Grade overview
Alloy 20 composition and metallurgy
The nominal composition of Carpenter 20Cb-3 is 35Ni-35Fe-20Cr with copper, molybdenum and niobium additions. The function of each addition is set out below.
- Copper, 3 to 4 %. Improves resistance in non-oxidising acids, principally sulphuric and phosphoric acid. Conventional 300-series stainless steels contain no deliberate copper addition.
- Nickel, 32 to 38 %. At this level the alloy is not susceptible to chloride stress corrosion cracking under normal process conditions. 316L, at 10 to 14 % nickel, falls within the susceptible range.
- Chromium 19 to 21 % with molybdenum 2 to 3 %. Provide general passivity together with pitting and crevice corrosion resistance. The resulting PREN is 26 to 31.
- Niobium, with carbon restricted to 0.07 % maximum. Stabilises the alloy against chromium carbide precipitation at grain boundaries, so welded fabrications are normally placed in service without post-weld heat treatment.
Alloy 20 was developed in the 1950s for dilute sulphuric acid duty, which remains its principal application. It resists stress corrosion cracking in boiling sulphuric acid across the 20 to 40 % concentration range.
Hot working. The hot working range for Alloy 20 is narrower than for carbon and low alloy steel. The alloy work hardens rapidly and is liable to crack if forging continues below 980 °C. Jiangyin Jiangnan Metal forges UNS N08020 in controlled reheats rather than in a single extended operation. Full practice is given under forging and heat treatment.
Cross-reference
Equivalent designations for Alloy 20
The designations below all refer to the same material.
| System | Designation | Notes |
|---|---|---|
| Trade name | Carpenter 20Cb-3® | Carpenter Technology name. The Cb-3 suffix denotes columbium (niobium) stabilisation |
| Common name | Alloy 20 / Carpenter 20 | Generic industry usage |
| UNS | N08020 | Unified Numbering System |
| Werkstoff | 2.4660 | German material number |
| EN | NiCr20CuMo | European designation |
| AFNOR | Z2 NCUD 31-20 AZ | French designation |
| Forgings specification | ASTM B462 / ASME SB-462 | Flanges, forged fittings, valves and parts |
| Bar and wire | ASTM B473 | Also covers N08024 and N08026 |
| Plate, sheet, strip | ASTM B463 | |
| Seamless pipe and tube | ASTM B729 | Welded: B464 pipe, B468 tube |
Carpenter 20Cb-3® is a registered trademark of CRS Holdings, Inc., a Carpenter Technology Corporation company. Jiangyin Jiangnan Metal Co., Ltd. is not affiliated with, endorsed by or licensed by Carpenter Technology. The name is used only to identify the alloy grade specified on a customer drawing. Material is supplied to UNS N08020 chemistry in accordance with ASTM B462.
Chemistry
Chemical composition of UNS N08020
Each heat is verified against the limits below by spectrographic analysis. Ladle analysis and product analysis are both reported on the mill certificate.
| Element | Minimum | Maximum | Function |
|---|---|---|---|
| Nickel (Ni) | 32.00 | 38.00 | Austenite stability, resistance to chloride SCC |
| Chromium (Cr) | 19.00 | 21.00 | Passive film, resistance in oxidising acids |
| Copper (Cu) | 3.00 | 4.00 | Resistance in sulphuric and phosphoric acid |
| Molybdenum (Mo) | 2.00 | 3.00 | Pitting and crevice corrosion resistance |
| Niobium + tantalum (Nb+Ta) | 8 × C | 1.00 | Stabilisation against sensitisation on welding |
| Carbon (C) | 0.07 | Restricted to limit carbide precipitation | |
| Manganese (Mn) | 2.00 | Deoxidation, sulphur control | |
| Silicon (Si) | 1.00 | Deoxidation | |
| Phosphorus (P) | 0.045 | Residual | |
| Sulphur (S) | 0.035 | Residual, low S assists hot workability | |
| Iron (Fe) | Balance, 29.85 min | Matrix | |
Copper and niobium are specified elements of UNS N08020. A composition table that does not list Cu 3.00 to 4.00 % and Nb+Ta stabilisation is incomplete. Heat analysis should be obtained before order placement.
