Alloy 20 contains 32–38% nickel and 31–44% iron, placing it on the boundary between austenitic stainless steel and nickel alloy. ASTM classifies it under Committee B-2 (Nonferrous Metals nickel alloys), not Committee A-1, which classifies it as steel. When purchasing, treat it as a nickel alloy. The 'N' prefix in the UNS system designates nickel and nickel-based alloys.
ASTM B729 Alloy 20 (UNS N08020) Seamless Pipe & Tube Standard
ASTM B729 is the standard specification for seamless UNS N08020, UNS N08026, and UNS N08024 nickel-alloy pipe and tube, covering the Alloy 20 family of materials. Alloy 20 (UNS N08020), also known as Carpenter 20Cb-3, is designed specifically for surviving sulfuric acid. It performs better than any other alloy in its price range.
The specification defines chemical composition, mechanical properties, heat treatment conditions, hydrostatic or non-destructive electric testing, and product marking for three related Ni-Fe-Cr-Mo-Cu-Nb alloys. In most mixed-acid and chloride environments, corrosion resistance is greater than that of 316L stainless steel but less than that of Incoloy 825. They offer better sulfuric acid resistance than 316L at a lower cost than 825, with niobium stabilisation that prevents intergranular corrosion in the as-welded condition.
ASTM B729 Alloy 20 pipe and tube is specified in sulfuric acid plants, phosphoric acid evaporators, chemical process heat exchangers, pharmaceutical reactors, food processing equipment, pickling systems, and FGD scrubbers. This guide covers every part of B729, from the three alloy grades, their chemical composition, mechanical properties, heat treatment differences, dimensional tolerances, hydrostatic testing, key applications, and practical FAQs.
ASTM B729 PDF Free Download
ASTM B729 PDF Free Download
What Is ASTM B729?
ASTM B729 is the ASTM International standard specification for seamless UNS N08020, UNS N08026, and UNS N08024 nickel-alloy pipe and tube. It falls under ASTM Committee B-2 on Nonferrous Metals and Alloys, Subcommittee B02.07 on Refined Nickel and Cobalt and Their Alloys.
All three alloys share the same nickel-iron-chromium-molybdenum-copper-niobium base chemistry, but each has a different balance of elements and heat treatment for specific corrosive environments. General requirements are governed by ASTM B829.
The Alloy 20 family has a combination of copper (for sulfuric acid), molybdenum (for pitting and crevice corrosion), and niobium (for intergranular corrosion prevention). The niobium content, calculated as 8 times the carbon content, up to 1.00% max, ties up carbon as niobium carbides (NbC) instead of chromium carbides (Cr₂₃C₆), preventing the chromium depletion at grain boundaries that causes sensitisation. This means Alloy 20 can be used in the as-welded condition without solution annealing, a major fabrication advantage.
The ASME equivalent is SB-729, listed in the ASME Boiler and Pressure Vessel Code, Section II, Part B. Materials meeting ASTM B729 can carry the SB-729 designation for code-stamped pressure equipment. ASME limits the maximum design temperature to 1000°F (538°C) for Alloy 20 in pressure service.
Chemical Composition of ASTM B729
All three ASTM B729 alloys share the Ni-Fe-Cr-Mo-Cu structure but differ in the balance of these elements, particularly molybdenum, copper, and nitrogen. N08020 has the highest copper (3–4%) for sulfuric acid; N08026 has the highest molybdenum (5–6.70%) and nitrogen (0.10–0.16%) for chloride pitting; N08024 bridges the two. The table below lists the full composition requirements.
| GRADE | C | Mn | P | S | Si | Cr | Ni | Mo | Ti | Cu | Al | Other |
| Alloy 20 (N08020) | 0.07 | 2.00 | 0.045 | 0.035 | 1.00 | 19.0–21.0 | 32.0–38.0 | 2.0–3.0 | - | 3.0–4.0 | - | Nb+Ta: 8C-1 |
| Alloy 20Mo-6 (N08026) | 0.03 | 1.00 | 0.03 | 0.03 | 0.50 | 22.0–26.0 | 33.0–37.2 | 5.0–6.7 | - | 1.0–3.5 | - | Fe: BAL |
| Alloy 20Mo-4 (N08024) | 0.03 | 1.00 | 0.035 | 0.035 | 0.50 | 22.5–25.0 | 35.0–40.0 | 3.5–5.0 | - | 0.50–1.50 | - | Fe: BAL; Nb+Ta: 8C–0.50 |
Mechanical Properties of ASTM B729
All three alloys in ASTM B729 share the same minimum mechanical properties: 80,000 psi (550 MPa) tensile strength, 35,000 psi (240 MPa) yield strength, and 30% elongation. This uniform requirement simplifies material selection. The choice between N08020, N08026, and N08024 is driven entirely by corrosion performance, not mechanical strength.
| Grade | Condition & Size | Standard | Heat Treating Temp. | Tensile Strength Min. MPa |
Yield Strength Min. MPa |
Elongation Min. % |
| Alloy 20 (N08020) |
Annealed | B729 | 925–1010°C | 550 | 240 | 30 |
| Alloy 20Mo-6 (N08026) |
Annealed | B729 | 1065–1120°C | 620 | 275 | 30 |
| Alloy 20Mo-4 (N08024) |
Annealed | B729 | 1010–1065°C | 586 | 241 | 30 |
Note: These are room-temperature minimum values.
