Formation of bainite in ductile iron

The subject of the present investigation was the standard Ni-Mn-Cu-Mo alloyed ductile iron. The kinetics of bainite transformation under isothermal conditions was measured by conventional dilatometry. The results of these experiments show that there are two C-shaped curves, which correspond to the formation of upper bainite (500-390 C) and lower bainite (390-250 C), on the TTT diagram. Optical microscopy shows that the morphology of the bainite changes from feathery-like in the upper temperature range to plate-like in the lower one. These facts are, probably, a result of different crystallographic shears during formation of upper and lower bainites. X-ray measurements were done on specimens cooled from isothermal temperatures just after ceasing of the bainite reaction. Comparison of dilatometrical data with X-ray results shows that bainite transformation ceases when the carbon content in low carbon austenite reaches a certain value. Probably, this concentration corresponds to the T0 curve and the composition of the high carbon austenite is closer to the Ae3 curve. The other fact is that an increase of the maximal volume fraction of bainitic α-phase promotes an increase in the volume fraction of high carbon austenite. (orig.)