List of the Most Common Types of Stainless Steel

Amongst the most widely employed metal materials, stainless steel is in high demand for its distinct property of being non-corrosive, strong, and highly durable and aesthetically pleasing. Right from kitchen utensils to healthcare equipment and chemical industries to building construction, every application uses the material in vital ways.

Stainless Steel Grades and Families

Basically, stainless steel can be described as an iron-base alloy with a minimum of 10.5% chromium content. The high concentration of chromium enables the creation of a thin oxide layer on the surface. Moreover, not all stainless steel is of the same type. Rather, variations based on alloys and microstructure result in different families of stainless steel with different properties.

Stainless steels can be divided mainly on the basis of metallurgical properties, which depend on the chemical composition of the steel and the process of heat treatment. These four main types: austenitic, ferritic, martensitic, and duplex, cover the majority of the total stainless steel used globally. These types of steels provide different properties of resistance, hardness, weldability, and price.

Knowledge about these families enables the engineer, designer, or buyer to identify the most appropriate stainless steel for the intended task, as opposed to the use of a one-size-fits-all stainless steel.

Common Types of Austenitic Stainless Steel

Austenitic stainless steels are the most used type, representing well over half the total production of stainless steel worldwide. They are known for their excellent corrosion resistance, superior formability, and good resistance to a range of temperatures.

The most popular varieties of austenitic stainless steel are 300 series, which include both 304 and 316 grade stainless steel. They contain chromium, nickel, and in the case of 316, molybdenum, which offers protection against chlorides and harsh chemicals.

Austenitic stainless steel is non-magnetic when annealed and cannot be hardened. However, they are hardened through cold working. Due to their ductility, austenitic stainless steel is very easy to work with and can be formed, bent, or welded. Thus, austenitic steel is prominently used in applications such as food processing equipment, medical instruments, storage tanks for chemicals, heat exchangers, and architectural structures.

One of the major benefits of austenitic stainless steel is that it is resistant to corrosion in various environments. However, this comes at a higher cost due to the presence of nickel in austenitic stainless steel, making it more expensive than ferritic stainless steel.

Ferritic Stainless Steel

Ferritic stainless steel contains chromium but very little, if any, nickel. Their structure is based on ferrite, making them magnetic and relatively cheaper compared to austenitic stainless steel.

Some common examples of ferritic stainless steel grades are 409, 430, and 446. They exhibit good resistance to oxidation as well as moderate corrosion resistance. Although they lack corrosion resistance compared to austenitic stainless steel, they are reliable at higher temperatures.

Ferritic stainless steel finds wide applications in exhaust systems of automobiles, household appliances, decorative trimwork, and industrial machinery. This type of steel boasts favourable values of thermal conductivity and low expansion properties.

However, ferritic stainless steels tend to have lower toughness and lower weldability compared to their austenitic counterparts. They also tend to have lower formability values, especially when it comes to thick sections. This makes them suitable when cost is a consideration but where high levels of corrosion resistance are not necessary.

Martensitic Stainless Steel

Martensitic stainless steels have high strength and hardness. Unlike austenitic and ferritic types, martensitic stainless steels can be hardened. Therefore, they can be used in applications where wear resistance and strength are required.

Some common martensitic grades are: 410, 420, and the 440 series stainless steel. These martensitic stainless steel series contain a higher concentration of carbon. The presence of higher amounts of carbon makes the stainless steel harder but less corrosion resistant.

Martensitic stainless steel is magnetic with low corrosion-resistance capabilities and can serve suitably under mildly corrosive conditions. It is commonly used for applications such as knives, surgical instruments, turbine blades, valve parts, and shafts requiring high strength and wear resistance.

The primary drawback of martensitic stainless steel relative to other types of stainless steel is reduced ductility and weldability. However, martensitic grades are still very important in their respective service because of their priority for mechanical properties against corrosion-resistance properties.

Duplex Stainless Steels

Duplex stainless steels contain a characteristic combination of the properties of both austenitic and ferritic stainless steel grades, which provide a microstructure consisting of almost equal amounts of both. Their unique microstructure is a key factor in conferring strength, corrosion resistance, as well as stress corrosion crack resistance.

Common duplex types are 2205 and 2507. Chromates, moderate amounts of nickel, and at times molybdenum and nitrogen are found in such alloys. Due to such composition, duplex stainless steel exhibits better resistance to aggressive environments, especially when chlorides are involved.

Duplex stainless steel has wide usage in oil and gas processing plants, chemical plants, desalination plants, marine structures, and pressure vessels. Higher strength enables the designers to work with reduced thicknesses, thereby providing cost advantages in larger structures.

Though duplex stainless steel provides excellent performance, it demands stricter weldability and fabrication process controls. Unwanted heat input can upset the phase balance, which may impact the mechanical and corrosion-resistant properties.

Data Table: Properties of Different Types of Stainless Steels

For more corrosion-resistant materials, please check Stanford Advanced Materials (SAM).

Conclusion

Stainless steel is not a material; it is rather a group of various alloys that have been designed for different task criteria. Austenitic stainless steels dominate applications requiring corrosion resistance and formability, while ferritic grades offer cost-effective solutions for moderate environments. Martensitic stainless steels excel in strength and wear resistance, and duplex stainless steels provide an advanced balance of strength and corrosion resistance for the most demanding conditions.