We have examined spheroidization and coarsening of cementite in an initially pearlitic 2C-4Cr ultrahigh carbon steel containing a cementite network. Coarsening kinetics of spheroidized cementite and growth of denuded zones adjacent to the cementite network were investigated by analyzing particle sizes from digital micrographs of water-quenched steel etched with Nital. Denuded zones grew at a rate proportional to t1/4–t1/5. Spheroidization of pearlite was completed within 90 minutes at 1073 K and 1173 K (800 °C and 900 °C), and within 5 minutes at 1243 K (970 °C). Bimodal particle size distributions were identified in most of the samples and were more pronounced at higher temperatures and hold times. Peaks in the distributions were attributed to the coarsening of intragranular and grain boundary particles at different rates. A third, non-coarsening peak of particles was present at 1073 K (800 °C) only and was attributed to particles existing prior to the heat treatment. Particle sizes were plotted vs time to investigate possible coarsening mechanisms. The coarsening exponent for the growth of grain boundary carbides was closest to 4, indicating grain boundary diffusion control. The coarsening exponent was closest to 5 for intragranular carbides, indicating suppression of volumetric diffusion (possibly due to reduced effective diffusivity because of Cr alloying) and control by dislocation diffusion.

Titanium nitride precipitation on a primary inclusion particle during solidification of bearing steel has been tracked by varying temperature in a confocal scanning violet laser microscope. Upon precipitation, an obvious growth of titanium nitride on a primary inclusion particle was observed due to the rapid solute diffusion in liquid steel. The onset of titanium nitride precipitation did not change with primary inclusion particle size, but the time of growth was greater for a smaller primary inclusion particle. Meanwhile, the particle size displayed little influence on the total precipitated amount of titanium nitride on it under the same conditions. At the later period of solidification, almost no change occurred in inclusion size, but the inclusion shape varied from circle to almost square in two-dimension, or cubic in three-dimension, to attain the equilibrium with steel.