TRANSMISSION ELECTRON-MICROSCOPY STUDY OF HIGH-TEMPERATURE BAINITIC TRANSFORMATION IN 1-WT-PERCENT-MN DUCTILE IRON

A 1 wt.% Mn ductile iron austenitized at 900 degrees C for 90 min and austempered at 375 degrees C for different periods was used to study s ome aspects of bainitic reaction in high Mn austempered ductile iron w ith reference to carbide precipitation in bainitic ferrite. Transmissi on electron microscopy (TEM) energy-dispersive X-ray analysis (EDXA) s tudy shows that precipitation of carbide in the ferritic component of bainite is a function of the local concentration of alloying elements. In other words, in the region near graphite where Si segregates and t here is negative Mn segregation as well as carbon, the bainitic ferrit e is carbide free. However, in the intercellular region where Mn segre gates and Si is depleted, the ferritic component of bainite occurs tog ether with very fine and almost uniformly distributed carbide. Further more, TEM-EDXA results show that the increase in Mn content not only d elays stage I (the initial transformation of austenite to ferrite and retained austenite) of the bainitic reaction, but also delays stage II (decomposition of retained austenite to ferrite and carbide).