On-line Control of Processing Atmospheres for Proper Sintering of Oxidation-Sensitive PM Steels

Journal article, 2012

Changes of atmosphere composition during sintering of water atomized powder prealloyed with Mn and Cr (up to 2% of both) were studied. Increasing sensitivity to atmosphere purity with increasing alloying elements content was registered. Continuous monitoring of sintering atmosphere composition (CO/CO2/H2O) indicates three critical stages during the heating up to final sintering temperature: the importance of rapid atmosphere purification after lubricant decomposition and removal; the reduction of the iron oxide layer by hydrogen at temperatures up to ~500°C and by carbon at temperatures around ~720°C; the reduction of the spinel oxides on the powder surface at above 900°C and further reduction of thermodynamically stable surface oxides and mixed internal oxides close to the sintering temperature. The measured ratio of CO/CO2 indicates favorable thermodynamic conditions for reduction of stable oxides as (Cr,Mn)xOy close to sintering temperature (1120C) for the applied sintering conditions. The experimental results were confirmed by modeling the metal-gas interactions using the thermodynamic/thermochemical softwares ThermoCalc and HSC Chemistry. The modeling indicate the significance of maintaining a sintering atmosphere with high reducing potential during heating stage for minimizing oxidation before high-temperature carbothermal reduction starts.