NAS 335X at a glance
- Grade
- NAS 335X (also written NAS335X, NAS 335-X)
- UNS number
- N08020
- Generic name
- Alloy 20; Carpenter 20Cb-3®; Incoloy® alloy 20
- Producer designation
- NAS 335X is the grade name used by Nippon Yakin Kogyo Co., Ltd. (Japan)
- Alloy family
- Nickel-iron-chromium, austenitic, solid solution, not hardenable by heat treatment
- Nominal chemistry
- 20Cr, 33Ni, 2.5Mo, 3.5Cu, 0.4Nb, balance iron
- Forging standard
- ASTM B462 / ASME SB-462 (forged flanges, fittings, valves and parts)
- Bar standard
- ASTM B473 / ASME SB-473
- Delivery condition
- Solution annealed 925 to 1010 °C (1700 to 1850 °F), rapidly cooled
- Principal property
- Resistance to sulfuric acid over a wide concentration and temperature range
- Density
- 8.08 g/cm³ (0.292 lb/in³)
- Magnetic response
- Essentially non-magnetic in the annealed condition
- Certification
- EN 10204 3.1 as standard; 3.2 with third-party witness on request
- Forged by
- Jiangyin Jiangnan Metal Co., Ltd., Jiangyin City, Jiangsu Province, China
What NAS 335X is, and when to specify it
NAS 335X was developed for sulfuric acid service. Type 304 and type 316 stainless steels corrode in dilute and intermediate sulfuric acid, particularly when the acid is hot or aerated. Nickel-base alloys resist the same acid but cost several times more per kilogram. NAS 335X carries enough nickel for austenite stability, enough copper for passivity in reducing acid, and an iron balance that holds the price below the nickel-base grades.
The four significant additions and their functions:
- Copper, 3 to 4%. Shifts the corrosion potential in reducing acids. This is the main addition for sulfuric acid resistance.
- Nickel, 32 to 38%. Stabilises the austenitic structure and provides resistance to chloride stress-corrosion cracking.
- Molybdenum, 2 to 3%. Improves resistance to pitting and crevice attack when the acid contains chlorides.
- Niobium plus tantalum, 8 x C to 1.00%. Combines with carbon as niobium carbide so that chromium carbides do not form at grain boundaries. Welds and heat-affected zones therefore resist intergranular attack without a post-weld solution anneal.
NAS 335X is normally specified when the process fluid is sulfuric acid or a mixed acid of reducing character, when the part is welded and cannot be solution annealed afterwards, or when chloride stress-corrosion cracking has caused a failure in 304 or 316. It is not suitable for hydrochloric acid, hydrofluoric acid or wet chlorine. It is a corrosion-resistant alloy rather than a heat-resistant one, and service above about 400 °C (750 °F) is uncommon.
Chemical composition of NAS 335X (UNS N08020)
Composition limits are those of UNS N08020 in ASTM B462 and ASTM B473, in weight percent. The highlighted rows are the two additions that separate NAS 335X from a conventional austenitic stainless steel.
| Element | Minimum % | Maximum % | Function |
|---|---|---|---|
| Nickel (Ni) | 32.00 | 38.00 | Stabilises austenite; resistance to chloride stress-corrosion cracking |
| Chromium (Cr) | 19.00 | 21.00 | Forms the passive film; general corrosion and oxidation resistance |
| Iron (Fe) | — | balance | Base element; holds cost below the nickel-base grades |
| Copper (Cu) | 3.00 | 4.00 | Passivity in reducing acid, in particular sulfuric acid |
| Molybdenum (Mo) | 2.00 | 3.00 | Pitting and crevice resistance in chloride-bearing acid |
| Niobium + Tantalum (Nb+Ta) | 8 x C | 1.00 | Stabilisation; prevents chromium-carbide precipitation and intergranular attack |
| Carbon (C) | — | 0.07 | Held low; the niobium ratio is calculated from the measured carbon content |
| Manganese (Mn) | — | 2.00 | Deoxidiser; combines with sulfur |
| Silicon (Si) | — | 1.00 | Deoxidiser |
| Phosphorus (P) | — | 0.045 | Residual limit |
| Sulfur (S) | — | 0.035 | Held low for hot workability and toughness |
The niobium requirement is a ratio rather than a fixed figure. At 0.05% carbon the minimum niobium plus tantalum is 0.40%; at the 0.07% carbon limit it is 0.56%. Heat analysis is checked by spectrometer before forging and reported on the mill certificate issued by Jiangyin Jiangnan Metal Co., Ltd.
