On the Controlling Factors of the Hardness of Quenched-and-Tempered Steels with the Spheroidized Carbide Structure

The inter-relation between the microstructural factor and hardness has been investigated on the quenched-and-tempered structure of 0.1∼0.6% C low-alloy steels under various tempering conditions above 550°C. The radius of carbide and ferrite grain size were determined by electron-microscopy and the multi-regression analysis was conducted on the relation of these factors to hardness. The following empirical formula was obtained.Hv=0.8893λ−1⁄2+0.6884l−1⁄2+8.02wc+41where Hv, λ, l and wc are the Vickers hardness, the intercarbide distance(cm), the mean free-ferrite-path from grain boundary to grain boundary (cm) and the amount of carbide (wt %), respectively. It was shown that the response of a low-alloy steel to softening on tempering is understood by the effect of the alloying element on λ and l.