Cementite occurs in steels, in meteorites, possibly at the core of the Earth and has uses in its pure form. It's composition can deviate from Fe3C${\mathrm{Fe}}_3\mathrm{C}$, but not by much because the Fe–C bond contributes to its cohesion. Its crystallographic unit cell is orthorhombic and primitive, with large lattice parameters, explaining its hardness. Many of its properties are anisotropic. Its single-crystal elastic properties have been investigated using first-principles calculations and by clever experiments. The iron atoms in the cell occupy two types of positions with different point symmetries; the four carbon atoms lodge within prismatic interstices. The structure can develop defects such as dislocations, faults and vacancies. Cementite is metallic and ferromagnetic with a Curie temperature of about 187${}^{\circ }$C. When alloyed, metallic solutes substitute on to the iron sites; smaller atoms such as boron replace carbon at interstitial sites. This review focuses on cementite as a single phase.

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渗碳体

渗碳体存在于钢中，也可能存在于地球的陨石中，并以其纯净形式存在。它的成分可能会偏离铁3C${\mathrm{铁}}_3\mathrm{C}$但不是很多，因为Fe–C键有助于其内聚。其晶体学晶胞为正交晶且原始，具有较大的晶格参数，说明其硬度。它的许多特性是各向异性的。使用第一性原理计算和聪明的实验研究了它的单晶弹性性能。单元中的铁原子占据两种类型的点对称性不同的位置。这四个碳原子位于棱柱状间隙中。该结构会产生诸如位错，断层和空位之类的缺陷。渗碳体是金属的和铁磁性的，居里温度约为187${}^{\circ }$C.合金化时，金属溶质替代铁质位点；较小的原子（例如硼）会在间隙位置取代碳。这篇评论集中于渗碳体作为一个单一的阶段。