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ASTM B668 is the standard specification for "UNS N08028 Seamless Tube". (In industrial contexts, "tube" and "pipe" from this standard are often used for pressure piping applications).
This standard defines the requirements for seamless, cold-worked, or hot-finished pipes/tubes made from UNS N08028, an austenitic stainless steel designed for extreme corrosive environments. UNS N08029 is a closely related, enhanced version of N08028 and is often accepted or specified within the framework of this standard due to its superior properties.
The standard governs the chemical composition, mechanical properties, dimensions, and testing for these pipes.
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Products chemical requirements
Products tensile requirements
Key Characteristics and Composition
The core difference lies in the optimized chemistry of N08029, which enhances its performance.
| Feature | UNS N08028 (Alloy 28) | UNS N08029 (Alloy 29) |
|---|---|---|
| Key Composition | ~31% Ni, 27% Cr, 3.5% Mo, ~1% Cu | ~31% Ni, 26% Cr, 4.0% Mo, ~1% Cu |
| Key Additions | ~0.02% N | ~0.25% N |
| PREN | ~38 | ~42 |
| Key Characteristics | Excellent resistance to sulfuric acid, phosphoric acid, and good chloride resistance. | An upgrade to N08028 with higher Mo & N, resulting in better strength and pitting resistance. |
| Primary Advantage | A proven, cost-effective solution for severe sulfuric acid service. | Improved pitting/crevice resistance and higher mechanical strength than N08028. |
| Relationship | The original/base alloy. | A modern, enhanced version with optimized chemistry. |
Why the Nitrogen (N) in N08029 is Critical:
The significant addition of 0.25% Nitrogen provides two major benefits:
Solid Solution Strengthening: Dramatically increases yield and tensile strength in the annealed condition.
Enhanced Pitting Resistance: Directly boosts the PREN value, making it more resistant to chloride-induced pitting and crevice corrosion.
Products Properties and Applications
1. Corrosion Resistance:
Sulfuric Acid Resistance: This is the primary application for these alloys. They offer outstanding resistance to sulfuric acid across a wide range of concentrations and temperatures, outperforming standard stainless steels like 316L significantly.
Phosphoric Acid: Excellent resistance, even in contaminated and fluorinated environments found in fertilizer production.
Chloride Stress Corrosion Cracking (SCC): Very good resistance due to the high Nickel content (~31%).
Pitting & Crevice Corrosion: Good to very good resistance, with N08029 being measurably superior due to its higher Molybdenum and Nitrogen content.
2. Mechanical Properties (per ASTM B668 - typical for annealed condition):
Tensile Strength: 85 - 110 ksi (590 - 760 MPa)
Yield Strength (0.2% Offset): 40 - 60 ksi (275 - 415 MPa)
Elongation: 35% minimum (in 2 inches / 50mm)
(N08029 will consistently achieve values at the higher end of this strength range due to nitrogen strengthening.)
3. Common Industrial Applications:
Chemical Processing: Piping, heat exchangers, and reactors in sulfuric acid and phosphoric acid production plants.
Petrochemical Industry: Handling aggressive process streams.
Pulp & Paper Industry: Digesters and bleach plant equipment.
Pollution Control: Scrubbers, ducting, and outlet stacks in Flue Gas Desulfurization (FGD) systems.
Oil & Gas: Downhole and surface components in sour service (H₂S containing) environments.
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