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UNS S32550 Nut
QS Fastener: UNS S32550 Nut
Name: UNS S32550 Nut
Standard: ANSI, DIN, UNS, ASTM
Material: S32550
Size: 5/16 to 4 inch
Grade: A
Class: 12.9
Application: Marine engineering, Chemical and Petroleum Industries, Desalination and Water Treatment, Energy Industry, Food and Pharmaceutical Industry
Standard: ANSI, DIN, UNS, ASTM
Material: S32550
Size: 5/16 to 4 inch
Grade: A
Class: 12.9
Application: Marine engineering, Chemical and Petroleum Industries, Desalination and Water Treatment, Energy Industry, Food and Pharmaceutical Industry
Quantity
UNS S32550 Nut is manufactured from high-strength super duplex stainless steel, complying with ANSI, DIN, UNS, and ASTM standards. Available in sizes ranging from 5/16 to 4 inches, it meets Grade A classification with a Class 12.9 performance rating. Engineered for exceptional corrosion resistance and mechanical strength, it is specifically designed for harsh environments and widely used in marine engineering, petrochemical, desalination and water treatment, energy industries, as well as food and pharmaceutical sectors—making it an ideal choice for durability and reliability.
The superior performance of UNS S32550 Nut (Ferrallium 255 duplex stainless steel nut) begins with stringent raw material selection and advanced smelting processes. Its chemical composition and microstructure directly influence the mechanical strength, corrosion resistance, and high-temperature stability of the final product. Below is a detailed analysis of the key production stages:
(1) Raw Material Selection: Precise Alloy Formulation
The alloy design of UNS S32550 Nut is based on the"austenite-ferrite duplex balance"principle, requiring precise control of the following elements:
- Primary Components:
- Chromium (Cr, 24-27%): Provides passive film formation, essential for the fastener's corrosion resistance.
- Nickel (Ni, 4.5-6.5%): Stabilizes the austenite phase, improving the nut's low-temperature toughness.
- Molybdenum (Mo, 2.9-3.9%): Enhances resistance to pitting and crevice corrosion, ensuring reliability in Cl⁻ environments.
- Copper (Cu, 1.5-2.5%): Improves resistance to sulfuric and phosphoric acid corrosion, making it suitable for chemical equipment.
- Nitrogen (N, 0.10-0.25%): Promotes austenite formation, increases strength, and suppresses σ-phase precipitation.
The alloy design of UNS S32550 Nut is based on the"austenite-ferrite duplex balance"principle, requiring precise control of the following elements:
- Primary Components:
- Chromium (Cr, 24-27%): Provides passive film formation, essential for the fastener's corrosion resistance.
- Nickel (Ni, 4.5-6.5%): Stabilizes the austenite phase, improving the nut's low-temperature toughness.
- Molybdenum (Mo, 2.9-3.9%): Enhances resistance to pitting and crevice corrosion, ensuring reliability in Cl⁻ environments.
- Copper (Cu, 1.5-2.5%): Improves resistance to sulfuric and phosphoric acid corrosion, making it suitable for chemical equipment.
- Nitrogen (N, 0.10-0.25%): Promotes austenite formation, increases strength, and suppresses σ-phase precipitation.
- Impurity Control:
- Carbon (C≤0.04%): Low carbon content minimizes chromium carbide precipitation during hot working or welding, preventing intergranular corrosion in UNS S32550 Nut.
- Sulfur (S≤0.02%), Phosphorus (P≤0.03%): Ultra-low impurities ensure hot workability and reduce the impact of non-metallic inclusions on fatigue life.
- Carbon (C≤0.04%): Low carbon content minimizes chromium carbide precipitation during hot working or welding, preventing intergranular corrosion in UNS S32550 Nut.
- Sulfur (S≤0.02%), Phosphorus (P≤0.03%): Ultra-low impurities ensure hot workability and reduce the impact of non-metallic inclusions on fatigue life.
Raw materials undergo Optical Emission Spectroscopy (OES) and Inert Gas Fusion (IGM) testing to verify compliance with ASTM A276 standards for UNS S32550 Nut.
(2) Smelting Process: AOD+VOD Duplex Refining Technology
To achieve high purity, UNS S32550 Nut employs a multi-step smelting process:
To achieve high purity, UNS S32550 Nut employs a multi-step smelting process:
1) Primary Melting (Electric Arc Furnace, EAF):
- Scrap steel and ferroalloys are melted in an EAF, with preliminary alloy composition adjustments.
