| The steel with high strength and good toughness can be realized by the refinement of its microstructures. However, the properties of weld joint of the steel are tended to be deteriorated due to the coarsening of microstructures during welding of the steel. In this thesis, the characterization, Electron Back Scattered Diffraction (EBSD) analysis and micro mechanical properties of weld joint microstructures of a ultra fine-grained high strength steel were studied, which was processed by a TMCP (Thermomechanical Control Process)-RPC (Relaxation-Precipitation-Controlled transformation) technique.Microstructural observations of weld joint of the steel were conducted by means of optical microscopy and scanning electron microscopy. The weld joint consisted of weld metal, fusion region, coarse-grained region, fine-grained region and incomplete recrystallization region. The weld metal was mainly made up of fine acicular ferrite. The fusion region was mainly made up of bainitic ferrite. The coarse-grained region was mainly made up of bainitic ferrite packets which consisted of parallel bainitic ferrite laths or plates. The fine-grained region consisted of fine polygonal ferrite grains. The incomplete recrystallization region was made up of the mixture of fine recrystallized polygonal ferrite grains and fine lath-like or plate-like bainitic ferrite matrix.SEM-EDS (Scanning Electron Microscopy-Electron Dispersed Spectrum) observations and analysis of weld metal was conducted on a field emission scanning electron microscope. Results showed that several ferrite laths or plates were nucleated on one inclusion. The inclusion consisted of an Al2O3 core and an outer layer of titanium oxide. The nucleation of intragranular ferrite was promoted by the formation of Mn depleted zone, probably due to Mn diffusion into titanium oxide by means of cation vacancies, or to the solubility difference of Mn in ferrite matrix and in inclusion.EBSD analysis was conducted with a FE-SEM to investigate the crystallographic orientation of weld joint microstructures. Results indicated that the crystallographic orientation of acicular ferrite was not completely random. A certain crystallographic orientation was dominant for acicular ferrite. Acicular ferrite was separated by high angle boundaries. Some acicular ferrite laths or plates which were formed on one inclusion and grew in opposite directions had the same crystallographic orientation, presumably because it had some crystallographic orientation relations with the prior austenite. In coarse-grained region, some bainitic ferrite packets grew in a certain crystallographic orientation were dominant among the packets in a prior austenite. Bainitic ferrite packets grew in different directions were separated by high angle boundaries. The acicular ferrite laths or plates in coarse-grained region which was formed prior to bainitic ferrite packets sectioned the prior austenite into many small zones so that fine-grained mixed microstructures could thus be obtained. It was found that some banitic ferrite laths or plates which were adjacent to acicular ferrite laths or plates had the same or similar crystallographic orientation with them. These banitic ferrite laths or plates might be formed by sympathetic nucleation on the pre-formed acicular ferrite laths or plates.The Vickers hardness and nano hardness measurements were carried out on BUEHLER MICROMET 5101 and Nano Indenter II (MTS), respectively. Generally speaking, the value of Vickers hardness and nano hardness were both in agreement with microstructure change in the weld joint. The hardness of weld metal was the highest and coarse-grained region lowest among the different regions in weld joint. The hardness of coarse-grained region was reduced by grain coarsening and the weakening or disappearing of matrix strengthening approaches during welding. The hardness of fine-grained region was lower, because the microstructure in this region was made up of fine polygonal ferrite grains.
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