What Is a Seamless Alloy 20 Tube?
Seamless Alloy 20 Tube — UNS N08020 — is a niobium-stabilized nickel–iron–chromium–molybdenum–copper alloy (32.0–38.0% Ni, 19.0–21.0% Cr, 2.0–3.0% Mo, 3.0–4.0% Cu) produced to ASTM B729 from a solid billet with no weld seam.
Alloy 20 was engineered for sulfuric acid across a wide range of concentrations and temperatures, and its balanced Ni-Cr-Mo-Cu system also handles phosphoric, nitric and mixed-acid streams with chloride contamination. The niobium stabilization (8×C minimum) resists sensitization, so as-welded equipment keeps full corrosion performance. With only 32–38% nickel it delivers nickel-alloy capability at a fraction of the cost of higher-nickel grades.
Key Features and Advantages
Seamless UNS N08020 tube pairs sulfuric-acid expertise with niobium-stabilized weldability:
Sulfuric Acid Specialist: low corrosion rates across dilute to mid-concentration H2SO4, including contaminated streams where stainless steels fail quickly.
Niobium Stabilized (8×C–1.00%): intergranular-corrosion resistance survives welding and thermal cycling without re-annealing in most fabrications.
Excellent Chloride Stress-Corrosion Resistance: the nickel base combined with balanced Cr-Mo-Cu resists chloride SCC that cracks austenitic stainless steels.
Balanced Cost-Performance: 32–38% nickel gives nickel-alloy performance while keeping material cost well below Inconel and Hastelloy grades.
Versatile Across Mixed Acids: reliable in phosphoric, nitric, acetic and organic-acid systems with chloride impurities.
Product Specifications and Data
To provide a comprehensive overview of this high-performance material, here are the detailed technical specifications and performance data for UNS N08020 seamless tube.
Equivalent Grade
| UNS | Common grade name | EN material number | EN steel name | JIS grade | GB/T grade |
|---|---|---|---|---|---|
| N08020 | Alloy 20 / 20Cb-3 | 2.4660 | NiCr20CuMo | Not listed in grade index | NS1403 / NS143 (legacy) |
Grade correspondence does not make ASTM, ASME, EN, JIS and GB delivery requirements interchangeable. Confirm chemistry, heat treatment, mechanical properties, dimensions, testing and certification against the purchase specification. Approximate or unconfirmed correspondences retain their qualification. See the grade designation index.
Chemical Composition
UNS N08020 seamless tube chemistry is tightly controlled so the corrosion and mechanical performance below is repeatable:
| Grade | C | Mn | P | S | Si | Ni | Cr | Mo | Cu | Nb+Ta | Fe |
|---|---|---|---|---|---|---|---|---|---|---|---|
| UNS N08020 | ≤0.07 | ≤2.00 | ≤0.045 | ≤0.035 | ≤1.00 | 32.0–38.0 | 19.0–21.0 | 2.0–3.0 | 3.0–4.0 | 8×C–1.00 | remainder |
Chemical requirements per ASTM B729-20, Table 1. Single values are maximum content unless a range is indicated; iron is determined arithmetically by difference. The niobium minimum is defined as eight times the carbon content (8×C–1.00%), which ties stabilizer level to the actual carbon of each heat and prevents intergranular attack after welding.
Typical Mechanical Properties
Values below are specification minimums for the delivery condition; they define the acceptance basis for UNS N08020 seamless tube.
| Condition | Tensile Strength, min, ksi (MPa) | Yield Strength (0.2% Offset), min, ksi (MPa) | Elongation in 2 in. (50.8 mm) or 4D, min, % |
|---|---|---|---|
| Annealed | 80 (550) | 35 (240) | 30.0 |
Mechanical properties per ASTM B729-20, Table 2, in the annealed condition. Tube is annealed by heating to 982 – 1010 °C (1800 – 1850 °F) followed by water quenching or other rapid cooling. Each pipe and tube is hydrostatically (fiber stress 20 000 psi / 138 MPa) or nondestructive electrically tested per the general requirements of ASTM B829.
