Abstract
The interplay between carbon and ferrite-stabilizing alloying elements during the peritectic transformation in high-alloy steels has been studied in unidirectionally solidified samples. During the experiments the samples were quenched in order to freeze the microstructure present during the unidirectional solidification. The sequence of solidification and transformation could thus be studied metallographically and by microprobe analysis. It was found that ferrite-stabilizing elements can segregate strongly to the ferrite during the austenite → ferrite transformation. This segregation can be described by the normal laws of segregation. It was further shown that the peritectic transformation in a steel containing a ferrite-stabilizing alloying element can occur in two different ways. The new phase, austenite, grows into ferrite as well as into the liquid phase. In one case, the growth into the liquid phase occurs by a eutectic transformation. In the other case, the growth into ferrite occurs by a metatectic transformation. This is explained theoretically and it is shown how the carbon content and the partition coefficient of the alloying element between ferrite and austenite govern the choice between the two types of reaction. As expected, the peritectic reaction temperature decreased with increasing cooling rate. It was found that the extent of the peritectic transformation increased with increasing cooling rate.
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