Automatic Reconstruction Approach to Characterization of Prior-Austenite Microstructure in Various Japanese Swords

Abstract

Each Japanese sword is unique by its microstructure resulted by the steel composition and heat treatment. In this study, the automatic reconstruction method was applied for characterizing the prior-austenite microstructure at the sharp edge of three Japanese swords made in different time periods. The reconstructed prior-austenite microstructure was compared with that of three carbon steels, in order to clarify the variations of carbon content and heating temperatures of the swords. It was found that in carbon steels, the austenite microstructure is probably affected by initial microstructures, which is predominantly determined by carbon content. The three swords have similar carbon content, but their prior-austenite microstructures are considerably different, probably due to their heating temperatures. The strength at the sharp edge of the modern sword is significantly higher than that of the old sword. It may be due to the fine-grained prior-austenite in the modern sword compared with the abnormally coarse grains observed in the old sword. In order to obtain fine-grained austenite along with high strength and hardness of the cutting edge, it is recommended that the carbon content of Japanese sword is 0.60.7 mass%, and the heating temperature is from 750 to 800°C.

A novel approach to study the effect of heating temperature on prior-austenite microstructure of a Japanese sword

“…Moreover, the initial microstructure, which mainly determined by C content, also has strong effect on the development of PAM in C steel during heating [11,[15][16][17]. Those matters will be investigated and discussed in this section.…”

“…To be distinguished from specimen of original sword, those specimens will be referred as reheated specimens. The lower limit 740 o C of heating temperature was chosen, because the Ae 3 critical temperature of steel at the sharp edge with reported 0.71 mass% C was calculated to be approximately 734 o C [11]. The microstructure of the specimens in normalized condition was observed by optical microscope.…”

“…After reconstruction, the PAM was characterized by calculation of prior-austenite grain (PAG) size, fraction of primary twin boundary (Σ3) and fraction of secondary twin boundary (Σ9). The calculation methods have been detailed in [11].…”

“…Some details of our reconstruction method and software have been reported in [11][12]. After reconstruction, the PAM was characterized by calculation of prior-austenite grain (PAG) size, fraction of primary twin boundary (Σ3) and fraction of secondary twin boundary (Σ9).…”

“…A typical austenite grain map contains both finegrained and coarse-grained regions, which correspond to the fine ferrite-pearlite matrix and coarse pearlite islands observed in the initial microstructure. The reason has been discussed in [11] that during heating, in C steel with sufficient amount of ferrite, the PAG size is significant finer compared with the steels with low amount of ferrite or without ferrite in their initial microstructure.…”