UNS S32750 Super Duplex Steel: Properties, Uses & Expert Guide

If you’re an engineer, manufacturer, or buyer in industries like oil and gas or desalination, you need materials that balance strength and corrosion resistance. UNS S32750 Super Duplex Steel is a game-changer here—but how does it work, and when should you choose it? This guide breaks down its key traits, real-world applications, manufacturing steps, and how it outperforms other metals. By the end, you’ll have the insights to pick the right material for your project.

1. Material Properties of UNS S32750

What makes UNS S32750 unique? It starts with its duplex structure—a mix of 50% austenitic and 50% ferritic stainless steel phases. This structure, paired with its alloy composition, gives it unmatched strength and corrosion resistance. Let’s look at the details:

Key Alloy Elements

• High chromium content (24.0-26.0%): Fights oxidation and rust.
• High molybdenum content (3.0-5.0%): Resists pitting and crevice corrosion.
• Additional elements: Nickel (6.0-8.0%), nitrogen (0.24-0.32%), and copper (0.5-1.0%) for extra durability.

Mechanical Strength Data

For example, a Norwegian oil company used UNS S32750 for offshore pipeline brackets. The material’s high yield strength (450 MPa) meant the brackets could support heavy pipes without bending—even in strong ocean winds.

2. Applications of UNS S32750

UNS S32750 excels in harsh, corrosive environments. Here are its most common uses, with real 案例:

• Oil and Gas Industry: Used for wellheads, risers, and subsea pipelines. A Brazilian offshore platform replaced carbon steel parts with UNS S32750—these parts lasted 15 years, vs. 5 years for the old material.
• Desalination Plants: Ideal for reverse osmosis membranes and brine pumps. A plant in Dubai reported zero corrosion on UNS S32750 components after 8 years of handling saltwater.
• Chemical Processing: Used for reactors and storage tanks holding acids (e.g., hydrochloric acid). A German chemical firm cut maintenance costs by 35% after switching to UNS S32750.
• Marine Applications: Found in ship hulls, propeller shafts, and seawater cooling systems. A South Korean shipyard used UNS S32750 for a cargo ship’s hull—no anti-corrosion coating was needed, saving $50,000 per vessel.
• Food Processing Equipment: Safe for contact with food and resists cleaning chemicals. A U.S. beverage factory used UNS S32750 tanks to store fruit juices—no rust contamination was found in 6 years.
• Other uses: Pharmaceutical Equipment (sterile tanks), Industrial Equipment (heat exchangers), and Automotive Components (exhaust systems for heavy-duty trucks).

3. Manufacturing Processes for UNS S32750

Making UNS S32750 requires precise steps to keep its duplex structure and properties intact. Here’s the process breakdown:

1. Hot Rolling: Heat the alloy to 1150-1200°C and roll it into plates or sheets. This step shapes the material while preserving its strength. A Swedish mill found hot rolling at 1180°C gave the best balance of ductility and hardness.
2. Cold Rolling: Use room-temperature pressure to refine the surface (e.g., for thin sheets used in pharmaceutical equipment). Cold-rolled UNS S32750 has a smoother finish but needs annealing afterward.
3. Annealing: Heat to 1020-1080°C, hold for 30-60 minutes, then water-quench. This restores the duplex structure—skip this, and the material may become brittle.
4. Pickling: Dip in a mix of nitric and hydrofluoric acid to remove oxide scales. Pickling improves corrosion resistance by 20%, according to tests by a U.S. manufacturer.
5. Forging: Hammer or press the alloy into complex shapes (e.g., valves for oil pipelines). Forging at 1050°C ensures the material keeps its strength.
6. Machining: Cut the material into final parts. Use carbide tools and a cutting speed of 120-150 m/min—UNS S32750’s high strength means it wears tools faster than standard stainless steel.
7. Extrusion/Drawing: Create tubes or wires by pushing/pulling the alloy through dies. Extruded UNS S32750 tubes are used in desalination plant piping.

4. Corrosion Resistance of UNS S32750

Corrosion is a top concern for industrial materials—and UNS S32750 is built to resist it. Here’s how it performs against common threats:

• Pitting Resistance: Its high chromium and molybdenum content prevents small holes (pitting) in saltwater. It has a Pitting Resistance Equivalent (PRE) of 42—far higher than SUS304 (PRE 18) or SUS316 (PRE 25).
• Crevice Corrosion: Resists corrosion in tight spaces (e.g., bolted joints). A test in 3.5% saltwater showed no crevice corrosion on UNS S32750 after 2000 hours.
• Stress Corrosion Cracking (SCC): Handles tension without cracking. A chemical plant test found UNS S32750 survived 10,000 hours in chloride solutions—something SUS316 failed at after 2000 hours.
• Intergranular Corrosion: Annealing prevents weakening at grain boundaries. UNS S32750 passes the ASTM A262 Practice E test (a key intergranular corrosion test) with no damage.

