Alloy 20 Weld Neck Flanges (UNS N08020 / DIN 2.4660 / WP20CB) are precision-forged pipe flanges manufactured from one of the most important corrosion-resistant alloys ever engineered — an iron-nickel-chromium-molybdenum-copper (Fe-Ni-Cr-Mo-Cu) alloy that was developed specifically to withstand the corrosive attack of sulphuric acid (H₂SO₄) across virtually the full range of concentrations. The alloy's systematic name, "20", traces back to its development history, while its trade designations — Carpenter 20, Incoloy 20, Nicrofer 3620Nb — are all the same UNS N08020 material.
The weld neck design is the preferred flange type for Alloy 20 piping because it provides the strongest, most radiography-friendly joint available under ASME B16.5. The tapered hub transitions stress gradually from the flange body to the pipe wall, and the butt weld at the neck can be 100% radiographically or ultrasonically inspected — critical in acid-service piping where an undetected defect could cause a release of corrosive fluid and a process safety incident.
Tesco Steel & Engineering manufactures Alloy 20 weld neck flanges from ASTM B462-compliant forgings in all ASME B16.5 pressure classes and face types, supplying pharmaceutical plants, sulphuric acid manufacturers, fertiliser complexes, electroplating shops, and chemical EPC projects across 96+ countries.
Alloy 20 is an iron-base, austenitic, Fe-Ni-Cr-Mo-Cu alloy developed in the early 1950s by W. L. Silence at Carpenter Technology Corporation to provide a commercially viable solution to sulphuric acid corrosion in chemical plant equipment. Before Alloy 20, engineers had to choose between inadequate stainless steels that corroded rapidly in H₂SO₄ and far more expensive pure nickel or nickel-copper alloys (Monel).
The alloy bridges the composition gap between standard austenitic stainless steels (316L at ~12% Ni) and true nickel alloys (Monel 400 at ~67% Ni), sitting at 32–38% nickel. Four compositional features work in concert to deliver its performance:
| Element | Min % | Max % | Role in Alloy Performance |
|---|---|---|---|
| Iron (Fe) | Balance | Balance | Matrix metal; austenite former; cost-effective base vs. pure Ni alloys |
| Nickel (Ni) | 32.0 | 38.0 | Core corrosion resistance in reducing acids, H₂SO₄, and chloride environments |
| Chromium (Cr) | 19.0 | 21.0 | Oxidation and oxidising acid resistance; contributes to passive film stability |
| Copper (Cu) | 3.0 | 4.0 | Key element — reduces corrosion rate in H₂SO₄ across 10–98% concentration range |
| Molybdenum (Mo) | 2.0 | 3.0 | Improves pitting and crevice corrosion resistance; enhances reducing acid performance |
| Niobium (Nb) | 8×C min | 1.00 | Stabiliser — forms NbC carbides, prevents sensitisation and intergranular corrosion in welds |
| Carbon (C) | — | 0.07 | Low carbon reduces susceptibility to sensitisation; Nb stabilises residual C |
| Manganese (Mn) | — | 2.00 | Austenite stabiliser; deoxidiser |
| Silicon (Si) | — | 1.00 | Deoxidiser; minor corrosion resistance contribution at high temperatures |
| Phosphorus (P) | — | 0.045 | Controlled low; can cause hot cracking if elevated |
| Sulphur (S) | — | 0.035 | Controlled low; prevents hot cracking during welding |
