Formation of intergranular M23C6 carbides and thereafter Cr-depletion zones in commercially available type-347 stainless steels were observed by optical microscopy, conventional transmission electron microscopy and analytical transmission electron microscopy. At the early stage of sensitization, only NbC carbides were observed both intragranularly and intergranularly. At the later stage, formation of intergranular M23C6 carbides and intergranular corrosion were found. A model is proposed to describe the formation kinetics of intergranular precipitation of NbC and of M23C6, as well as Cr-depletion zones, thermodynamically. Evolution mechanisms of intergranular precipitates were found in the order of 1) the formation of intergranular NbC, 2) the coarsening of NbC, 3) the formation of M23C6, then 4) the coarsening of M23C6 and formation of Cr-depletion zones.

A large number of morphological studies of intergranular carbides in steels have always been carried out in two dimensions without considering their dispersion manners. In this article, focused ion beam serial-sectioning tomography was carried out to study the correlation among the grain boundary characteristics, the morphologies and the dispersions of intergranular carbides in 347 austenitic stainless steel. More than hundred intergranular carbides were characterized in three dimensions and finally classified into three different types, two types of carbides probably semi-coherent to one of the neighboring grains with plate-type morphology, and one type of carbides incoherent to both grains with rod-type morphology. In addition, the rod-type carbide was found as the largest number of carbides among three types. Since large numbers of defects, such as misfit dislocations, may be present at the grain boundaries, which can be ideal nucleation sites for intergranular rod-type carbide precipitation.

Dispersion behavior of intragranular NbC precipitates in Nb added austenitic stainless steel were investigated via nanoscopic characterization in detail, FIB-SEM slice-sectioning tomography, orientation image microscopy, energy-dispersive X-ray spectrometry (EDS), selected area electron diffraction pattern (SAEDP) and transmission electron microscopy (TEM). The heterogeneous dispersion of fine intragranular NbC precipitates were visualized, and in particular, it was found that they were on the {111} slip plane and associated with dislocations.

Synopsis : Dispersion behavior of intragranular NbC precipitates them in Nb added austenitic stainless steel were investigated via nanoscopic characterization, FIB-SEM slice-sectioning tomography, energy-dispersive X-ray spectrometry (EDS), selected area electron diffraction pattern (SAEDP) and transmission electron microscopy (TEM). The heterogeneous dispersion of fine intragranular NbC precipitates were visualized, and in particular, it was found that they were on the {111} slip plane and interacted with <110> dislocations.