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

There is uncertainty about heating temperature of final heat treatment and its effect on microstructure of Japanese sword, since the temperature is controlled entirely by experiences and skills of the swordsmith. This study proposed a novel approach to clarify how heating temperature affects prior-austenite microstructure of a Japanese sword. A fragment of the sword was normalized at 800°C to recover its initial microstructure. Specimens were cut and quenched, after being reheated at temperatures from 740 to 860°C with increment step of 20°C. Their prior-austenite microstructure was reconstructed from martensite by using our newly developed automatic reconstruction method. In normalized microstructure of the sword, coarse pearlite islands of 0.70–0.76 mass% C steel were found within a fine ferrite-pearlite matrix of 0.52–0.56 mass% C steel. The prior-austenite grains of the medium C matrix remained fine (11.6 - 15.1 μm) over the temperature range, while abnormal coarse grains (73.9 - 123.7 μm) developed from high C islands in the specimens reheated above 780°C. The prior-austenite microstructure of the original sword was closely similar to that of the specimen reheated at 760°C. It was able to estimate the heating temperature of the original sword to be approximately 760 ± 10°C