Duplex Steel Pipes: A Comprehensive Guide

Duplex Steel Pipes have gained significant popularity in recent years due to their impressive durability, strength, and corrosion resistance. They are an excellent choice for various applications in the oil and gas, chemical, and petrochemical industries. If you are considering using duplex steel pipes for your project, there are some key points you should be aware of. In this blog post, we will cover everything you need to know about duplex steel pipes.

What is Duplex Steel?

Duplex steel is an alloy with approximately 50% iron content, also known as austenitic-ferritic stainless steel. It is characterized by its high strength, excellent corrosion resistance, good weldability, and outstanding formability. Additionally, duplex steel remains tough even at cryogenic temperatures and retains its mechanical properties in acidic and alkaline environments up to 700°C. These attributes make it an ideal material for industrial applications such as heat exchangers, tanks, and pipes in the chemical processing industry.

Why Use Duplex Pipes?

Duplex pipes offer numerous advantages over other types of pipes, making them a popular choice in various industries. Their high strength-to-weight ratio makes them ideal for offshore applications where weight is a concern. Additionally, their exceptional corrosion resistance makes them well-suited for corrosive environments, such as chemical plants and oil refineries.

Duplex steel pipes perform exceptionally well in salty, acidic, and caustic environments where other metals may fail. They are highly resistant to cracking and pitting while maintaining high strength levels even at cryogenic temperatures. Their excellent mechanical properties, combined with high flexibility and toughness, help reduce fatigue in dynamically loaded components like shafts and housings.

Moreover, their good weldability enhances the material’s fabrication properties, making them an ideal choice for applications such as oil and gas pipelines, water treatment systems, and process piping networks. Duplex steel pipes also have excellent fatigue resistance, enabling them to withstand repeated stress without cracking or breaking. This, combined with their corrosion resistance, makes them perfect for high-pressure applications.

Benefits of Duplex Steel Pipes

• High Strength-to-Weight Ratio: Duplex stainless steel pipes are strong yet lightweight, which is particularly beneficial for offshore and other weight-sensitive applications.
• Excellent Corrosion Resistance: These pipes are highly resistant to corrosion, making them ideal for harsh environments like chemical plants and oil refineries.
• Good Mechanical Properties: They offer high flexibility and toughness, reducing fatigue in dynamically loaded components, such as shafts and housings. This makes duplex steel pipes suitable for high-stress applications.

Manufacturing Process of Duplex Pipes

Duplex steel pipes are renowned for their strength and corrosion resistance, but their manufacturing process is more complex and costly compared to other stainless steel pipes like 304 or 316 grades.

Cold Forming for Duplex Pipes

Duplex steel is challenging to work with due to its high strength. During manufacturing, cold working techniques such as cold rolling or cold drawing are commonly used. For larger pipes, hot forming may be employed. The process requires more force than shaping standard stainless steel, increasing production costs due to the additional force needed to work with this stronger, harder material.

Cold Forming for Duplex Pipes

Duplex steel is challenging to work with due to its high strength. During manufacturing, cold working techniques such as cold rolling or cold drawing are commonly used. For larger pipes, hot forming may be employed. The process requires more force than shaping standard stainless steel, increasing production costs due to the additional force needed to work with this stronger, harder material.

Duplex Pipes Physical Properties

Strengths and Weaknesses of Duplex Stainless Steel

Standard austenitic steels like ferritic 430 and austenitic 1.4301 (304) are relatively simple to manufacture, as they generally consist of a single phase—either ferrite or austenite. While these steels are widely used and effective, they have individual weaknesses:

Austenitic Steels:

• • Low Strength: Approximately 200 MPa in solution-annealed condition.
  • Low Resistance to Stress Corrosion Cracking: This can be a significant drawback in certain applications.
  • Price Fluctuations: The nickel content in austenitic steels can lead to unpredictable price changes.

Ferritic Steels:

• • Low Strength: Slightly higher than austenitic steels, around 250 MPa.
  • Weldability Issues: Difficult to weld in thick sections.
  • Poor Low-Temperature Toughness: Not suitable for very low temperatures.

