General Properties
Alloy 625 (UNS N06625) is an austenitic nickel base superalloy possessing excellent resistance to oxidation and corrosion over a broad range of corrosive conditions, including jet engine environments and many other aerospace and chemical process applications. The alloy has outstanding strength and toughness at temperatures ranging from cryogenic temperature to 2000°F (1093°C). Alloy 625 also has exceptional fatigue resistance.

Alloy 625 derives its strength from the solid solution strengthening effects of molybdenum and columbium on the nickel-chromium matrix. These elements also contribute to the alloy’s outstanding corrosion resistance. Although the alloy was developed for high temperature strength, its highly alloyed composition provides a high level of general corrosion resistance to a wide range of oxidizing and nonoxidizing environments. The levels of chromium and molybdenum provide excellent resistance to chloride ion pitting, and the high level of nickel provides resistance to chloride stress corrosion cracking.

The material possesses a high degree of formability and shows better weldability than many highly alloyed nickel-base alloys. The alloy is resistant to intergranular corrosion even in the welded condition.

Alloy 625 can be produced by vacuum induction melting or AOD refining. Consumable electrode remelting procedures may be used to further refine the material.

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Applications

• Seawater applications
• Aerospace components
• Chemical processing equipment
• Nuclear water reaction components

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Standards
AMS........... 5599
ASTM.......... B 443
ASME.......... SB 443

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Chemical Analysis
Typical Analysis (Weight %)

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Corrosion and Oxidation Resistance
The high level of chromium and molybdenum in Alloy 625 provides a high level of pitting and crevice corrosion resistance to chloride contaminated media, such as seawater, neutral salts, and brines.

Typical Data in Chloride Solutions

Panel Exposures in Seawater

The alloy is resistant to a variety of corrosive media from highly oxidizing to moderately reducing.

Tests in geothermal brines indicate Alloy 625 is highly resistant to hot geothermal fluids comparable to Titanium Grade 2.

Tests in simulated flue gas desulfurization environments show Alloy 625 highly resistant to the environment in comparison to alloys such as Alloy 316 and comparable to Alloy 276.

The following data are illustrative. Typical corrosion rates are in mils/year (mm/a).

Boiling Organic Acid Solutions

Dilute Reducing Acids — Boiling Solutions*

* Sulfuric acid test samples activated before tests and hydrochloric acid test samples tested without activation.

Miscellaneous Environments

Chloride Stress Corrosion Cracking Resistance

Oxidation Resistance
Alloy 625 has excellent oxidation and scaling resistance at temperatures up to 2000°F (1093°C). It is superior to many other high temperature alloys under cyclic heating and cooling conditions. The following graph compares the weight loss of several stainless steel alloys to Alloy 625 under cyclic oxidation at 1800°F (982°C).

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Formability
Alloy 625 is capable of being formed like the standard austenitic stainless steels. The material is considerably stronger than conventional austenitic stainless steels and consequently requires higher loads to cause the material to deform. During cold working, the material work hardens more rapidly than austenitic stainless steels. The combination of high initial strength and work hardening rate may necessitate the need for intermediate anneals if the cold deformation is extensive.

Effect of Cold Reduction on Properties of Plate Annealed at 2150°F (1177°C)

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Mechanical Properties
Typical Short Time Tensile Properties as a Function of Temperature

Typical room temperature tensile properties of material annealed at 1920°F (1065°C) follow.

The typical room temperature tensile properties of material solution annealed at 2150°F (1177°C) follow.

The short time elevated temperature tensile properties of Alloy 625 annealed at 1950°F (1066°C) are shown in the following graph.

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Welding
Alloy 625 can be readily welded by conventional processes used for austenitic stainless steel, including fusion and resistance methods. The material should be in the mill annealed condition and thoroughly descaled and cleaned before welding. Preheating is not required and post-weld treatment is not needed to maintain or restore corrosion resistance.

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Heat Treatment
Alloy 625 is furnished with one heat treatment for optimum properties up to 1200°F (649°C) and another for optimum properties above 1200°F (649°C). The standard anneal at a minimum of 1600°F (871°C) is used for service temperatures up to 1200°F (649°C). When optimum high temperature creep and rupture properties are required, as for service above 1200°F (649°C), a solution anneal at 2000°F (1093°C) minimum is used. In the solution annealed condition, a subsequent stabilization anneal at 1800°F (982°C) minimum is sometimes specified to further increase resistance to sensitization.

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Physical Properties
Density:
0.305 Ib/in³
8.44 g/cm³

Specific Gravity:
8.44

Melting Range:
2350°-2460°F
1280°-1350°C

Magnetic Permeability:
75°F, 200 oersted 1.0006

Specific Heat:
0.098 Btu/lb.-°F
410 Joules/kg-°K

Electrical Resistivity:
0.098 Btu/lb.-°F
410 Joules/kg-°K

Thermal Properties

(a) Average coefficient from 70°F (21°C) to temperature shown.
(b) Measurements made at Battelle Memorial Institute.
(c) Material annealed 2100°F (1149°C).

Modulus Data

(a) Poisson’s ratio (m) computed from the relation: µ = E-2G/2G

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Impact Resistance
Alloy 625 maintains high impact resistance at low temperatures as shown below.

Typical Alloy 625 Impact Properties

(a) Charpy Keyhole Specimens (Mean Value of 3 Tests)

Impact properties may be expected to decrease with extended service in the 1200° to 1600°F (649° to 871°C) range.

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