THE INFLUENCE OF Mn ADDITIONS ON THE THICKNESS OF GRAIN BOUNDARY CARBIDES IN FERRITE/PEARLITE STEELS

The growth of grain boundary carbides (GBC) has been followed after heating to 920 C /min to room temperature in plain carbon steels at two Mn levels. In ferrite/ pearlite steels, the GBC form the tails to the pearlite colonies. The thickness of GBC is related to the pearlite reaction, since the temperature at which this occurs controls (a) the thickness of carbide nuclei and (b) the amount of carbon available for precipitating out onto these tails. Increasing cooling rate and Mn level decreases the transformation temperature, and leads to finer carbides. The pearlite nose transformation temperature should be ≤600C° in order to produce fine carbides (<0.2 micron). The gamma grain size which controls the pearlite colony size is also very important in determining the carbide thickness, since the finer the gamma, the greater is the carbide density, and for a given amount of carbon available for precipitation the finer are the resulting carbides. Both faster cooling and high Mn levels refine the pearlite colony size leading to finer carbides.