- Temperature is maintained at 1600–1650°C to prevent excessive oxidation of Mn, Cr, and other elements.
- Scrap steel and ferroalloys are melted in an EAF, with preliminary alloy composition adjustments.
- Temperature is maintained at 1600–1650°C to prevent excessive oxidation of Mn, Cr, and other elements.
2) Argon Oxygen Decarburization (AOD):
- Decarburization: Argon-oxygen gas mixtures are blown into the melt to reduce carbon content to ≤0.04% via the [C]+[O]→CO↑ reaction, ensuring ultra-low carbon characteristics.
- Reduction Phase: Ferrosilicon (FeSi) is added for deoxidation, and ferrochromium (FeCr) compensates for oxidation losses.
- Advantage: Precise control of C and O content while minimizing oxidation losses of precious metals (e.g., Ni, Mo).
- Decarburization: Argon-oxygen gas mixtures are blown into the melt to reduce carbon content to ≤0.04% via the [C]+[O]→CO↑ reaction, ensuring ultra-low carbon characteristics.
- Reduction Phase: Ferrosilicon (FeSi) is added for deoxidation, and ferrochromium (FeCr) compensates for oxidation losses.
- Advantage: Precise control of C and O content while minimizing oxidation losses of precious metals (e.g., Ni, Mo).
3) Vacuum Oxygen Decarburization (VOD):
- Deep Degassing: The molten steel is transferred to a vacuum chamber (pressure ≤1 mbar) to further remove [H] (≤2 ppm) and control [N], enhancing purity.
- Fine-Tuning: Aluminum (Al) and calcium (Ca) are added under inert atmosphere for final deoxidation, forming fine Al₂O₃-CaO inclusions (improving hot workability).
- Deep Degassing: The molten steel is transferred to a vacuum chamber (pressure ≤1 mbar) to further remove [H] (≤2 ppm) and control [N], enhancing purity.
- Fine-Tuning: Aluminum (Al) and calcium (Ca) are added under inert atmosphere for final deoxidation, forming fine Al₂O₃-CaO inclusions (improving hot workability).
4) Continuous Casting or Ingot Casting:
- Electromagnetic Stirring (EMS) ensures uniform solidification, minimizing centerline segregation.
- Slow cooling prevents excessive δ-ferrite precipitation, preserving material properties.
- Electromagnetic Stirring (EMS) ensures uniform solidification, minimizing centerline segregation.
- Slow cooling prevents excessive δ-ferrite precipitation, preserving material properties.
(3) Quality Control Critical Points
- Phase Balance: Feritscope verification ensures an austenite/ferrite ratio near 50:50. Deviations >55% require heat treatment adjustments.
- Inclusion Rating: Per ASTM E45, Type A (sulfides) and B (oxides) ≤1.5; Type D (globular oxides) are excluded to guarantee fatigue resistance.
- Macrostructure Inspection: Acid-etched samples must be free of cracks/porosity, with ≥90% equiaxed grains for machining stability.
- Phase Balance: Feritscope verification ensures an austenite/ferrite ratio near 50:50. Deviations >55% require heat treatment adjustments.
- Inclusion Rating: Per ASTM E45, Type A (sulfides) and B (oxides) ≤1.5; Type D (globular oxides) are excluded to guarantee fatigue resistance.
- Macrostructure Inspection: Acid-etched samples must be free of cracks/porosity, with ≥90% equiaxed grains for machining stability.
(4) Comparative Advantages of Smelting Processes
| Process | Carbon Control | Impurity Level | Cost | Applicable Steel Grades |
| AOD+VOD | ≤0.04% | S/P≤0.03% | High | UNS S32550 Nut, high-alloy steels |
| ESR | ≤0.03% | S≤0.01% | Very High | Nuclear/aerospace grades |
| Conventional EAF | ≥0.06% | S/P≤0.05% | Low | Carbon/low-alloy steels |
Conclusion: The AOD+VOD process for UNS S32550 Nut achieves ultra-low carbon and high-purity molten steel, laying the foundation for subsequent hot working and corrosion resistance. This reflects the production philosophy of premium stainless steels—"composition is the core, purity is the key"—ensuring outstanding performance in demanding environments.
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