Placive Phase Transformation and Surface Effects Associated with Confo- Cal Laser Scanning Microscopy, Submitted

This work is to the best of my knowledge original, except where acknowledgements and references are made to previous work. Neither this, nor any substantially similar dissertation has been or is being submitted for any other degree, diploma or other qualification at any other university. This dissertation does not exceed 60,000 words in length. Abstract iii DFT Pak, Jun Hak 20080959 Coalesced structure in bainite and martensite for low–carbon steels, Department of Ferrous Technology Abstract This work aims to illuminate the formation principle of coalesced structure in bainite and martensite of low–carbon steels. It was also tried to suggest a solution to avoid the phenomenon efficiently. There were four kinds of specific work included in the thesis. Firstly, coalescence in martensitic structure for investigated four alloys was identified by its bimodal distribution of grain size, which is never explained by simple geometrical sectioning effect. Considering martensite transformation proceeds as temperature decreases, the related free energy change between austenite and ferrite increases, which led to the coales-cence in martensitic structure due to the resultant large driving force. Co-alescence in bainite occurred at low–transformation temperatures, where carbon partitions laggingly and consequently the formation of austenite film becomes retarded. These enhance the adjacent martensitic ferrites to meet. The critical driving force for coalescence was estimated for a spe-Abstract iv cific alloy by tracing the transformation temperature where no coalescence occurred. Secondly, misorientation within coalesced martensite was examined using transmission electron microscopy. Considering that austenite accommodates the associated shape deformation plastically, the bending of corresponding slip planes relates to the rotation of austenite crystal. This affected the orientation of martensitic ferrites which formed successively. Two active slip systems were specified using well established Taylor theory combined with literature data on the growth of bainitic ferrite. Thirdly, effect of external force on coalescence was studied using confo-cal laser scanning microscopy and thermo–mechanical simulation. Marten-site transformation was promoted near to free surface, which is consistent with theoretical calculation to predict large strain energy relaxation there. Interestingly, coalesced martensite tended to form near to free surface as a same manner. Direct tensile stress during isothermal bainite transformation induced the coalescence in bainitic ferrites, however, it occurred selectively in terms of crystal orientation of bainitic ferrites. Finally, it was attempted to block the coalescence by fragmenting the crystallographically homogeneous domains. Intercritical annealing after martensite transformation was designed to construct substitutional elements– enriched area using a program DICTRA version …