Properties
Alloy 625 (UNS N06625) is a low-carbon Ni-Cr-Nb alloy which shows excellent resistance to a variety of corrosive media.
Due to its low carbon content and stabilising heat treatment, alloy 625 shows little tendency to sensitisation even after 50 hours at temperatures in the range 650-900°C (1200-1650°F).
The alloy is supplied in the soft-annealed condition for applications involving wet corrosion, and is approved by TUV for pressure vessels in the temperature range -196-450°C (-321-840°F).
For high-temperature applications a modified version of alloy 625 is available on request.
The mechanical properties of alloy 625 can be increased by age-hardening.
This alloy is characterised by:
- Outstanding resistance to pitting, crevice corrosion, impingement corrosion and intergranular attack
- Almost complete freedom from chloride-induced stress-corrosion cracking
- Good resistance to mineral acids, such as nitric, phosphoric, sulphuric and hydrochloric acids
- Good resistance to alkalis and organic acids
- Good mechanical properties
In special high-temperature applications where very high strength and creep values are required, the high-carbon, solution treated version (Alloy 625, grade 2) should be used.
Designation and Standards
| Country National Standards |
Material Designation |
Specification | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Chem. Compo- sition |
Tube & Pipe | Sheet & Plate |
Rod & Bar |
Strip | Wire | Forgings | |||
| Seamless | Welded | ||||||||
| France AFNOR | NC 22 DNb | ||||||||
| Germany DIN VdTÜV |
W.-Nr . 2.4856 NiCr22Mo9Nb |
17744 499 |
17751 | 17750 499 |
17752* 499 |
17750 499 |
|||
| United Kingdom BS |
NA 21 | 3072 | 3076 | ||||||
| USA ASTM ASME AMS ASME Code Case |
UNS N06625 Grade 1 |
B444 SB444 5581 1935 |
B704/705 5581 |
B443 SB444 5581 1935 |
B444 SB443 5599 1935 |
B443 SB443 5599 1935 |
5837 | B564 5666 |
|
| ISO | NiCr22Mo9Nb | ||||||||
*Solution treated condition only
Chemical Composition (average %)
| Alloy 625 | Ni | Cr | Fe | C | Mn | Si | Mo | Al | Ti | Nb | P | S |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Min | bal | 21.0 | - | - | - | - | 8.0 | - | - | 3.2 | - | - |
| Max | bal | 23.0 | 4.0 | 0.025 | 0.40 | 0.40 | 10.0 | 0.40 | 0.40 | 3.8 | 0.010 | 0.010 |
Mechanical Properties
The following properties are applicable to alloy 625 in the hot or cold formed and soft-annealed condition and in the indicated ranges. Specified properties of material outside these ranges are subject to special enquiry.
| Form | Dimensions | Tensile Strength |
0.2% Yield Strength |
Elong A5 % |
Brinell Hardness HB |
|||
|---|---|---|---|---|---|---|---|---|
| mm | in | N/mm2 | Ksi | N/mm2 | Ksi | |||
| Sheet, Strip cr | 830 | 120 | 415 | 60 | 30 | 150 - 220 | ||
| Sheet, Plate hr | ≤ 75 | ≤ 3 | 760 | 110 | 380 | 55 | ||
| Rod, Bar | ≤ 100 | ≤ 4 | 830 | 120 | 415 | 60 | ||
| >100 | >4 | 760 | 110 | 345 | 50 | |||
| Tube, Pipe | 830 | 120 | 415 | 60 | ||||
Minimum mechanical properties at room temperature according to ASTM. Please Note: The figures quoted are intended for guidance only. For further information, please refer to the standards listed or contact our sales or QA department.
Metallurgical Structure
Alloy 625 has a face-centred cubic structure. After long exposures at a temperature of about 650°C (1200°F) the alloy precipitates some isolated carbides and a metastable tetragonal phase which later changes to a stable orthorhombic phase Ni3 (Nb, Ti). The mechanical properties are enhanced by
the molybdenum and niobium content through solid-solution hardening of the nickel-chromium matrix. There is, however , a loss in ductility.
Corrosion Resistance
Alloy 625 shows excellent corrosion resistance in a wide range of media:
- Outstanding resistance to pitting and crevice corrosion in chloride-bearing media and to impingement corrosion or intergranular attack
- High resistance to corrosive attack by mineral acids, such as nitric phosphoric, sulphuric and hydrochloric acids, as well as to alkalis and organic acids in both oxidising and reducing conditions
- Virtual immunity to chloride-induced stress-corrosion cracking
- Practically no corrosive attack in marine and industrial atmospheres. High resistance to seawater and brackish water , even at high temperatures
- No sensitisation during welding
- Good resistance to carburisation and to oxidation under static and cyclic conditions, and to chlorine containing gases
Applications
The soft annealed, low carbon alloy 625 is widely used in chemical process technology, as its good corrosion resistance and high strength permit the use of thin structural parts. Alloy 625 is used for structures in contact with seawater and subject to high mechanical stresses.
Typical applications include:
- Flue gas scrubber components
- Chimney linings
- Superphosphoric acid production equipment
- Nuclear waste reprocessing equipment
- Sour gas production tubes
- Product piping systems and sheathing of risers
- Offshore industry, marine equipment
For high-temperature applications, up to about 1000°C (1830°F), the solution- annealed, high carbon version (alloy 625, grade 2) is recommended, due to its excellent creep properties.
