In this paper, the morphologies of martensite and retained austenite for 1.5 mass%C-12 mass%Cr and 1 mass%C-8 mass%Cr steels were observed by means of optical microscopy, XRD, SEM/EBSD and TEM. The amount of retained austenites was quantitatively investigated and compared with XRD, EBSD and TEM observation methods. The retained austenites were distributed in the form of a block type and a film type in the martensite structures. For the 1.5 mass%C-12 mass%Cr steel, the amount of block type retained austenite and film type one are almost equal in three distinct regions of carbide; primary carbide, secondary carbide and without carbide. In the case of 1 mass%C-8 mass%Cr steel, as to film type retained austenite there are not so much differences among those regions. However, block type retained austenites are distributed with much amount, especially in the region around primary carbide.

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Abstract. Tatara is traditional iron making method developed around San-in area in Japan. Typical product of Tatara is Japanese sword, which is tough and strong and is also known as an art object. It is said that Japanese sword cannot be produced without Tatara iron. Thus microscopic observations of Japanese sword attracted researchers and were performed for example by Tawara with optical microscopy at the beginning of 1900s. Since then a few microscopic observations have been performed but not so many. A lot of knowledge of steels has been accumulated and new equipments have been also developed, which give information on atomistic scale. Thus the precise microstructural observations of Japanese sword using such as SEM/EBSD have been made and are compared with ordinary steel. The preliminary experiments indicated that the average martensite block thickness is little longer than that of ordinary steels and crystal orientation distribution is different from ordinary one.

Japanese sword has finer grain size and lath martensite in the microstructure of sharp edge amazingly. Nowadays these structures are considered to be one of the ideal structure at which are greatly aimed to strengthen or improve toughness of steels. Though the carbon content of its sharp edge is 0.70 mass %, there are no lenticular martensite and no micocracking in that area. As a result of bending test by actual sword specimen, one sword was finally bent, the other sword was broken. However it is found the sharp edge in Japanese sword has such a large bending strength 2500, 4600MPa respectively as modern, high performance tool steels and the difference of crack propagation under bending depends on the microstructure distribution and the grain size in cross section of Japanese sword.