Abstract
Effects of quenching rate from austenite temperature on microstructures in the high chromium cast steel containing 1.6%C, 0.6%Si, 0.7%Mn, 0.5%Ni, 12%Cr and 1.5%Mo were investigated. The specimens were destabilized at 1000 ºC for 7.2 ks and then cooled to room temperature at quenching rates ranging from 50 to 0.05 ºC/s. Detailed analysis of temperature-dilatation diagrams with referring to microstructural observations on the specimens etched using LePera etchant lead to define a very narrow bainite region in a CCT diagram. Bainite was formed along grain boundaries when the specimen was quenched at 0.2 to 1.0 ºC/s. The temperature range for bainite transformation was about from 230 ºC to 260 ºC. Grain boundary bainite showed little effect on hardness of the specimen.
Hardness and microstructures were also investigated on the specimens quenched at 0.2 ºC/s to 400 ºC followed by quenching at various cooling rates to room temperature and tempering at 550 ºC for 1.8 ks, in order to optimize heat treatment conditions for large practical products. Hardness of the specimen quenched at 0.006 ºC/s in the second stage quenching was increased with tempering. The increase in hardness was interpreted as a result of enrichment of carbon content in austenite due to formation of grain boundary bainite, that is, retained austenite containing high carbon content transformed into martensite with tempering.
