What is Duplex Steel and its Features

 

What is Duplex Steel and its Features

Duplex stainless steels are called “duplex” because they have a two-phase microstructure consisting of grains of ferritic and austenitic stainless steel. The picture shows the yellow austenitic phase as “islands” surrounded by the blue ferritic phase. When duplex stainless steel is melted it solidifies from the liquid phase to a completely ferritic structure. As the material cools to room temperature, about half of the ferritic grains transform to austenitic grains (“islands”). The result is a microstructure of roughly 50% austenite and 50% ferrite.

Duplex stainless steel is not a modern day concept. In fact, the idea of creating duplex stainless steel goes back to the early 20th century. The first cast was manufactured in Avesta in Sweden in the year 1930. With that being said, duplex steel has rose to prominence only in the last 3 decades. The popularity that it has achieved is based on the fact that steel making techniques are being improved with each day, especially when it comes to controlling the nitrogen content of the stainless steel. Duplex stainless steel is indeed a much talked about subject. Technical papers are presented in special conferences on duplex stainless steel that are held every 2-3 years. These particular stainless steel grades have also been subjected to tremendous marketing. As years roll by, people are coming up with ideas for new grades.

The duplex structure of this stainless steel gives rise to a combination of desirable properties. These include:

1) Strength

Duplex stainless steels have double the strength of ordinary austenitic or ferritic stainless steels. The structure facilitates reduced thickness and thus reduced weight. Due to this property, duplex stainless steels can be useful for application such as pressure vessels, storage tanks and bridges.

2) Toughness and Ductility

The toughness and ductility of duplex stainless steels is comparatively much better than ferritic grades. With that being said thought, it is important to bear in mind that the toughness and ductility of duplex stainless steel is not quite as good as the austenitic grades.

3) Corrosion Resistance

This is a common property that you will find in stainless steels. Duplex stainless steel is therefore no exception. The level of corrosion resistance of stainless steel is heavily reliant on the composition of the steel itself. If you want chloride pitting and crevice corrosion resistance, chromium, molybdenum and nitrogen content is essential. Duplex stainless steel grades do not have a fixed corrosion resistance. They vary, similar to how the corrosion resistance of austenitic stainless steels vary.

4) Stress Corrosion Cracking Resistance

Stress corrosion cracking resistance of duplex stainless steel is quite impressive. It is said that the property has been derived from the ferritic side. For standard austenitic such as Types 304 and 316, stress corrosion cracking resistance can turn out to be a bit of a problem under certain circumstances. These include presence of chlorides, humidity and elevated temperature.

5) Cost

The quantity of nickel and molybdenum in duplex stainless steel is relatively lower than that of their austenitic brethren of analogous corrosion resistance. Since the alloying content is low, the cost of the duplex stainless steel can be curbed down, especially during times when ally costs are spiraling upwards.

Types of Duplex Stainless Steel

Duplex Stainless Pipe

The internal micro structure of 2205 Duplex Stainless Tubing is a two phase one. This microstructure is a mix of both ferrite and austenitic in an equal ratio in the Duplex 2205 Stainless Steel Pipe.

Duplex Stainless Tube

Duplex Stainless Steel Tubes Manufacturer and Supplier in India. We Manufacture and supply a comprehensive range of products, including Duplex Tubing , Duplex ‘U’ Shaped Tubing and Duplex Welded Tubing as the leading stockholder of Duplex Stainless Steel Tubes. Philips Metal should be contacted to check availability and pricing.

Duplex Stainless Buttweld Fitting

Buttweld fittings are pipe fittings that change the pipeline’s path. These fittings are crafted to be welded on-site and allow for changes in direction, pipe diameter, branching, and end. Buttweld fittings are used for piping systems larger than 2 inches in diameter. A pipe fitting improves a system for safely and efficiently transporting liquids (oil, gas, steam, chemicals) over short or long distances.

