 80%: 2.2-6 mm/rok

3-year exposure test to stagnant seawater showed almost no corrosion, with a maximum pit depth of 0,025mm.

Pitting in chloride oxidizing environment - critical pitting temperature in 6% FeCl for 24h equals 35-40°C.

Dynamic oxidation resistance - test in high-velocity combustion gas stream showed relatively poor oxidation resistance of alloy 625 at a temperature above 1050°C, which may be attributed to niobium content. The sample was consumed after 500h of testing at 1090 °C. On the other hand, when subjected to a combustion gas stream at 980°C, alloy 625 lost only 0.12-0.19 mm after 1000h of testing.

Carburization resistance - 24 test in Ar-5H₂-5CO-5CH₄ at 1090°C showed low carbon absorption of only 9,9 mg/cm²

High-temperature nitridation resistance - after 168 h of testing in ammonia at 650°C, alloy 625 exhibited low nitrogen absorption of 0.9 mg/cm², with a depth of nitride penetration of  0.01 mm. When tested at 980°C, the nitrogen absorption equaled 2.5 mg/cm², with the depth of nitride penetration 0.17 mm. However with the temperature increased to 1090°C, the penetration exceeded 0.56 mm.

High-temperature oxygen-chlorine environments resistance is comparatively poor, which is evident from the weight-loss experienced after following tests:

• Ar-30Cl₂ 500 h; 400°C: 0.7
• Ar-30Cl₂ 500 h; 500°C: 7
• Ar-30Cl₂ 500 h; 705°C: 180
• Ar-20O₂-0.25Cl₂ 400 h; 900°C: 99.07
• Ar-20O₂-0.25Cl₂ 400 h; 1000°C: 220.09

In a simulated sour gas well environment, cold worked Inconel 625 showed stress-corrosion craking at 191°C.

Weldability

Selection of welding consumables for N06625:

• Coated electrodes: ENiCrMo3
• Filler metals: ERNiCrMo-3

Electropolishing and thermal spray coatings

Electropolishing - electrolyte composition: 37 ml of H₃PO₄, 56 ml of glycerol. Platinum cathode. 1.2-1.6 A/cm².

Thermal spray coatings - for corrosion-resistant applications, alloy 625 can be deposited using various thermal spray methods, including flame spraying, high-velocity oxyfuel (HVOF) spraying, and plasma spraying.

Heat treatment and working

Recommended working and heat treatment parameters for Alloy 625:

• Forging: 1000-1175 °C; 2h
• Soft annealing: 980-1150 °C; 30 - 60 min
• Solutioning: 1150 °C; 2h

Forging of alloy 625 requires heating it to the temperature very close to 1175°C but no higher. Forging is done from this temperature down to 1010°C. Whenever the temperature drops below 1010°C, the piece must be returned to the furnace and reheated to 1175°C. To gueard against duplex grain structure, give uniform reductions. For open-die work, final reductions of a minimum of 20% are recommended. The cooling rate is not of critical matter.

We deliver this steel alloy as:

• Pipes
• Welding fittings
• Strips
• Bars
• Welded pipes
• Seamless pipes
• Forgings

Replacements, equivalents and other designations:

Alloy 625, Bohler L625, 2.4856, NiCr22Mo9Nb, Nicrofer S6020, Na21, Haynes 625, NCF 625, Sanicro 60, 1N12, 2563, 467AC, AMS 5401, AMS 5402 B, AMS 5854, 62Ni-21.5Cr-9.0Mo-3.65Nb, A 494, N26625, N06625, A5.11, ENiCrMo-3, F 467, SA5.14, TNi 6625-xy, H22, NC6625, NW6625

Variants with different chemical composition, properties and applications:

• Alloy 625 LCF - more resistant to low-cycle fatigue. Can also be used for bellows.
• Alloy 725, Alloy 625Plus - higher Ti content. Much higher low-temperature strength. Can be age hardened. Used in oil and gas production.
• Alloy C22, alloy C276, alloy C-4, alloy 59, alloy 686, alloy C-2000 - higher Mo and W content. Resistant to highly aggressive oxidizing and reducing aqueous environments. Used in chemical processing.