Growth of grain boundary carbides in C–Mn steels

Abstract

The growth of grain boundary carbides has been followed during continuous cooling from the austenite using cooling rates of 7 and 150 K min⁻¹ and Mn levels of 0·5 and 1 wt-%. The isothermal growth of grain boundary carbides in the temperature range 600–650°C has also been examined and growth was found to obey a (time)^1/4 relation which is very slow. The activation energy for growth increased from 241 kJ mol⁻¹ for the lower Mn containing steel to 293 kJ mol⁻¹ for the higher Mn steel. For continuous cooling conditions growth was very rapid. Grain boundary carbides started to form at the time of the pearlite reaction and their initial thickness corresponded to the thickness of lath carbides in the pearlite colonies. The grain boundary carbides are believed to form the tails of the pearlite colonies and probably initiate the pearlite reaction. Increasing the cooling rate or Mn content reduced the lath carbide thickness within the pearlite colonies. Subsequent growth of the grain boundary carbides is believed to occur by the excess C rejected on cooling diffusing to the ferrite grain boundaries and the activation energy for growth in this case may well be that for C diffusion in ferrite (84 kJ mol⁻¹). The amount of C available for precipitation in the form of grain boundary carbides is that present at the time of the pearlite reaction. The start of the pearlite reaction is suppressed by Mn and the amount of C available to form carbides is also reduced, hence producing finer carbides. By increasing the cooling rate, the pearlite transformation was suppressed to lower temperatures reducing the amount of C as well as increasing the number of nucleation sites for precipitation, both of which favour fine carbides. Except for very slow cooling rates ‘isothermal’ growth is not likely to be important.

MST/868