Mechanical data
Mechanical properties, solution annealed
The values below are specification minima for forgings in the solution annealed condition. Room temperature tensile testing is carried out on every heat lot and reported on the EN 10204 certificate.
| Property | Metric, min. | Imperial, min. | Test method |
|---|---|---|---|
| Ultimate tensile strength | 551 MPa | 80 ksi | ASTM A370 / ISO 6892-1 |
| Yield strength (0.2 % offset) | 241 MPa | 35 ksi | ASTM A370 / ISO 6892-1 |
| Elongation in 50 mm (2 in) | 30 % | 30 % | ASTM A370 |
| Reduction of area | 50 % | 50 % | Where stated on the purchase order |
| Hardness, annealed | 160 to 190 HB | 83 to 92 HRB | Typical value, not a specification limit |
Impact testing, elevated temperature tensile testing and grain size determination are available on request and can be added to the certificate.
Physical data
Typical physical properties
These are published typical values for UNS N08020 in the annealed condition, given for design calculation. They are not specification limits and are not certified values.
| Property | Metric | Imperial |
|---|---|---|
| Density | 8.08 g/cm³ | 0.292 lb/in³ |
| Melting range | 1425 to 1445 °C | 2600 to 2635 °F |
| Modulus of elasticity (20 °C) | 193 GPa | 28 × 10⁶ psi |
| Thermal conductivity (20 °C) | 12.6 W/m·K | 87 Btu·in/h·ft²·°F |
| Mean thermal expansion (20 to 100 °C) | 14.9 µm/m·K | 8.3 × 10⁻⁶ in/in·°F |
| Specific heat (20 °C) | 448 J/kg·K | 0.107 Btu/lb·°F |
| Electrical resistivity (20 °C) | 1.08 µΩ·m | 108 µΩ·cm |
| Relative magnetic permeability | 1.004 | Essentially non-magnetic |
Corrosion
Corrosion resistance and alloy selection
Alloy 20 is selected for four reasons: sulphuric acid service, phosphoric acid service, resistance to chloride stress corrosion cracking, and weldability without post-weld heat treatment. In strongly oxidising or mixed acid environments a higher molybdenum alloy such as Hastelloy C-276 or Alloy 31 gives better performance, at a higher material cost.
| Alloy | Ni % | Cr % | Mo % | Cu % | PREN (Cr + 3.3Mo) | Principal application |
|---|---|---|---|---|---|---|
| 316L | 10 to 14 | 16 to 18 | 2 to 3 | nil | 24 to 26 | General stainless duty. Limited in H₂SO₄ and hot chlorides |
| Alloy 20 (N08020) | 32 to 38 | 19 to 21 | 2 to 3 | 3 to 4 | 26 to 31 | Dilute to mid-concentration sulphuric acid, chloride SCC resistance |
| Alloy 825 (N08825) | 38 to 46 | 19.5 to 23.5 | 2.5 to 3.5 | 1.5 to 3 | 30 to 32 | Sour service, mixed reducing and oxidising duty |
| 904L (N08904) | 23 to 28 | 19 to 23 | 4 to 5 | 1 to 2 | 34 to 36 | Higher pitting resistance, lower nickel for SCC resistance |
| Alloy 31 (N08031) | 30 to 32 | 26 to 28 | 6 to 7 | 1 to 1.4 | 50 to 54 | Concentrated sulphuric and phosphoric acid plant |
| Hastelloy C-276 (N10276) | 57 min | 14.5 to 16.5 | 15 to 17 | nil | 68 to 76 | Severe mixed acids and wet chlorine. Highest cost |
Compositions are nominal ranges taken from the governing ASTM specifications. PREN is a screening index only. It does not predict behaviour in reducing acids and does not replace corrosion testing in the actual process fluid. Bars are scaled to the highest PREN in the table.
Service conditions
- Sulphuric acid. Resistant to stress corrosion cracking in boiling concentrations of 20 to 40 %. Used across dilute concentrations and at ambient temperature into the high concentration range.
- Phosphoric acid. Resistance derives from the copper addition.
- Chloride bearing water and brines. Not susceptible to chloride SCC in conditions where 316L cracks, typically above 60 °C.
