Q: What are the uses of Duplex Stainless Steel?
A: Due to their excellent corrosion resistance, increased strength, and affordable pricing, duplex stainless steels are popular in a variety of industries and markets, including:
Offshore and near-shore operations such as oil drilling, desalination, water treatment, and other industrial operations.
Chemical and liquid processing.
Naval parts and components.
Pollution control equipment.
Pulp and paper production.
Construction.
Hot water and brewing tanks.
Q: What is Duplex Stainless Steel?
A: Duplex stainless steels are extremely corrosion resistant, work hardenable alloys. Duplex stainless steel plate contains relatively high levels of chromium (between 18% and 28%) and low to moderate amounts of nickel (between 1.5% and 8%). The high corrosion resistance and excellent mechanical properties of duplex stainless steels can be attributed to their chemical composition and balanced (duplex) microstructure of approximately equal volume percentages of ferrite and austenite. Because of this duplex nature, it will display properties characteristic of both austenitic and ferritic stainless steels. Duplex stainless steels are in most cases, tougher than ferritic stainless steels. Strengths of duplex stainless steels can in some cases be double that for austenitic stainless steels.
Q: What should you pay attention to with Duplex Stainless Steel?
A: Duplex stainless steels can also form a number of unwanted phases if steel is not given the correct processing, notably in heat treatment. Phases like sigma phase leads to embrittlement, meaning the loss of impact toughness, but sigma phase also reduces corrosion resistance.
The formation of sigma phase is most likely to occur when the cooling rate during manufacture or welding is not fast enough. The more highly alloyed the steel, the higher the probability of sigma phase formation. Therefore, super duplex stainless steels are most prone to this problem. Another form of embrittlement occurs above 475°C, and it can still form at temperatures as low as 300°C. This leads to the design limitations on the maximum service temperature for duplex stainless steels.
Q: What are the properties of Duplex Stainless Steel?
A: Good weldability
Duplex stainless steel grades generally have good weldability and they can be welded using most of the welding methods used for austenitic stainless steels:
Shielded metal arc welding (SMAW).
Gas tungsten arc welding TIG (GTAW).
Gas metal arc welding MIG (GMAW).
Flux-cored arc welding (FCW).
Plasma arc welding (PAW).
Submerged arc welding (SAW).
Laser welding.
Resistance welding.
High-frequence welding.
Superior corrosion resistance.
Duplex stainless steel grades are superior to standard austenitics with regard to stress corrosion cracking resistance. They are also good in a number of acidic and alkaline environments.
Outokumpu's duplex offering is unique: for a given austenitic stainless steel grade there is a duplex counterpart that not only provides similar corrosion resistance but also twice the strength.
Q: What is the difference between stainless steel and duplex stainless steel?
A: Stainless steels encompass several classes of alloys on the basis of their crystalline microstructure. One such classification is the duplex variety. The four main classes of stainless steel include austenitic, ferritic, martensitic as well as a duplex. While austenitic stainless steels have an FCC microstructure, ferritic classes have a BCC structure, martensitic has a body-centered tetragonal form crystalline microstructure. On the other hand, the microstructure of a duplex stainless steel alloy includes both austenite and ferrite in a 50% ratio, which is what gives these steels their unique mechanical and corrosion resistance properties.
Galvanic corrosion between stainless steel and duplex stainless steel
bimetallic corrosion, or otherwise known as galvanic corrosion is an electrochemical process that occurs when two dissimilar metals are submerged in a conductive solution and the metals are electrically connected. In this process, while one metal is protected, the other one gets corroded. It is possible to prevent galvanic corrosion by practicing several rules. Materials with similar corrosion potentials must be selected, the electrical connection must be broken down, by means of insulation, and lastly, the application of a coating to both the materials. The application of proper coating prevents the stripping of metals from either the cathode or anode, thereby improving their resistance to corrosion and effectively protecting both the metals.
