UNS N08825 Seamless Pipe for Chemical Processing
Chemical plants need piping that holds up when the process stream changes character from oxidising to reducing within the same circuit. UNS N08825, the nickel-iron-chromium alloy with molybdenum, copper and titanium additions, was developed for exactly that duty and is supplied as seamless pipe under ASTM B163. The specification controls chemistry, dimensions, mechanical minimums and heat treatment, and the usual delivery condition is annealed or annealed and pickled because the alloy cannot be strengthened by heat treatment.
The combination of roughly 40 % nickel with molybdenum and copper is what separates the grade from ordinary stainless steel: it tolerates strong acids, resists pitting and crevice corrosion, and withstands chloride induced stress corrosion cracking that would crack a 316L line in the same service.
Chemical Composition and Stabilising Additions
| Element | Requirement, % | Element | Requirement, % |
|---|---|---|---|
| Nickel | 38.0-46.0 | Carbon | 0.05 max |
| Chromium | 19.5-23.5 | Manganese | 1.0 max |
| Iron | 22.0 min, balance | Sulfur | 0.03 max |
| Molybdenum | 2.5-3.5 | Silicon | 0.5 max |
| Copper | 1.5-3.0 | Aluminium | 0.2 max |
| Titanium | 0.6-1.2 | Others | total 3.15-4.15 |
Titanium at 0.6-1.2 % with carbon held at 0.05 % max keeps carbon tied up as stable carbides, so a welded joint retains its corrosion resistance without a solution anneal after fabrication. Molybdenum and copper together provide the resistance to localised attack, and the high nickel level is responsible for the resistance to chloride stress corrosion cracking.
Typical Mechanical Properties by Condition
| Form | Condition | 0.2 % proof stress | Tensile strength | Elongation |
|---|---|---|---|---|
| Tube | annealed | 440 MPa | 770 MPa | 35 % |
| Tube | cold drawn | 880 MPa | 1000 MPa | 15 % |
The table reports typical room temperature results for the two supply conditions rather than specification minimums: cold drawing more than doubles the proof stress but cuts elongation sharply, so cold worked pipe is reserved for applications where the strength gain justifies the loss of formability. The specified minimums for the annealed condition remain the governing values for design, and at elevated temperature creep and stress rupture data should be used instead of short term tensile figures.
Corrosion Behaviour in Chemical Media
Mixed media: resistance to both oxidising and reducing agents suits the grade to streams containing sulphuric acid, nitric acid and sodium hydroxide, as found in nuclear fuel reprocessing equipment.
Acids: particularly resistant to sulphuric and phosphoric acid, which makes it a cost effective selection for sulphuric acid circuits.
Localised attack: molybdenum and copper give good resistance to pitting and crevice corrosion in chloride bearing solutions.
Stress corrosion cracking: the high nickel content resists cracking initiated by chloride ions.
Performance still depends on concentration, temperature, aeration and flow. The alloy is not a universal answer to every acid duty, so a corrosion review against the actual process conditions remains part of correct grade selection.
Hot Working, Fabrication and Applications
The hot working temperature range for the alloy is 1600 to 2150 degF, equivalent to 870 to 1180 degC. For the best corrosion resistance the final hot working temperature should fall between 1600 and 1800 degF, that is 870 to 980 degC, and material worked outside that finishing window should be given a further anneal before service. Service temperature is generally limited to about 550 degC (1022 degF), and pressure vessel codes approve the alloy for wall temperatures up to about 450 degC (842 degF).
Typical duties include acid handling equipment, heat exchangers, evaporators, scrubbers and piping in acid production, oil and gas gathering lines, pollution control equipment such as scrubbers and flue gas desulphurisation systems, marine hardware, and nuclear fuel reprocessing plant.
Frequently Asked Questions
Q: Which specification applies to UNS N08825 seamless pipe?
ASTM B163 covers seamless nickel alloy tubing, and the ASME equivalent is used for pressure vessel work.
Q: What condition is the pipe supplied in?
Annealed, annealed and pickled, or bright annealed. It cannot be hardened by heat treatment.
Q: Why is titanium added to the chemistry?
Titanium at 0.6-1.2 % stabilises carbon and prevents sensitisation of welds, protecting intergranular corrosion resistance.
Q: What is the hot working window?
870 to 1180 degC, with a finishing range of 870 to 980 degC for optimum corrosion resistance.
Q: What is the maximum service temperature?
About 550 degC for general service, with pressure vessel approval normally limited to 450 degC wall temperature.
Q: Is cold drawn pipe stronger?
Yes, typical proof stress rises from about 440 MPa annealed to about 880 MPa cold drawn, but elongation falls from 35 % to 15 %.