- Nitric and mixed process acids. Good general resistance. Dedicated nitric service alloys may be preferred at high concentration and temperature.
Corrosion behaviour depends on concentration, temperature, aeration, velocity and contaminants acting in combination. Published data should be used for screening only. Coupon testing in the actual process stream is recommended before final material selection.
Production route
Forging and heat treatment practice
The sequence below is the production route used at Jiangyin Jiangnan Metal for UNS N08020 orders.
-
Melting and remelting
Ingot or billet is produced by AOD refining or vacuum induction melting. Vacuum arc remelting or electroslag remelting is applied where the specification requires improved cleanliness and reduced segregation.
-
Discard and billet preparation
A sufficient discard is taken from each ingot end to remove piping and undue segregation, as required by ASTM B462. Billet surfaces are conditioned before heating.
-
Heating to 1150 to 1230 °C (2100 to 2250 °F)
Soaked heating with calibrated pyrometry. Soak time is set by section thickness. Alloy 20 is sensitive to overheating and to uneven through-thickness temperature.
-
Open-die forging or ring rolling
Cogging, upsetting, punching and ring rolling to shape. Forging is stopped and the workpiece reheated before it falls below 980 °C (1800 °F). Working below that temperature risks cracking and produces a coarse, uneven grain structure.
-
Solution annealing at 940 to 1010 °C (1725 to 1850 °F)
Held for a minimum of 30 minutes per 25 mm (1 in) of section thickness, followed by water quench. Solution annealing dissolves carbides and restores the corrosion resistance reduced by hot work. Forgings that have not been correctly annealed and quenched will not give the expected performance in acid service.
-
Rough machining and dimensional inspection
Machined to the customer drawing with the agreed machining allowance. Dimensions are recorded against the drawing revision.
-
Non-destructive testing
Ultrasonic testing to EN 10228-3, SEP 1921 or ASTM A388 as specified, with liquid penetrant examination where required. See testing and certification.
-
Certification, marking and packing
EN 10204 3.1 or 3.2 certificate issued with heat number, chemical analysis, mechanical results, heat treatment record and NDT report. Parts are hard stamped or vibro-etched with the heat number for traceability to the certificate, then packed for sea or air freight.
Capability
Carpenter 20Cb-3 forged product forms
All forms below are produced in UNS N08020 to customer drawing or to standard dimensions.
Seamless rolled rings
Ring rolled, giving continuous circumferential grain flow.
OD 200 to 4,000 mmForged flanges
Weld neck, slip-on, blind, lap joint, threaded, socket weld and long weld neck.
1/2" to 80", Class 150 to 2500Discs and blanks
Solid and bored discs, tube sheets and blind plates.
Diameter to 2,500 mmBars, shafts and spindles
Round, square and stepped sections, eccentric shafts and crankshafts.
Dia 80 to 1,200 mm, L to 8,000 mmSleeves and bushings
Bored and rough machined hollow sections.
To drawingValve components
Bodies, bonnets, seat rings, stems, gates and wedges.
To drawingPipe and tube blanks
Forged and bored hollows, nozzles and reducers.
To drawingOther open-die shapes
Blocks, rolls, gear blanks, manifolds and crankshafts.
10 kg to 20,000 kgSingle piece weight ranges from 10 kg to 20,000 kg. Send a drawing or dimensioned sketch to sales@steelforgepieces.com for confirmation of feasibility, weight and price.
Quality
Testing, NDT and certification
Every Carpenter 20Cb-3 forging is supplied with documentation traceable to the heat number. State the required level on the enquiry and it will be included in the quotation.
