What is the difference between Ferritic, Austenitic and Martensitic Stainless Steels?
Crystalline Structure of Stainless Steels
The vast majority of metals have a crystalline structure in their solid state, meaning that they are made up of crystallised lattice structures of atoms. By definition, all steels, including stainless steels, are primarily made up of crystallised iron atoms with the addition of carbon. The iron in steel can exist in several different crystalline structures, dependent on the conditions of its creation. Ferrite, austenite, and martensite are all examples of iron’s crystal structures, and all are found within different types of steel. One of the defining differences between these crystal structures is the amount of carbon they can absorb - a greater carbon content generally, though not always, makes a steel harder, but more brittle.
As a liquid, molten iron is not crystalline, and crystals are only formed when the material cools. When the material cools, steel solidifies as individual crystals forming gradually, which can mean that any one type of steel is actually made up of several crystal types as the metal slowly forms crystals through multiple temperature stages. This means that regardless of their defining crystal structure, it is not uncommon for steels to contain small mixed amounts of ferrite, austenite, and cementite.
While the information below covers ferritic, austenitic, and martensitic steels, almost all of Accu's stainless steel components have an austenitic crystalline structure. For more specific information on exact austenitic steel grades, please see our article on the many different grades of austenitic stainless steel.
Ferritic Stainless Steel
Ferritic steels are made up of ferrite crystals, a form of iron which contains only a very small amount (up to 0.025%) of carbon. Ferrite absorbs such a small amount of carbon because of its body centred cubic crystal structure - one iron atom at each corner, and one in the middle. This central iron atom is what gives ferritic stainless steels their magnetic properties.
Ferritic stainless steels are less widely-used due to their limited corrosion resistance and average strength and hardness.
Austenitic Stainless Steel
Austenitic stainless steels contain austenite, a form of iron which can absorb more carbon than ferrite. Austenite is created by heating ferrite to 912 degrees C, at which point it transitions from a body centred cubic crystal structure to a face centred cubic crystal structure. Face centred cubic structures can absorb up to 2% carbon.
When austenite cools, it generally reverts back to its ferrite form, which makes austenite difficult to utilise at anything below the extreme temperatures of a smelting furnace. Austenite can be forced to retain its crystal structure at low temperatures with the inclusion of chemical additives, such as the nickel and manganese found in many austenitic stainless steels.
Austenitic stainless steels cannot be significantly hardened by heat treatment, but can be hardened by cold working. Austenitic stainless steels are widely used, particularly in stainless steel screws, due to their excellent resistance to corrosion.
Cementite is a form of iron which contains even more carbon than ferrite and austenite. Cementite contains up to 6.67% carbon. Because of its increased carbon content, cementite is hard and brittle, and its presence is usually a byproduct, rather than by design. Cementite commonly occurs in steels when excess carbon, such as left-over carbon which cannot be absorbed into ferrite, must be used for the formation of crystals.
As iron cools, austenite crystals transition back into ferrite crystals, losing excess carbon which cannot be properly absorbed by the newly formed ferrite. The excess carbon creates patches of crystals with a mixture of low-carbon ferrite and leftover high-carbon cementite, and these mixed crystals are known as pearlite.
This video from Real Engineering does a great job of explaining how the crystalline structure of steel affects the physical properties of the material - the key points are discussed at 2:55:
Martensitic Stainless Steel
Martensitic stainless steel is formed by the creation of martensite. Martensite has been a key element of quenched steel for hundreds of years, but was officially named in the 20th century after the metallurgist Adolf Martens (1850 - 1914).
Martensite is a body centred cubic form of crystallised iron which is created when heated austenite is rapidly cooled by quenching. The increased rate at which Martensite crystals are created prevents cementite from being formed, and causes carbon atoms to become unnaturally trapped in crystals which would ordinarily expel excess carbon during gradual cooling.
Martensitic stainless steels can be heat treated and hardened, but have reduced chemical resistance when compared to austenitic stainless steels. Martensitic stainless steel is often used when hardness is critical, such is in knives, where surface hardness creates a sharper blade.