Advanced microscopy techniques resolving complex precipitates in steels

Scanning electron microscopy as well as analytical transmission electron mi croscopy techniques such as high resolution, electron diffraction, energy d ispersive X-ray spectrometry (EDX), parallel electron energy loss spectrosc opy (PEELS) and elemental mapping via a Gatan Imaging Filter (GIF) have bee n used to study complex precipitation in commercial dual phase steels micro alloyed with titanium. Titanium nitrides, titanium carbosulfides, titanium carbonitrides and titanium carbides were characterized in this study. Both carbon extraction replicas and thin foils were used as sample preparation t echniques. On both the microscopic and nanometric scales, it was found that a large amount of precipitation occurred heterogeneously on already existi ng inclusions/precipitates. CaS inclusions (1 to 2 mu m), already present i n liquid steel, acted as nucleation sites for TiN precipitating upon the st eel's solidification. In addition. TiC nucleated on existing smaller TiN (a round 30 to 50 nm). Despite the complexity of such alloys, the statistical analysis conducted on the non-equilibrium samples were found to be in rathe r good agreement with the theoretical equilibrium calculations. Heterogeneo us precipitation must have played a role in bringing these results closer t ogether.