Austenizing gray cast iron

Abstract

1. Austenizing of cast iron is related to the redistribution of silicon between the austenite formed and the remaining ferrite. The most intense redistribution of silicon occurs in ferritic cast irons with large graphite inclusions. The redistribution of silicon between phases is very rapid during the α→γ transformation, because the diffusion rate of silicon in a single-phase matrix is considerably slower than the diffusion rate of carbon. Therefore, we can ascertain that the main factor slowing down the austenizing of ferritic cast iron containing large graphite inclusions is the difficulty with which the ferrite with a high concentration of silicon is carburized. 2. Austenizing of ferritic cast iron, particularly in the case of rapid heating to high temperatures, leads to a microliquation of silicon entirely different from the original one. This affects the formation of the structure of the matrix during subsequent cooling of cast iron and also repeated austenizing. 3. The heating of ferritic cast iron containing large graphite inclusions leads first to the formation of austenite with a concentration of carbon considerably lower than the eutectic concentration (0.1–0.2%). The carburizing of austenite is very slow. Therefore, most of the austenite is transformed into ferrite when cast iron is cooled in air. A considerable part of the austenite is transformed into ferrite even when small samples are quenched in brine. The austenite is rapidly carburized when pearlitic cast iron or cast iron with highly branched graphite inclusions is heated. In all cast irons the high-silicon areas of austenite are carburized slowest — the dendrite branches and the central areas of the colonies. This is due to the fact that silicon increases the activity of carbon and therefore slows down its diffusion into austenite and ferrite. Silicon increases the rate of the diffusion of carbon from ferrite or austenite.