ASTM B729 / ASME SB729 is the standard specification by ASTM International covering seamless nickel alloy pipe and tube in UNS N08020 (standard Alloy 20), UNS N08026, and UNS N08024 for general corrosive service. The specification covers cold-worked and hot-finished conditions. Alloy 20 is the industry standard for mixed acid environments (simultaneous nitric acid and sulfuric acid) where standard austenitic stainless steels fail prematurely due to stress corrosion cracking or pitting. It is one of the most widely specified nickel alloys for chemical process plant construction.

Zhengzhou Huitong (HT Pipe) supplies ASTM B729 seamless pipe in all specified UNS grades with full Mill Test Certificates (MTC) per EN 10204 3.1. 

1. Standard Scope & Designation

ASTM B729 covers seamless nickel alloy pipe and tube in three UNS grades. All alloys share the same iron-nickel-chromium base with molybdenum and copper additions for acid resistance. The specification applies to inch-pound and SI unit systems with tolerances per ASTM B829.

2. Grades Covered by ASTM B729

3. Chemical Composition — ASTM B729 Table 1

Values are maximum unless shown as a range. Heat and product analysis per ASTM B729-20.

Note: The copper content (3-4% in N08020) is the defining feature that distinguishes Alloy 20 from standard 18Cr-8Ni austenitic stainless steels. This copper addition provides superior resistance to sulfuric acid attack, which is the primary reason Alloy 20 was developed.

4. Mechanical Property Requirements — ASTM B729 Table 2

All values are minimum requirements per ASTM B729-20. Testing per ASTM E 8. Elongation measured over 2 in. (50 mm) gauge length.

5. Heat Treatment Requirements

6. Testing & Inspection Requirements

7. Pipe Dimensions — ASTM B729 (per ASME B36.19M)

Alloy 20 pipe dimensions follow ASME B36.19M (Stainless Steel Pipe). Standard schedules: Sch 5S, 10S, 40S, 80S. Tolerances per ASTM B829.

Note: Cold-drawn tube tolerances: OD ±0.13 mm (D<15.9mm); wall ±10% average. Hot-finished pipe tolerances: OD ±0.38–0.79 mm per OD range; wall nominal ±12.5%. Dimensions per ASME B36.19M. For tube sizes (≤2 in OD), common walls: 0.035", 0.049", 0.065", 0.083".

8. Industries & Applications

9. ASTM B729 vs ASTM B423 vs ASTM B407 — Nickel Alloy Pipe Comparison

10. Related Standards

11. How to Order — Required Information

12. Frequently Asked Questions

Q: Why is Alloy 20 specified instead of 316L stainless steel?

316L stainless steel fails in mixed acid environments (simultaneous nitric acid and sulfuric acid) due to stress corrosion cracking. Alloy 20 (UNS N08020) was developed specifically for this service. The key differentiator is the copper content (3-4%) which provides superior resistance to sulfuric acid — the primary acid in most chemical processing mixed acid streams. PREN of Alloy 20 (~34) is approximately 40% higher than 316L (~24), translating to significantly longer service life in aggressive media.

Q: What is the difference between Alloy 20 and Hastelloy G-30?

Both are high-performance nickel alloys for chemical processing, but in different performance tiers. Hastelloy G-30 (UNS N06030) has significantly higher chromium (28-31% vs 19-21%), molybdenum (4-6% vs 2-3%), and tungsten additions, providing superior resistance to wet-process phosphoric acid and highly concentrated acids. G-30 is specified when Alloy 20 experiences corrosion or when acid concentrations exceed Alloy 20's practical service limits. G-30 is approximately 40-60% more expensive than Alloy 20. Specify G-30 only when Alloy 20 performance has been confirmed inadequate by corrosion testing.

Q: Can ASTM B729 pipe be welded without post-weld heat treatment?

Yes. Like Incoloy 825, Alloy 20 does not require post-weld heat treatment. The niobium (columbium) addition in Alloy 20 provides stabilization against intergranular corrosion, similar to how titanium stabilizes Incoloy 825. ERNiCrMo-3 (Inconel 625) filler is recommended for maximum corrosion resistance in the weld zone. GTAW (TIG) with 100% argon shielding gas is preferred for the root pass. Interpass temperature: max 150°C (300°F).

Q: What is the PREN of Alloy 20 and why does it matter?

Alloy 20 has a PREN (Pitting Resistance Equivalent Number) of approximately 34, calculated as Cr + 3.3Mo + 16N. For comparison: 316L has PREN ~24, 904L has PREN ~34, and UNS N08367 (AL-6XN) has PREN ~45-48. PREN predicts resistance to pitting and crevice corrosion in chloride-containing environments. A higher PREN means better performance in seawater and halide-containing acids. For typical chemical processing mixed acid service (low chloride), PREN 34 is generally adequate.

Q: When should I specify UNS N08026 instead of standard N08020?

Specify UNS N08026 when the application involves seawater, brackish water, or higher chloride concentrations in the acid stream. The nitrogen addition (0.10-0.20%) increases yield strength (same minimum 240 MPa, but typically higher actual values) and improves pitting resistance. N08026 also has lower maximum carbon (0.03% vs 0.07% for N08020), which provides better resistance to sensitization in the weld heat-affected zone. For critical heat exchanger tube applications where both corrosion resistance and mechanical strength are important, N08026 is preferred.

Request a Quote — ASTM B729 Alloy 20 Seamless Pipe

Zhengzhou Huitong (HT Pipe) supplies ASTM B729 / ASME SB729 seamless pipe in UNS N08020 (Alloy 20), N08026, and N08024. Full EN 10204 3.1 CMTRs and competitive lead times for mixed acid, sulfuric acid, and chemical processing applications.