CALPHAD based interstitial solid solution thermodynamic modeling was used to determine carbon solubility in the presence of select carbide phases and the eutectoid temperatures of 15-5 PH and 17-7 PH stainless steels under paraequilibrium conditions. Predictions using CALPHAD parameters from different sources in the literature were compared. Resulting values for the eutectoid temperature in 15-5 PH stainless steel varied nominally 100K depending upon the choice of Cr-Fe-C CALPHAD parameters employed.
By converting from a dilute solution to a CALPHAD model, Cu-C interaction parameters were derived and applied to the numerical prediction. CALPHAD based thermodynamic modeling also was used to predict the effects of Cr and Ni on the solubility in Fe-based bcc and fcc matrices of graphite and paraequilibrium M3C, M7C3, and M23C6 carbides. The solubility of graphite increases with increasing Cr, and decreases with increasing Ni contents in both bcc and fcc matrices. For the Fe-Ni-C system, CALPHAD modeling of compositions up to 40 wt. pct. Ni found that the formation of paraequilibrium conbides increases the solubility of carbon relative to graphite for both bcc and fcc matrices at 700K.
The Swagelok low-temperature carburization process was applied to 15-5 PH stainless steel over a temperature range from 380 to 450C. Through this process, hardened cases 8-12 µm thick were produced having a carbon content of 8-10 at. pct. as determined using Auger electron spectroscopy (AES). Near surface microstructures were examined using optical microscopy and scanning electron microscopy (SEM). Microhardness testing measured case hardness values of 950-1100 HV which are much higher than the core hardness values of approximately 500 HV.