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 16 mass% Cr but reduced it in 26 mass% Cr cast irons. The volume fraction of retained austenite (V_γ) was positively correlated with alloy content except for V addition and it was high at elevated austenitizing temperatures. Tempered micro-hardness curves showed secondary hardening and the degree of secondary hardening (ΔH_D) was greater in alloyed specimens comparing with alloy-free specimen. The ΔH_D was closely related to V_γ in as-hardened state, and the more the V_γ, the greater the ΔH_D. 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 V_γ was less than 20%. The H_MTmax values of Mo and V containing specimens increased with the V_γ in the as-hardened state. The highest value of H_MTmax 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.