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2. (2) Electroheat Technologies: “Simultaneous Dual-Frequency Gear Hardening”, http://www.industrialheating.com/ih/cda/articleinformation/features/bnp_features_item/0,,27824,00+en-uss_01dbc.html
3. (3) Eldec Induction USA: “SDF Induction Heating Provides Accurate Contour Hardening of PM Parts”, http:/www.industrialheating.com/ih/cda/articleinformation/features/bnp_features_item/0,,99599,00+en-uss_01dbc.html
4. (4) Neturen. Co., Ltd. : http://ns.k-neturen.co.jp/gijutsu/gijutsu.html