ASTM B729 Tolerances
ASTM B729 tolerances define the permissible variations in outside diameter, wall thickness, and length for cold-worked and hot-finished nickel alloy pipe, as specified in the standard.
| STANDARD | OD(D) | TOLERANCE(MM) | THICKNESS(S) | TOLERANCE | LENGTH | TOLERANCE |
| MM | COMMON HIGH | MM | MM | MM | ||
| B729 (Cold-worked) |
D≤10 | ±0.1 | D<16, S(Ave) | ±15% | D≤50.8 | +3.2/0 |
| 10<D≤16 | ±0.13 | 16<D≤114, S(Ave) | ±10% | |||
| 16<D≤38 | ±0.19 | 114<D≤219, S(Ave) | ±12.5% | |||
| 38<D≤76 | ±0.25 | 219<D≤610, S(Ave) | ±15%/-12.5% | |||
| 76<D≤114 | ±0.38 | D≤16, S(Min) | +30%/0 | |||
| 114<D≤152 | ±0.51 | 16<D≤114, S(Min) | +22%/0 | D≥50.8 | +4.8/0 | |
| 152≤D≤168 | ±0.64 | 114<D≤219, S(Ave) | +28%/0 | |||
| 168<D≤219 | ±0.79 | 219<D≤610, S(Ave) | +30%/0 | |||
| 219<D≤356 | +1.57/-0.79 | |||||
| 356<D≤610 | +3.18/-0.79 | |||||
| B729 (Hot-finished) |
19≤D≤38 | -0.5 | All size, S(Ave) | ±12.5% | D≤50.8 | +3.2/0 |
| 38.1<D≤102 | ±0.8 | D≥50.8 | +4.8/0 |
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ASTM B729 Alloy Grades
ASTM B729 covers three UNS designations. N08020 (Alloy 20 / Carpenter 20Cb-3) accounts for over 90% of B729 production by volume. N08026 is a higher-molybdenum, nitrogen-bearing variant for more aggressive environments. N08024 sits between the two with moderate molybdenum and tighter chemistry control. The table below summarises the key differences.
| Parameter | N08020 (Alloy 20) | N08026 | N08024 |
| Trade Name | Carpenter 20Cb-3 | — | — |
| Nickel, % | 32–38 | 33–37.20 | 35–40 |
| Chromium, % | 19–21 | 22–26 | 22.5–25 |
| Molybdenum, % | 2–3 | 5–6.70 | 3.5–5 |
| Copper, % | 3–4 | 2–4 | 0.5–1.5 |
| Nb + Ta | 8 × C to 1.00 | — | 0.15–0.35 |
| Nitrogen, % | — | 0.10–0.16 | — |
| Carbon, % max | 0.07 | 0.03 | 0.03 |
| Condition | Stabilised-annealed | Solution-annealed | Annealed |
| Anneal Temp, °F (°C) | 1800–1850 (982–1010) | 2050–2200 (1121–1204) | 1925–1975 (1052–1079) |
| Sulfuric Acid Service | Primary application | Superior (higher Mo) | Moderate |
| Chloride Pitting | Comparable to 316L | Superior (higher Cr+Mo+N) | Better than N08020 |
UNS N08020
Alloy 20 contains Ni (32–38% Ni), 20% Cr, 2.5% Mo, and 3.5% Cu and is specifically designed for sulfuric acid service. Copper provides strong resistance across a wide sulfuric acid concentration range, while niobium stabilisation prevents intergranular corrosion. It is supplied in the stabilised-annealed condition, suitable for use in the as-welded state without post-weld heat treatment.
UNS N08026
UNS N08026 is the higher-alloyed version with 22–26% Cr, 5–6.7% Mo, and added nitrogen for enhanced corrosion resistance. It offers superior pitting and crevice corrosion resistance in hot chloride and mixed acid environments. It is solution-annealed at higher temperatures to maximise alloy performance in aggressive sulfuric acid–chloride conditions.
UNS N08024
UNS N08024 is the intermediate grade with higher Ni and Cr than N08020 and moderate molybdenum content. It provides improved general corrosion resistance and better performance in mixed-acid environments. It is stabilised and annealed with controlled carbon content for reliable weldability and structural integrity.
Heat Treatment Requirements for ASTM B729
Each ASTM B729 alloy requires a different heat treatment to achieve its specified corrosion resistance. The main distinction is between stabilised annealing (N08020), solution annealing (N08026), and standard annealing (N08024). All three require rapid cooling through water quenching or equivalent after annealing to prevent carbide reprecipitation.
