Advanced microscopy techniques resolving complex precipitates in steels^*

¹ Laboratoire de Métallurgie, EDIFIS (UMR 6518 du CNRS), case 511, Faculté des Sciences et Techniques de St. Jérôme, 13397 Marseille Cedex 20, France
² CRPC – SOLLAC, bâtiment DB26, 13776 Fos-sur-Mer Cedex, France

Corresponding author: charai@caedifis.u-3mrs.fr

Received: 3 July 1998
Revised: 29 September 1998
Revised: 25 January 1999
Accepted: 16 March 1999
Published online: 15 June 1999

Abstract

Scanning electron microscopy as well as analytical transmission electron microscopy techniques such as high resolution, electron diffraction, energy dispersive X-ray spectrometry (EDX), parallel electron energy loss spectroscopy (PEELS) and elemental mapping via a Gatan Imaging Filter (GIF) have been used to study complex precipitation in commercial dual phase steels microalloyed 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 techniques. On both the microscopic and nanometric scales, it was found that a large amount of precipitation occurred heterogeneously on already existing inclusions/precipitates. CaS inclusions (1 to 2 μm), already present in liquid steel, acted as nucleation sites for TiN precipitating upon the steel's solidification. In addition, TiC nucleated on existing smaller TiN (around 30 to 50 nm). Despite the complexity of such alloys, the statistical analysis conducted on the non-equilibrium samples were found to be in rather good agreement with the theoretical equilibrium calculations. Heterogeneous precipitation must have played a role in bringing these results closer together.