THERMODYNAMIC AND KINETIC ASPECTS OF OXIDE TRANSFORMATION DURING SINTERING OF Cr-PREALLOYED PM STEELS

摘要

Despite the high purity of the commercially available chromium-prealloyed powder grades, factors affecting stable oxide formation during the sintering process are not fully understood. Hence, thermodynamic and kinetic simulation of oxide formation/transformation has been performed on the prior powder surfaces and the developed inter-particle necks, for the full duration of the sintering process. Thermodynamic and kinetic modeling (Thermo-Calc and HSC Chemistry) was performed based on the results from the analysis of the amount, morphology, and composition of the oxide phases inside the inter-particle necks (HRSEM+EDX and XPS). Possible scenarios for oxide reduction/formation/transformation for iron-chromium-(manganese)-carbon powder systems were evaluated with respect to processing parameters (sintering atmosphere composition, temperature profile, etc.). Results indicate that oxide transformation occurs in accordance with the thermodynamic stability of oxides where the spinel, MnCr2O4, was identified as the most stable oxide phase at the applied sintering conditions. Controlled conditions during the heating stage minimize the formation of stable oxide products and produce oxide-free sintered parts.