COMPREHENSIVE TECHNICAL ANALYSIS AND DATASHEET OF ASTM F593 ALLOY 20 (UNS N08020) FASTENING SYSTEMS
ASTM F593 Alloy 20 (officially designated as UNS N08020 and widely recognized by its original trade name, Carpenter 20 / 20Cb-3) is a highly alloyed austenitic stainless steel/nickel superalloy. It was engineered with one primary objective: maximum resistance to acid attack, particularly from hot sulfuric acid (H₂SO₄). This is achieved through a massive foundation of Nickel (up to 38%) and Chromium (20%), fortified with significant additions of Copper and Molybdenum.
Under the ASTM F593 bolting specification, Alloy 20 is classified as Alloy Group 7. Beyond its exceptional sulfuric acid resistance, the alloy is stabilized with Columbium (Niobium) to prevent chromium carbide precipitation during welding or high-temperature exposure. This structural stability, combined with its near-immunity to chloride stress-corrosion cracking, makes F593 Alloy 20 the premier fastening solution for severe chemical processing, synthetic rubber manufacturing, and acid pickling operations where standard 316L bolts rapidly dissolve.
■ Product Overview & Specifications
Alloy 20 fasteners are supplied in either Condition A (Machined from Annealed stock) or Condition CW (Cold Worked) depending on the required mechanical yield strength and diameter.
| Material Classification | Austenitic Nickel-Iron-Chromium Alloy (F593 Alloy Group 7) |
|---|---|
| ASTM Specification | ASTM F593 (Bolts, Hex Cap Screws, Studs) |
| UNS Designation | N08020 |
| Magnetic State | Strictly Non-Magnetic |
| Density | ~8.05 g/cm³ (0.291 lb/in³) |
| Temperature Limits | Up to +1000°F (+538°C) |
| Size Range | Metric: M6 to M36 | Imperial: 1/4" to 1-1/2" Custom U-bolts and acid tank anchor studs available. |
| Thread Types | UNC, UNF, 8UN, Metric Coarse, Metric Fine |
■ Proprietary Datasheet Download (GATED)
Access professional-grade technical data for ASTM F593 Alloy 20, including specific iso-corrosion charts for Sulfuric and Phosphoric acid across various temperatures and concentrations.
ASTM F593 Alloy 20 (N08020) — Complete Technical Datasheet
Contains specific high-temperature design stress limits, Condition CW strain-hardening reduction charts, and fluid-velocity erosion data in acidic environments.
⬇ DOWNLOAD DATASHEET■ MTC Integration Section
For lethal service in chemical mixing and acid containment, Ananka Group provides absolute material traceability and metallurgical verification.
- EN 10204 Type 3.1: Detailed reporting of actual chemical analysis (specifically verifying the precise Copper and Columbium additions), mechanical testing by Condition (A or CW), and solution treatment temperatures.
- EN 10204 Type 3.2: Third-party witness testing facilitated for critical petrochemical reactors and pharmaceutical acid-wash systems.
■ Chemical Composition (UNS N08020)
The defining characteristic of Alloy 20 is its aggressive use of Copper (3-4%) in combination with Molybdenum to resist sulfuric acid, while Columbium (Niobium) stabilizes the matrix against intergranular corrosion.
| Nickel (Ni) | Chromium (Cr) | Copper (Cu) | Moly (Mo) | Columbium + Tantalum | Carbon (C) | Iron (Fe) |
|---|---|---|---|---|---|---|
| 32.00 – 38.00 | 19.00 – 21.00 | 3.00 – 4.00 | 2.00 – 3.00 | 8 x (Carbon %) Min (1.00% Max) |
0.07 Max | Balance (~35%) |
*Manganese (2.00 Max) and Silicon (1.00 Max) are tightly controlled. Note: Columbium (Cb) is the traditional ASTM term for Niobium (Nb).
■ Mechanical Properties (F593 Alloy Group 7)
ASTM F593 dictates mechanical properties based on the condition of the metal. Condition A provides maximum ductility, while Condition CW (Cold Worked) increases the yield strength significantly for structural bolting applications.
| Property | Condition A (Annealed) | Condition CW (1/4" to 5/8" Dia) | Condition CW (3/4" to 1-1/2" Dia) |
|---|---|---|---|
| Tensile Strength (Min) | 80 ksi (550 MPa) | 100 ksi (690 MPa) | 85 ksi (585 MPa) |
| Yield Strength (0.2% Offset, Min) | 35 ksi (240 MPa) | 65 ksi (450 MPa) | 45 ksi (310 MPa) |
| Elongation in 4D (Min) | 30% | 20% | 25% |
| Hardness (Typical Max) | ~95 HRB | ~32 HRC | ~32 HRC |
■ Equivalent Grades & Designations
| Standard / System | Designation |
|---|---|
| ASTM Specification | ASTM F593 Alloy Group 7 (Bolts) |
| UNS Designation | N08020 |
| Common Trade Name | Alloy 20, Carpenter 20®, 20Cb-3® |
| European Equivalent | EN 1.4519 / NiCr20CuMo |
■ Corrosion & Environmental Resistance Profile
Alloy 20 was invented for a single, brutal environment—hot sulfuric acid—but excels in many others:
- Sulfuric Acid (H₂SO₄): Exceptional resistance to boiling sulfuric acid across a wide range of concentrations (up to 40%). It is the undisputed standard for sulfuric acid processing.
