Abstract
The hardness of quenched and tempered tool steels fluctuates with the cooling rates in quenching. Hereunder are the effect of the cooling rate on two types of high-speed tool steel, AISI M2 and the matrix alloy version of it.
(1) At lower cooling rates above 600°C the coarse carbides precipitate during quenching, so that they suppress the precipitation hardening by fine ones on tempering and reduce the hardness of tempered materials.
(2) As the cooling rate becomes lower below 550°C, the quenched matrix contains more retained austenite, whose decomposition on tempering enhances the secondary hardening and raises slightly the hardness of tempered steels. And the increment of hardness on tempering, H in Rockwell C scale, is directly proportional to the amout of retained austenite in materials as quenched, Vγ in beyond 12%, as follows: H=0.5Vγ+Const.
