How does ferrite content affect stainless steel?

Ferrite Testing

When you look at the crystalline structure of steel you will see a lattice with different crystals of iron atoms. Depending on the conditions when making the alloy, the lattices can be different. For example they could be ferrite, austenite, or cementite. It is important to test the material to see which of these are present as they can affect the final properties. Format NDT Ltd can help by providing ferrite testing.

What is ferritic stainless steel?

Sometimes the steel will contain an abundance of ferrite crystal lattices. Here the structure is one iron atom in the centre and iron atoms at each corner. This structure makes the metal magnetic. It also means there is only a very small amount of carbon in the alloy.

In terms of industrial usage, ferritic stainless steel is not very popular. This is because the crystal structure means it has less corrosion resistance than other types. The hardness is also lower and the magnetic properties can cause problems for some applications.

What about austenitic stainless steel?

Ferrite becomes austenite when you heat it to 912 degrees Celsius. This changes the crystal structure from body centred to face centred, as a result it can absorb more carbon. The austenitic steel has much higher corrosion resistance, so it is more popular than ferritic.

The thing to remember here though is that austenite can turn back into ferrite when it cools. This can mean there is more in the alloy than people expect if there is a problem with the cooling. To avoid this, manufacturers add things like nickel and manganese to austenitic steels. This retains the crystal structure.

Cementite, pearlite and martensite

There are three more type of crystal you can find within the stainless steel. Firstly is the high carbon cementite. If there is too much carbon and it cannot absorb into the ferrite, it can form the cementite crystals. They are hard and brittle so can cause flaws in the steel.

The second crystal is pearlite. This is a combination of low carbon ferrite and high carbon cementite.

Martensite crystals form when a manufacturer quenches austenitic steel to rapidly cool it. This prevents the formation of cementite by trapping carbon atoms in the crystal. The end result is harder martensitic steel. However, it does have lower chemical resistance.

Do you need ferrite testing?

Format NDT Ltd understands how important it is to know how much ferrite is in different alloys of stainless steel. We can help clients here, offering reliable testing for various forms of the metal. The tests can give you a clearer idea of what properties the steel will have.

So, if you want to arrange ferrite testing, please contact us. We will help you to ensure there are no surprises in terms of the hardness, corrosion resistance, and more.