Mechanical properties of NAS 335X forgings
Specified minimums, solution annealed
These are the values a mill certificate must meet for UNS N08020 in the annealed condition. They are minimum values, not typical results.
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength, minimum | 551 MPa | 80 ksi |
| Yield strength, 0.2% offset, minimum | 241 MPa | 35 ksi |
| Elongation in 50 mm (2 in), minimum | 30 % | 30 % |
| Hardness, maximum | 217 HB | 217 HB (approx. 95 HRB) |
Typical as-delivered values
Solution annealed NAS 335X forgings normally test above the specified minimums. The ranges below are representative of production and vary with section size, reduction ratio and cooling rate.
| Property | Typical range (metric) | Typical range (imperial) |
|---|---|---|
| Tensile strength | 580 to 690 MPa | 84 to 100 ksi |
| Yield strength, 0.2% offset | 260 to 360 MPa | 38 to 52 ksi |
| Elongation | 35 to 50 % | 35 to 50 % |
| Reduction of area | 50 to 70 % | 50 to 70 % |
| Hardness | 140 to 190 HB | 140 to 190 HB |
Typical values are given for design screening. The mill test certificate supplied with each forging governs acceptance. Elevated-temperature tensile, impact and stress-rupture testing can be added to the test plan.
Physical properties of NAS 335X
| Property | Metric | Imperial |
|---|---|---|
| Density | 8.08 g/cm³ | 0.292 lb/in³ |
| Melting range | 1370 to 1430 °C | 2500 to 2600 °F |
| Modulus of elasticity | 193 GPa | 28 x 106 psi |
| Specific heat, 20 to 100 °C | 500 J/kg·K | 0.12 Btu/lb·°F |
| Thermal conductivity, 100 °C | 11.9 W/m·K | 6.9 Btu·ft/ft²·h·°F |
| Mean expansion, 20 to 100 °C | 14.8 µm/m·K | 8.2 x 10-6 in/in·°F |
| Electrical resistivity | 1.08 µΩ·m | 108 µΩ·cm |
| Magnetic permeability | 1.003 | essentially non-magnetic |
Physical properties are nominal published values for UNS N08020. They are not certified on the mill certificate unless separately agreed.
Corrosion behaviour of NAS 335X
The pitting resistance equivalent number, calculated as Cr + 3.3 x Mo, is approximately 26 to 31 depending on where the heat falls within the composition range. This is higher than 316L, lower than 904L and well below the 6% molybdenum grades. The alloy is selected for resistance to reducing acids rather than for chloride pitting resistance.
| Medium | Behaviour of NAS 335X |
|---|---|
| Sulfuric acid | The design duty. Resistant over a wide range of concentration and temperature, including hot dilute and intermediate concentrations in which 316L fails. Aeration and oxidising contaminants generally improve performance. High velocity and abrasive solids reduce it. |
| Phosphoric acid | Used for wet-process phosphoric acid plant equipment, including contaminated commercial acid. |
| Nitric acid | Good resistance in most commercial concentrations. |
| Chloride-bearing waters | The nickel content gives good resistance to stress-corrosion cracking, which is the usual failure mode for 304 and 316. Pitting resistance is moderate, so check chloride level and temperature. |
| Organic acids, solvents, pharmaceuticals | Resistant. Also used where iron pick-up would contaminate the product. |
| Welded assemblies | Niobium stabilisation allows the heat-affected zone to resist intergranular attack without a post-weld solution anneal. |
| Hydrochloric acid, hydrofluoric acid, wet chlorine | Not suitable. Use a nickel-molybdenum alloy such as Hastelloy B-2, B-3 or C-276. |
| Sustained service above 400 °C | Outside the intended service range. For hot gas or creep duty use Incoloy 800H, 800HT, Inconel 617 or Haynes 230. |
Corrosion guidance is qualitative and does not replace a corrosion test in the actual process fluid. For critical duty, request intergranular corrosion testing to ASTM A262 or ASTM G28 Method A on the delivered forging. Jiangyin Jiangnan Metal Co., Ltd. includes either test in the test plan on request.
