PURPOSE: To prevent the unevenness of strain in the heat stress caused by difference in cooling speed at the time of cooling in high temp. while restraining lowering of the hardness in the inner part, at the time of executing quenching.
CONSTITUTION: A material to be quenched is dipped into high temp. coolant of mineral oil at the temp. C just below the starting temp. of martensitic transformation and thereafter, cooled by low temp. coolant of mineral oil at the temp. D lower than the temp. of the high temp. coolant. As the high temp. coolant uses the mineral oil having comparatively small heat capacity and the cooling speed is not so fast, partial temp. difference is small and the unevenness of the strain caused by the heat stress is small. Further, as the temp. of the high temp. coolant is set at the temp. C just below the starting temp. of the martensitic transformation, the cooling speed is not so low and lowering of the hardness can be restrained. Further, as this material is cooled by the low temp. coolant, without receiving the influence from the outer part, the deviation in strain caused by the martensitic transformation is small.
YAMAMOTO YUKIHISA
GOTO SHIGERU
