Abstract
The cooling curve at any point in the quenched specimen depends not only on the cooling characteristics of quenchant but also on thermal constants,size and shape of the specimen. The effects of thermal constants, size and shape of steel and measuring position in it on the cooling curve were examined quantitatively. Then, the curve which showed only the cooling characteristics of quenchant and was not affected by the other factors was derived for several quenchants respectively from the results of experiments in this work and in the 8th report by the authors. Such a curve was called as “Mother Cooling Curve” of a quenchant. The mother cooling curve was obtained by converting the measured T-t curve into T-t/(sρDn) curve, where T was the temperature at the centre of specimen quenched, t was the time after quenching, s and ρ were the specific heat and the density of specimen, respectively, and n was the size factor. The average value of n was 1.35 (1.34 for silver and 1.37 for steel). The conception of the mother cooling curve may be the most reasonable one to express the relationship among hardening factors such as hardenability and size of specimen and cooling ability of quenchant for hardening steel. The mother cooling curve indicates the cooling ability of quenchant and the continuous cooling transformation diagram indicates the hardenability of the steel. The hardening behavior of steel parts, therefore, may be found by comparing the cooling curve, which can be derived from the mother cooling curve and the size factor, with the continuous cooling transformation diagram of the steel.
