Crystal Structure of Hard Steel

Abstract

Using an x-ray spectrometer with ionization chamber and reflecting Mo $\mathrm{K}\ensuremath{\alpha}$ and $\mathrm{K}\ensuremath{\beta}$ rays from small blocks of steel, spectra were determined for 0.80 and 1.31 per cent carbon steels both in coarse and in fine martensitic condition. The same lines were found as in the spectrum of carbon free iron, but they were only one-third as intense and were somewhat broader and slightly shifted. Lattice of martensite. These results confirm the previously reported conclusion that martensite has a body centered lattice like that of $\ensuremath{\alpha}$ iron but show that the lattice is contracted about 0.4 per cent on a side and is also distorted. This distortion, which changes the shape of the lines, disappears largely however after heating the martensite at 260\ifmmode^\circ\else\textdegree\fi{}C for 30 min., although the hardness is not much decreased. Nature of martensite. From a comparison of the densities of the specimens with those of $\ensuremath{\alpha}$ iron and of cementite, it is concluded that martensite is a mixture (1) of a solid solution of carbon in iron, the C atoms replacing the Fe atoms, and (2) of finely dispersed cementite. The solid solution is unstable even at room temperature. Both constituents seem to contribute to the hardness, the solid solution by distorting the lattice and the cementite by being so fine grained.