Effects of Graphene Oxide Addition and Carbonitriding on Metal Injection Molding Low Carbon Steel

The effect of adding 1 wt.% graphene oxide (GO) to a low-carbon steel feedstock processed by metal injection molding (MIM) was investigated in terms of its thermal behavior, chemical composition after sintering, microstructure, and response to carbonitriding followed by quenching and tempering. Differential scanning calorimetry (DSC) revealed a shift in the melting peak of the polymeric binder system from 158 to 161 °C and a decrease in its apparent melting enthalpy from 112 to 98 J/g, indicating changes in the thermal behavior of the feedstock rather than melting or phase transformation of the steel matrix. Thermogravimetric analysis showed a shift in the onset of mass loss from approximately 325 to 310 °C, with additional decomposition events occurring between approximately 420 and 450 °C. After sintering, the carbon content increased from 0.118 to 0.219 wt.%, accompanied by higher Cr, Ni, and Cu contents. The GO-containing material also exhibited a finer ferritic grain structure and a higher pearlite fraction, together with indications of increased residual porosity and local microstructural heterogeneity. Following carbonitriding, quenching, and tempering, surface microhardness was substantially higher than core microhardness, and the 5% polymer solution produced the highest cooling intensity and surface microhardness among the evaluated quenching media. Overall, the results indicate that GO addition modified the thermal behavior of the polymeric binder system and was associated with changes in the chemical composition, microstructure, and response of sintered MIM low-carbon steel to carbonitriding and subsequent heat treatment.