WHAT IS DUPLEX STAINLESS STEEL?

There are various grades of stainless steel that are manufactured in mass scale today in the world. Among them, duplex stainless steel grades have been rising in popularity. The major stainless steel mills have now turned their attention towards producing more duplex stainless steel. Duplex steel gets its name from the fact that they have a two-phase microstructure that comprises of grains of ferritic and austenitic stainless steel. Upon melting, the duplex stainless steel solidifies from the liquid state to an entirely ferritic structure. As the heat escapes from the substance and the material begins to cool down gradually to room temperature, about 50% of the ferritic grains take the form of austenitic grains. As a result, you have a microstructure that consists of approximately 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.

Limitation of Duplex Stainless Steel

Given all these attractive properties, one may think that duplex stainless steel is highly marketable. However, these grades of steel have certain limitations that stun their chances of becoming as profitable as mainstream grades of steel. Duplex stainless steel may have high strength, but when you consider factors such as formability and machinability, you realize that this advantage actually turns out to be quite a big disadvantage. In addition to that, one needs to consider the fact that metallurgy of duplex stainless steel is a lot more complex than that of austenitic or ferritic stainless steel. The complex structure makes it difficult for them to be manufactured conveniently. Fabrication too is a steep task. Last but not the least, if you do not give the steel the correct processing, then it will develop unwanted phases. Once these phases are developed, the stainless steel will become brittle and non-functional.

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