| Item | Standard | Scope |
|---|---|---|
| Chemical analysis | ASTM B462 / ASTM E1473 | Ladle and product analysis, reported per heat |
| Tensile testing | ASTM A370 / ISO 6892-1 | Per heat lot, room temperature |
| Ultrasonic testing | EN 10228-3, SEP 1921, ASTM A388 | Acceptance class to purchase order |
| Liquid penetrant | ASTM E165 / EN ISO 3452 | On request or where specified |
| Intergranular corrosion | ASTM G28 method A | On request. Recommended for acid service |
| Positive material identification | XRF / OES | On request, per piece |
| Mill certificate | EN 10204 3.1 | Issued by the works inspection department |
| Third party certificate | EN 10204 3.2 | Witnessed by TÜV, SGS, BV, DNV or Lloyd's Register |
Service
Applications for Carpenter 20Cb-3 forgings
| Industry | Forged components | Reason for selection |
|---|---|---|
| Sulphuric acid production | Pump casings and shafts, valve bodies, flanges, heating coils, pickling tank fittings | Developed for dilute H₂SO₄. SCC resistance at 20 to 40 % boiling |
| Chemical and petrochemical | Reactor internals, mixing tank flanges, process piping flanges, bubble caps, tube sheets | General corrosion resistance with good weld fabricability |
| Pharmaceutical and fine chemicals | Agitator shafts, seal housings, nozzles, discs | Cleanability and resistance across varied acid streams |
| Fertiliser and phosphates | Pump components, rolled rings, sleeves | Phosphoric acid resistance from the copper addition |
| Oil, gas and marine | Seawater pump shafts, gate valve components, ballast system valves, expansion joint pins and hangers | Chloride SCC resistance and low temperature toughness |
| Pickling and surface treatment | Racks, drums, coils, tank fittings | Sustained acid exposure at temperature |
| Synthetic fibre and plastics | Spinnerette housings, process rolls, manifolds | Resistance in aggressive polymer process chemistry |
| Food and beverage processing | Valve parts, fittings, shafts | Resistance to organic acids and CIP chemicals |
Supplier
Jiangyin Jiangnan Metal Co., Ltd.
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, in the Yangtze River delta forging cluster approximately 150 km from the Port of Shanghai. The works produces open-die forgings and seamless rolled rings in carbon steel, alloy steel, tool steel, stainless steel and nickel alloys, including Carpenter 20Cb-3 (Alloy 20, UNS N08020), Inconel, Incoloy, Hastelloy, Monel and Nimonic grades.
Work is produced to customer drawing and shipped worldwide with EN 10204 3.1 or 3.2 material certification. Technical enquiries in English or Chinese are handled by sales@steelforgepieces.com and +86 189 2135 9659, which is also the WhatsApp and WeChat number.
Ordering information
Request a quotation for Alloy 20 forgings
Quotations are normally issued within one working day when the following information is provided.
- Grade: Carpenter 20Cb-3 / Alloy 20 / UNS N08020, and the governing specification
- Drawing or dimensions, with machining allowance if the part is rough machined
- Quantity and required delivery date
- Heat treatment condition, normally solution annealed and water quenched
- Testing scope: UT acceptance class, PT, ASTM G28, impact testing
- Certification level: EN 10204 3.1 or 3.2, and the third party if applicable
- Destination port and Incoterm
- Company
- Jiangyin Jiangnan Metal Co., Ltd.
Open-Die Forging Factory - Address
- No.1 Chengxiqiao Road,
Zhouzhuang Town, Jiangyin City,
Jiangsu Province, China - Telephone / WhatsApp
- +86 189 2135 9659
Questions
Frequently asked questions about Alloy 20 forgings
What is Carpenter 20Cb-3?
Carpenter 20Cb-3 (Alloy 20, UNS N08020, Werkstoff 2.4660) is an austenitic nickel-iron-chromium alloy containing 32 to 38 % nickel, 19 to 21 % chromium, 2 to 3 % molybdenum and 3 to 4 % copper, stabilised with niobium. It was developed for dilute sulphuric acid service and falls between the stainless steels and the nickel alloys in composition and cost.
Are Carpenter 20Cb-3, Alloy 20 and UNS N08020 the same material?
Yes. Carpenter 20Cb-3 is the Carpenter Technology trade name. Alloy 20 and Carpenter 20 are the generic names. UNS N08020 is the Unified Numbering System designation and Werkstoff 2.4660 (NiCr20CuMo) is the European equivalent. Whichever name appears on the drawing, heat analysis should be verified against the chemistry limits of ASTM B462 or ASTM B473.
Which standard covers Alloy 20 forgings?
ASTM B462, adopted by ASME as SB-462, covers forged or rolled UNS N08020 pipe flanges, forged fittings, valves and parts for corrosive high-temperature service. ASTM B473 covers bar and wire, ASTM B463 covers plate, sheet and strip, and ASTM B729 covers seamless pipe and tube.