N08020’s lower annealing temperature (1800–1850°F vs 2050–2200°F for N08026) is possible because its niobium stabilisation provides intergranular corrosion protection even at lower temperatures. N08026 needs the higher temperature to dissolve its elevated molybdenum and chromium content into solid solution; without this, Mo-rich intermetallic phases can precipitate and reduce corrosion resistance.
Testing and Inspection Requirements
ASTM B729 requires chemical analysis, tension testing, and either hydrostatic or non-destructive electric (NDE) testing on every lot. These requirements work with the general testing framework of ASTM B829.
Chemical Analysis
One test per lot (one heat). Product (check) analysis is the purchaser’s responsibility and must conform to the required tolerances.
Tension Test
One test per lot. Full tubular specimens are preferred; longitudinal strip or round specimens are used when full tubular testing is not possible. Testing follows ASTM E 8.
Hydrostatic or Non-Destructive Electric Test
Each pipe and tube is tested by either a hydrostatic pressure test or a non-destructive electric test (eddy current) at the manufacturer’s option. The purchaser may specify which test is to be performed.
Frequently Asked Questions
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Is Alloy 20 a Stainless Steel or a Nickel Alloy?
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How Does Alloy 20 Compare to 316L for Sulfuric Acid?Alloy 20 significantly outperforms 316L in sulfuric acid service. In 20% H₂SO₄ at 175°F (80°C), 316L corrodes at roughly 50–100 mpy (1.3–2.5 mm/a) as it is verified against mill data for design use, while Alloy 20 stays below 5 mpy (0.13 mm/a). The 3.5% copper in Alloy 20 creates a protective CuSO₄ surface film that significantly slows the dissolution rate. 316L has no copper and relies solely on its Cr₂O₃ passive film, which breaks down in reducing acid environments.
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How Does Alloy 20 Compare to Incoloy 825 (ASTM B423)?Incoloy 825 has overlapping molybdenum content (2.5–3.5% vs 2–3% for Alloy 20) and higher nickel (~42%), with titanium stabilisation instead of niobium. Unlike Alloy 20's niobium stabilisation, which permits as-welded use, 825's titanium stabilisation requires a controlled stabilising anneal. This gives 825 broader corrosion resistance in most environments, but at 15–25% higher material cost. Alloy 20 is the better value when the primary corrosive agent is sulfuric acid at moderate temperatures. For mixed-acid environments with significant chloride exposure, ASTM B423 may be suitable.
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Can Alloy 20 Be Used in Hydrochloric Acid?Alloy 20 can be used only in very dilute HCl (under 1–2%) at room temperature. It is not recommended for concentrated or hot HCl service. Its 2.5% Mo provides limited reducing-acid resistance compared to Hastelloy B-2 (28% Mo) or Hastelloy C-276 (16% Mo).
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What Welding Filler Metal Is Used for Alloy 20?The standard filler metal for Alloy 20 is ERNiCrMo-3 (Inconel 625 filler, AWS A5.14) for GTAW, or ENiCrMo-6 (AWS A5.11) for SMAW. The matching filler wire ER320 (AWS A5.9) can also be used, but is less commonly specified because ERNiCrMo-3 provides higher corrosion resistance in the weld deposit. No preheating is required. Interpass temperature should be maintained below 300°F (150 °C). The niobium stabilisation in the base metal means PWHT is generally not needed.
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What Is the ASME Equivalent of ASTM B729?ASME SB-729 is the identical specification adopted into ASME Boiler and Pressure Vessel Code, Section II, Part B. Materials meeting ASTM B729 can carry the SB-729 designation for code-stamped pressure vessels, heat exchangers, and piping. ASME limits the maximum design temperature to 1000°F (538°C) for Alloy 20.
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What Is the Difference Between N08020 and N08026?N08026 has roughly double the molybdenum (5–6.70% vs 2–3%), higher chromium (22–26% vs 19–21%), and intentional nitrogen (0.10–0.16%). This gives N08026 superior pitting and crevice corrosion resistance and better performance in hot chloride-bearing sulfuric acid. N08026 is solution-annealed at 2050–2200°F (much higher than N08020’s 1800–1850°F). Use N08026 when standard Alloy 20 reaches its corrosion limits, particularly in chloride-contaminated acid environments.
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Does Alloy 20 Resist Stress Corrosion Cracking?Yes. The 35% nickel content makes Alloy 20 resistant to the two failure modes that destroy 304 and 316 stainless steels: chloride SCC and polythionic acid SCC. Alloys with more than 25–30% nickel are generally considered immune to chloride SCC. Alloy 20 exceeds this threshold. Alloy 20 may be qualified under NACE MR0175/ISO 15156 for sour service, subject to hardness limits and environmental conditions defined in the standard. Consult the standard directly before specifying for H₂S service.
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What Is the Maximum Service Temperature for Alloy 20?ASME limits Alloy 20 to 1000°F (538°C) for pressure service. Above this temperature, the alloy loses its corrosion advantage and does not have the creep strength of high-temperature alloys like Incoloy 800H (ASTM B407).