Relationship between Residual Stress Distribution near Blade Surface and Cross Sectional Microstructure of Japanese Sword

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  • 日本刀表層の残留応力分布と断面微構造の関係
  • ニホントウ ヒョウソウ ノ ザンリュウ オウリョク ブンプ ト ダンメン ビコウゾウ ノ カンケイ

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Abstract

The Japanese sword “WAKIZASHI” used in this study was made by sword craftsman. The surface of blade was finished by rough grinding just before final hand polishing process. In this study, the sword was cut using a wire-electrical discharge machine. Using a cut specimen, microstructure, hardness and carbon content of cross section of blade were measured experimentally. Surface residual stress distributions from the front edge “HASAKI” to the ridge “MUNE” of blade were measured by x-ray stress measurement. The diffraction from 211 plane of ferrite or martensite by Cr-Kα radiation was used. As a result, the edge of blade “HA” had martensite structure. The area of “HA” was 10% of the total cross-sectional area. Other part remained pearlite and ferrite structures. The carbon density between two different carbon steels, the surface layer and core materials, changed continuously by diffusion. The hardness distribution coincided with the carbon content distribution. Biaxial principal compressive residual stresses were found to be generated and have constant stress gradients in depth on the ground blade surface because the ψ-splitting was not observed and the measured 2θ vs. sin2ψ relations could be approximated as a parabolic curve. Large compressive residual stresses more than -1.0 GPa were distributed from “HASAKI” to “HAMON” on the edge of blade. On the other hand, compressive residual stresses diminished gradually to -500 MPa from “HAMON” to “MUNE”. The surface compressive residual stress distribution directly depended on the cross sectional microstructure of sword. Additional compressive residual stress field induced by rough grinding was superimposed on the residual stress field after quenching and tempering process.

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