Why is copper added to Alloy 20?
The 3 to 4 % copper addition improves resistance in non-oxidising acids, principally sulphuric and phosphoric acid. This is why Alloy 20 performs better than 316L and 317L in dilute to mid-concentration sulphuric acid service.
At what temperature is Carpenter 20Cb-3 forged?
Heat to 1150 to 1230 °C (2100 to 2250 °F) under controlled conditions and stop forging before the workpiece falls below 980 °C (1800 °F). After hot working, solution anneal at 940 to 1010 °C (1725 to 1850 °F) for a minimum of 30 minutes per 25 mm of thickness, followed by water quench.
Is Alloy 20 magnetic?
No. Alloy 20 is fully austenitic and is essentially non-magnetic in the solution annealed condition, with relative magnetic permeability of approximately 1.004. A slight magnetic response can develop after heavy cold work. Material with a strong magnetic response is not UNS N08020.
When should 316L be upgraded to Alloy 20?
Alloy 20 is specified in place of 316L where the service involves sulphuric or phosphoric acid, or where chloride stress corrosion cracking is a risk above approximately 60 °C. At around 33 % nickel Alloy 20 is not susceptible to chloride SCC, whereas 316L at 10 to 14 % nickel falls in the susceptible range. Where pitting rather than SCC governs, PREN values should also be compared, and 904L or Alloy 31 may be more suitable.
Does Alloy 20 require post-weld heat treatment?
Normally no. Carbon restricted to 0.07 % maximum together with niobium stabilisation suppresses chromium carbide precipitation, so welded fabrications are usually placed in corrosive service without post-weld heat treatment. Confirm against the project specification and design code. Typical filler metals are ER320LR for GTAW and GMAW and E320LR for SMAW.
Who forges Carpenter 20Cb-3 rings and flanges in China?
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, producing Carpenter 20Cb-3 (Alloy 20, UNS N08020) seamless rolled rings, flanges, discs, shafts, sleeves, tube sheets and bars to ASTM B462. Contact sales@steelforgepieces.com or +86 189 2135 9659.
What certification is supplied with Alloy 20 forgings?
EN 10204 3.1 mill certificates as standard, or EN 10204 3.2 witnessed by a third party such as TÜV, SGS, BV, DNV or Lloyd's Register. Ultrasonic testing is carried out to EN 10228-3, SEP 1921 or ASTM A388 as stated on the purchase order. ASTM G28 intergranular corrosion testing is available on request.
What is the largest Alloy 20 forged ring available?
Seamless rolled rings in UNS N08020 are produced from 200 mm to 4,000 mm outside diameter, with single piece weights from 10 kg to 20,000 kg. Send a drawing to confirm feasibility for a specific geometry.
What is the delivery time for Alloy 20 forgings?
Typical lead time is 25 to 45 days from order confirmation, depending on raw material availability, forging size, heat treatment cycle and the extent of machining and testing required.
Sources
Referenced standards
- ASTM B462. Standard Specification for Forged or Rolled Nickel Alloy Pipe Flanges, Forged Fittings, and Valves and Parts for Corrosive High-Temperature Service, covering UNS N08020. Adopted by ASME as SB-462.
- ASTM B473. Standard Specification for UNS N08020, UNS N08024 and UNS N08026 Nickel Alloy Bar and Wire.
- ASTM B463 plate, sheet and strip. ASTM B729 seamless pipe and tube. ASTM B464 and B468 welded pipe and tube.
- ASTM A370 and ISO 6892-1. Mechanical testing methods.
- EN 10228-3, SEP 1921 and ASTM A388. Ultrasonic examination of forgings.
- ASTM G28 Method A. Detecting susceptibility to intergranular attack in wrought nickel-rich alloys.
- EN 10204. Types of inspection document, 3.1 and 3.2.
Property data on this page is compiled from the governing ASTM specifications and published mill data for UNS N08020. Specification minima are binding. Values stated as typical are for guidance only and are not certified. Design to the current issue of the applicable standard and confirm requirements on the purchase order.
Maintained by the technical sales department of Jiangyin Jiangnan Metal Co., Ltd. Last reviewed .