Changes in Carbon Content of Materials of the Japanese Sword under Traditional Forging Process

The Japanese sword has excellent strength-toughness balance given by two traditional processes. One process is compounding of inner tough iron and outer strong steel. And the other process is cooling-rate control quenching by using thickness of clay-coating before quenching. However, it is a precondition for acquiring high strength-toughness that carbon content of sword parts are regulated in proper quantities. Carbon content of sword parts is very important and controlled under forging process which contains TSUMI-WAKASHI (forge-welding piled steel chips) work and fold-forging. Therefore, changes in carbon content of TAMA-HAGANE steels and carbon steels under traditional forging process were investigated in this study. Changes in carbon content depended on enclosing decarburized or carburized surface in steel as forge-weld interface. Total changes in carbon content were proportional to total layers of included surface in steel. Changes in carbon content per one forge-welded layer. “d (mass%/layer)” was ruled by carbon content of raw materials “C0 (mass%)”. When C0 is over 0.2 mass%, carbon content of forged steel is decreasing and the more C0, the more |d |. C0 is under 0.2 mass%, carbon content is increasing. The mass of forged steels is 4% increasing per one fold-forging cycle, but d is not affected by changes in mass of them.