ASTM B163 Scope for UNS N08825 Seamless Tubing
ASTM B163 is the standard specification for seamless nickel and nickel alloy tubing. For UNS N08825, the nickel-iron-chromium alloy with molybdenum, copper and titanium additions, the specification fixes chemical limits, dimensional tolerances, mechanical property minimums and the required heat treatment condition. Refinery pipe is normally ordered in the annealed condition because the alloy cannot be hardened by heat treatment. Annealing after cold drawing restores ductility, relieves the stresses left by reduction, and prepares the surface for pickling or bright annealing.
Because the chemistry balances nickel with controlled chromium and molybdenum, the alloy performs in both moderately oxidising and moderately reducing media. That balance is the reason it is chosen for refinery piping that carries mixed acid streams, where a standard austenitic stainless steel would pit, crack, or sensitise in the heat affected zone after welding.
Chemical Composition of UNS N08825 Seamless Pipe
| 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 |
The titanium range of 0.6-1.2 % is the decisive stabilising addition. Together with a carbon ceiling of 0.05 % max, it binds carbon as stable titanium carbides, so welded joints keep their corrosion resistance instead of sensitising. Copper at 1.5-3.0 % and molybdenum at 2.5-3.5 % deliver the resistance to reducing acids and to pitting in chloride bearing streams, while chromium at 19.5-23.5 % provides the oxidising side of the balance.
Mechanical and Physical Properties
| Property | Requirement |
|---|---|
| Tensile strength, annealed, room temperature | 586 MPa min |
| Yield strength, 0.2 % offset, room temperature | 241 MPa min |
| Elongation in 2 in | 30 % min |
| Density | 8.14 g/cm3, 0.294 lb/in3 |
| Melting range | 1385 degC, 2525 degF |
| Recommended maximum service temperature | about 550 degC, 1022 degF |
These figures are specification minimums for annealed tubing at room temperature. Cold drawn temper raises tensile and proof strength considerably but lowers elongation, so cold worked tube is selected only where the extra strength is needed and the fabrication route allows for the reduced ductility. For sustained high temperature, high stress duty, creep and stress rupture data should be reviewed instead of relying on short term tensile values.
Heat Treatment, Finish and Dimensional Control
Tube is supplied annealed, annealed and pickled, or bright annealed. Annealing is performed in the range that dissolves carbides and is followed by rapid cooling; the alloy is not hardenable by thermal treatment, so no ageing step is used for strengthening. Pickling strips the oxide scale left by hot working and annealing and leaves a uniform, passive surface that is easier to examine and to weld.
Condition: annealed, annealed and pickled, or bright annealed
Tolerances: outside diameter and wall thickness variation controlled by the specification
Inspection: chemical analysis, tensile testing, flattening or flaring where applicable, hydrostatic or non destructive examination
Traceability: grade, specification and heat number marked and backed by a mill test report
Refinery and Chemical Service Conditions
UNS N08825 pipe serves sulphuric and phosphoric acid circuits, heat exchanger and condenser tubing in acid plants, evaporators, scrubbers, flue gas desulphurisation lines, and feed or product piping that sees mixed corrosive streams. The high nickel content also gives useful resistance to chloride stress corrosion cracking, a common failure mode for 300 series stainless steel in wet, chloride bearing refinery service.
Corrosion rate depends on acid concentration, temperature and aeration. In hot concentrated sulphuric acid the alloy performs best where the acid is either fairly dilute or moderately concentrated; between those ranges, and at the highest temperatures, a dedicated corrosion review is needed. Isolated systems must also be protected from contact with carbon steel or copper alloys in moist acidic conditions.
Frequently Asked Questions
Q: What does ASTM B163 cover?
It is the standard specification for seamless nickel and nickel alloy tubing, covering chemical composition, dimensions, mechanical property minimums and heat treatment.
Q: Can this pipe be hardened by heat treatment?
No. The alloy is austenitic and is strengthened only by cold work. Heat treatment is used for annealing and stress relief, not for hardening.
Q: What is the maximum service temperature?
About 550 degC (1022 degF) for continuous service, while pressure vessel codes commonly approve it for wall temperatures up to roughly 450 degC (842 degF).
Q: Which finishes are available?
Annealed, annealed and pickled, and bright annealed. Pickled tube is the usual choice when the surface has to be inspected or welded on site.
Q: Why is titanium specified in the chemistry?
Titanium at 0.6-1.2 % stabilises carbon as titanium carbides, so welds resist intergranular corrosion without a post weld heat treatment.
Q: Is UNS N08825 suitable for chloride service?
Yes. The high nickel content gives good resistance to chloride stress corrosion cracking, and molybdenum plus copper limit pitting and crevice attack in chloride bearing streams.
