Effect of molybdenum content on heat treatment behavior of multi-alloyed white cast iron

Abstract

Multi-alloyed white cast iron contains several kinds of carbide forming elements namely V, Cr, Mo and W to form their own carbides during solidification. In such alloying elements, Mo is very useful one which not only forms eutectic carbide but also dissolves into matrix to improve the characteristics of heat treatment. In order to investigate the effect of Mo content on the heat treatment behavior, the multi-alloyed white cast iron varying Mo content from nil to 7.66 mass% were prepared. After annealing, test pieces in disk shape were austenitized at 1,373K for 3.6 ks in vacuum furnace and then, hardened by liquid nitrogen. The hardened specimens were tempered at various temperatures between 673 and 873K for 12 ks. As Mo content of the specimen increased, the macro- and micro-hardness in as-hardened state increased progressively. In tempered state, hardness curves showed secondary hardening, i.e. with increasing of tempering temperature; the hardness rose first to the maximum value and then decreased. The reason why secondary hardening takes place is mainly due to the precipitation of special carbides from martensite by carbide reaction and the martensite transformation from the rest of austenite during post cooling. The maximum tempered hardness (HTmax) was obtained at 798K tempering, regardless of Mo content of specimens and the highest HTmax was 946 HV30 in specimen with 4.98%Mo. Volume fraction of retained austenite (Vγ) in as-hardened state decreases gradually with an increase in Mo content. In tempered state, the Vγ began to decrease remarkably as the tempering temperature got over 748K in all the specimens. The Vγ values at HTmax were less than 10% and the Vγ at HTmax decreased proportionally from 10% to 2% as the Mo content is increased.