Comparison between duplex stainless steel and 316 austenitic steel
SS 316 VS Duplex Steel
Composition
The 316 is an austenitic stainless steel that has chromium, nickel and molybdenum in its composition. There are three major variations of the 316 steel mainly based on their carbon content. The ones high in carbon are called the 316H, the ones low in carbon are called the 316L and the standard 316 grade which has a medium level of carbon. They have manganese 2% max, phosphorus 0.045% max, sulfur 0.03% max, silicon 0.75% max, chromium 16-18%, nickel 10-14%, molybdenum 2-3%, nitrogen 0.1% max and the rest with iron with the addition of carbon in different quantities. The 316 grade has 0.08% max of carbon, the 316L grade has 0.03% carbon and the 316H has 0.04-0.1% carbon.
The duplex is a group of stainless steel materials that are both austenitic and ferritic in their metallurgic microstructure. This is the reason how they got the name “Duplex” stainless steels. There are three major groups; the standard duplex, the super duplex and the lean duplex. The composition includes carbon, silicon, manganese, phosphorus, silicon, nitrogen, chromium, copper, molybdenum and nickel in different quantities depending on the grade of duplex. The amount of chromium and nickel are the major deciding factors in the grades. The chromium content can be as high as 30% as in X2CrNiMoN29-7-2 grade and the nickel can be as high as 9.5% in X2CrNiMoCoN28-8-5-1 grade of duplex.
Physical mechanical and chemical properties
Both these type of materials have different chemical, mechanical and physical properties. The 316 grade of steel has 515MPa tensile strength which means it would permanently deform after that level of force. The duplex grade easily range from 620MPa to 770MPa of tensile strength. So they are stronger, and are hard to break than the 316 steel. The 316 is corrosion resistant especially to chloride ion rich solution and prevents chloride ion corrosion stress cracking. Also it is resistant to mild acids and oxidation under room temperature. The duplex grades on the other hand have high oxidation resistance due to the increased amount of chromium and nickel. Their strength differs from grade to grade. Some grades trade off corrosion resistance for strength. The lean duplex grades could corrode at a rate closer to the ordinary steels, but the hardness and strength make them applicable to some complex building and construction industrial applications.
The 2205 is called the standard duplex and it is the most used of duplex grade. Lean duplex is only made on demand because it is not a fast moving item; and stocks would mean unnecessary operating cost. the 316 is sometimes called the food grade of steel. This is the second most used of all steel types in the world. Optimum corrosion resistance, oxidation resistance and strength makes the 316 steel be used over a host of applications.
Applications
Applications of 316 steel is seen in transportation, food processing industries, marine industries, sea water treatment, water desalination plants, power plants, oil and has industries an in other appropriate applications. Duplex is used on a variety of occasions where the other grades of steels cannot satisfy the customers. The Duplex steels are used in engineering applications like valves, fittings, springs and pump, cars, buses and other transportation vehicles, water desalination plants, pressure vessels, distillation columns and heat exchangers, oil and gas industry, pulp and paper industries.
They can operate under elevated temperatures as well. The S32760 grade is formed at 1230 – 1000 degrees Celsius. The high temperature tolerance, high resistance and high ductility and hardness make the duplex steels suitable for hot water piping, oil and many others. These two materials can sometimes be interchangeable. When the strength is considered in a short run, the duplex steels are good in corrosion resistance. Some of the grades have tungsten as an addition so that they are stabilized and the ferritic and austenitic microstructures can easily blend in.