Effects of Carbon Balance and Heat Treatment on Hardness and Volume Fraction of Retained Austenite of Semi-multi-alloyed White Cast Iron

Abstract

The heat treatment behavior of semi-multi-alloyed white cast iron with relatively lower carbide-forming elements compared with regular multi-alloyed white cast iron was investigated. The semi-multi-alloyed white cast iron with carbon balance (Cbal) of − 0.68 to + 0.53 wt% was prepared by varying chromium (Cr) content from around 3–9 wt% under the fixed contents of 2 wt% molybdenum (Mo), 1 wt% tungsten (W) and 5 wt% vanadium (V). After annealing, the test specimens were hardened after austenitizing at 1050 °C and 1100 °C. The tempering was carried out between 400 and 600 °C. In as-hardened state, the macro-hardness and micro-hardness increased greatly as the Cbal rose from − 0.68 to − 0.01 wt% and then decreased gradually with an increase in the Cbal value. The volume fraction of retained austenite (Vγ) increased continuously as the Cbal increased, and more Vγ was obtained in the case of hardening from higher austenitizing temperature. In tempered state, the hardness curve showed an evident secondary hardening due to the precipitation of fine secondary carbides during holding and the transformation of retained austenite to martensite during post-cooling. The maximum tempered macro-hardness (HTmax) was obtained by tempering at 500–550 °C. The highest values, 862 HV30 and 795 HV0.1, were obtained in the specimen with − 0.01 wt%Cbal where the Vγ was about 2%. As the Cbal value increased, the maximum tempered micro-hardness (HTmax-M) increased remarkably toward − 0.01 wt%Cbal and then it decreased regardless of austenitizing temperature. At the same Cbal value, the HTmax-M of specimens hardened from 1100 °C were larger than those hardened from 1050 °C. It was found that the 10–50% Vγ in as-hardened state was necessary to obtain hardness over 800 HV30 and 750 HV0.1 by tempering. The degree of secondary hardening (ΔHs) increased with an increase in the Cbal and the Vγ in the as-hardened state.