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A Comprehensive Guide to Duplex Stainless Steel
Duplex stainless steels combine the characteristics of both the austenitic and ferritic family of alloys, offering both benefits with fewer downsides, and often at a lower cost than other steel alloys. However, cost is just one part of the bigger picture. Choosing the suitable stainless steel also depends on how the steel will perform for your preferred use. Can duplex stainless steel work in your next project? Let’s take a closer look at how and when duplex steel might be a good fit for your demands.
What is Duplex Stainless Steel
Duplex stainless steels are highly corrosion-resistant, work-hardenable alloys. They contain relatively high levels of chromium (18% -28%) and low to moderate amounts of nickel (1.5% – 8%). The superb corrosion resistance and mechanical properties of duplex stainless steels are due to their chemical composition and balanced (duplex) microstructure, consisting of approximately equal percentages of ferrite and austenite. Due to this duplex nature, they exhibit properties characteristic of austenitic and ferritic stainless steels. In most cases, duplex stainless steels are tougher than ferritic stainless steels, and in some cases, their strengths can be double that of austenitic stainless steels.
History of Duplex Stainless Steel
Duplex stainless steel was first conceptualized in the 1930s. During this time, maintaining its chemistry was challenging due to manufacturing constraints. However, it was still used for a few applications. With technological advancements, particularly the introduction of decarburization in the 1960s, manufacturing duplex stainless steel became practically doable. Duplex 2205, which is thought to be the workhorse of duplex grades, was developed during this period. In the 1980s, superior Super and Hyper duplex grades were introduced. These new grades can withstand the most corrosive of environments. The oil and gas industry was introduced to duplex stainless steel with the development of these superior Super and Hyper duplex grades.
Duplex Stainless steel Grades
The most common grades are 2205, 2304, 2101, and 2003. Other grades include 2507, 2202, and Z100. Let’s delve into each common grade.
Type 2205: It was upgraded to a duplex SS grade due to its unique combination of properties: high strength, good weldability, and excellent anti-corrosive properties. Additionally, it boasts a higher yield strength than conventional austenitic steel grades.
Type 2304: The most commonly used grade is 2304, which is a combination of 2% molybdenum, 4% nickel, and 23% chromium. This combination provides excellent strength compared to traditional austenitic and ferritic grades.
Type 2101: The 2101 stainless steel is classified as a duplex grade due to its combination of austenitic and ferritic microstructures. This unique combination balances strength and ductility, making it an excellent choice for chemical processing, oil and gas, and food processing industries.
Type 2003: It is classified as a duplex grade due to its high chromium, nickel, and molybdenum content. This composition makes it highly resistant to pitting and crevice corrosion. Moreover, its ability to resist chloride-induced stress corrosion cracking makes it suitable for use in environments exposed to chlorides, like seawater and brackish water.
Chemical Composition of Duplex Stainless Steel
The composition of steel determines its properties and performance in various applications.
| Element | Percentage |
| Chromium (Cr) | 18–20% |
| Nickel (Ni) | 8–10% |
| Manganese (Mn) | 0.02 |
| Silicon (Si) | 0.01 |
| Molybdenum (Mo) | 2–3% |
| Nitrogen (N) | 0.08–0.20% |
| Phosphorus (P) | 0.00045 |
| Carbon (C) | 0.0003 |
| Iron (Fe) | Balance |
The Properties of Duplex Stainless Steel
Physical Properties of Two Duplex Stainless steel
| Property | 2205 | UR52N+ |
| Density (g.cm3) | 7.805 | 7.81 |
| Modulus of Elasticity (GPa) | 200 | 205 |
| Electrical Resistivity (Ω.m) | 0.85 x 10-6 | 0.85 x 10-6 |
| Thermal Conductivity (W/m.K) | 19 at 100 °C | 17 at 100 °C |
| Thermal Expansion (m/m.K) | 13.7 x 10-6 to 100 °C | 13.5 x 10-6 to 200 °C |
Mechanical Properties of Two Duplex Stainless steel
| Grade | 2205 | UR52N+ |
| Tensile Strength (MPa) | 620 | 770 |
| Proof Stress 0.2% (MPa) | 450 | 550 |
| Elongation A5 (%) | 25 | 25 |
Some other Characteristics of Duplex Stainless Steel
Corrosion Resistance of Duplex Stainless Steel
Duplex stainless steels are highly corrosion resistant. They possess strong resistance to intergranular corrosion. Even in sulfide and chloride environments, duplex stainless steels demonstrate very high resistance to stress corrosion cracking. The super duplex grades are even more corrosion-resistant.
