What is the difference between ferritic, martensitic, and austenitic stainless steel?
Stainless steels in their solid state have a microscopic crystalline structure. The terms “ferritic, martensitic, and austenitic” refer to the crystal structure of the metal. Each has a different metallurgical phase which affects the mechanical and corrosive properties of the metal.
Ferritic Stainless Steel
The shape of a ferrite crystal structure allows it to only contain a minimal amount of carbon. Therefore, ferritic stainless steel is soft and ductile with limited corrosion resistance and only average durability. Regular steel (non-stainless) is considered “ferritic” because the room-temperature microstructure has a mix of ferrite and pearlite. These steels are magnetic and aren’t recommended for use in high-temperature settings.
Martensitic Stainless Steel
Martensite is a meta-stable phase formed when high-temperature austenite is quickly quenched below a critical temperature (that changes depending on chemistry). During the quenching process, carbon atoms are trapped in the crystalline structures. Martensitic stainless steel is characterized by its extremely high strength, low fracture resistance, and low ductility. It can be held at an intermediate temperature for various times, in a process called tempering, to reduce strength while vastly improving toughness and ductility. Martensite can be achieved in both alloy and stainless steel and is magnetic. Cutlery and knives are often made of martensitic steel. The aerospace and medical device industries also often use this type of steel.
Austenitic Stainless Steel
Austenite is a high-temperature phase of plain steel, which recrystallizes into ferrite/pearlite around 1425°F (depending on chemistry), below which ferrite becomes the more stable phase. Certain alloying elements, most notably nickel, can stabilize the austenite phase down to room temperature. The alloying elements are the significant difference between 300 and 400 series stainless steel; 300-series grades contain enough nickel to stabilize austenite at room temperature and are termed “austenitic” grades. Austenitic stainless steels have superior corrosion resistance and are the most common type of stainless steel.