Structural characterization of rapidly solidified white cast iron powders

Abstract

Three white cast iron alloy powders (2.4% C, 3.0% C, and 3.0% C + 1.5% Cr) manufactured by a rapid solidification processing technique were investigated. It was found that the microstructures of all three alloy powders were similar. The primary constituent of these powders was found to be retained austenite. Only small amounts of carbide and martensite were found in the rapidly solidified white cast iron powders. The primary austenite cells and dendrites that nucleate and grow from the melt are retained upon cooling to room temperature with little carbide precipitation. The low volume fraction of marensite found was due to the high carbon concentration of the austenite. A fine dispersion of carbides in an austenite matrix is formed as a result of the solidification of the eutectic liquid in the intercellular and interdendritic regions. The relative proportion of primary austenite to eutectic can be explained by the carbon segregation that occurs during the solidification of the primary austenite. Annealing of the powders at 650° C transforms the metastable austenite into alpha iron and carbide. The carbides have a bimodal distribution with small carbides precipitating within the primary austenite cells and dendrites and large carbides precipitating within the intercellular and interdendritic regions.