NAS 335X equivalent designations
| System | Designation |
|---|---|
| UNS (USA) | N08020, the designation to use on a drawing or purchase order |
| Generic industry name | Alloy 20; Alloy 20Cb-3 |
| Nippon Yakin Kogyo (Japan) | NAS 335X |
| Werkstoff Nr. (Germany) | 2.4660 (nearest, NiCr20CuMo) |
| ASTM / ASME | B462 / SB-462 forgings; B473 / SB-473 bar; B463 plate; B729 seamless pipe |
| Trade names | Carpenter 20Cb-3®, Incoloy® alloy 20, Nicrofer® 3620 Nb, Sandvik Sanicro® 20 |
Designations marked as nearest are close but not identical, and national standards do not apply the same limits to trace elements. Where a design registration or code case applies, order to the UNS number and check equivalence against the mill certificate. 20Cb-3 is a registered trademark of Carpenter Technology Corporation, Incoloy of Special Metals Corporation, Nicrofer of VDM Metals and Sanicro of Sandvik. Jiangyin Jiangnan Metal Co., Ltd. is not affiliated with those companies. The names are used only to identify the equivalent material specification.
NAS 335X compared with 316L, 904L, Incoloy 825 and Hastelloy C-276
These five grades are often considered for the same enquiry. The table lists the differences that affect selection.
| Criterion | 316L | NAS 335X / Alloy 20 | 904L | Incoloy 825 | Hastelloy C-276 |
|---|---|---|---|---|---|
| UNS | S31603 | N08020 | N08904 | N08825 | N10276 |
| Nickel, % | 10 to 14 | 32 to 38 | 23 to 28 | 38 to 46 | balance |
| Molybdenum, % | 2 to 3 | 2 to 3 | 4 to 5 | 2.5 to 3.5 | 15 to 17 |
| Copper, % | — | 3 to 4 | 1 to 2 | 1.5 to 3 | — |
| Sulfuric acid | Poor | Very good | Good | Good | Excellent |
| Chloride pitting (PREN) | 24 | 26 to 31 | 34 | 30 | 65 and above |
| Chloride SCC | Vulnerable | Resistant | Resistant | Very resistant | Immune in practice |
| Hydrochloric acid | No | No | No | Limited | Yes |
| Relative cost | 1x | 3x | 3.5x | 4.5x | 9x |
| Select it when | General service, no strong acid | Sulfuric or phosphoric acid duty at moderate cost | Chloride pitting governs | Mixed acid with chloride SCC risk | Hydrochloric acid, wet chlorine |
Relative cost is an approximate indication of forged material cost against 316L and moves with nickel and molybdenum prices. Ask Jiangyin Jiangnan Metal Co., Ltd. for a current comparison on your part before selecting the grade.
Applicable standards by product form
| Product form | ASTM | ASME |
|---|---|---|
| Forged flanges, fittings, valves and parts | B462 | SB-462 |
| Bar and wire | B473 | SB-473 |
| Plate, sheet and strip | B463 | SB-463 |
| Seamless pipe and tube | B729 | SB-729 |
| Welded pipe | B464 | SB-464 |
| Welded tube | B468 | SB-468 |
| Factory-made wrought fittings | B366 | SB-366 |
Allowable stresses for UNS N08020 in pressure-retaining design are tabulated in the ASME Boiler and Pressure Vessel Code, Section II Part D. Check the maximum design temperature for the product form against the current edition. If the part falls under NACE MR0175 / ISO 15156, state this on the enquiry so that the delivery condition and hardness limit are agreed before forging.
