| Summary: | There are many potential advantages to welding structural ferritic steels with a power-beam process such as electron beam welding. These include low distortion, high welding speed, and computer control. The microstructure developed by a typical EB weld is poor from a mechanical properties perspective, however, and this limits the applications in which it is used. This work investigates the microstructures developed in an EB welded structural steel with the aim of further understanding and promoting a sound microstructure. A single steel composition was welded using a wide range of conditions. This resulted in the full gamut of possible microstructures being produced. These welds indicated that there may be a connection between the solidification microstructure and the room temperature microstructure in this steel. Further experiments were carried out that varied certain weld conditions and the link was confirmed. Two hypotheses were suggested to explain the observed microstructures and these were tested by replicating the EB weld microstructure with an A-TIG weld and then quenching to 'freeze' the partially developed microstructure. It was discovered that the poor microstructure in this particular steel was the result of Mn segregation, which caused lines of preferential nucleation through out the weld and thus, an aligned ferritic microstructure. The A1:O ratio in the weld metal is known to be a particular problem for EB welds and, in the final experimental chapter, an experiment was designed to test the hypothesis that excess soluble A1 in the steel prevents the nucleation and growth of acicular ferrite.
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