Given its assets of strength, corrosion resistance, and easy weldability, it’s no wonder stainless steel is so widely used across many industries. Each stainless steel family – austenitic, martensitic, ferritic, duplex, and precipitation hardening – is determined by the alloying elements in its chemical structure and the crystal structure of the steel itself. While all stainless steels share the same basic characteristics, each family has an asset making it particularly good for certain uses. For example, the low carbon found in ferritic stainless steel makes it highly malleable and excellent for welding. On the other hand, stainless steels in the martensitic family contain a high percentage of carbon, greatly increasing its tensile strength and impact resistance.
Duplex steel is unique because it combines two of those families, austenitic and ferritic, to create a stainless steel with the benefits of both. Austenitic stainless steel has good corrosion and impact resistance, but the nickel used in its alloy adds to the production cost. Ferritic stainless steel is known for its ability to resist stress cracking, but the alloys used in ferritic steel’s composition make it more susceptible to corrosion. By combining these to make duplex steel, you can reap the performance benefits of both types while keeping costs down.
How is Duplex Steel Made?
Duplex steel production begins like all stainless steel: the iron ore is smelted and combined with alloying elements such as nickel, molybdenum, chromium, and others. However, duplex steel’s chemical composition has a higher level of chromium than most stainless steels (typically 25% or more) and a lower percentage of nickel (less than 9%). It also contains a small amount of nitrogen, which helps improve the weldability of the finished steel.
Once combined, the molten steel alloy is forged and left to cool. As the steel solidifies, the grain structure of the steel is formed into ferritic crystals; at this point, the entire product is ferritic steel. However, as it continues to cool to room temperature, more and more grains will begin to convert to austenitic steel until it becomes roughly 50-50 of each type. The reason duplex steel is so strong is due to the intermixing of these grains, much like the fat marbling on a prime steak. If the austenitic and ferritic steel were in distinct layers, that would leave one type more prone to cracking or corrosion, which risks weakening the other. In duplex steel, the interspersing of the grains means they reinforce the steel’s strength while minimizing each other’s weaknesses.
Advantages of Duplex Stainless Steel
While duplex steel makes up a fairly small part of the global stainless steel market, it has some clear advantages compared to other families of stainless steel:
• Strength: Stainless steel is already a strong material on its own, but duplex steel can be 2 to 3 times stronger than austenitic or ferritic steels.
• Corrosion resistance: Duplex stainless steel has comparable or even higher levels of corrosion resistance than austenitic steels. Additionally, its high level of chromium provides more protection against crevice corrosion and pitting.
• Cost: Perhaps the greatest appeal of duplex steel is that it offers so many benefits while requiring less molybdenum and nickel, which keeps costs low. The strength and durability of duplex steel can also contribute to a longer lifetime use with less required maintenance, which helps money.
Disadvantages of Duplex Stainless Steel
You might wonder, with all the benefits of duplex steel, why is it not in greater demand? There are a few disadvantages which limit its use:
• Production: Due to its unique chemical composition, advanced knowledge is needed to produce this steel.
• Performance at low temperatures: Duplex steel has excellent impact resistance, except at very low temperatures. The ferrite in the duplex steel becomes brittle and prone to cracking.
• Performance at high temperatures: The structure of duplex steel can be negatively altered by very high temperatures, even with brief exposure. This includes the heat produced during welding, although the addition of nitrogen and a skilled welder can help prevent damaging the steel.