Characterization and Kinetics of Chromium Carbide Coatings on AISI O2 Tool Steel Performed by Pack Cementation

Chromium carbide coatings were grown on AISI O2 cold work tool steel via pack cementation method at the temperature of 900, 1000, and 1100 degrees C for 4, 6, 8h. The resulting coatings were examined using light microscopy LM, scanning electron microscopy/energy dispersive spectroscopy SEM/EDS, and X-ray diffraction XRD characterization techniques. The process produced chromium carbide coatings on a substrate with a thickness of up to 50 mu m. The results indicated that the phase and layer thickness of the chromium coatings are highly dependent on the holding time and temperature. At low temperature (900 degrees C), the carbides coating appeared as one layer mainly consisting of Cr7C3 phase with thickness up to 13 mu m. As the temperature rises to 1000 and 1100 degrees C, the carbides coating becomes two sublayers mainly consisting of Cr7C3 and Cr23C6 phases with thickness up to 50 mu m. The microhardness value of the coating reached 1750 +/- 45 HV0.05 (17.16 GPa), which was higher than the 291 +/- 2 HV0.05 and 632 +/- 4 HV0.05 for the uncoated/annealed and the quench/tempered specimens, respectively. The activation energy for the process is 187 kJ/mol, and the kinetics of chromizing coating by pack cementation method revealed a parabolic relationship between carbide layer thickness and treatment time.