| Property | Value (Min / Typical) | Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 551 MPa (80 ksi) | ASTM B462 / ASME SB-462 |
| 0.2% Proof Stress (YS) | ≥ 241 MPa (35 ksi) | ASTM B462 |
| Elongation | ≥ 30% (in 50 mm) | ASTM B462 |
| Hardness (Brinell) | ≤ 217 HB (typical) | ASTM E10 |
| Density | 8.08 g/cm³ | — |
| Melting Range | 1357–1412 °C (2475–2573 °F) | — |
| Modulus of Elasticity | 196 GPa (28.4 × 10⁶ psi) | — |
| Thermal Expansion (20–100°C) | 14.8 μm/m·°C | — |
| Thermal Conductivity (at 100°C) | 12.5 W/m·K | — |
| Trade Name | UNS | Werkstoff Nr. | ASTM (Forgings) | ASTM (Fittings) | AFNOR | EN |
|---|---|---|---|---|---|---|
| Alloy 20 / Carpenter 20 / Incoloy 20 / Nicrofer 3620Nb | N08020 | 2.4660 | ASTM B462 (F20) | ASTM B366 (WP20CB) | Z2NCUD31-20AZ | NiCr20CuMo |
| Alloy 20 (UNS N08020) Weld Neck Flanges — Complete Range | |
|---|---|
| Material Standard | ASTM B462 / ASME SB-462 (Forgings, UNS N08020) · ASTM A182 Grade F20 · DIN 2.4660 |
| Dimensional Standards | ASME B16.5 · ASME B16.47 Series A (MSS SP-44) & Series B (API 605) · DIN EN 1092-1 · BS 4504 · JIS B 2220 |
| Size Range | ½″ NB to 60″ NB (DN 15 to DN 1500) |
| Pressure Classes (ASME) | Class 150, 300, 400, 600, 900, 1500, 2500 |
| Pressure Ratings (DIN/EN) | PN 6, PN 10, PN 16, PN 25, PN 40, PN 64, PN 100, PN 160, PN 250, PN 320, PN 400 |
| Schedule / Wall Thickness | SCH 10, 20, 40, STD, 80, XS, 100, 120, 140, 160, XXS |
| Face Types | Raised Face (RF / WNRF) · Flat Face (FF) · Ring Type Joint (RTJ) · Tongue & Groove (T&G) · Male & Female (M&F) |
| Manufacturing Process | Open-die forging · Rolled ring forging · CNC machined |
| Heat Treatment | Solution annealed at 1066–1121 °C (1950–2050 °F) with rapid quench, per ASTM B462 |
| Surface Finish | Mill finish · Pickled & passivated · Electropolished · Custom Ra per specification |
| Special Services | Hot-Dip Galvanizing · Sand Blasting (Sa 2.5) · Shot Peening · Epoxy Coating · FBE Coating · Electropolishing |
Sulphuric Acid (H₂SO₄) — Primary Application
Alloy 20 was designed for H₂SO₄ service and remains the benchmark material for this medium. It provides acceptable corrosion rates across a wide range of concentrations — from dilute (5–10%) through to concentrated (93–98%) H₂SO₄ — at temperatures up to approximately 65–70 °C. At higher temperatures, Hastelloy C276 or alloy upgrades are required. The 3–4% copper content is the critical enabling element: copper shifts the electrochemical potential of the alloy into the passive region in the presence of SO₄²⁻ ions, reducing the corrosion rate by an order of magnitude compared to copper-free stainless steels.
Phosphoric Acid (H₃PO₄)
Alloy 20 weld neck flanges perform well in wet-process phosphoric acid (WPA) production, where dilute H₂SO₄, fluorosilicic acid, and phosphoric acid coexist. The combined chromium-molybdenum-nickel-copper composition provides corrosion rates low enough for long service life in phosphate rock digesters and clarifier piping — environments that rapidly corrode 316L stainless.
Mixed Acids (H₂SO₄ + HNO₃)
The high chromium content (19–21%) allows Alloy 20 to resist the mixed nitric-sulphuric acid environments encountered in nitration reactors, TNT production, and pharmaceutical API synthesis. This is a key advantage over Monel alloys, which are attacked by HNO₃.
