DURING SINTERING, compacted metal powders are bonded or sintered by heating in a furnace to a temperature that is usually below the melting point of the major constituent. Sintering occurs in a series of overlapping but balanced phases, all of which depend on temperature, time, and atmospheric composition, flow, circulation, and direction.
Furnaces used for sintering typically are similar to protective atmosphere furnaces used in brazing and heat treating of steel. However, the significant differences between heat treating and sintering require careful selection of furnace atmospheres for sintering. Primarily, P/M compacts are porous; therefore, a much greater surface area is exposed to the furnace atmosphere than with solid parts. Sintering temperatures are considerably higher than heat treating temperatures (1120 ¡ãC, or 2050 ¡ãF, for iron and steel compared to 900 ¡ãC, or 1650 ¡ãF, for carburizing and neutral hardening of steel).
This combination of greater exposed surface area and higher temperatures enhances chemical reactivity between the surface and atmosphere during sintering. The trend toward higher sintering temperatures also places additional requirements on furnace capability and tight atmosphere control.