Changes in Microstructure of Ferrous Powder Metal Compacts During High Temperature Sintering

Samples of commercial powders were sintered at the conventional sintering temperature of 1120°C (2050°F) and also at high temperatures of 1232°C and 1288°C (2250°F and 2350°F) using different time cycles in an industrial walking-beam furnace. Pore morphology is shown in a series of photomicrographs. The spheroidization of pores is rapid at high temperatures and the rate of spheroidization increases with the complexity of powder particles. Pore rounding is also associated with the elimination of prior particle identity as shown in a series of fractographs. High temperature sintered parts, due to improved impact strength and ductility, have potential for expanding the field of powder metallurgy, because parts so processed can be substituted for all but the most demanding forgings and also for nodular iron castings. A nitrogen-base atmosphere of 96:4 N₂/H₂ composition with a small addition of CO gas performed satisfactorily during high temperature sintering. Dimensional changes were studied and problems connected with dimensional control are discussed.