ASTM B464 covers welded pipe of UNS N08020 (commonly known as Alloy 20, Carpenter 20, or CN7M) intended for general corrosive service and low- or high-temperature service. This specification covers alloy 20 pipes suitable for handling sulfuric acid, nitric acid, and other aggressive chemicals.
Quick Specifications
Material: UNS N08020 (Alloy 20, Carpenter 20, CN7M)
Product Form: Welded Pipe
Manufacturing: Welded, solution-annealed
Applications: Chemical processing, sulfuric acid service, heat exchangers
Chemical Composition (ASTM B464 - UNS N08020)
| Element |
UNS N08020 (Alloy 20) % |
| Nickel (Ni) |
32.0 - 38.0 |
| Chromium (Cr) |
19.0 - 21.0 |
| Iron (Fe) |
Balance |
| Molybdenum (Mo) |
2.0 - 3.0 |
| Copper (Cu) |
3.0 - 4.0 |
| Nitrogen (N), max |
0.045 |
| Carbon (C), max |
0.07 |
| Manganese (Mn), max |
2.0 |
| Silicon (Si), max |
1.0 |
| Sulfur (S), max |
0.035 |
| Phosphorus (P), max |
0.045 |
| Niobium (Nb), max |
0.035 (added as stabilizer) |
Mechanical Properties (Solution-Annealed Condition)
| Property |
UNS N08020 (Alloy 20) |
| Tensile Strength, min (MPa) |
550 |
| Yield Strength (0.2% offset), min (MPa) |
240 |
| Elongation in 2 in. (50 mm), min (%) |
30 |
| Hardness, max (HB) |
217 |
Available Products (ASTM B464)
Alloy 20 (UNS N08020) Welded Pipe
Size Range: 1/2" - 24" (OD 21.3mm - 610mm)
Wall Thickness: SCH 5S - SCH 160
Length: Random 5-7m or Fixed 6m
Surface: Solution annealed, pickled, or bright annealed
Alloy 20 (UNS N08020) Welded Tube
Size Range: 1/2" - 8" (OD 21.3mm - 219.1mm)
Wall Thickness: 0.5mm - 10mm
Heat exchanger tubes available
Coiled or straight lengths
Supplementary Products
Alloy 20 Fittings (Elbows, Tees, Reducers)
Alloy 20 Flanges
Alloy 20 Fasteners
Key Features of Alloy 20 (UNS N08020)
Excellent Sulfuric Acid Resistance: Outstanding performance in sulfuric acid environments
Resists Pitting and Crevice Corrosion: High Cr, Mo, and Cu content
Good Formability: Can be cold worked and formed
Weldable: Good weldability with proper filler metals
Temperature Resistance: Up to 1000°F (538°C) in oxidizing atmospheres
Corrosion Resistance
ASTM B464 Alloy 20 provides exceptional resistance to:
Sulfuric Acid: Excellent resistance across wide concentration and temperature range
Nitric Acid: Good resistance, especially in intermediate concentrations
Phosphoric Acid: Good resistance
Chlorides: Resists pitting and crevice corrosion
Intergranular Corrosion: Niobium stabilization minimizes sensitization
Applications
Chemical Processing
Sulfuric acid production and handling
Chemical reactors and vessels
Heat exchangers and condensers
Storage tanks and piping
Petrochemical Industry
Hydrocarbon processing
Sour gas handling (with limitations)
Pharmaceuticals
Processing equipment
Storage and transfer systems
Power Generation
Auxiliary systems in corrosive environments
Pollution control equipment
Manufacturing Process
Melting: AOD (Argon Oxygen Decarburization) or VOD (Vacuum Oxygen Decarburization)
Forming: Plate rolled into pipe shape
Welding: TIG or plasma welding of longitudinal seam
Solution Annealing: 920-1150°C (1688-2102°F), rapid cooling
Pickling: Remove weld scale and oxides
Non-Destructive Testing: Eddy current, radiographic, ultrasonic
Final Inspection: Dimensional, visual, and documentation
Heat Treatment
Solution Annealing:
Temperature: 920-1150°C (1688-2102°F)
Cooling: Rapid water quenching (critical for corrosion resistance)
Purpose: Dissolve carbides and intermetallic phases, optimize corrosion resistance
Note: Alloy 20 must be solution-annealed after welding for optimum corrosion resistance in aggressive environments.
