Abstract The carbides of lower bainite is main constituent of bainite structure. The purpose of this paper is to clarify that where the lower bainitic carbides come from. Therefore ,three things have been completed in this paper:1. By using three group of experiment,the results indicate that the lower bainitic carbides can nucleate in the g side of a/g interface or between two bainitic ferrite sub-units and grow towards austenite. The carbides can exist across the interface of a/g or a/a.2. The results of another experiment indicate that the essence of phenomenon that lower bainitic carbides exist inside the ferrite is lower bainitic carbides precipitate from the g side at a/g interface. The carbides grow competitively with the ferrite,during the whole transformation process. Meanwhile the growing rate of ferrite is higher than that of carbides. As a result, the carbides are surrounded by ferrite which may give us a false concept that the lower bainitic carbides are precipitated from bainitic ferrite.3. The nucleation and growth model of lower bainitic carbides is proprosed on the basic of thermodynamics and the ledge-wise growth theory.
From what has been mentioned above,it is suggested that lower bainitic carbides precipitate from carbon-enriched retained austenite rather than supersaturated ferrite. The essential of this paper is to identify whether existing the carbon-supersaturated bainitic ferrite , which directly concerning the mechanism of bainitic transformation.
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