Electron Diffraction on Strained Cementite in Steels

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Abstract
In order to investigate the mutual relation between strained cementite and X-carbide, observations by transmission electron microscopy were made on internal defect structures and crystallographic natures of cementite in cold-rolled high carbon steels containing 0.86% and1.25% carbon. Thermomagnetic and X-ray diffraction measurements were also made for comparison's sake. The main results obtained are summarized as follows; 1) The strained cementite in cold-rolled high carbon steels contains dislocation and the other defect structures of various types. The cementite which was suffered an extremely heavy straining has also fine strain markings and moire fringes of nearly 100Å in spacing. 2) The electron diffraction pattern of strained cementite generally distorts as compared with that of cementite free from strain. The distorted electron diffraction pattern is, however, never agreeable with that of monoclinic X-carbide, showing that the strained cementite is not identified with X-carbide. 3) Internal defect structures, the distortion of electron diffractin pattern, and the tangential spread ofdiffraction spot disappear by the annealing for 5 hr at 700°C. 4) Raise in the curie point of cementite with the degree of cold-rolling is detected by the present thermomagnetic measurements. The result of X-ray diffraction of cementites in the cold-rolled steels shows peak shift, lowering of peak height, and broadening of diffraction line. Any peak from xcarbide is not detected. These results are consistent with the above electron microscopic observations. 5) The strained cementite showing the distorted electron diffraction pattern is also observed in the tempered high carbon steels. From these experimental results, it is suggested that the strained cementite suffering some constraints from ferrite matrix in tempered steels is not regarded as X-carbide.