300 series vs duplex: which should I choose?

300 series vs duplex: which should I choose?
The large stainless tank in the middle left is a duplex alloy, replacing a carbon steel tank (left). The higher strength of duplex made for a much thinner wall thickness. The smaller tanks on the right were made of an austenitic stainless steel, as the higher strength of duplex did not offer significant advantage.

The duplex alloys are “problem-solving alloys” for good reason, they have been successfully used in many places where carbon steels and standard austenitic alloys have failed. Just like with the austenitic family, there are many different duplex alloys to choose from, with corrosion resistance varying from moderate with the lean duplex alloys to very high with the superduplex alloys.

By Gary Coates, Nickel Institute

Many of us are familiar with the advantages of duplex stainless steel – higher strength, better fatigue properties, high resistance to chloride stress corrosion cracking, higher corrosion resistance in certain other media, and lower cost of alloying elements. The duplex alloys are “problem-solving alloys” for good reason, they have been successfully used in many places where carbon steels and standard austenitic alloys have failed. Yet currently, the production of duplex stainless steels is less than 1% of total stainless production, whereas the 300 series austenitic grades represent about 55% of total production and austenitics as a family around 75%. Why has the volume of duplex grades not increased significantly over these many years of availability with all their advantages?Firstly, higher strength is not always advantageous. It can make forming processes more difficult. Higher strength and hardness also impact the ability to produce a wide range of surface finishes. The high ductility of the 300 series is exceptional and allows for even extreme forming operations such as deep drawing. Secondly, the two-phase structure of duplex stainless steel means that ensuring quality of weldments is challenging. In contrast, 300 series alloys are much more forgiving during the welding process.
Another major difference is that austenitic alloys are suitable for an extremely wide temperature range whereas duplex alloys have limitations at both low and high temperatures. While the cost of the alloying elements in duplex alloys are lower than for a roughly equivalent austenitic alloy, the production cost of duplex is likely to be higher. Both families are often used under severe corrosive conditions, and it is more critical that the metallurgical quality of the delivered duplex components is assured by additional testing. Duplex alloys are also more likely to have surface defects that will need conditioning or repair. In addition, the availability of any one duplex grade will be poorer. So where many product forms and sizes are required, it may be a challenge to find a complete bill of material.
Duplex alloys have their niche applications where they are excellent choices with proven performance and durability, including thicker-walled tanks and pressure vessels, rotating equipment, piping systems where CUI (corrosion under insulation) is an issue, and many more. It is important, however, when considering a duplex alloy to not only understand all their advantages, but also their limitations, comparing them to 300 series alloys with their advantages and limitations.

About the author
Gary Coates is Manager, Market Development & Technical at Nickel Institute, responsible for market development projects. He provides technical nickel-related education and training internationally for various industries including the chemical, petrochemical, food and beverage, pharmaceutical, water and engineering sectors. Gary also provides metallurgical support to the organisation worldwide.

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