n. A very hard carbide of iron, approximately of the composition Fe24C, formed in the recalescenee of steel at 850° C. in cooling from a temperature of 1,000° C. or over. It remains unchanged if the metal is then suddenly cooled, as by plunging it into cold water, but on slow cooling is decomposed into iron and the carbide Fe3C. On the other hand, it appears that this latter compound, known as cementite, may split into martensite and carbon in the form of graphite.