INFLUENCE OF MICROALLOYING ADDITIONS ON THICKNESS OF GRAIN-BOUNDARY CARBIDES IN FERRITE PEARLITE STEELS

For a series of plain C and microalloyed steels at two levels of Mn, t he growth of grain boundary carbides has been monitored after heating to 920-degrees-C and cooling at 40 and 150 K min-1 through the austeni te-ferrite/pearlite transformation down to room temperature. In pearli te free steels, on cooling to room temperature, all the C in solution in the ferrite is able to precipitate as carbides at the boundaries an d the grain boundary carbide thickness is dependent on the number of n ucleation sites for precipitation. Increasing the cooling rate increas es the number of sites and reduces the carbide thickness. In ferrite-p earlite steels, the grain boundary carbides form the 'tails' to the pe arlite colonies. The thickness of the grain boundary carbide is relate d to the pearlite reaction, since the temperature at which this occurs controls both the thickness of the carbide nuclei and the amount of C available for precipitating out on these tails. Increasing the coolin g rate and Mn content causes a decrease in the transformation temperat ure and leads to finer carbides. The pearlite nose transformation temp erature must be less-than-or-equal-to 600-degrees-C to produce fine (l ess-than-or-equal-to 0.2 mum) carbides. The austenite grain size, whic h controls the pearlite colony size, is also very important in determi ning the thickness of carbides, since the finer the grain size, the gr eater the carbide density and, for a given amount of C available for p recipitation, the finer the resulting carbides. Faster cooling or a hi gher Mn content refine the pearlite colony size leading to finer carbi des. Compared with C-Mn-Al steels, Nb and Ti microalloying additions r esult in coarser carbides and higher carbide densities. The increased carbide density is due to the finer austenite grain size and the coars er carbides are due to the finer grain size raising the transformation temperature. The implications of these observations on impact behavio ur are discussed.