Duplex Stainless Steel: Grades, Properties, and Hardening Methods

History of Duplex Stainless Steels

The history of duplex stainless steel dates back to the 1930s in Sweden, but its industrial significance has grown significantly over the last 30 years. Early versions of duplex steels had limitations, particularly in as-welded conditions. The invention of argon oxygen decarburization (AOD) allowed the addition of nitrogen as an alloying element, leading to improved austenite stability and reducing the formation of harmful intermetallic phases. This innovation marked the development of the second generation of duplex steels, widely used in industries like offshore oil and gas due to their high strength and excellent corrosion resistance.

The Second Generation and Beyond

In the late 1970s, duplex grades gained traction, particularly in industries like oil and gas. Their high mechanical strength and superior corrosion resistance allowed them to replace traditional materials, such as carbon steel and austenitic stainless steels, in demanding environments. Since then, five distinct types of duplex grades have been developed, each optimized for specific industrial needs based on their corrosion resistance and mechanical properties.

Types of Duplex Stainless Steel Grades

Different types of duplex stainless steel have been developed to meet specific industrial demands. They are typically categorized based on their resistance to corrosion and the proportion of key alloying elements like chromium, molybdenum, and nickel.

Standard Duplex Grades

Standard duplex grades, such as S31803 (1.4462), are the most commonly used. These grades contain around 20% chromium, 5% nickel, and 3% molybdenum. They provide a good balance of strength and corrosion resistance, making them ideal for various applications where these properties are essential.

Lean Duplex Grades

Lean duplex grades, such as S32001 and S32101, are cost-effective alternatives to standard duplex. With lower nickel and molybdenum content, they still offer good strength and moderate corrosion resistance. They are often used in the construction industry, where their mechanical properties can help reduce material costs while maintaining performance.

Super Duplex Grades

Super duplex grades, including S32750 (1.4410), have a higher alloy content, which gives them superior corrosion resistance and mechanical strength compared to standard duplex. They are commonly used in highly corrosive environments like seawater desalination and chemical processing.

Hyper Duplex Grades

Hyper duplex grades are designed for the most extreme conditions, with even higher pitting resistance equivalent numbers (PREN) than super duplex. These grades are typically used in deep-sea drilling and other aggressive environments where corrosion resistance is paramount.

Properties of Duplex Stainless Steels

The properties of duplex grades are what make them valuable across many industries. Their duplex structure allows them to combine the benefits of both ferritic and austenitic steels, giving them a range of mechanical and corrosion-resistant properties.

Chemical Composition

Duplex stainless steel grades typically feature a balance of ferrite and austenite phases, with chromium, molybdenum, nitrogen, and nickel as key alloying elements. This balance provides both excellent strength and resistance to corrosion, particularly in chloride-rich environments where austenitic stainless steels might fail.

Mechanical Properties

One of the significant advantages of duplex grades is their excellent mechanical properties. Compared to austenitic stainless steels, duplex grades offer much higher yield strength, allowing them to be used in thinner sections while still maintaining the required structural integrity. This not only saves material costs but also reduces weight, which is essential in industries like oil and gas or construction.

Corrosion Resistance in a Wide Range of Environments

Duplex grades are known for their superior resistance to corrosion in a wide range of environments, including both oxidizing and reducing acidic conditions. They outperform many austenitic grades, such as 304 and 316, especially in applications that involve exposure to chlorides, making them ideal for industries such as chemical processing and marine engineering. Their high chromium and molybdenum content enhance their performance in these harsh conditions.

Applications of Duplex Stainless Steels

Duplex grades are widely used across many industries due to their durability and cost-effectiveness. Their excellent mechanical properties and resistance to corrosion make them ideal for various demanding applications.

Pulp and Paper Industry

In the pulp and paper industry, duplex grades are commonly used for bleaching equipment, storage tanks, and other machinery that must withstand corrosive environments. Duplex steels have largely replaced carbon steel and austenitic stainless steels due to their superior corrosion resistance and longer lifespan.

Oil and Gas Industry

The oil and gas industry relies heavily on duplex grades, particularly the 2205 grade, for pipelines, pumps, and other equipment that must endure harsh conditions. Super duplex grades are often used for components requiring high fatigue resistance and excellent corrosion protection in offshore environments.

Construction Industry

Duplex grades play a significant role in construction, especially in the building of bridges and other large structures. Their combination of strength and corrosion resistance makes them ideal for long-lasting infrastructure projects. Notable examples include the Hong Kong Stonecutter Bridge and Singapore’s Helix Bridge, both built with duplex stainless steel.

Hardening Methods for Duplex Stainless Steel

While duplex stainless steel offers many advantages, enhancing its mechanical properties through heat treatment can be challenging. Standard heat treatments may negatively affect corrosion resistance, but advanced methods such as the BORINOX® process overcome these challenges.

The BORINOX® Method

The BORINOX® process is a low-temperature diffusion treatment specifically designed for hardening duplex stainless steel without compromising its corrosion resistance. This method increases surface hardness while maintaining the steel’s protective properties, making it ideal for critical components that need to withstand both mechanical stress and corrosive environments.