Martensite is a very unique form of steel and its properties have made it invaluable in many modern applications. The martensite transformation temperature (Mf) is the temperature below which it is possible for martensite to transform from ferrite or austenite to its more brittle form. This transformation requires a large amount of energy which can be provided by either mechanical or thermal means. The mechanical method involves the use of extreme cold, while the thermal method uses electrical heating.
At room temperature, steel is usually in the form of ferrite or austenite; however, when quenched (cooled rapidly) to a temperature below its Mf, martensite is formed. The martensite transformation begins to occur at a rate of 1⁰C per second or slower, depending upon the temperature gradient, and is irreversible at room temperature.
Martensite has a Sub-Microscopic structure, which gives it an increased hardness, strength and elasticity compared to most other metals. Its formation also causes small amounts of distortion in the crystal structures of the metal. The combination of these features makes it an ideal choice for many engineering applications.
The most common application of martensite is in tool and knife steels, where it is used to provide additional strength and wear resistance. The Firth Stirling process, which is used to produce this type of steel, requires the steel to be quenched to a temperature of -35⁰C to -40⁰C and held at this temperature for a few hours before being cooled, so as to reach its Mf.
Austenitic steel, which is the most common type of steel available, cannot be hardened by quenching and due to its higher Mf, is not suitable for tools and knives. Therefore, tools which require tempering are usually made from a combination of martensite and austenitic steel, as a small amount of martensite provides the necessary hardness whilst the large amount of austenitic steel increases its toughness and ductility.
Martensite is also used to harden certain mechanical components in order to increase their wear resistance and durability. Examples include bearing raceways and chassis components. The process of carburising, which involves heating the component to a high temperature and then cooling it quickly so as to reach its Mf, is used to harden these components in the same way as for tool and knife steels.
In addition to its use in steel, martensite can be used to produce highly resistant ceramics. This is achieved through the process of quenching, where the ceramic material is rapidly cooled to a temperature below its Mf and held at this temperature for a period of time. The resulting ceramic material has an increased hardness, improved wear resistance and higher temperature resistance, making it perfect for many engineering applications.
Whilst martensite is usually associated with steel and ceramics due to its toughness and resistance to wear, it can also be used in other applications. It can be used to create complex shapes in sheet metal and can even be used in the production of flat-panel displays.
On the whole, martensite is an incredibly useful form of steel and its use has resulted in many modern-day applications being both easier and more cost-effective to produce. With further research into its properties, it is likely that new applications that increase its toughness and resistance will continue to be discovered.
