PURPOSE:To prevent the generation of defectively hardened products by the change in the viscosity of a quenching medium in the stage of hardening metal with a polymer as the quenching medium by monitoring the viscosity of the quenching medium at all time and maintaining the quenching medium at the specified viscosity during the use. CONSTITUTION:The cooling capacity increases and the metal is overhardened when the viscosity of the quenching medium 31 decreases in the stage of using the polymer such as PE glycol as the quenching medium 31 and hardening the metal in a hardening tank 1. Quenching cracks are then generated. The medium 31 is maintained always at the specified viscosity in order to prevent such overhardening so that the metal is hardened always under the finest condition. The measurement of the viscosity of the quenching medium is executed by supplying the medium 31 in the tank 1 into a cylinder 103 and measuring the gravity-group speed of a weight 107 in the cylinder by upper and lower switches 22. The quenching media of different concns. are supplied from a stock tank 12 and dilution tank 13 for the quenching medium through respective nozzles 11, 10 into the tank 1 to adjust the concn., i.e., viscosity of the medium 31 at the normal level. The always adequate quenching is thus executed.
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