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

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\ub0C and by carbon at temperatures around ~720\ub0C; the reduction of the spinel oxides on the powder surface at above 900\ub0C 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 (1120C) 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