Introduction
Generally speaking, martensite is a type of crystalline structures of certain metals and alloys, formed by cooling rapidly. Steel is one of martensite forming materials, especially in the stainless steel grades. Martensitic steels, such as 3Cr13, inherit impressive strength due to the formation of martensite during the rapid quenching process. 3Cr13 steel has an austenite decomposition temperature of 772 to 842 °C (1400 to1550 °F) and due to its content of chromium, nickel and molybdenum, it is one of the most common stainless steels.
For 3Cr13 steel it is custom that it is heat treated with a quenching temperature of 1100 °C (2012 °F) with an oil quenching medium and an aging temperature of 580 °C (1076 °F). There are two major microstructural changes in 3Cr13 stainless steels that are present during oil quenching and aging process, rapidly cooling and fast heating. These two processes together forms austenite to martensite and martensite to martensite.
Microstructure
Once 3Cr13 steel has been oil quenched and tempered at 580°C, the microstructure of the steel will consist of martensite as the major phase, circled with ferrite and austenite. The microstructure formed by the quenching and tempering is determined by the cooling rate of the 3Cr13 steel, which will determine the amount of resulting martensite and austenite. In general, a faster cooling rate will produce more martensite and less austenite, while a slower cooling rate will produce more austenite and less martensite. This martensitic matrix is characterized by small, elongated grain boundaries. With high hardness and strength, the martensitic matrix can resist deformation and abrasion, which is why 3Cr13 stainless steel is so popular in knife blades.
Martensite
Martensite can be observed in the microstructure of the 3Cr13 steel after the quenching and tempering. Martensite is a type of lath (plate-like) structure formed when steel is cooled quickly (quenched) from an austenite state. It is a transformation of austenite to a low carbon supersaturated solid solution, usually in the form of tempered martensite. It also forms a very hard and brittle final product when heat treated without tempering.
Ferrite
In the microstructure of 3Cr13 steel after quenching and tempering, ferrite can be observed as well. Ferrite is a solid solution of carbon in iron, and it is a crystal hexagonal lattice with no net magnetization. It can be formed at room temperature. Compared to the austenite, ferrite is softer and more ductile. In 3Cr13 steel, its presence is beneficial as it can increase the toughness of the steel and reduce the brittleness of martensite.
Austenite
Austenite can also be observed in the microstructure of 3Cr13 steel after quenching and tempering. Austenite is an FCC (Face-Centered Cubic) metastable structure at room temperature, which originates from the parent austenite structure. It is formed when steel is heated or cooled slowly, and it is stable at temperatures above 723.9°C. Austenite is a supersaturated solid solution of carbon in gamma iron, or FCC iron. It is beneficial for increasing the wear resistance of 3Cr13 steel.
Conclusion
3Cr13 steel’s microstructure is predominantly martensite, with ferrite and austenite present as well. Martensite is a type of lath-like structure which is formed when steel is rapidly cooled from an austenite state. It exhibits high hardness and strength and serves as a major wear resistance. Ferrite is a solid solution of carbon in iron, and it increases the toughness and reduces the brittleness of the steel. Austenite is present as well and it is a FCC metastable structure which is beneficial for increasing wear resistance. Thus, through the oil quenching and tempering process, 3Cr13 steel obtains an impressive hardness and strength and is suitable for many applications.
