This technique, which provides valuable information about grain boundaries, is advantageous for refined microstructures to confirm constituents such as acicular ferrite, bainite, and martensite. Recently, in addition to the previously mentioned techniques, Electron Back Scattering Diffraction (EBSD) has also been used to enable a more effective analytical procedure. The use of scanning electron microscopy as an auxiliary technique to optical microscopy has been successfully used for many decades to study C-Mn and low alloy weld metals, mainly when refined microstructures are to be assessed. This situation is more relevant for high strength steel weld metals, where very refined microstructures may not be adequately resolved, resulting in misidentification of the microstructure. Thus, to consider the influence of all these parameters, the method proposed by the International Institute of Welding (IIW) is not sufficiently comprehensive and so complementary techniques are necessary. Parameters such as the recrystallized fraction, the presence of micro-phases and inclusions may also have a relevant role. The common practice that associates this property with the microstructure of the last bead of multipass weldments has been proven to be unsatisfactory, because the amount of acicular ferrite, the most desirable constituent, may not always be the main contributor to toughness. Prediction of impact toughness based on the microstructural characteristics of steel weld metals is complicated due to the innumerous parameters involved.
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