The principal component of hard steel, formed by quenching from high temperatures. It consists of intergrown plate-like crystals with a distorted cubic structure arising from the presence of carbon atoms in the iron structure.
| Iron alloy phases |
|---|
|
Austenite (γ-iron; soft) |
| Types of Steel |
|
Plain-carbon steel (up to 2.1% carbon) |
| Other Iron-based materials |
|
Cast iron (>2.1% carbon) |
Martensite, named after the German metallurgist Adolf Martens (1850-1914), is any crystal structure that was formed by displacive transformation, as opposed to much slower diffusive transformations. "Martensite" most commonly refers to a form of ferrite supersaturated with carbon found in very hard steels, for use in such products as springs and piano wire. The martensite is formed by rapid cooling (quenching) of austenite which traps carbon atoms that do not have time to diffuse out of the crystal structure.
In the 1890s, Martens studied samples of different steels under a microscope, and found that the hardest steels had a regular crystalline structure.
Martensite has a very similar crystalline structure to austenite, and identical chemical composition. Differential quenching causes martensite to form predominantly in the edge of the blade rather than the back;
Martensite is usually considered to be a grain structure not a phase.
Since chemical processes accelerate at higher temperature, martensite is easily destroyed by the application of heat. Since quenching can be difficult to control, most steels are quenched to produce an overabundance of martensite, then tempered to gradually reduce its concentration until the right structure for the intended application is achieved. Too much martensite leaves steel brittle, too little leaves it soft.
Martensitic Transformation: Mysterious Properties Explained
The difference between austenite and martensite is, in some ways, quite small: while the average unit cell of austenite is, on average, a perfect little cube, the transformation to martensite sees this cube distorted by interstitial carbon atoms that do not have time to diffuse out during displacive transformation, so that it is a tiny bit longer than before in one dimension and a little bit shorter in the other two. Unlike cementite, which has bonding reminiscent of ceramic materials, the hardness of martensite is difficult to explain in chemical terms.
Shape memory alloy also has surprising mechanical properties, that were eventually explained by an analogy to martensite.
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