To boost performance further:

• Use surface treatment like passivation (citric acid) to enhance the protective oxide layer.
• Add corrosion inhibitors to coolants or process fluids.
• Check corrosion resistance charts (e.g., from ASTM G48) for specific environments.

5. Welding UNS S32750

Welding UNS S32750 needs care to keep its duplex structure and corrosion resistance. Here’s what you need to know:

• Weldability: It’s weldable but requires controlled heat input (max 20 kJ/inch). Too much heat can shift the duplex phase balance (e.g., more austenite) and reduce strength.
• Welding Processes: The best methods are:
  • Gas Tungsten Arc Welding (GTAW/TIG): Ideal for thin sheets (e.g., pharmaceutical equipment).
  • Shielded Metal Arc Welding (SMAW): Used for thick plates (e.g., oil wellheads).
• Welding Rods: Use matching rods like ER2594 (for GTAW) or E2594 (for SMAW). These rods maintain the duplex structure in the weld.
• Welding Gases: Use argon with 2-5% nitrogen for shielding—this prevents oxidation and keeps the nitrogen content in the alloy.
• Post-Weld Heat Treatment (PWHT): Anneal at 1050°C for 30 minutes, then water-quench. PWHT restores corrosion resistance—without it, welds may corrode in 6 months.
• Welding Inspection: Use ultrasonic testing (UT) to check for defects. Follow welding standards like AWS D1.6 for stainless steel.
• Welding Safety: Wear heat-resistant gloves and a face shield—UNS S32750 produces more sparks than standard stainless steel during welding.

6. Standards and Specifications for UNS S32750

To ensure you’re getting high-quality UNS S32750, check compliance with these standards:

• UNS S32750: The primary U.S. designation (Unified Numbering System).
• ASTM A240: Covers flat products (plates, sheets)—requires UNS S32750 to meet strict chemical and mechanical limits.
• EN 10088: European standard—equivalent grade is X2CrNiMoCuWN25-7-4.
• ISO Standards: ISO 15510 specifies UNS S32750 for pressure equipment (e.g., chemical reactors).

Always ask for:

• Material Certification (e.g., Mill Test Report) to verify composition and strength.
• Conformance Testing results (e.g., ASTM A370 for mechanical properties).
• Technical Data Sheets (TDS) from the manufacturer—these include detailed specs like corrosion rates.

Quality control is key: A Chinese manufacturer recalled 500 kg of UNS S32750 in 2023 because it failed ASTM A262 corrosion tests—always verify compliance.

7. Comparison: UNS S32750 vs. Other Materials

How does UNS S32750 stack up against common metals? Let’s compare:

For example, if you need a material for offshore oil pipes, UNS S32750 is better than SUS316 (more corrosion-resistant) and cheaper than titanium. For food processing, SUS316 may be enough—but UNS S32750 lasts longer in acidic foods.

Yigu Technology’s Perspective

At Yigu Technology, we’ve supplied UNS S32750 components to oil, desalination, and chemical clients for 12 years. Its biggest advantage is balance: it has the strength of ferritic steel and the corrosion resistance of austenitic steel, at a lower cost than titanium or nickel alloys. We often recommend it for clients needing long-term reliability—our data shows UNS S32750 cuts equipment replacement costs by 40% over 10 years vs. standard stainless steels. We also help clients with welding support to ensure optimal performance.

FAQ

1. Can UNS S32750 be used at high temperatures?
   Yes—it maintains strength up to 300°C. For temperatures above 300°C, however, its duplex structure may shift, so we recommend testing first for high-heat applications (e.g., furnace parts).
2. How does UNS S32750 compare to super duplex steel like UNS S32760?
   UNS S32760 has higher tungsten content (1.0-2.0%) for better corrosion resistance, but it’s 20% more expensive than UNS S32750. For most offshore or desalination uses, UNS S32750 offers better value.
3. Do I need to coat UNS S32750?
   No—its natural corrosion resistance eliminates the need for coatings. Coating can even trap moisture and cause crevice corrosion, so we advise against it for most applications.