Chloride Environments
Alloy 20 is significantly more resistant to chloride-induced pitting and stress corrosion cracking than 304 or 316L stainless steel, thanks to its elevated nickel content (32–38%) and 2–3% molybdenum. It is suitable for environments containing moderate chloride concentrations, though it should not be specified for aggressive seawater service where Super Duplex or Inconel 625 is required.
Sensitisation & Intergranular Corrosion Resistance
The niobium stabilisation of Alloy 20 ensures that the heat-affected zone (HAZ) of welds on weld neck flanges is resistant to intergranular attack. In the as-welded condition — without any PWHT — Alloy 20 passes the ASTM A262 Practice E (Strauss Test) for intergranular corrosion resistance. This is a significant practical advantage over unstabilised alloys like 316 (non-L) in acid piping systems where PWHT is impractical.
| Property | Alloy 20 (N08020) | 316L SS (S31603) | Incoloy 825 (N08825) | Hastelloy C276 (N10276) |
|---|---|---|---|---|
| Nickel % | 32–38% | 10–14% | 38–46% | ≥57% |
| Copper % | 3–4% ✓ | None ✗ | 1.5–3% ✓ | None ✗ |
| Molybdenum % | 2–3% | 2–3% | 2.5–3.5% | 15–17% |
| H₂SO₄ Resistance (10–70%) | Excellent | Poor above 10% | Very Good | Excellent |
| H₂SO₄ Resistance (70–98%) | Good (up to ~65°C) | Poor | Good | Excellent |
| HCl Resistance | Moderate | Poor | Moderate | Excellent |
| HNO₃ Resistance | Good (Cr-rich) | Good | Good | Moderate |
| Chloride SCC | Resistant | Susceptible | Resistant | Excellent |
| Stabilised (no PWHT needed) | Yes — Nb ✓ | No (L grade helps) ✓/✗ | Yes — Ti ✓ | Yes — Mo-rich ✓ |
| Relative Cost | Moderate | Low | Moderate-High | Very High |
| Primary Industry Use | H₂SO₄, pharma, electroplating | General process, dairy, pharma | Sour oil & gas, H₃PO₄, H₂SO₄ | HCl, mixed acids, oxidising + reducing |
Sulphuric Acid Manufacturing & Handling
Alloy 20 is the workhorse material in H₂SO₄ plants — from absorption tower piping and acid cooler connections to storage tank nozzles and transfer pump flanges. Weld neck flanges in Alloy 20 provide the leak-free butt-welded connection essential in acid piping where flange-to-pipe joint integrity is a process safety requirement. Sulphuric acid is used in fertiliser production (superphosphates, ammonium sulphate), metal pickling, petroleum refining (alkylation units), and explosives manufacturing.
Pharmaceutical Manufacturing
Pharmaceutical API (active pharmaceutical ingredient) synthesis routinely involves acid hydrolysis, acid wash steps, and acid-catalysed reactions. Alloy 20 weld neck flanges are specified in GMP-compliant piping systems for sulphuric acid, phosphoric acid, and mixed acid service — providing long service life without product contamination from corrosion products. The alloy's smooth internal bore, when electropolished, meets FDA cleanability standards.
Fertiliser & Agrochemical Industry
Wet-process phosphoric acid (WPA) plants, superphosphate granulation units, and ammonium sulphate crystallisers all handle aggressive acid slurries. Alloy 20 weld neck flanges are used throughout these circuits — in reactor outlet piping, flash cooler connections, filter header flanges, and crystalliser nozzles.
Mining & Hydrometallurgy
Acid leaching circuits in copper, nickel, zinc, and uranium ore processing use dilute to moderate H₂SO₄ at elevated temperatures. Alloy 20 flanges are used in leach solution piping, solvent extraction (SX) plant connections, and electrowinning cell header flanges where the combination of acid, oxidising conditions, and chloride ions creates a challenging corrosion environment.
Electroplating & Metal Finishing
Sulphuric acid and chromic acid plating baths, bright dipping solutions, and acid-cleaning systems all see Alloy 20 in flange and fitting applications — where its superior acid resistance and resistance to product contamination from corrosion products are critical.
