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.
Ferritic stainless steel is a type of stainless steel alloy that belongs to the chromium family. It is known for its high chromium content, typically ranging from 10% to 30%, which imparts excellent corrosion resistance properties to the material. Ferritic stainless steel is magnetic and has a relatively low carbon content compared to other stainless steel types.
The microstructure of ferritic stainless steel is characterised by a body-centred cubic crystal structure, which gives it its magnetic properties. This structure is formed by the addition of chromium to the steel alloy, which enables the formation of a protective chromium oxide layer on the surface of the material. This oxide layer acts as a barrier, preventing corrosion and providing resistance to oxidation, even in high-temperature environments.
One of the notable advantages of ferritic stainless steel is its excellent resistance to stress corrosion cracking and chloride-induced corrosion. This makes it suitable for applications in environments where exposure to chlorides or other corrosive agents is a concern. Ferritic stainless steel is commonly used in automotive exhaust systems, architectural applications, and household appliances such as refrigerators, dishwashers, and ovens.
In addition to its corrosion resistance, ferritic stainless steel offers good formability and weldability. It can be easily formed into various shapes and structures, making it suitable for manufacturing processes such as bending, rolling, and deep drawing. Welding ferritic stainless steel requires proper techniques and filler materials to maintain its corrosion resistance properties and avoid sensitisation issues.
However, it’s important to note that ferritic stainless steel has some limitations compared to other stainless steel types. It exhibits lower toughness and strength properties, particularly at lower temperatures. Additionally, it is susceptible to sensitisation and inter-granular corrosion when exposed to high temperatures for extended periods. Therefore, ferritic stainless steel is typically not recommended for applications requiring high strength or operating in highly demanding conditions.
In summary, ferritic stainless steel is a corrosion-resistant alloy with a magnetic microstructure and a relatively low carbon content. Its high chromium content provides excellent resistance to corrosion and oxidation, making it suitable for various applications where durability and aesthetic appeal are essential. While it may have limitations in terms of strength and toughness, ferritic stainless steel remains a popular choice in industries such as automotive, architecture, and household appliances.
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.