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By extrapolation, we estimated cementite's elastic constants. Altogether, we studied seven Fe-C binary alloys varying from 0 to 17.3 atomic percent carbon. Thus, we measured hypoeutectoid, near eutectoid, and hypereutectoid alloys. For cementite, we report a complete set of quasiisotropic (polycrystal) elastic constants. If texture occurred, we removed its effect by using a Voigt-Reuss-Hill method. Surprisingly, for the dilatational modes, the cementite's elastic constants differ little from those of pure b.c.c. iron and are about ten percent higher for a shear mode such as the Young modulus. This indicates a tendency toward Fe-C covalent bonding. The decrease in Poisson ratio supports covalent bonding, but the small decrease indicates smaller covalent-bonding effects than expected by Pauling . Contrary to all previous reports, we find Fe3C stiffer than pure α-iron.