Food Processing
In food-grade acid processing (citric acid, acetic acid, lactic acid fermentation), Alloy 20 flanges offer long service life without iron or chromium contamination that could affect product quality. The alloy is approved for food contact applications.
Chemical Processing
Chlorinated solvents, organic acid recovery, acetic anhydride, terephthalic acid (PTA) plants, and specialty chemical reactors are among the many chemical process environments where Alloy 20 weld neck flanges are the first choice over 316L.
Alloy 20 (N08020) is readily weldable by GTAW (TIG), GMAW (MIG), SMAW, and SAW using procedures qualified to ASME Section IX. Critical welding considerations:
Alloy 20 weld neck flanges follow the same ASME B16.5 dimensional tables as all other materials. The pressure-temperature (P-T) ratings for UNS N08020 are found under Material Group 3.3 of ASME B16.5 Table 2-1.1. Click below for complete dimension tables by class:
Q: What is Alloy 20 and why was it developed?
Alloy 20 (UNS N08020) is an iron-nickel-chromium-molybdenum-copper alloy developed specifically to resist sulphuric acid corrosion in chemical plant piping and equipment. Its 3–4% copper content is the critical design feature — copper is the most effective alloying addition for reducing corrosion rates in H₂SO₄ environments. The niobium stabilisation prevents intergranular corrosion in weld heat-affected zones, making it suitable for as-welded acid service without post-weld heat treatment.
Q: Is Alloy 20 a stainless steel or a nickel alloy?
Alloy 20 is technically neither. It is an iron-base alloy (Fe is the balance element) with 32–38% nickel — placing it compositionally between standard austenitic stainless steels (~12% Ni) and true nickel alloys (Monel ~67% Ni, Inconel 625 ~62% Ni). It is classified as a corrosion-resistant alloy (CRA) and covered by its own ASTM standard — B462 for forgings — separate from both the stainless steel standards (ASTM A182) and the nickel alloy standards (ASTM B564). However, it is also referenced as Grade F20 in ASTM A182.
Q: What is the maximum concentration of H₂SO₄ that Alloy 20 can handle?
Alloy 20 provides acceptable corrosion rates across the full concentration range of sulphuric acid (0–98%) at ambient to moderate temperatures. At room temperature, it resists virtually all concentrations. At 50–70 °C, it performs well in most industrial concentrations (10–98%). Above 70–80 °C, corrosion rates increase significantly, particularly in intermediate concentrations (40–80% H₂SO₄) — at these conditions, Incoloy 825 or Hastelloy C276 is recommended. Always consult corrosion isocorrosion diagrams for your specific concentration-temperature combination before final material selection.
Q: Can Alloy 20 be used in pharmaceutical applications?
Yes — Alloy 20 is widely used in pharmaceutical manufacturing. It resists the dilute-to-concentrated acids used in API synthesis and acid wash steps, does not contaminate products with corrosion products at acceptable operating conditions, and can be electropolished to achieve smooth, cleanable internal surfaces meeting FDA and GMP standards. It is a standard alloy in pharma piping codes (ASME BPE-adjacent specifications) for acid service lines.
Q: Does Alloy 20 need PWHT after welding?
No — this is one of Alloy 20's most significant fabrication advantages. The niobium stabilisation prevents sensitisation in the weld heat-affected zone, so post-weld heat treatment (PWHT) is not required for corrosion resistance in acid service. The alloy is used in the as-welded condition in most industrial applications. Optional solution annealing (PWHT) at 1066–1121 °C may be performed for stress relief in highly restrained assemblies, but it is not necessary for maintaining corrosion performance.
Q: What filler metal is used to weld Alloy 20 weld neck flanges?
