Evidence of Lath Martensite in High-C Japanese Sword Produced from Tamahagane Steel by Tatara Process

Abstract

Field Emission Scanning Electron Microscopy with Electron Back-Scattering Diffraction (SEM-EBSD) and Optical microscopy were used to point out the microstructural features of a Japanese sword prepared from tamahagane steel using traditional method. A lath martensite structure, which is usually characterized by packet and block in a prior austenite grain, existed both on the surface and the cross-section of the sword. SEM-EBSD study revealed that the development of prior austenite grain and packet were not much distinctive but the blocks within the packets were fairly observed. It was found that the packet size increased with the prior austenite grain size but the increment was small. Vickers micro-hardness measurement revealed that the sharp end was comparatively harder than other sections of the sword. EPMA study showed that the average carbon content of the sword was around 1 mass% along with a variety of non-metallic inclusions. Formation of lath martensite structure in such high carbon steel is remarkable but comparable to 0.6 mass% carbon ordinary steel. It was realized that the traditional method of preparation using tamahagane as well as the higher content of carbon provided the extraordinary features to the Japanese sword different from the ordinary steel.