Heat Resistance of Duplex Stainless Steel
Duplex stainless steel contains a high chromium content, which protects against corrosion but can cause embrittlement at temperatures over 300°C.
At low temperatures, duplex stainless steels possess better ductility than the ferritic and martensitic grades, and they can be used at almost -50°C.
Fabrication of Duplex Stainless Steel
Remember only to use tools dedicated to stainless steel fabrication when working with stainless steel materials. Clean the tooling and work surfaces thoroughly before use to prevent cross-contamination by easily corroded metals, which could discolor the surface of the fabricated product.
Heat Treatment of Duplex Stainless Steel
Duplex stainless steel cannot be hardened through heat treatment but can be work-hardened.
Solution treatment or annealing can be achieved by rapidly cooling the steel after heating it to around 1100°C.
Machinability
Although machinable, the high strength of Duplex stainless steel makes machining difficult. For example, the machining of 2205 is around 20% slower than that of 304.
Machining can be improved by following these guidelines:
- Sharp cutting edges are essential to avoid excess work hardening caused by dull edges.
- Cuts should be light but deep enough to preclude work hardening by riding on the material’s surface.
- Chip breakers should be used to help ensure that the swarf is cleared from the workpiece.
- Low thermal conductivity of austenitic alloys leads to heat concentration at the cutting edges, necessitating the use of large quantities of coolants and lubricants.
Welding of Duplex Stainless Steel
Duplex stainless steel has good weldability, allowing all standard welding processes to be used. While not as easily welded as the austenitic grades, the low thermal expansion in duplex grades helps to reduce distortion and residual stresses after welding. For 2205 stainless steel, the recommended filler material is 2209.
Standard Designations of Duplex Stainless Steel
Some Standard Designations for Duplex Stainless Steel
| Euronorm | UNS | BS | En | Grade |
| 1.4462 | S31803, S32205 | 318S13 | – | 2205 |
| 1.4507 | S32520, S32550 | – | – | UR52N+ |
Common Forms of Duplex Stainless Steel
- Fittings & Flanges
- Pipe
- Plate
- Sheet
- Tube
- Bar
- Band
Applications of Duplex Stainless Steel
Pulp and Paper Industry
The use of Duplex stainless steels began in the 1930s in the sulfite paper industry. These steels are used in various pulp and paper processing components such as digesters, bleaching equipment, chip storage tanks, suction roll shells, and liquor storage containers. Duplex steels have replaced austenitic stainless steels and carbon steels due to their higher strength and better corrosion resistance, allowing for the use of thinner plates at the same pressure level. This thinner plate production reduces material costs and lowers welding time, shipping, and utility costs.
Desalination
This sector of application involves rigorous testing of materials under highly corrosive conditions with high chloride and elevated temperatures. The desalination industry heavily relies on materials production as desalination equipment suppliers and customers seek cost-effective options that offer corrosion resistance. Previously, desalination plants, such as multi-stage flash and multi-effect desalination plants, were built using mild steel for evaporators. Subsequently, MSF evaporators were coated with austenitic stainless steel 316L to enhance corrosion resistance while managing investment costs.
The advantages of duplex stainless steel for this application sector are significant. It offers almost twice the strength of traditional austenitic steels and excellent corrosion resistance. Therefore, evaporators made of Duplex steel grades can be constructed using thinner sheets, reducing costs and not requiring extensive welding.
Oil and Gas
Duplex stainless steel is crucial in operations at oil and gas plants due to its higher strength and superior pitting and crevice corrosion resistance compared to traditional austenitic stainless steel.
Duplex steels are mainly used in process pipes, flow lines, and equipment such as scrubbers, separators, and pumps. In subsea applications, these materials are used in downhole production pipes, tubes, flowlines, and pipelines for transporting corrosive oil and gas. Super duplex stainless steel, which contains 25% chromium, is known for its high strength and is used for making bars, sheets, plates, tubes, and more. It also provides excellent fatigue strength and is compatible with other highly alloyed steel grades.
Duplex stainless steels are commonly used as materials for umbilicals, which monitor well-top functions through hydraulic lines and chemical injection. Duplex umbilicals are also utilized in analyzing deep sea surfaces due to their excellent corrosion resistance and high strength.
