What is austenitic steel?
Austenitic stainless steel is a type of steel alloy which is composed of iron, carbon, chromium, nickel and other minor alloys. Austenitic steels can be divided into two groups: 300 series and 200 series. 300 series steels contain at least 8% nickel whereas 200 series steels replace portions of that nickel with cheaper alloys such as nitrogen or manganese. Austenitics are best known for its corrosion resistance which is why they are often used for stainless steel applications. Making up more than 70% of steel production, they are also one of the most commonly used stainless steels, with grade 304 being by far the most preferred type.
Austenitic stainless steels are defined by their face-centered cubic crystal structure. Their cubical unit cells have one atom at each corner and one atom on each face of the cube. This is different to ferritic steels, which have a body-centered cubic crystal structure.
300 series stainless steels are iron-based with high contents of nickel, meaning its nickel alloy amounts to at least 8% or more. The most standard type of the series is grade 304, containing 8% nickel and 18% chromium. Adding, increasing or reducing various alloys leads to different outcomes. In grade 302 of stainless steel, there’s a higher percentage of carbon, increasing strength but decreasing formability of the material. To achieve higher corrosion resistance, molybdenum can be added (Grade 316, 317).
200 series stainless steels have a lower content of nickel. Due to the high price of nickel, 200 series stainless steels are used in many areas of application to reduce production costs. It is partly replaced by other alloying elements that can produce austenitic steels, such as nitrogen, manganese, and copper. Nitrogen is a common alloy in 200 series stainless steel. However, its connection to chromium leads to chromium nitrides which increases gas porosity and reduces ductility. To increase nitrogen content without harming the levels of chromium, manganese can be added.
Differences between 300 and 200 series austenitic stainless steels
300 and 200 series stainless steels are both austenitic. Their different levels of nickel result in distinct properties. 300 series stainless steels show a higher corrosion resistance due to their higher contents of nickel. Furthermore, the 200 series stainless steels are defined by their reduced ductility compared to the 300 series. However, the increased nitrogen levels result in better impact resistance and strength. Both 200 and 300 series stainless steels are non-magnetic as well as not heat treatable. Cold working austenitic steels can improve hardness, stress resistance and strength.
Areas of Application
Austenitic steels can be found in many areas of applications, including:
Power Plant Engineering