Alloy 825 (UNS N08825) is a nickel-iron-chromium alloy with additions of molybdenum, copper and titanium. The combination produces a material that is used in chemical, nuclear and oil and gas equipment where high temperature, high pressure and corrosive media occur together. Its mechanical properties change markedly with temperature, so the allowable stress of a vessel, exchanger or pipeline depends on the design metal temperature rather than on a single room-temperature figure. The data below summarise how yield strength, tensile strength and ductility behave from cryogenic conditions to about 800 °C.
Composition and Why It Drives the Properties
| Element | Content | Function |
|---|---|---|
| Nickel (Ni) | 38 - 46 % | Chloride stress corrosion cracking resistance and matrix stability |
| Chromium (Cr) | 19.5 - 23.5 % | Oxidizing acid resistance and high-temperature scaling resistance |
| Iron (Fe) | Balance, approximately 22 - 33 % | Cost control while retaining an austenitic structure |
| Molybdenum (Mo) | 2.5 - 3.5 % | Resistance to reducing acids and to pitting |
| Copper (Cu) | 1.5 - 3.0 % | Additional resistance in sulfuric acid service |
| Titanium (Ti) | 0.6 - 1.2 % | Stabilisation against intergranular corrosion after welding |
The alloy is delivered in the annealed condition, which produces a single austenitic phase strengthened by solid solution rather than by precipitation. Product forms include seamless pipe and tube to ASTM B423 / ASME SB423, plate, sheet and strip to ASTM B424, and bar and wire to ASTM B425, with the European designation EN 2.4858 (NiCr21Mo).
Mechanical Properties at Room Temperature
| Property at 20 °C | Typical value |
|---|---|
| Yield strength, Rp0.2 | 240 - 290 MPa |
| Tensile strength, Rm | 550 - 690 MPa |
| Elongation | 30 - 40 % |
| Impact behaviour | High fracture toughness, no brittle fracture under normal design loads |
At ambient temperature the material combines a useful load-bearing capacity with enough ductility to accommodate plastic deformation during forming and installation, which is why it is used for pressure vessels, reactor internals and structural components that also see corrosive service.
Behaviour from 300 °C to 700 °C
Strength falls as temperature rises, while ductility increases. The change is gradual and predictable, which allows the alloy to be used for long-term service in the medium and high temperature range.
| Temperature | Yield strength | Tensile strength | Elongation |
|---|---|---|---|
| 20 °C | 240 - 290 MPa | 550 - 690 MPa | 30 - 40 % |
| 500 °C | 170 - 200 MPa | 450 - 500 MPa | Above 40 % |
| 700 °C | About 150 MPa | About 350 MPa | About 50 % |
Between 300 °C and 500 °C both the yield and the tensile strength begin to decline steadily. At 500 °C the material still shows good mechanical stability, so it is widely applied in medium and high temperature equipment. From 500 °C to 700 °C the trend continues; at 700 °C the alloy combines reduced strength with markedly improved plastic deformation capacity and good creep performance, which suits high-temperature pressure vessels and heat exchange equipment.
Response Above 700 °C
Above 700 °C both yield and tensile strength fall sharply and creep damage becomes the design limitation. At 800 °C the yield strength is around 100 MPa and the tensile strength is only about 250 MPa, so components at this temperature are designed as thin-walled, well-supported structures with generous allowance for creep strain:
Pipelines and manifolds handling high-temperature corrosive gas;
Furnace internals, radiant ducting and support systems;
Heat exchangers operating with a hot-side metal temperature near 800 °C.
The oxidation resistance and structural stability of the nickel-rich matrix still allow a long service life in these conditions, provided the mechanical design accounts for the reduced strength and for creep rather than relying on short-term tensile data.
Low-Temperature Strength and Toughness
Cold service is the other extreme, and here the alloy improves rather than embrittles. Below 0 °C, and even in the range approaching -100 °C, yield strength can exceed 350 MPa while tensile strength remains around 600 MPa. Impact toughness stays high and no cold brittleness develops. That behaviour supports use in deep sea equipment, polar installations and cryogenic storage tanks, where the material keeps both strength and toughness instead of failing by low-temperature embrittlement.
Typical applications that rely on this combination include reactors, pipelines and containers in chemical plants handling strong acids or hot media, sulfuric acid production equipment where reactor temperatures reach about 500 °C, and steam generator and heat exchanger components in the nuclear industry that must withstand temperatures above 700 °C while resisting corrosion and irradiation damage.
Frequently Asked Questions
Q: What is the yield strength of Alloy 825 at room temperature?
Typically 240 to 290 MPa at 20 °C, with a tensile strength of 550 to 690 MPa and elongation of 30 to 40 % in the annealed condition.
Q: How does temperature affect Alloy 825 strength?
Strength falls steadily as temperature rises: about 170 to 200 MPa yield strength at 500 °C, around 150 MPa at 700 °C and roughly 100 MPa at 800 °C, while elongation increases from about 30 % to 50 %.
Q: Is Alloy 825 suitable for cryogenic service?
Yes. At low temperature its yield strength exceeds 350 MPa, tensile strength stays near 600 MPa and impact toughness remains high, so no cold brittleness occurs.
Q: Does ductility increase at high temperature?
Yes. Elongation rises from 30 - 40 % at room temperature to about 50 % at 700 °C, which is why high-temperature components deform rather than fracture but must still be designed against creep.
Q: Which standards cover Alloy 825 products?
Seamless pipe and tube are covered by ASTM B423 / ASME SB423, plate, sheet and strip by ASTM B424, and bar and wire by ASTM B425, with EN 2.4858 as the European designation.
Q: What limits the use of Alloy 825 above 700 °C?
Creep rather than oxidation. Strength drops sharply above 700 °C, so the wall thickness, support spacing and strain allowance are set from creep data instead of room-temperature tensile values.
