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

Abstract

Abstract. 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 o C to recover its initial microstructure. Specimens were cut and quenched, after being reheated at temperatures from 740 to 860 o C with increment step of 20 o 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 o C. The prior-austenite microstructure of the original sword was closely similar to that of the specimen reheated at 760 o C. It was able to estimate the heating temperature of the original sword to be approximately 760 ± 10 o C.

<i>In Situ</i> EBSD Observation of Pearlite-Austenite Reverse Transformation in Eutectoid Steel

“…The previous studies have reported the nucleation sites of γ and the possible orientation relationship between the γ and the pearlite constituents. However, the currently obtained knowledge is not sufficient to explain our recent observation, in which the coarse pearlite grains transformed to coarse γ grains and vice versa [9]. It seems that more constraints, which limited the number of nucleation sites and the selection of γ orientations, were involved in the formation of γ from pearlite.…”

“…The growth of this γ is faster in multiple directions so that it often has a coarse globular shape. This nucleation mode along with the selection of unique γ orientation by double-OR relation probably leads to the formation of coarse γ grains, which are often observed in high-C steel with the presence of coarse pearlite structure [9].…”