- Stress Corrosion Cracking (SCC): With roughly 35% Nickel, Alloy 20 is virtually immune to chloride-ion stress corrosion cracking, a failure mode that destroys standard 304 and 316 stainless steels.
- Intergranular Corrosion: The Columbium stabilization prevents sensitization ("weld decay"), allowing the fasteners to be used in high-heat zones without losing their localized corrosion resistance.
■ Applications & Industries
ASTM F593 Alloy 20 is specified wherever aggressive acidic solutions would rapidly degrade standard steel fasteners:
- Chemical Manufacturing: Mixing tanks, heat exchangers, and piping flanges handling sulfuric, phosphoric, and nitric acids.
- Pharmaceuticals: High-purity reactor bolting where contamination from fastener corrosion is unacceptable.
- Synthetic Rubber & Plastics: Processing equipment subjected to highly corrosive chemical catalysts.
- Metal Cleaning: Tie-rods, fasteners, and structural hardware used in industrial acid pickling racks.
■ Compatible Fastener Assemblies
To avoid galvanic corrosion in highly conductive acid electrolytes, Alloy 20 bolts must strictly be mated with identical Alloy 20 hardware.
| External Threads (ASTM F593 Alloy 20) | Recommended Mating Nuts (ASTM F594) |
|---|---|
| F593 Alloy Group 7 (Alloy 20) Hex Bolts | ASTM F594 Alloy Group 7 (Alloy 20) Hex Nuts |
| F593 Alloy Group 7 (Alloy 20) Studs | ASTM F594 Alloy Group 7 (Alloy 20) Heavy Hex Nuts |
■ Torque & Installation Guidelines
Like all high-nickel austenitic alloys, Alloy 20 is highly susceptible to thread galling (cold-welding). The application of premium, chemically compatible anti-seize paste is strictly mandatory. Caution must be taken to ensure the anti-seize itself will not react with the process acids.
| Nominal Diameter | Threads Per Inch (UNC) | Target Torque (Condition CW) — Lubricated (ft-lbs) |
|---|---|---|
| 1/2" | 13 | 35 - 50 |
| 5/8" | 11 | 75 - 95 |
| 3/4" | 10 | 110 - 140 |
| 1" | 8 | 250 - 320 |
■ Why Choose Ananka Group?
Ananka Group guarantees the flawless execution of advanced chemical-grade metallurgy, providing the petrochemical industry with infallible fastening packages.
- Condition CW Expertise: We expertly control the cold-drawing process to ensure our Condition CW Alloy 20 fasteners meet the strict yield requirements without fracturing the material.
- Chemical Verification: Strict adherence to ASTM chemical limits ensures that the Columbium successfully ties up the carbon, and the Copper content is perfectly calibrated for sulfuric acid defense.
- Rolled Threads: External threads are rolled rather than cut, significantly increasing fatigue resistance and minimizing the risk of galling during installation.
- 100% Traceability: Every fastener is marked with the specific F593 alloy group identification, backed by a comprehensive EN 10204 3.1 MTC.
■ Frequently Asked Questions
1. What makes Alloy 20 so resistant to sulfuric acid?
The secret is the combination of ~35% Nickel and 3-4% Copper. Copper greatly enhances the alloy's resistance to reducing acids (like sulfuric), while the high nickel content prevents the metal from suffering stress-corrosion cracking in these aggressive environments.
2. Is Alloy 20 a stainless steel or a nickel alloy?
It occupies a middle ground. Because it contains more than 50% combined Iron (~35%) and Chromium (20%), it is technically highly alloyed austenitic stainless steel. However, because its Nickel content is so high (~35%), it is often classified and sold as a Nickel Superalloy (UNS N08020).
3. What does "20Cb-3" mean?
20Cb-3 is the registered trademark of Carpenter Technology Corporation, who developed the modern version of the alloy. The "Cb" stands for Columbium (Niobium), the element added to stabilize the alloy against intergranular corrosion. The "3" signifies the third major revision to the alloy's chemistry.
4. Is Alloy 20 magnetic?
No. Due to its highly stable Face-Centered Cubic (FCC) austenitic microstructure, reinforced by massive amounts of nickel, Alloy 20 remains completely non-magnetic even after extensive cold working.