General Properties
Alloy 625 (UNS N06600)is an austenitic nickel-chromium-molybdenum-niobium alloy that offers a unique combination of exceptional corrosion resistance and high strength across a wide temperature range.The strength of Alloy 625 is achieved through solid-solution hardening of the nickel-chromium matrix by the presence of molybdenum and niobium. This eliminates the need for precipitation-hardening treatments, simplifying the fabrication process.The chemical composition of Alloy 625 contributes to its outstanding corrosion resistance in various severe operating environments. It also exhibits resistance to oxidation and carburization at high temperatures. The alloy demonstrates resistance to pitting corrosion, crevice corrosion, impingement corrosion, and intergranular attack. Additionally, it is highly resistant to chloride stress corrosion cracking, making it nearly immune to this form of corrosion.
Alloy 625 exceptional corrosion resistance and high strength make it suitable for a wide range of applications. It is commonly used in industries such as chemical processing, oil and gas, marine engineering, aerospace, and power generation. The alloy is well-suited for environments where exposure to corrosive media, elevated temperatures, and mechanical stresses are expected.
Applications
- Aerospace Components – bellows and expansion joints, ducting systems, engine thrust-reversers, turbine shroud rings
- Air Pollution Control – chimney liners, dampers, flue gas desulfurization (FGD) components
- Chemical Processing – equipment handling both oxidizing and reducing acids, super-phosphoric acid production
- Marine Service – steam line bellows, Navy ship exhaust systems, submarine auxiliary propulsion systems
- Nuclear Industry – reactor core and control rod components, waste reprocessing equipment
- Offshore Oil and Gas Production – waste flare gas stacks, piping systems, riser sheathing, sour gas piping and tubing
- Petroleum Refining – waste flare gas stacks
- Waste Treatment – waste incineration components
Standards
ASTM..................B 443
ASME..................SB 443
AMS ...................5599
Weight % (all values are maximum unless a range is otherwise indicated)
|
|
|
|
|
|
Nickel
|
58.0 min.
|
Silicon
|
0.50
|
|
Chromium
|
20.0 min.-23.0 max.
|
Phosphorus
|
0.015
|
|
Molybdenum
|
8.0 min.-10.0 max.
|
Sulfur
|
0.015
|
|
Iron
|
5.0
|
Aluminum
|
0.40
|
|
Niobium (plus Tantalum)
|
3.15 min.-4.15 max.
|
Titanium
|
0.40
|
|
Carbon
|
0.10
|
Cobalt (if determined)
|
1.0
|
|
Manganese
|
0.50
|
|
|
Physical Properties
Density
0.305 lbs/in3
8.44 g/cm3
Specific Heat
0.102 BTU/lb-°F (32-212°F)
427 J/kg-°K (0-100°C)
Modulus of Elasticity
30.1 x 106 psi
207.5 GPa
Thermal Conductivity 200°F (100°C)
75 BTU/hr/ft2/ft/°F
10.8 W/m-°K
Melting Range
2350 – 2460°F
1290 – 1350°C
Electrical Resistivity
50.8 Microhm-in at 70°C
128.9 Microhm-cm at 210°C
|
Mean Coefficient of Thermal Expansion
Temperature Range
|
|
°F
|
°C
|
in/in/°F
|
cm/cm°C
|
|
200
|
93
|
7.1 x 10-6
|
12.8 x 10-6
|
|
400
|
204
|
7.3 x 10-6
|
13.1 x 10-6
|
|
600
|
316
|
7.4 x 10-6
|
13.3 x 10-6
|
|
800
|
427
|
7.6 x 10-6
|
13.7 x 10-6
|
|
1000
|
538
|
7.8 x 10-6
|
14.0 x 10-6
|
|
1200
|
649
|
8.2 x 10-6
|
14.8 x 10-6
|
|
1400
|
760
|
8.5 x 10-6
|
15.3 x 10-6
|
|
1600
|
871
|
8.8 x 10-6
|
15.8 x 10-6
|
|
1700
|
927
|
9.0 x 10-6
|
16.2 x 10-6
|
Typicals Values at 68°F (20°C)
|
Yield Strength
0.2% Offset
|
Ultimate Tensile
Strength
|
Elongation
in 2 in.
|
Hardness
|
|
psi (min.)
|
(MPa)
|
psi (min.)
|
(MPa)
|
% (min.)
|
(max.)
|
|
65,000
|
448
|
125,000
|
862
|
50
|
200 Brinell
|
Corrosion Resistance
The highly alloyed chemical composition of Alloy 625 provides outstanding corrosion resistance in various severely corrosive environments. Here are some key points about Alloy 625's corrosion resistance:Immunity to Attack in Mild Conditions:Alloy 625 is virtually immune to attack in mild conditions such as the atmosphere, fresh and sea water, neutral salts, and alkaline solutions.Nickel and chromium in the alloy contribute to its resistance to oxidizing solutions.The combination of nickel and molybdenum offers resistance in non-oxidizing environments.Resistance to Pitting and Crevice Corrosion:Alloy 625 is resistant to pitting corrosion, which is localized corrosion that can cause small holes or pits in the material's surface.It also resists crevice corrosion, which occurs in confined spaces or crevices.Intergranular Cracking Prevention:Niobium, present in Alloy 625, acts as a stabilizer during welding, preventing intergranular cracking. Intergranular cracking can occur along the boundaries of the metal grains during welding.Immunity to Chloride Stress Corrosion Cracking:The high nickel content of Alloy 625 makes it virtually immune to chloride stress corrosion cracking. Chloride stress corrosion cracking is a type of corrosion that occurs in the presence of chlorides and tensile stress.Resistance to Mineral Acids, Alkalis, and Organic Acids:Alloy 625 resists attack by mineral acids such as hydrochloric, nitric, phosphoric, and sulfuric acids.It also shows resistance to alkalis and organic acids in both oxidizing and reducing conditions.
