Factors Affecting Eddy Current Hardness Measurement of Spheroidal Graphite Cast Iron

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  • 球状黒鉛鋳鉄の渦電流による硬さ測定に影響する因子
  • キュウジョウ コクエン チュウテツ ノ ウズ デンリュウ ニ ヨル カタサ ソクテイ ニ エイキョウスル インシ

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  Several factors affecting eddy current hardness of spheroidal graphite cast iron were examined. Sensitivity of the eddy current hardness measurement was influenced by the coil turn number and measured frequency. In case of a 22mm diameter 400-turn coil, the applied frequency of 6kHz showed highest correlation with hardness, and 30kHz showed lowest correlation. Applying this result, we fabricated a prototype eddy current hardness tester, by which we evaluated the effects of heat treatment and surface machining methods, milling and grinding. At first, the eddy current hardness tester was calibrated on a grinding surface of ten specimens with different hardness. Each side of the same cast iron block specimens was machined by either milling or grinding. On a grinding surface, measured eddy current hardness well agreed with Brinell hardness. On the other hand, measurement on a milled surface clearly showed higher eddy current hardness than on a grinding surface. Vacuum heat treatment at 973K for one hour followed by furnace cooling clearly reduced eddy current hardness. Heating temperatures ranging from 673 to 873K for one hour had little effect on eddy current hardness, but additional heating at 733 to 833K for one hour clearly reduced eddy current hardness. Finally, the effect of heat treatment on cold rolled low carbon steel was examined. Heat treatment of the steel at 673 to 973K for one hour reduced eddy current hardness, which showed good correlation with Vickers hardness.

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