NAS 335X forgings we produce
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory. Each item below is forged to the customer drawing or dimensional sketch. There is no fixed catalogue, no tooling charge and no minimum piece quantity for open-die work.
Ring-rolled with no weld seam. Rectangular or profiled section.
Weld neck, slip-on, blind, long weld neck and orifice, to ASME B16.5 / B16.47 or drawing.
Forged and peeled or rough turned, cut to length.
Upset-forged discs for covers, blind ends and tube sheets.
Straight, stepped and eccentric shafts; pump and agitator shafts.
Hollow forged, trepanned or bored, thin or heavy wall.
Forged and drilled tube sheets for shell-and-tube exchangers.
Trepanned or expanded hollows for heavy-wall pipe sections.
Bodies, bonnets, seat rings, stems, gate and plug blanks.
Forged nozzle bodies, headers, blocks and custom shapes.
Size and weight capability
| Product form | Dimensional range | Unit weight |
|---|---|---|
| Seamless rolled rings | OD 200 to 3,000 mm, height to 800 mm | 20 to 8,000 kg |
| Round bars | 80 to 800 mm dia., length to 6,000 mm | 20 to 6,000 kg |
| Discs and tube sheets | 200 to 2,500 mm dia., thickness to 600 mm | 30 to 10,000 kg |
| Shafts, sleeves and blocks | to 6,000 mm long | to 10,000 kg |
Machining allowance is normally 6 to 20 mm per surface depending on section size, unless the drawing states otherwise. Finish-machined parts to print are also supplied. Plant equipment includes 1, 3, 5 and 9 tonne forging hammers, hydraulic presses to 5,000 tonnes and 3 m and 6 m ring-rolling mills.
How NAS 335X forgings are made
- MeltingNAS 335X stock is melted by EAF with AOD or VOD refining. ESR, or VIM plus ESR plus VAR, is used where the specification calls for improved cleanliness and reduced segregation.
- Heat verificationHeat chemistry is checked by spectrometer against the UNS N08020 limits, including the niobium plus tantalum ratio calculated from the measured carbon content, before any material is charged into the furnace.
- Open-die forgingForging starts at about 1180 °C and finishes at not less than 950 °C. The reduction ratio is controlled to close porosity and produce grain flow that follows the part contour. Rings are ring-rolled seamless. Material is reheated rather than forged below the finishing temperature.
- Solution annealingParts are held at 925 to 1010 °C (1700 to 1850 °F) and cooled rapidly, normally by water quench where section size allows. This redissolves carbides and restores corrosion resistance. The alloy is not precipitation hardened, so no ageing step follows.
- Rough or finish machiningTurned, bored, drilled and faced to the drawing, with an agreed machining allowance or to final dimensions.
- Non-destructive testingUltrasonic examination to EN 10228-3, SEP 1921 class C, D or E, ASTM A388 or the nominated customer class. Liquid penetrant and dimensional inspection as required.
- Certification and despatchEN 10204 3.1 mill certificate as standard, 3.2 witnessed by TUV, BV, LR, SGS or DNV on request. Parts are hard stamped, preserved and packed for sea or air freight.