Typical Physical Properties
These physical properties highlight the material’s ability to perform reliably across a range of environments.
| Density | Melting Range | Thermal Expansion | Thermal Conductivity | Modulus of Elasticity | Electrical Resistivity | Magnetic Permeability |
|---|---|---|---|---|---|---|
| 8.08 g/cm3 | 1370 – 1400 °C | 14.8 μm/m·°C (20–100°C) | 11.7 W/m·K (100°C) | 193 GPa | 1.08 μΩ·m (20°C) | ≈1.003 (annealed) |
Size Tolerance Chart (mm)
Permissible variations for seamless nickel alloy tube in accordance with ASTM B829-24, Table 2:
| Nominal Outside Diameter, mm | Permissible Variation in OD, mm | Average Wall, % | Minimum Wall |
|---|---|---|---|
| 10.0 to 15.9, excl | ±0.13 | ±15 | +30% / −0 |
| 15.9 to 38.1, incl | ±0.19 | ±10 | +22% / −0 |
| over 38.1 to 76.2, incl | ±0.25 | ±10 | +22% / −0 |
| over 76.2 to 88.9, incl | ±0.38 | ±10 | +22% / −0 |
Notes (dimensions per ASTM B829-24, Table 2, referenced by ASTM B729-20): tolerances apply to individual measurements and include ovality. For thin-wall tube (nominal wall 3% or less of the OD), the mean OD shall comply with the table and individual measurements are increased by 0.5% of the nominal OD. Wall variation in any cross section (eccentricity) shall not exceed ±10% of the actual average wall.
Length Tolerance
- When tube is ordered cut-to-length, length variation is +3.2 mm / −0 for OD under 50.8 mm and +4.8 mm / −0 for larger sizes (ASTM B829-24, Table 5); for lengths over 24 ft, an additional +3.2 mm per 10 ft applies, up to a maximum of +12.7 mm.
- Tube is normally supplied in mill lengths or specific cut lengths as agreed in the order.
Disclaimer: Data above are summarized from ASTM B729-20 (Standard Specification for Seamless Nickel-Iron-Chromium-Molybdenum-Copper Nickel Alloy Pipe and Tube), Tables 1 and 2, with dimensions per ASTM B829-24 as referenced by B729. The full and most current specification documents must always be consulted for official design and procurement purposes.
Common Industrial Applications
The combination of corrosion resistance, mechanical strength and manufacturing quality makes UNS N08020 seamless tube a preferred choice for:
Sulfuric Acid Production and Handling
Coolers, transfer lines and heat-exchanger tubing across concentration changes.
Petrochemical and Refining
Sour-crude processing equipment and acid lines exposed to H2S, chlorides and polythionic acids.
Pharmaceutical and Food Processing
Corrosion-resistant, easily cleaned tubing for citric and organic acid service.
Flue Gas Desulfurization
Scrubber and reagent circulation tubing in low-pH chloride-bearing slurries.
Ordering Guide
When specifying UNS N08020 seamless tube, please include: quantity, outside diameter and wall (nominal or minimum), delivery condition, length, and any third-party inspection or supplementary requirements — our sales team will confirm the production schedule and send a formal quotation.
Frequently Asked Questions
What sizes are available for Alloy 20 seamless tube?
The overall range is OD 6–257 mm; WT 0.5–35 mm. Requirements outside this range are reviewed against mill capability on request.
Which specification governs Alloy 20 seamless tube?
ASTM B729 / ASME SB729 is the governing specification for Alloy 20 seamless tube. The edition in force at order time is stated on the quotation and purchase order; chemistry, mechanical properties, heat treatment, dimensions and testing are reported against the ordered standard and material heat.
What inspection documents are provided?
Each order is documented with a mill test certificate stating heat chemistry and mechanical test results. EN 10204 3.1 certification, hydrostatic testing, eddy-current or ultrasonic inspection and PMI are performed when specified in the purchase order.
Ready to Request a Quotation?
Send the NPS and schedule (or actual OD and wall), quantity, length, end finish and the inspection items you need as acceptance criteria — you will get a written quotation stating the specification and edition, delivery condition, documents included and lead time.
