Effect of Alloying Elements on Variation of Micro-Hardness during Heat Treatment of Hypoeutectic High Chromium Cast Iron

摘要

Hypoeutectic cast irons containing 16 mass percent and 26 mass percent 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 percent Cr but reduced it in 26 mass percent Cr cast irons. The volume fraction of retained austenite (V_(gamma)) 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 (DELTA H_D) was greater in alloyed specimens comparing with alloy-free specimen. The DELTA H_D was closely related to V_(gamma)r in as-hardened state, and the more the V_(gamma), the greater the DELTA H_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_(gamma) was less than 20 percent . The H_(MTmax) values of Mo and V containing specimens increased with the V_(gamma) in the as-hardened state. The highest value of H_(MTmax) was obtained in those samples containing 3 mass percent 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.