Effect of alloying elements on variation of micro-hardness during heat treatment of hypoeutectic high chromium cast iron

Abstract

Hypoeutectic cast irons containing 16 mass% and 26 mass% Cr with single additions of Ni, Cu, Mo and V as well as without alloy addition were prepared to investigate variations of micro-hardness of matrix during heat treatment. In the as-hardened state. Ni and Cu decreased the micro-hardness but Mo increased it slightly. By contrast. V increased the micro-hardness in 16mass% Cr but reduced it in 26mass% Cr cast irons. The volume fraction of retained austenite (Vy) was positively correlated with alloy content except for V addition and it was high at elevated austcnitizing temperatures. Tempered micro-hardness curves showed secondary hardening and the degree of secondary hardening (ΔHD) was greater in alloyed specimens comparing with alloy-free specimen. The ΔHD was closely related to Vy in as-hardened state, and the more the Vy. the greater the ΔHD. The maximum tempered micro-hardness (H MTmax) was obtained in the specimen tempered at 698 to 873 K depending on the kind and the amount of alloying element where the Vy was less than 20%. The HMTmax values of Mo and V containing specimens increased with the Vy, in the as-hardened state. The highest value of HMTmax was obtained in those samples containing 3 mass% Mo in both series of the cast irons. The mechanism of secondary hardening in Mo and V containing cast irons was mainly by both the precipitation of special secondary carbide and the transformation of destabilized as-hardened retained austenite into martensite providing the high micro-hardness. © 2008 The Japan Institute of Metals.