Duplex Stainless Forged Fitting

2205 Duplex Stainless Steel,  Stainless Steel, Carbon Steel& Alloy SteelForged Fittings / Threaded Fitting. A rejection rate of less than one percent has consolidated the claim. The primary reason for this is the priority that we give to complete satisfaction of the customer. This includes our excellent after sales services,We have supplied  Duplex 2205,  Stainless Steel, Carbon Steel& Alloy SteelForged Fittings / Threaded Fitting. 

Duplex Stainless Flanges

The granular structure of duplex is a ferrite-austenite hybrid with a superior strength to weight ratio in comparison to 304 or 316. It balances the weldability of an austenitic with a pure ferritic and retains decent mechanical toughness in applications with exposure to colder ambient temperatures. Offshore/seawater applications are a common use for this type of steel due to its excellent stress corrosion cracking properties. Therefore, duplex is often found in desalination and pressure vessel equipment located in port cities.

Duplex Stainless Fasteners

Duplex 2205 and Super Duplex 2507 (UNS S32205 / UNS S32750) stainless steel fasteners are characterized by their high yield strength, which is twice that of annealed austenitic stainless steels, like 304 and 316. In addition, Duplex steels offer improved corrosion protection compared to 300 series stainless, especially in terms of localized corrosion such as crevice and pitting, as well as stress corrosion cracking.  In general, Duplex steel bolts are a great step up from 300 series stainless.

Duplex Stainless Sheet, Plate Coils

Duplex Stainless Steel Plate belongs to a class of steel known as the duplex steel. Duplex Steel Plate was derived to get benefits from both the ferritic and austenitic types of steel. Duplex Plate contains almost equal proportions of ferrite and austenitic metallic phases in its microstructure. Duplex Steel Plate lacks carbon and has the addition of molybdenum from 3% to 9%. It also contains chromium from 21% to 30%. The additions and quantities of these materials provide the Stainless Steel Duplex Plate an extra ordinary corrosion resistance and tensile strength.

Duplex Steel Grades

Duplex stainless steels have a higher chromium content, 20–28%; higher molybdenum, up to 5%; lower nickel, up to 9%, and 0.05–0.50% nitrogen as compared to austenitic stainless steels. For resistance against pitting corrosion, DSS material is an ideal selection.

Lean duplex grades (PREN range: 22–27): No deliberate Molybdenum addition. mainly used in building and construction industry for bridges, pressure vessels or tie bars. Example: S32001, S32101, S32304, S32202.

Standard duplex (PREN range: 28–38): The most widely used (More than 80%) duplex stainless steel material with mid-range properties. Example: S32003, S31803, S32205

Super duplex (PREN range: 38-45): Higher contents of Cr, Ni, Mo, N and even W.  Specifically designed for highly corrosive oil&gas and chemical industries. Example, S32750, S32760, S32950, S32808.

Hyper duplex (PREN >45): These are highly alloyed duplex stainless steels. Example S32707, S33207

Benefits of Duplex Stainless Steel

While duplex stainless steel only comprises a small percentage of the global stainless steel market, it has a range of benefits when compared to traditional austenitic stainless steel and ferritic stainless steel grades.

Improved Strength

Many duplex grades are as much as two-times stronger than austenitic and ferritic stainless steel grades.

High Toughness and Ductility

Duplex stainless steel is often more formable under pressure than ferritic grades and provides greater toughness. Though they often offer lower values than austenitic steels, the unique structure and characteristics of duplex steel often outweigh any concerns.

High Corrosion Resistance

Depending on the grade in question, duplex stainless steels offer comparable (or better) corrosion resistance as common austenitic grades. For alloys with increased nitrogen, molybdenum, and chromium, steels exhibit high resistance to both crevice corrosion and chloride pitting.

Cost Effectiveness

Duplex stainless steel offers all of the above benefits while requiring lower levels of molybdenum and nickel. This means that it is a lower-cost option than many traditional austenitic grades of stainless steel.The price of duplex alloys is often less volatile than other steel grades making it easier to estimate costs -- both at an upfront and lifetime level.The higher strength and corrosion resistance also means that many parts made using duplex stainless can be thinner than their austenitic counterparts providing lower costs.