What is stainless steel? How many groups of stainless steel exist and what can we use them for?

Stainless steel is not one type of steel, but an enormous range of alloys divided into five different main groups.

 

Austenitic steel is by far the most important group of stainless steels. Typically, the steel contains 16-20% chrome (Cr) and at least 8% nickel (Ni). The austinites are characterised by being excellent for all types of machining from bending to welding – all while the corrosion resistance of the steel oftentimes is great. The regular 304 and 316-steels belong amongst the austinites, which covers over 70% of the global market and are used for everything from kitchen sinks to silos.

 

Martensitic steel does not contain (apart from iron) much besides 11-15% Cr and a bit of carbon (C). These steels are hardenable and are therefore used for e.g. knife blades and other objects where a high degree of hardness is required. In contrast, the martensites can neither be bent nor welded and the corrosion resistance is low.

 

Ferritic steel is even simpler: 11-19% Cr in iron while carbon is low. Contrary to the martensites, the ferrites can therefore not be hardened. On the other hand, however, they are bendable and weldable and have a better corrosion resistance. Ferritic steel is often referred to as “chrome steel” and is often seen in products such as white goods, cooker hoods and other thin sheet items.

 

Duplex steel is an exciting, two-phased mixture of austenite and ferrite. With over 22% Cr and 2,5-4% molybdenum (Mo), they have a particularly excellent corrosion resistance paired with quite high strength. They only cover a few per cent of the global market and are particularly present in highly critical applications such as oil/gas and pharma.

 

Precipitation hardening steel (PH-steel) are exceedingly rare. They are thermally hardenable but normally less hard than the martensites. The PH-steels are often applied for e.g. the heads of golf clubs and axles – but apart from that, they are rare to come across.