Welding
ASTM B464 Alloy 20 is weldable using:
Shielded Metal Arc Welding (SMAW): Use ENiCrMo-3 (Inconel 112) or E320LR electrodes
Gas Tungsten Arc Welding (GTAW): Use ERNiCrMo-3 (Inconel 625) or ER320LR filler
Gas Metal Arc Welding (GMAW): Use ERNiCrMo-3 or ER320LR wire
Important Welding Note: Post-weld solution annealing is strongly recommended for corrosive service. Use low heat input to minimize carbide precipitation.
Testing and Inspection
Chemical Analysis: PMI (Positive Material Identification) on every heat
Tensile Test: ASTM E8/E8M
Flattening Test: ASTM A999/A999M
Flange Test: For pipes
Eddy Current Test: 100% of welded seam
Radiographic Test (RT): Or ultrasonic test for critical applications
Hydrostatic Test: ASTM A999/A999M
Intergranular Corrosion Test: ASTM G28 (Practice A)
Dimension Tolerances
| Nominal Pipe Size (NPS) |
OD Tolerance (mm) |
Wall Thickness Tolerance |
Length Tolerance |
| 1/2 - 1 1/2 |
±0.8 |
+20% / -12.5% |
+20mm / -0mm |
| 2 - 8 |
±0.8 |
| 10 and larger |
±1.6 |
Comparison: Alloy 20 vs 904L vs 316L
| Property |
Alloy 20 (N08020) |
904L (N08904) |
316L |
| Ni Content (%) |
32 - 38 |
23 - 28 |
10 - 14 |
| Cr Content (%) |
19 - 21 |
19 - 23 |
16 - 18 |
| Mo Content (%) |
2 - 3 |
4 - 5 |
2 - 3 |
| Cu Content (%) |
3 - 4 |
1 - 2 |
- |
| Sulfuric Acid Resistance |
Excellent |
Very Good |
Fair |
| Tensile Strength (MPa) |
550 min |
490 min |
485 min |
Alternative Designations for Alloy 20
UNS: N08020
Common Names: Alloy 20, Carpenter 20, Carpenter 20Cb-3®, CN7M
DIN: 2.4660
EN: Not standardized (often referred to as Alloy 20)
Old ASTM: ASTM A351 CN7M (cast version)
Related ASTM Standards
ASTM B729 - Seamless UNS N08020 Pipe
ASTM B463 - Alloy 20 Plate, Sheet, Strip
ASTM B473 - Alloy 20 Bar and Wire
ASTM B472 - Alloy 20 Forgings
Frequently Asked Questions (FAQ)
Q: What is the difference between ASTM B464 and ASTM B729?
A: ASTM B464 covers welded Alloy 20 pipe, while ASTM B729 covers seamless Alloy 20 pipe. Welded pipe is more cost-effective; seamless pipe is preferred for high-pressure or critical applications.
Q: Can Alloy 20 be used in seawater applications?
A: Alloy 20 has good resistance to seawater but is not as resistant as super austenitic or duplex stainless steels. For seawater applications, 904L, 2205 duplex, or super duplex are better choices.
Q: Is post-weld heat treatment required for Alloy 20?
A: Yes, solution annealing after welding is strongly recommended for optimum corrosion resistance in aggressive environments like sulfuric acid. Without PWHT, the weld heat-affected zone may be susceptible to corrosion.
Q: What is the maximum operating temperature for Alloy 20?
A: Alloy 20 can be used up to 1000°F (538°C) in continuous service. For intermittent service, it can withstand higher temperatures. However, prolonged exposure above 800°F may cause embrittlement.
Q: Can Alloy 20 be used interchangeably with 904L?
A: Not always. Alloy 20 is superior in sulfuric acid environments, while 904L is better in chloride-containing environments (better pitting resistance). Material selection should be based on the specific corrosive media.
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