Effect Of Austenitic Structure On The Bainitic Transformation Behaviour In The High-carbon Silicon Steel

The influence of austenitization temperature on characteristics of medium temperatureisothermal phase change curve in Fe-0.88C-1.45Si-1.10Cr-0.45Mn-0.25Co (wt.%) steel hasbeen investigated using X-ray diffraction (XRD), optical microscopy, transmission electronmicroscopy (TEM) and hardness test. The phase composition of the isothermal transformation(250℃) products after austenitizing at different temperatures, the microstructure (especiallymicrostructure of low-temperature bainite) characteristics and the formation mechanism oflow-temperature bainite were studied. It was found that the nose temperature of bainitetransformation curve turned to lower temperature and incubation period turned shorter withincreasing austenitizing temperature. Both changes are not increases linearly with theincreasing austenitizing temperature. After isothermal heat treatment at250℃for20mins, allsamples austenitized at different temperatures produced a bainitic structure, which consists ofpackets of parallel ferrite laths. However, notable differences in sub-structure were foundamong these samples. When austenitizing at lower temperature(such as880℃),the majordifference lies in the edge boundary morphology. Bainitic laths formed in low-temperatureaustenitizing conditions have sharp saw-tooth edge boundaries, whereas bainite transformedfrom high-temperature austenitizing conditions have smooth wedge boundaries. The observedbainite packets do not possess the twin substructure of quenched martensite and itscorresponding lattice constant, microscopic morphology and strain accommodationmicrostructure are all different from those of acicular martensite structure which has theindependent distribution after direct quenching. The formation of packet bainitic structuresinvestigated in this work simply cannot be explained using martensitic displacivetransformation mechanism. The lattice constant and microscopic morphology of bunchybainite are different from ferrite and pearlite, so the formation of packet bainitic structurescannot be explained using diffusion phase change theory either. Further research is required toclarify the baintic transformation mechanism.