Where NAS 335X forgings are used
| Industry | Components forged in NAS 335X |
|---|---|
| Sulfuric acid plant and pickling | Pickling tank hardware, heating coil headers, racks and hangers, phosphate coating drum shafts, acid piping flanges and forged nozzles |
| Chemical and petrochemical processing | Process piping flanges, mixing and reactor vessel nozzles, bubble caps, agitator shafts, forged rings for vessel shells, heat exchanger tube sheets |
| Pharmaceutical, plastics and organic chemicals | Reactor and crystalliser components, forged rolls and wheels for synthetic rubber plant, product-contact shafts where iron pick-up must be avoided |
| Marine and seawater systems | Seawater pump shafts and casings, gate valve components for tanker ballast systems, deep-sea sampler and incubation chamber bodies, underwater manipulator parts |
| Oil and gas production | Forged bodies and blocks for oilfield and downhole equipment, wellhead and christmas tree components, forged crankshafts and eccentric shafts on process units |
| Valves and flow control | Valve bodies and bonnets, seat rings, stems, gate and plug blanks, forged manifolds for acid service |
| Structural and infrastructure | Pin and hanger expansion joint components for bridges, where corrosion resistance, galling resistance and low-temperature toughness are required in place of A36 and A588 carbon steel |
Most of these applications combine a reducing acid environment with a mechanical duty such as a rotating shaft, a sealing flange or a pressure-retaining tube sheet. Where the part only contains the fluid, a lined or clad construction may cost less. Where the part carries load, a solid NAS 335X forging is normally the simpler option.
Testing, inspection and certification
- Chemical analysis. Spectrometric heat analysis and, on request, product analysis on the finished forging.
- Mechanical testing. Room-temperature tensile, yield, elongation and hardness per ASTM B462. Elevated-temperature tensile and impact testing to the customer specification.
- Intergranular corrosion. ASTM A262 practice as agreed, or ASTM G28 Method A, in the delivered heat-treated condition.
- Ultrasonic testing. EN 10228-3, SEP 1921 class C, D or E, ASTM A388 or a nominated customer class.
- Surface NDT. Liquid penetrant examination per ASTM E165 or ASME Section V Article 6.
- Grain size and microstructure. ASTM E112 grain size and metallographic examination where specified.
- Certification. EN 10204 3.1 as standard; EN 10204 3.2 witnessed by TUV, BV, LR, SGS or DNV on request. Heat traceability, hard stamping and positive material identification before despatch where required.
Frequently asked questions about NAS 335X
What is NAS 335X?
NAS 335X is a nickel-iron-chromium alloy with copper and molybdenum additions. It corresponds to UNS N08020 and is known generically as Alloy 20 or 20Cb-3. The composition is 32 to 38% nickel, 19 to 21% chromium, 2 to 3% molybdenum, 3 to 4% copper and niobium plus tantalum at eight times the carbon content, with iron as the balance. NAS 335X is the designation used by Nippon Yakin Kogyo. The alloy was developed for sulfuric acid service and also resists chloride stress-corrosion cracking better than 304 and 316 stainless steel.
Is NAS 335X the same as Alloy 20 or UNS N08020?
Yes. NAS 335X, Alloy 20, 20Cb-3 and UNS N08020 describe the same chemistry. NAS 335X is the producer designation used by Nippon Yakin Kogyo, Alloy 20 is the generic industry name, 20Cb-3 is a Carpenter Technology trademark and N08020 is the UNS number. Use N08020 on the drawing or purchase order, because that is the designation the ASTM specifications reference.
Why does NAS 335X resist sulfuric acid?
Copper at 3 to 4% shifts the corrosion potential in reducing acids and is the main addition for sulfuric acid resistance. Molybdenum at 2 to 3% improves pitting and crevice resistance where the acid contains chlorides. Nickel at 32 to 38% stabilises the austenite and provides resistance to chloride stress-corrosion cracking. Niobium stabilisation prevents chromium-carbide precipitation, so welds and heat-affected zones do not suffer intergranular attack.
Which standard covers NAS 335X forgings?
For forged flanges, fittings, valves and parts the governing specification is ASTM B462 / ASME SB-462 for UNS N08020. Bar and wire fall under ASTM B473, plate, sheet and strip under ASTM B463, seamless pipe and tube under ASTM B729, welded pipe under ASTM B464 and welded tube under ASTM B468. Jiangyin Jiangnan Metal Co., Ltd. forges and certifies NAS 335X to ASTM B462 or to the customer specification.
What heat treatment do NAS 335X forgings receive?
