Application of orientation imaging microscopy to study phase transformations in steels

In this investigation, the methodology, advantages, and limitations of using Orientation Imaging Microscopy (OIM) to study microstructures in multiphase steels are examined. In particular, proeutectoid ferrite precipitation has been investigated in a hypoeutectoid steel (Fe–0.12%C–3%Ni), while the proeutectoid cementite transformation has been studied in a hypereutectoid steel (Fe–1.3%C–13%Mn). By utilizing misorientation maps and inverse pole figure maps to determine the orientation differences between individual crystals, the internal morphology of the solid-state precipitates has been determined. What appear by other types of imaging to be monolithic single crystals are shown to be aggregates of finer crystals with misorientations between them. The relative quality of the Electron Backscatter Diffraction (EBSD) patterns has been correlated to specific phases and to interphase and grain boundaries using pattern quality maps (also referred to as image quality maps). These techniques have been adapted for quantitative identification of different microconstituents, including volume fraction measurements. By making comparisons among optical micrographs, conventional SEM micrographs, pattern quality maps, inverse pole figure maps, and misorientation maps, the advantages and limitations of different types of OIM mapping for studying different aspects of phase transformations in steels are elucidated.