Pickling and passivation of duplex and stainless steel
Passivation is a phenomenon that occurs in stainless steels that contain higher contents of chromium in their alloy. Duplex steels have chromium ranging between 19% to about 32% in their alloys. Since duplex stainless steels contain high levels of chromium in their alloy, it is possible for the surface of these components to form a passivated oxide layer over the surface of the steel. However, some stainless steel alloys possess insufficient chromium content to form this self-repairing passivation oxide layer, leading to the material getting corroded by oxidation. Oxidation is a common problem associated with low chromium stainless steel. It is possible to improve the corrosion resistance properties of these stainless steel alloys by subjecting them to a passivation treatment.
Pickling is the elimination of scales caused by exposing the material to elevated temperatures. Scales that were also caused by an adjacent chromium layer of metal could be eliminated by pickling by means of chemical treatments from the external body of stainless steel alloys. The use of mechanical processes often harms or damages the surface of the metal. To avoid such injurious defects, it is easier to eliminate the excess metals via the use of chemical washes, as they effectively eliminate the extra layer from the surface of stainless steel. This process makes use of acidic solutions including nitric (HNO3) and hydrofluoric (HF) acids
Advantages and disadvantages of duplex and stainless steel
Since there are multiple grades of duplex and stainless steel grades, the advantages, and disadvantages associated with each grade are dependent on the application it is to be used for along with the chemical composition of the grade. For instance, austenitic stainless steel grades belonging to the 300 series have excellent ductility, improved resistance to corrosion, and good mechanical strength. Their chemistry could be altered by lowering or increasing the content of carbon for specific purposes such as welding or use in elevated temperatures. Those stainless steels belonging to the martensitic and ferritic classes have better hardness, and the corrosion resistance properties are lower in comparison to austenitic and the duplex variety of stainless steels. Because duplex stainless steels are highly alloyed, the cost of using these metals may be high. However, the cost of grades like 904L is higher than some standard duplex stainless steels. Aside from the cost factor, there are very few stainless steel alloys that could be used at elevated temperatures, unlike nickel alloys such as Monel, Inconel, etc. Another disadvantage of these alloys is their impact resistance, which is not very good at temperatures as low as -50°C.
Q: How Is Duplex Steel Made?
A: The production of duplex stainless steel is a multistage process of steelmaking and forming. It begins with the combination of raw materials containing: nickel, iron ore, chromium, molybdenum, and other alloying elements in an electric arc furnace, in which they are melted and mixed together. This melted steel is moved to an argon oxygen decarburization (AOD) converter, in which critical metallurgical processes take place. In the AOD converter, gasses are removed from the molten steel, and the final composition is adjusted by adding alloying elements. Afterwards, the steel is transferred to a ladle furnace, so that modifications can be made to its inclusions.
Once the molten steel is prepared, it can be cast into continuously cast blooms or solid ingots. The molten mixture of duplex steel initially solidifies as a completely ferritic solid. However, as it cools to room temperature, approximately half of the material undergoes a phase transformation from ferritic to austenitic.
These blooms or ingots are then processed further, typically by reheating and forging or rolling to achieve the required diameter. Afterward, solution annealing, a heat treatment process, is applied to ensure the consistency of the bar’s internal structure. Following annealing, the bars are quenched to achieve the desired properties. Finally, to provide a consistent surface finish, the bars undergo mechanical peeling to remove the irregular surface, resulting in the finished duplex stainless steel product.
Q: What Is the Carbon Content of Duplex Steel?
A: The carbon content of duplex steel is typically below 0.04%. The chromium (Cr) and nickel (Ni) content can be adjusted within the ranges of 20% to 30% and below 9.5%, respectively. Duplex stainless steel also contains minor alloying elements such as: molybdenum, nitrogen, tungsten, and copper.
Q: What Are the Common Forms of Duplex Steel Material?