The correct filler metal for Alloy 20 welding is ERNiFeCr-1 (AWS/SFA 5.14) for TIG/MIG welding and ENiFeCr-1 (AWS/SFA 5.11) for SMAW (stick). Using stainless steel filler metals (ER316L, ER309) is a common error that significantly reduces corrosion resistance at the weld joint in acid service — the weld metal will lack the copper and niobium of the base metal. Always use matching Alloy 20 consumables.
Q: What are the alternative names for Alloy 20?
Alloy 20 is known by several trade names, all referring to UNS N08020: Carpenter 20 (Carpenter Technology Corporation), Incoloy 20 (Special Metals), Nicrofer 3620Nb (VDM Metals), Sanicro 28 (Sandvik — note: Sanicro 28 is slightly higher-alloyed at 27% Ni, 31% Cr, 3.5% Mo, 1% Cu and is UNS N08028, distinct from N08020), and 20Cb-3 (Rolled Alloys). The ASTM forging designation is F20 (ASTM A182) and the fittings designation is WP20CB (ASTM B366).
Alloy 20 (UNS N08020) weld neck flange prices are influenced by nickel, chromium, molybdenum, and copper market rates, as well as flange type, NPS, pressure class, and quantity. As a specialty alloy at a moderate price point between stainless steel and high-nickel alloys, Alloy 20 offers excellent value for sulphuric acid and mixed-acid services where 316L is inadequate but full Hastelloy is over-engineered.
Tesco Steel & Engineering offers competitive factory-direct pricing with full documentation. Please use the Inquiry Form or contact our team on WhatsApp at +91 92233 66922 for an itemised quotation with current stock availability and lead times. Our technical sales team responds within 24 hours.
Countries We Export Alloy 20 Weld Neck Flanges To: Kuwait, UAE, Saudi Arabia, Germany, West Africa, Iraq, Congo, Mexico, Bahrain, Canada, Philippines, Thailand, Kenya, Oman, Malaysia, Turkey, Qatar, Sudan, Netherlands, Nigeria, Lithuania, Gabon, Russia, Vietnam, Angola, Bolivia, Indonesia, UK, Yemen, Italy, United States, Venezuela, Spain, Iran, Estonia, Kazakhstan, Algeria, Jordan, Ecuador, Portugal, Colombia, Libya, Chile, Peru, South Africa, Bangkok, Namibia, Jeddah, Afghanistan, Israel, Zambia, Macau, Morocco, Denmark, Taiwan, Norway, Belarus, North Macedonia, Lebanon, Sri Lanka, Bhutan, Bulgaria, Albania, Ukraine, Belgium, Finland, Slovakia, Tibet, Romania, France, Brazil, Trinidad & Tobago, Fiji, Tunisia, Gambia, Hungary, Zimbabwe, Mongolia, Ghana, Egypt, Czech Republic, Azerbaijan, Poland, Greece, Costa Rica, Kosovo, New Zealand, Croatia, Puerto Rico, Tanzania, Somalia, Australia, Singapore, Japan, South Korea.
Domestic Supply — Alloy 20 Weld Neck Flanges in India: Mumbai, Delhi, Bangalore, Hyderabad, Ahmedabad, Chennai, Kolkata, Surat, Pune, Jaipur, Lucknow, Kanpur, Nagpur, Visakhapatnam, Indore, Thane, Bhopal, Pimpri-Chinchwad, Patna, Vadodara, Ghaziabad, Ludhiana, Coimbatore, Agra, Andhra Pradesh, Arunachal Pradesh, Assam, Bihar, Chandigarh, Chhattisgarh, Goa, Gujarat, Haryana, Himachal Pradesh, Jammu & Kashmir, Jharkhand, Karnataka, Kerala, Madhya Pradesh, Maharashtra, Manipur, Meghalaya, Mizoram, Nagaland, Odisha, Punjab, Rajasthan, Sikkim, Tamil Nadu, Telangana, Tripura, Uttar Pradesh, Uttarakhand, West Bengal.