The strength of 61S-T which tempered immediately after quenching is stronger than that tempered after natural aging. This phenomenon is an example of the "SplitAging" named by H. K. Hardy. Effect of "heat-treating factors" (such as quenching temp., tempering temp., and temp of quenching water etc.) on this phenomenon were studied.
Results obtained are as follows,
(1) The difference of mechanical properties caused by "Split Aging Effect" are: Tensile strength 2-4kg/mm2 Proof strength 2-4kg/mm2 Elongation nearly zero. Hardness (V. H. N.) 10
(2) The strength of T6 state decreases with lengthening natural aging period before tempering. This decrease is most remarkable within imtial 24Hrs of natural aging and, after 2-4, days becomes negligibly small.
(3) The effect of natural aging on the strength of T6 state becomes larger as the quenching temp. increases up to 540°C, and above this temp. the effect is nearly constant.
(4) Suitable tempering temperature is 170-180°C, regardless of aging period before tempering.
(5) Hardness of specimen tempered immediately after quenching changes with tempering period more rapidly than that tempered after natural aging.
(6) Effect of temp. of quenching medium on the spilit aging is negligibly small.
(7) Preliminary tempering of short period at 175°C, or long period at 100°C, immediately after quenching counteracts the natural aging effect.
(8) Streching of quenched sheet has no effect on split aging.
(9) Corrosion resistivity of T6 condition is independent on the natural aging period befor tempering.