A: Duplex steel is available in various forms to meet diverse industrial and engineering needs including:
Fittings and Flanges
Duplex steel fittings and flanges are essential components in piping systems, providing robust connections that can withstand harsh environments. Their corrosion resistance and strength make them suitable for applications in industries such as: oil & gas, chemical processing, and marine engineering.
Pipe
Duplex steel pipes are widely used for conveying fluids and gasses in demanding settings. They offer superior resistance to corrosion, making them ideal for transporting corrosive substances in industries like: petrochemicals, pulp and paper, and desalination.
Plate
Duplex steel plates are flat, broad sheets of material that find applications in structural and industrial projects. Their high strength and corrosion resistance make them valuable for constructing pressure vessels, storage tanks, and various structural components.
Sheet
Duplex steel sheets are thinner than plates and are often used in applications requiring lightweight yet durable materials. They are commonly employed in architectural cladding, kitchen equipment, and decorative panels.
Tube
Duplex steel tubes are used for conveying liquids or gasses, especially in situations in which corrosion resistance is paramount. They are integral in industries like: chemical processing, offshore drilling, and heat exchanger manufacturing.
Bar
Duplex steel bars come in various shapes, including: round, square, and hexagonal, and are used in applications such as: shafts, fasteners, and valves. Their combination of strength and corrosion resistance makes them versatile in mechanical and industrial contexts.
Q: What Are the Advantages of Using Duplex Steel?
A: Duplex stainless steel offers many advantages compared to traditional austenitic and ferritic stainless steel grades. These are:
Often exhibit up to twice the strength of both austenitic and ferritic stainless steel grades.
Impressive formability under pressure, surpassing that of ferritic grades, and offers superior toughness.
Depending on the specific grade, duplex stainless steels provide comparable or even superior corrosion resistance when compared to common austenitic grades. Alloys with elevated levels of nitrogen, molybdenum, and chromium exhibit remarkable resistance to crevice corrosion and chloride pitting.
Offers superior mechanical properties when compared to ferritic stainless steel, especially in terms of plastic toughness.
Exhibits significantly improved cold-working and cold-forming capabilities compared to ferritic stainless steel.
Generally, no preheating or post-weld heat treatment is required, simplifying the welding process.
The versatility of duplex stainless steel extends to a broader range of applications compared to ferritic stainless steel.
Q: What Is the Difference Between Duplex Steel and 304 Stainless Steel?
A: Duplex stainless steel, particularly duplex 2205 stainless steel, stands apart from austenitic 304 stainless steel primarily in three key aspects. First, its yield strength is notably higher—more than double that of regular austenitic stainless steel. This elevated strength allows for the construction of storage tanks and pressure vessels with thinner walls, ultimately reducing material costs. Second, it boasts exceptional resistance to stress corrosion cracking, surpassing the capabilities of austenitic stainless steel in this regard. Finally, its mechanical properties are superior, with the capacity to absorb more energy under dynamic or static loading conditions, making it more resilient in scenarios involving sudden accidents like collisions or explosions.
However, it's essential to consider some drawbacks when comparing duplex stainless steel to austenitic stainless steel. Duplex steel has limited versatility in applications, necessitating temperature control below 250 °C. Its plastic toughness is lower, resulting in inferior cold and hot processing and forming performance compared to austenitic stainless steel. Furthermore, it has a medium-temperature brittle zone, requiring strict control in heat treatment and welding processes to prevent harmful phase formation and performance deterioration.
Q: What Is the Difference Between Duplex Steel and 316 Stainless Steel?
A: The main difference between duplex and 316 stainless steel is in their composition. Duplex stainless steel has more chromium, molybdenum, and nitrogen, making it stronger and corrosion-resistant at higher temperatures. While 316 stainless steel exhibits excellent resistance to corrosive environments within the 300 family, duplex stainless steel offers significantly greater resistance to corrosive pitting, especially in the presence of chloride. Duplex steel is also more ductile, allowing for complex shaping.
