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Bainitic reaction kinetics in ductile iron contained according to Ukrainian standard (weight %) 3.60-3.80 carbon, 2.60-2.80 Si, -0.12 Mn, -0.60 Cu and additionally alloyed by Mo (0.15-0.20) has been studied. It was found that the overall transformation kinetics becomes slower as transformation temperature increase. This is because more intensive redistribution of carbon into austenite at higher temperatures. Two austenites wiith different carbon content have been fixed and kinetics of their lattices parameters has been studied. EXPERIMENTALThe subject of present investigation is ductile iron containing (weight percent): 3.60-3.80 carbon, 2.60-2.80 Si, -0.12 Mn, -0.60 Cu and additionally alloyed by . Content of sulphur, magnesium and phosporus corresponds to Ukrainian standard (GOST 7293-85). Specimens austenitisation has been done in an electrically heated furnace with protective argon atmosphere at 9 5 0 '~ during one hour. Austempering was performed by rapid transferring of specimens to salt bath with mixture ( K N a , N W 3 ) and holding at 380°C, 350°C, 3 0 0 '~ for times from 5 to 120 minutes. M e r austempering the specimens were quenched into water. The temperature has been controlled automatically within accuracy range f 3 '~. Austempered specimens have been ground and polished with diamond paste. The study has been realised by methods of optical microscopy, scanning electron microscopy, X-ray difiaction. Before optical and scanning electron microscopy specimens was etched by 5% nital. MICROSTRUCTURAL OBSERVATIONAt the initial stages of bainitic reaction at 3 8 0 '~ (5 minutes holding) a-phase has been mainly formed around graphite nodules (Fig. la). The amount of bainitic ferrite is much smaller in the peripherical volume of austenite. Bainitic sheaves in the periphery of eutectic austenite are much shorter and h e r than that located in austenite near graphite nodules. This microstructure is typical for up to 30 minutes austempering at 380'~. The amount of austenite has been reduced with austempering time increasing due to transformations in peripherical austenite volumes. Untransformed austenite fields have existed only in thin zone in the periphery of eutectic cells -so-called "white spot etching" (Fig. lb). At the austempering time increasing to 45 minutes (Fig. lc), amount of bainitic fenite also increases due to transformation in peripherical austenite volume. Increasing of austempering time in the interval 45-120 minutes did not lead to the essential alteration of matrix structure. The examination of bainitic sheaves morphology shown that each sheaf consists of elipsoidal so-called sub-units, which have the same crystallographic orientation (Fig. ld) [I]. This morphology is specific for upper bainite and looks like bainite morphology in low carbon steels [I].Bainite reaction at 3 5 0 '~ follows by the same way as at 380'~. At the initial step of austempering bainite reaction has been mainly realised in the austenite volume neq graphite nodules (during 5 minutes). The formation of bainite...

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