Abstract





This paper reviews the subject of carbonitride precipitation during the thermomechanical processing of microalloyed steels, with particular emphasis on the observation and identification of small particles in the transmission electron microscope. Precipitation may occur in austenite during hot rolling, along the gamma/alpha phrase boundary during transformation. or in ferrite during final cooling, with different effects on steel properties. Each precipitation mode can be identified by its characteristic crystallographic orientation of the carbonitride particles with respect to the ferrite mátrix, and composite electrón diffraction patterns have been constructed which allow rapid identification when the sample is tilted toward standard ferrite orientations. Diffraction analysis has been applied to a number of commercial hot strip steels, and the different precipitation strengthening potential. In addition, particle contrast and practical resolution limits for carbonitride observation in the transmission electron microscope are discussed. Finally, some characteristic artifacts from sample preparation are presented which could lead to false interpretations about the formation of small carbonitride particles.