In terms of strength and durability, duplex steel outperforms 316, due to its resistance to various corrosive environments. It's ideal for high-stress applications like valves and pumps that need longevity.
Q: What Is the Difference Between Duplex Steel and 201 Stainless Steel?
A: Duplex steel and 201 stainless steel exhibit distinct differences in composition, properties, and applications. Duplex stainless steel is characterized by its dual-phase microstructure, comprising both austenitic and ferritic phases, typically in a 50-50 ratio. It contains higher levels of chromium, nickel, molybdenum, and nitrogen, providing exceptional corrosion resistance, especially in chloride-rich environments, and superior mechanical properties, including strength and toughness. Consequently, duplex steel finds its niche in demanding applications like: chemical processing, marine environments, and structural components in which both corrosion resistance and strength are critical.
In contrast, 201 stainless steel is an austenitic stainless steel with a simpler alloy composition, primarily consisting of iron, chromium, and nickel. While it offers good general corrosion resistance, it doesn't match the corrosion resistance and mechanical properties of duplex stainless steel. As a result, 201 stainless steel is commonly used in less-demanding applications, including: kitchen appliances, cookware, utensils, and decorative trim, for which corrosion resistance and mechanical strength requirements are less stringent.
Q: Why is nitrogen added to Duplex Stainless Steel?
A: Austenite Stabilization: Nitrogen is an alloying element that helps stabilize the austenite phase in duplex stainless steels. Compared to ferrite alone, this phase improves strength, toughness, and corrosion resistance.
Corrosion Resistance: Nitrogen enhances the resistance of duplex stainless steels to various forms of corrosion, such as pitting and crevice corrosion. Nitrogen promotes the formation of stainless steel's renowned passive protective film on the surface, which helps resist attack from corrosive environments.
Strength and Toughness: Nitrogen strengthens the alloy matrix by solid solution strengthening, which improves the mechanical properties of duplex stainless steels. It enhances the material's yield strength, tensile strength, and impact toughness, making it suitable for demanding structural applications.
Cost-Effective Additions: Nitrogen alloying offers a cost-effective alternative to increasing the alloy's performance compared to other alloying elements like nickel or molybdenum. It allows foundries to create alloys with desirable properties without significantly increasing the material cost.
Q: What grades of Duplex Stainless Steel are there and where are they generally used?
A: 2205 (UNS S32205/S31803)
This grade is known for good general corrosion resistance, offering superior performance to 316L in most environments.
Its high chromium, molybdenum, and nitrogen content resist pitting, crevice, and general corrosion.
However, it's unsuitable for applications requiring extended exposure to temperatures above 300°C due to precipitation of brittle microstructures.
Typical applications include:
Chemical processing, transport, and storage: Grade 2205 is used in pressure vessels, tanks, piping, and heat exchangers in the chemical industry.
Oil and gas exploration and processing: It is suitable for offshore platforms, pipelines, and equipment exposed to corrosive environments in the oil and gas industry.
Desalination plants: Grade 2205 is used in seawater desalination plants due to its resistance to chloride corrosion.
Marine applications: It is utilized in shipbuilding, offshore structures, and marine equipment. It is utilized in shipbuilding, offshore structures, and marine equipment.
Pulp and paper industry: Grade 2205 is used in pulp and paper production equipment, where corrosion resistance is crucial.
2507 (UNS S32750)
This super duplex grade has very high pitting and crevice corrosion resistance thanks to its high molybdenum and chromium content.
It offers high strength but can also undergo embrittlement if exposed to high temperatures for an extended time.
Grade 2507 duplex stainless steel is frequently used in extreme environments like desalination plants, seawater systems, and petrochemical industries.
Typical applications include:
Chemical processing equipment: Grade 2507 is suitable for equipment used in producing and handling chemicals, acids, and solvents.
Oil and gas industry: It is used in offshore platforms, subsea equipment, and pipelines due to its resistance to corrosion in seawater and harsh environments.