In contrast, duplex steel is designed to balance austenite and ferrite phases, offering several advantages:

Higher Strength: Duplex grades typically range from 400 to 550 MPa in 0.2% PS, allowing for reduced section thickness and weight. This is particularly beneficial for:
Pressure Vessels and Storage Tanks
Structural Applications (e.g., bridges)

Improved Weldability: While not as straightforward as austenitic steels, duplex steels are easier to weld than ferritic steels, especially in thick sections.

Enhanced Toughness: Duplex steels perform better at low temperatures compared to ferritic steels, with toughness extending to as low as minus 50°C and sometimes reaching minus 80°C.

Resistance to Stress Corrosion Cracking: Duplex steels offer better resistance to stress corrosion cracking than standard austenitic steels, making them suitable for:

• • Hot Water Tanks
  • Brewing Tanks
  • Process Plants
  • Swimming Pool Structures

Duplex Stainless Steel Pipe Grades

• Grade 2205: 2205 Grade is the most common duplex  steel grade, renowned for its excellent corrosion resistance and high strength. It is widely used in various applications due to its balanced properties.
• Grade 2507: Often referred to as super duplex,2507 grade offers superior corrosion resistance and strength, making it ideal for extremely harsh environments and high-pressure applications.
• Grade 2304: A lean duplex grade, 2304 provides good performance at a more economical cost. It is suitable for applications where moderate corrosion resistance and strength are required.

Duplex and Super Duplex Stainless Steel: Corrosion Resistance and Applications

Corrosion Resistance

Duplex and super duplex stainless steels are highly resistant to corrosion. However, welding these materials can be challenging. The Heat-Affected Zone (HAZ) may experience reduced toughness, increased risk of cracking, and decreased corrosion resistance due to the high temperatures involved in welding.

To achieve strong and corrosion-resistant welds, use suitable techniques such as Submerged Arc Welding (SAW), Tungsten Inert Gas (TIG) welding, or Metal Inert Gas (MIG) welding. Proper post-welding treatment is also crucial to maintain the durability and performance of these alloys.

Cutting Speed for Duplex and Super Duplex Steel Pipe 

Cutting duplex and super duplex stainless steels can be difficult due to their high strength and tendency to harden. Effective machining requires robust, rigid machines and high-performance cutting tools. Adequate coolant is necessary to manage heat and ensure smooth cutting. Using the correct cutting speeds and feeds is essential for efficient machining and maintaining the quality of the cut.

Industrial Applications

• Duplex Stainless Steel: Valued for its high strength and lightweight properties, duplex stainless steel is ideal for applications such as pressure vessels and structural components. Its excellent resistance to stress corrosion cracking makes it suitable for processing plants, water tanks, and various chemical processing environments. Other applications include heat exchangers, power generation, pharmaceuticals, and petrochemicals.
• Super Duplex Stainless Steel: Offering even greater strength and corrosion resistance, super duplex stainless steel is used in demanding environments like power plants, chemical industries, heat exchangers, boilers, and pressure vessels. Its superior durability makes it a reliable choice for extreme conditions where both high strength and exceptional resistance are essential.

Applications of Duplex Steel Pipes

Duplex steel pipes are highly versatile and find application across a range of industries due to their exceptional durability and corrosion resistance. Their robust properties make them suitable for:

• Oil and Gas Extraction: Duplex steel pipes are commonly used in the extraction and transportation of oil and gas, where their strength and resistance to harsh environments are crucial.
• Chemical and Petrochemical Industries: They are used in chemical production equipment and heat exchangers for petrochemical plants, where durability and resistance to corrosive substances are essential.
• Food Processing: Duplex steel pipes are utilized in food processing systems due to their ability to withstand rigorous conditions and maintain hygiene standards.
• Marine Applications: Their resistance to seawater corrosion makes them ideal for offshore drilling platforms and marine vessels.
• Power Generation: They are used in turbines and other components of power generation systems where high strength and resistance to high temperatures are required.

Conclusion

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