Different types of stainless steel

Stainless steel is usually divided into 5 types:

Ferritic: Ferritic stainless steels are initially based on chromium and then have a small amount (usually less than 0.10%) of carbon. They are magnetic and have a microstructure that is comparable to carbon and low alloy steel and subsequently are usually only used in applications where thin sections are required as they have been found to have a lack of toughness in welds. When welding is not required, they can prove to be very useful. Ferritic steels have good ductility and are regularly chosen for their ability to withstand stress corrosion cracking.

Austenitic: Austenitic stainless steels are the most common type. They have added Nickel, Manganese and Nitrogen and their microstructure gives them both weldability and formability. Their ability to resist corrosion can be strengthened by adding Chromium, Molybdenum and Nitrogen.

Martensitic: Martensitic stainless steels are similar to ferritic steels as they are based on chromium, however, they do have higher carbon levels, up to 1%. This addition of carbon allows them to be hardened and tempered and therefore they can be used where high strength, but moderate corrosion resistance is necessary.

Duplex: Duplex stainless steels were developed in the 1960s and get their name as a consequence of being made up of approximately 50% ferritic and 50% austenitic stainless steels. The resultant metal has better strength than either of them on their own. They are also resistant to stress corrosion cracking, have better weight saving and often a better price stability as well.

Precipitation Hardening (PH): Precipitation Hardening – or PH – stainless steels can be developed to have extremely high tensile and yield strength when elements like copper, niobium and aluminium are added. When they undergo aging heat treatments, fine particles form in the matrix of the steel which subsequently gives it the extra strength. They can be machined into many different shapes and are regularly used in the oil and gas, nuclear and aerospace industries where a combination of high strength, corrosion resistance and a generally low but acceptable degree of toughness is required.

How to choose the right stainless steel

Stainless steel is one of the most durable materials and therefore is in high demand. However, as has been covered, there are many different types to choose from which all have different properties. If the correct stainless steel is chosen, with an appropriate surface finish, design and grade to its environment, and if it receives the correct maintenance then its appearance should remain virtually the same throughout its length of service, whether that be months or years.

Realistically, it will be a specialist engineer who will be making the final decision on what stainless steel is right for a project, but it is important to consider factors such as how corrosiveness and temperature of the environment, what strength is required, whether welding is required and what the required life-cycle is.

There are two factors that have a particular influence on what type of stainless steel should be used in a specific application, these being the grade of the metal and the surface metal.

Grade: The grade of stainless steel has a major influence on its performance and needs to be matched to the environment. Broadly speaking, the different grades simply represent a difference is composition. As an example, the only practical difference between 304 / 316 and 304L / 316L is carbon content; with the [L] representing the lower carbon content. These lower carbon content variants were developed to overcome the risk of intercrystalline corrosion

Surface Finish: A smooth surface is often essential in helping to reduce the risk of corrosion as it reduces the ability of corrosive substances to adhere to a surface. A smooth surface finish makes it difficult for contaminants to stick and also increases the effectiveness of manual and heavy rain-washing, therefore reducing staining.