Solution annealing at 925 to 1010 °C (1700 to 1850 °F) followed by rapid cooling, normally a water quench where section size allows. This dissolves carbides and restores corrosion resistance. NAS 335X is a solid-solution alloy and is not age hardened, so no ageing step follows. Slow cooling through the sensitisation range should be avoided even though the niobium stabilisation provides a margin of safety.
NAS 335X, 904L or Incoloy 825: which should be specified?
For sulfuric acid at low to intermediate concentration, NAS 335X is usually the most economical choice because of its copper content. Where chloride pitting and crevice attack govern, 904L or a 6% molybdenum grade has a higher pitting resistance number. Incoloy 825 carries more nickel and titanium stabilisation and is used where acid resistance and chloride stress-corrosion resistance are both required. For hydrochloric acid or strongly reducing conditions none of the three is suitable and a nickel-molybdenum alloy such as Hastelloy B-2 or C-276 is required.
Can NAS 335X be welded?
Yes. NAS 335X is welded by GTAW, GMAW and SMAW. The alloy is niobium stabilised, so sensitisation of the heat-affected zone is not normally a problem and post-weld heat treatment is not required for corrosion reasons. Matching Alloy 20 consumables such as ER320LR and E320LR are used. The LR grades have controlled niobium and lower carbon for better resistance to hot cracking. Keep heat input low and control interpass temperature.
What sizes of NAS 335X forgings can you supply?
Jiangyin Jiangnan Metal Co., Ltd. produces NAS 335X seamless rolled rings from 200 to 3,000 mm outside diameter, round bars from 80 to 800 mm diameter and up to 6,000 mm long, discs and tube sheets to 2,500 mm diameter and 600 mm thick, and shafts, sleeves and blocks up to 6,000 mm long, with single-piece weights up to 10,000 kg. Open-die forging requires no dies, so one-off and prototype quantities are economic. There is no tooling charge and no minimum piece quantity.
What certification is supplied?
Each NAS 335X forging ships with an EN 10204 3.1 mill test certificate as standard, showing heat number, chemical analysis, mechanical results, heat-treatment record and NDT results, with heat traceability and hard stamping. EN 10204 3.2 certification witnessed by a third party such as TUV, BV, LR, SGS or DNV is available when requested at order stage. Intergranular corrosion testing to ASTM A262 or ASTM G28 can be added to the test plan.
Who forges NAS 335X (UNS N08020) parts 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. The factory forges NAS 335X into seamless rolled rings, flanges, round bars, discs, shafts, sleeves, bushings, tube sheets and valve components to customer drawings, and exports worldwide. Enquiries: sales@steelforgepieces.com or +86 189 2135 9659.
What is the lead time for a NAS 335X forging?
Lead time depends on whether the grade is in raw stock and on the size of the part. Rings, discs and bars from stock material normally run 25 to 40 days. Parts that require a new melt, ESR remelting, heavy machining or third-party witnessed inspection take longer. Send the drawing and a firm date is confirmed with the quotation.
Request a quotation for NAS 335X forgings
Send a drawing, a sketch, or the dimensions and quantity. NAS 335X open-die forgings are quoted with no tooling charge and no minimum piece quantity.
Useful details, if available: grade and standard (NAS 335X / UNS N08020 / ASTM B462), product form, dimensions with machining allowance, quantity, delivery condition, NDT class, certificate type (EN 10204 3.1 or 3.2) and required delivery date.
- Company
- Jiangyin Jiangnan Metal Co., Ltd.
- Business
- Open-die forging factory
- Address
- No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China
- Telephone / WhatsApp
- +86 189 2135 9659 (0086-189-2135-9659)
- sales@steelforgepieces.com
- Website
- www.steelforgepieces.com
Jiangyin Jiangnan Metal Co., Ltd. (2026). NAS 335X (UNS N08020) forgings: composition, properties, standards and forged forms. Jiangyin, Jiangsu, China. https://www.steelforgepieces.com/Nickel-Alloy/NAS-335X.html