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°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.
Highlights
Results and DiscussionsIt is well known that the austenite grain in carbon and low-alloy steels grows with increase of heating temperature [13,14]
1 Introduction obtains its final microstructure and mechanical properties
It is well known that the austenite grain in carbon and low-alloy steels grows with increase of heating temperature [13,14]
Summary
It is well known that the austenite grain in carbon and low-alloy steels grows with increase of heating temperature [13,14]. The initial microstructure, which mainly determined by C content, has strong effect on the development of PAM in C steel during heating [11, 15,16,17]. In order to estimate the C content of the fine- grained matrix and coarse pearlite islands, the area fraction of ferrite as grains of white contrast in the appropriate region on micrographs of two normalized specimens was calculated. Because of lower C content in the matrix compared with the expected value of 0.71 mass% C, for which the lower limit of heating temperature was determined, there is 0.11 fraction of retained ferrite in 740oC reheated specimen. When heating temperature is equal or higher 780oC the abnormally coarse austenite grains, which probably formed from coarse pearlite islands, are observed. The austenite grains, which correspond to the fine ferritepearlite matrix, do not grow significantly over the range of heating temperatures
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