Alloy 625 corrosion resistance allows it to be used in a wide range of applications where exposure to harsh environments and corrosive substances is expected. However, it's important to consider specific operating conditions and consult with materials engineers or alloy manufacturers to ensure the alloy's suitability for a particular application.
Corrosion-resistance of nickel alloys in 24-hour tests in boiling 40% formic acid.
|
Alloy
|
Corrosion Rate
|
|
|
mpy
|
mm/a
|
|
Alloy 825
|
7.9
|
0.2
|
|
Nickel 200
|
10.3-10.5
|
0.26-0.27
|
|
Alloy 400
|
1.5-2.7
|
0.038-0.068
|
|
Alloy 600
|
10.0
|
0.25
|
|
Alloy G-3
|
1.8-2.1
|
0.046-0.05
|
|
Alloy 625
|
6.8-7.8
|
0.17-0.19
|
|
Alloy C-276
|
2.8-2.9
|
0.07-0.074
|
Corrosion-resistance of nickel alloys in four 24-hour tests in boiling acetic acid
|
Alloy
|
Acetic Acid
Concentration
|
Corrosion/Erosion Rate
|
|
mpy
|
mm/a
|
|
Alloy 825
|
10%
|
0.60-0.63
|
0.0152-0.160
|
|
Alloy 625
|
10%
|
0.39-0.77
|
0.01-0.19
|
|
Alloy C-276
|
10%
|
0.41-0.45
|
0.011-0.0114
|
|
Alloy 686
|
80%
|
<0.1*
|
<0.01*
|
Resistance of nickel alloys to impingement attack by seawater at 150 ft/sec (45.7 m/s)
|
Alloy
|
Corrosion/Erosion Rate
|
|
|
mpy
|
mm/a
|
|
Alloy 625
|
Nil
|
Nil
|
|
Alloy 825
|
0.3
|
0.008
|
|
Alloy K-500
|
0.04
|
0.01
|
|
Alloy 400
|
1.5-2.7
|
0.038-0.068
|
|
Alloy 600
|
0.4
|
0.01
|
|
Nickel 200
|
40
|
1.0
|
The comparative PREN number for alloy 625 is shown in the table below.
Pitting resistance equivalency numbers (PREN) for corrosion resistant alloys
|
Alloy
|
Ni
|
Cr
|
Mo
|
W
|
Nb
|
N
|
PREN
|
|
316 Stainless Steel
|
12
|
17
|
2.2
|
—
|
—
|
—
|
20.4
|
|
317 Stainless Steel
|
13
|
18
|
3.8
|
—
|
—
|
—
|
23.7
|
|
Alloy 825
|
42
|
21.5
|
3
|
—
|
—
|
—
|
26.0
|
|
Alloy 864
|
34
|
21
|
4.3
|
—
|
—
|
—
|
27.4
|
|
Alloy G-3
|
44
|
22
|
7
|
—
|
—
|
—
|
32.5
|
|
Alloy 625
|
62
|
22
|
9
|
—
|
3.5
|
—
|
40.8
|
|
Alloy C-276
|
58
|
16
|
16
|
3.5
|
—
|
—
|
45.2
|
|
Alloy 622
|
60
|
20.5
|
14
|
3.5
|
—
|
—
|
46.8
|
|
SSC-6MO
|
24
|
21
|
6.2
|
—
|
—
|
0.22
|
48.0
|
|
Alloy 686
|
58
|
20.5
|
16.3
|
3.5
|
—
|
—
|
50.8
|
Oxidation Resistance
The oxidation and scaling resistance of Alloy 625 is superior to a number of heat resistant austenitic stainless steels such as 304, 309, 310 and 347 up to 1800°F (982°C) and under cyclic heating and cooling conditions. Above 1800°F (982°C), scaling can become a restrictive factor in service.
Fabrication Data
Alloy 625 can be easily welded and processed by standard shop fabrication practices, however because the high strength of the alloy, it resists deformation at hot-working temperatures.
Hot Forming
The hot-working temperature range for Alloy 625 is 1650 – 2150°F (900 – 1177°C). Heavy working needs to occur as close to 2150°F (1177°C) as possible, while lighter working can take place down to 1700°F (927°C). Hot-working should occur in uniform reductions to prevent duplex grain structure
Alloy 400 is readily cold worked by virtually all cold fabrication methods. Cold working should be performed on annealed material. The alloy has a somewhat higher work hardening rate than carbon steel, but not as high as 304 stainless steel.
Cold Forming
Alloy 625 can be cold-formed by the standard shop fabrication practices. The alloy should be in the annealed condition. Work hardening rates are higher than the austenitic stainless steels.
Welding
Alloy 625 can be readily welded by most standard processes including GTAW (TIG), PLASMA, GMAW (MIG/MAG), SAW and SMAW (MMA). A post weld heat treatment is not necessary. Brushing with a stainless steel wire brush after welding will remove the heat tint and produce a surface area that does not require additional pickling.
Machining
Alloy 625 should preferably be machined in the annealed condition. Since Alloy 625 is prone to work–hardening, only low cutting speeds should be used and the cutting tool should be engaged at all times. Adequate cut depth is necessary to assure avoiding contact with the previously formed work-hardened zone.