Desalination plants: The alloy's corrosion resistance makes it suitable for use in desalination plants where it comes into contact with chloride-rich seawater.
Pulp and paper industry: Grade 2507 is used in equipment such as digesters, bleach washers, and evaporators due to its resistance to corrosive chemicals used in the papermaking process.
LDX 2101 (UNS S32101)
As a lean duplex stainless steel, LDX 2101 offers good general corrosion resistance, superior to 304 but not as good as 316.
It's characterized by high strength and good resistance to stress corrosion cracking.
It's less expensive because it uses less nickel and molybdenum but can be harder to source depending on market conditions.
Common applications include:
Chemical processing: LDX 2101's corrosion resistance makes it suitable for equipment and components used in chemical processing plants.
Oil and gas industry: LDX 2101 can be used in offshore platforms, pipelines, and other equipment exposed to corrosive environments in the oil and gas industry.
Desalination plants: LDX 2101's resistance to corrosion and chloride stress-corrosion cracking makes it suitable for desalination plants where seawater is processed.
Food processing: LDX 2101's corrosion resistance and good machinability make it suitable for food processing and storage equipment.
Construction: LDX 2101 can be used in structural applications requiring corrosion resistance, such as bridges and buildings.
2304 (UNS S32304)
Another lean duplex, 2304 offers corrosion resistance similar to 316L but with higher strength.
However, it's susceptible to pitting corrosion in chloride-rich environments.
It's often used in construction, architecture, and in the pulp and paper industry.
Some ideal specific applications include:
Chemical Processing: The construction of chemical tankers, cargo tanks, and storage tanks due to grade 2304's corrosion resistance properties.
Oil and gas industry: Offshore platforms, pipelines, and subsea equipment in the oil and gas industry.
Pulp and paper industry: Heat exchangers, digesters and bleach towers, recovery boilers, piping systems, and other equipment.
Construction: Architectural structures and components, including facades, beams, columns, trusses, handrails, decorative elements, roofing, and cladding.
ZERON 100 (UNS S32760)
Like 2507 (UNS S32750), this super duplex grade offers high pitting and crevice corrosion resistance. It also provides good resistance to sulphide stress corrosion cracking.
However, like other super duplex grades, it's susceptible to embrittlement at high temperatures.
Typical applications include:
Oil and gas industry: Zeron 100 is commonly used in offshore oil and gas production, providing reliable corrosion resistance in seawater and sour gas environments.
Chemical processing: It is used in chemical processing plants throughout piping systems, heat exchangers, valves and fittings, pressure vessels, and chemical storage tanks.
Desalination plants: It can withstand the corrosive effects of seawater, making it ideal for use in reverse osmosis (RO) equipment, filler vessels, sulphate removal units, and flue gas desulphurization (FGD) equipment.
Petrochemical, paper and pulp, and marine industries: This grade is often found in heat exchangers, pressure vessels, and piping systems.
Q: What are the welding methods of Duplex Stainless Steel?
A: Skilled welders can weld Duplex stainless steel sheets or duplex stainless steel profiles using the common welding methods for high-alloyed steels. Thereunder one can find: submerged welding, tungsten inert gas welding, plasma welding, active gas metal arc welding, laser welding and electro-beam welding. The cooling process after welding takes significant influence in the austenite-ferrite ratio of the material at room temperature. Like this, a delayed cooling results in a higher austenite content and in general a quenching to a lower austenite content. The choice of the suitable welding process, as the correct seam geometry, ensures ease in adjusting quite precisely the austenite-ferrite ratio. For that matter, it is important to ensure a slow cool down rate between the temperatures of 1.100 degrees Celsius and 1000 degrees Celsius. The weld’s temperature shall pass from 950 to 700 degrees Celsius within two minutes, in order to avoid unwanted precipitations. A fast cooling speed should be kept until the temperature of 300 degrees Celsius. Super Duplex is also weldable similar to a high-alloyed steel, therefore with good results.
