THERMAL-EXPANSION OF CEMENTITE IN HYPEREUTECTOID IRON-CARBON ALLOY

Iron forms a number of carbides, of which cementite Fe3C has been stud ied best in steels. However, the nature of cementite has not been dete rmined completely. It is known that carbon atoms can be arranged in tw o possible ways in the cementite lattice, namely, at the centers of tr igonal prisms with iron atoms at the vertexes and at the centers of sl ightly deformed octahedrons (octahedral surroundings of iron atoms). T he first variant is considered to be the most probable and is characte rized by a lamellar structure that causes strong anisotropy of the phy sical properties of cementite. In the case of octahedral surroundings the structure should be quasi-isotropic with weak anisotropy of the ce mentite properties. In the present work the method of x-ray dilatometr y is used to study the thermal expansion of cementite extracted from a hypereutectoid steel containing 1.72% C.