Effect of furnace atmosphere on sintering process of chromium-containing steel via powder metallurgy

During a powder metallurgy process such as sintering, the primary role played by the atmosphere in furnace is to prevent an excessive oxidation of powder compacts in case of the formation of oxides as residuals on powder surfaces. In particular, the adjustment of furnace atmosphere is the key to eliminate the phenomenon “decarburization” likely to occur in carbon-containing compacts. A continuous belt furnace was used to stabilize the potentials of carbon and oxygen in zones divided by sintering, delubrication, and cooling. Chromium and manganese, which are sensitive to oxygen, were added to improve mechanical properties in a cost-effective way. Powders of steel containing chromium were sintered in an atmosphere composed of CO, O₂, and H₂. The effects of atmosphere, lubricant, and graphite on oxidation (or reduction) and decarburization (or carburization) were investigated. Superior quality was achieved under the control of delubrication atmosphere. It is indicated that in a protective atmosphere, the chemical reactions occurring at various stages took remarkable effect on the quality of sintered compact. The potentials of oxygen and carbon in a continuous belt furnace were monitored and analyzed using an on-line thermal measuring unit consisting of thermocouple, oxygen probe, and carbon monoxide sensor. The avoidance of oxidation and decarburization promises desired microstructure and carbon content and satisfactory properties through the adjustment of technical parameters, e.g., the composition of gases in delubrication and various sintering zones, the rate of gas inlet, and cooling rate.