| 摘要 |
PROBLEM TO BE SOLVED: To relieve the centrifugal stress during the operation of the impeller and improve the strength thereof, by heating the entire impeller and then quickly cooling only the circumferential portion of the core plate, generating local elastic deformation in the inner peripheral portion by the heat stress which is generated during the quick cooling, thereby generating residual stress of compression. SOLUTION: In a device that applies residual stress, an inner peripheral portion of an impeller comprising a core plate 1 and a vane 2 is cooled by cooling water passed by a piping 3 inserted therein. A jig 4 is mounted near the outer peripheral portion of the core plate 1 for heating it with a high frequency current, and a jig 5 is mounted near the vane 2 for heating it with a high frequency current. When the inner peripheral portion of the impeller is cooled and the outer peripheral portion is heated with this device, temperature difference occurs between the inside and outside of the impeller. As results, a tensile stress is generated in the inner peripheral portion, while a thermal stress of compression stress is generated in the outer peripheral portion. With increased difference in temperature, the tensile stress in the inner peripheral portion exceeds the yield point, and an elastic deformation occurs. If the temperature is lowered to the ordinary temperature in this state, a residual stress of compression remains in the inner peripheral portion.
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