Food Industry
Duplex stainless steel is commonly utilized in the food and beverage industry. Steel grades 2205 and 2507 are mainly utilized in food and liquor storage units due to their high performance and cost-effectiveness, making them a valuable alternative to steel types 304 and 304L.
Architecture
Duplex stainless steel plays a crucial role in manufacturing bridges that require excellent resistance to corrosion in saline conditions and high load-bearing strength. Duplex stainless steel 2205 pipes and plates are commonly used in bridge construction in various countries, particularly in the form of sheets and pipes. These steels are also utilized in constructing airport roofs, providing the necessary strength and cost-effectiveness compared to other types of steel.
Advantages & Disadvantages
Advantages
While duplex stainless steel makes up only a tiny percentage of the global stainless steel market, it offers a range of benefits exceeding traditional austenitic stainless steel and ferritic stainless steel.
- Improved Strength: Numerous duplex grades are up to two times stronger than austenitic and ferritic stainless steels.
- High Toughness and Ductility: Duplex stainless steels are usually more formable under pressure than ferritic grades and provide more excellent toughness. Although they often offer lower values than austenitic steels, duplex steels’ unique structure and characteristics usually outweigh any concerns.
- High Corrosion Resistance: According to the grade in question, duplex stainless steels provide comparable (or better) corrosion resistance compared to common austenitic grades. Alloys with increased nitrogen, chromium, and molybdenum exhibit high resistance to both chloride pitting and crevice corrosion.
- Heat Resistance: Duplex stainless steel has lower thermal expansion and higher heat conductivity. Duplex grades can even be used at temperatures of -50°C because they have better ductility than ferritic steel grades at low temperatures.
- Cost Effectiveness: Duplex stainless steel provides the abovementioned benefits while requiring lower levels of nickel and molybdenum. This makes it a more cost-effective option than many traditional austenitic stainless steel grades. The price of duplex alloys is often more stable than other steel grades, making it easier to estimate costs upfront and over the product’s lifetime. Additionally, duplex stainless steel’s higher strength and corrosion resistance means that parts made using this material can be thinner than their austenitic counterparts, resulting in lower costs.
Disadvantages
Although duplex steel is gaining immense popularity due to its unique properties, it has downsides. The following are some of the disadvantages of utilizing this specific steel:
- High Cost: The metal is significantly more expensive than other types of stainless steels because of its high alloy content and the difficulty in manufacturing it.
- Difficult to Weld: Welding this steel is more challenging due to its high alloy content, which poses issues for specific applications.
- Low Temperature: You should not use it below -50°C, which may pose a problem for specific applications, like in cryogenic systems.
- Hot Tearing: Hot tearing can occur during the welding process of stainless steel due to its high alloy content, which causes the weld center to solidify faster than the edges.
- Toughness: Duplex stainless steel is a very tough material and can be challenging to machine, which may result in longer processing times and increased production costs.
Comparison
Duplex Stainless Steel VS. 304 Stainless Steel
Duplex stainless steel consists of two or more phases, offering a high strength-to-weight ratio. It is particularly valuable for applications requiring excellent corrosion resistance and strength, like oil and gas pipelines.
Compared to 304 stainless steel, duplex steels have a higher strength-to-weight ratio and better weldability. Additionally, they are more resistant to stress corrosion cracking, which is a common problem in specific environments. However, duplex steel is more expensive than 304. Despite the higher cost, its superior properties make it worthwhile. Duplex steel is an excellent choice for applications requiring superior strength and weldability. It serves as a great alternative to 304 SS when cost is a primary factor.
Duplex Stainless Steel VS. 316 Stainless Steel
Duplex stainless steel is composed of austenitic and ferritic steel.
316 SS is an austenitic stainless steel composed of iron, nickel, chromium, and molybdenum. This alloy is widely used due to its excellent anti-corrosive properties and strength. It is highly resistant to pitting, which can occur when specific chemicals come into contact with the steel surface.
The main difference between the two types of steel is their composition. Duplex stainless steel combines ferritic and austenitic steel, while 316 stainless steel is made up of only austenitic steel. This composition makes duplex stainless steel more corrosion-resistant than 316 stainless steel. However, 316 stainless steel is stronger and more pitting-resistant than duplex stainless steel.
Summary
We can draw a conclusion that duplex stainless steels are advantageous for most projects because they have unique advantages and drawbacks compared to other stainless steels. The information provided above is undoubtedly helpful in determining whether it suits your applications.
