Materials Science Forum Vols. 539-543

Abstract: Continuous cooling and isothermal dilatometry was performed for a binary Fe-0.3C alloy and a ternary Fe-0.3C-1.0Mn alloy at slow (< 0.1 oC/s) cooling rates and isothermally at temperatures below the equilibrium eutectoid reaction temperature but above the bainite start temperature (625 to 715 oC). Some of the test conditions produced an unusual morphology in which fine scale ‘sub-grains’ are decorated with carbide, with additional discrete carbide particles inside the ‘sub-grains’. A detailed investigation into the network carbide formation indicates formation during austenite decomposition, as opposed to a post lamellar transformation coarsening or spheroidization reaction, but only for select temperatures, and apparently only during isothermal conditions.

Abstract: The effect of hot rolling parameters on graphitization of a spray formed ultra high carbon steels(UHCSs) was described. The number of graphite stringers and graphite area fractions increased with the increase of rolling reduction. Graphite stringers nucleated at small pores and grew by carbon diffusion from adjacent austenite during hot rolling. Alloy contents, pores and hot deformation atγ+Fe3C phase range are the key factors for graphitization.The graphite stringers of UHCSs have little effect on tensile strength, but reduce the ductility at room temperature.

Abstract: The microstructural evolution and the softening behavior of hot rolled and 60% cold rolled 0.85wt% carbon pearlitic steels during spheroidization annealing have been investigated by using the textural and microstructural information contained in the Orientation Imaging Microscopy (OIM) scans. The local boundary energy map, recently suggested by the present authors, is used to monitor the changes of stored plastic strain energy distribution in ferrite during the annealing process, which shows that the spheroidization process of cementite is finished before the completion of recrystallization of the 60% cold-rolled high carbon pearlitic steel.

Abstract: By employing the new developed relaxation-precipitation-controlling phase transformation (RPC) technique in large scale production the bainitic steels with ultra fine bainite has been obtained. These bainitic steels have good synergistic properties. With the aid of thermal simulation the refinement mechanism of RPC technique has also been investigated. The optical microscope, scanning electron microscope, transmission electron microscope and Electron back scattering diffraction technique were employed to study the features of microstructure produced by RPC technique, precipitation and the evolution of dislocation configuration during the relaxation. The results show that when produced by RPC technique the microstructure of the steel is mainly ultra-fine lath bainite packets, and these bainite packets block each other. It is also found that during the relaxation the dislocation cells form and strain induced precipitation occurs, the dislocation cell pinned by the precipitates can confine the bainite transformation. After the relaxation during the cooling the acicular ferrite forms at first and in succeeding the bainite transformation is blocked by the acicular ferrite and the bainite is refined effectively.

Abstract: This paper addresses the physical mechanisms of the transformation of deformed austenite into allotriomorphic ferrite and Widmanstätten ferrite. The possible implementation of deformation in currently available transformation models for allotriomorphic ferrite and Widmanstätten ferrite is discussed based on a series of deformation dilatometry experiments. It is concluded that a small amount of deformation already gives significantly faster transformation kinetics and a significant decrease in fraction Widmanstätten ferrite, and that the key to understanding the effect of deformation on transformation lies in the nucleation of allotriomorphic and Widmanstätten ferrite. For Widmanstätten ferrite also the growth needs further study.

Abstract: Three-dimensional observations of proeutectoid ferrite formed at grain boundary in an Fe-0.09%C-1.48%Mn vanadium microalloyed alloy was revealed by techniques of serial sectioning along with computer-aided reconstruction. The ferrite allotriomorphs nucleated at grain boundary edges were approximately prolate ellipsoids. Not all the ferrite allotriomorphs formed at grain boundary faces were oblate ellipsoids. The growth kinetics of ferrite allotriomorphs nucleated at grain boundary edges was greater than that of ferrite allotriomorphs nucleated at grain boundary faces.

Abstract: The model of the γ→α transformation calculated with the CATRAN program is described. The expected effect of former austenite grain size on the transformation kinetics as well as the final microstructure is analyzed and compared with experimental data. The effect of austenite grain size distribution spread on the transformation kinetics is also considered and must be included as the additional input parameter in the computer model of austenite decomposition. In particular, probable variations of grain size non-uniformity should be taken into account when comparing transformation kinetics observed in the dilatometer experiment and under industrial conditions.