Formed during rapid cooling of austenite at the temperature rate higher than 500º F (260º C) per second. Such rapid cooling causes restructuring of crystalline lattice of gamma iron into crystalline lattice of alpha iron in which carbon is fully dissolved. Because only 0.04 percent carbon can be dissolved in alpha iron, the excessive amount of carbon transforms into supersaturated solution of carbon in alpha iron. This type of solution is called martensite, which is characterized by an angular needlelike brittle structure and high hardness (greater than 60 HRC).