Effect of Molybdenum Content on Heat Treatment Behavior of Multi-Alloyed White Cast Iron

The effect of the Molybdenum (Mo) content on the heat treatment behavior of multi-alloyed white cast iron was investigated. The cast iron with varying Mo contents from 0.12 to 7.66% under the basic alloy composition of 5% Cr, W and V each was prepared. After annealing at 1223 K for 18 ks, the test specimens were austenitized at 1373 K for 3.6 ks in a vacuum furnace and subsequently hardened by a jet-spray of liquid nitrogen. The tempering was carried out at temperatures from 673 to 873 K at 50 K intervals for 12 ks. It was found that the hardness in the as-hardened state was increased progressively with an increase in the Mo content. The volume fraction of the retained austenite (V.) decreased markedly when the Mo content was increased over 1.17%. In the tempered state, the hardness curve showed clear secondary hardening due to the precipitation of fine secondary carbides and a reduction of the V.. The V. value in each specimen decreased gradually as the tempering temperature was elevated, but reduced greatly when tempered at 748 to 823 K. The maximum tempered hardness (HTmax) was obtained in the specimen tempered at 798 K where the V. was less than 10%. The HTmax increased first, and then subsequently decreased with an increase in the Mo content. The highest HTmax value, 946 HV30, was obtained in the specimen with 4.98% Mo. It was found that the 15-37% V. in the as-hardened state was necessary to get the hardness over 900 HV30 by tempering. The degree of secondary hardening (Delta Hs) increased as the Mo content rose from 0.12 to 4.98% where the difference between the V-gamma in the as-hardened state and that at HTmax (Delta V-gamma) was 22-23%.