Effect of hydrogen gas content in sintering atmosphere on the amount of free graphite in sintered steel made of iron powder containing sulfur, copper powder and graphite powder

The solute carbon content and the ratio of pearlite in the sintered steel increased and the free graphite content and the sulfur content in the sintered steel decreased with the increase in hydrogen content in the sintered atmosphere during the sintering of green compact made of the iron powder containing sulfur, copper powder and graphite powder, resulting in the deterioration of machinability of the sintered steel.The amount of decrease in sulfur content during sintering was roughly the same as that calculated, on the assumption of hydrogen reduction of FeS in an equilibrium state.The Auger spectrum of the iron powder containing sulfur indicated that FeS segregated on the surface of the iron powder. The dilatometry study indicated that the carbon diffusing rate during sintering green compact containing iron powder with sulfur was smaller than that during sintering green compact containing commercially pure iron powder.These results showed that the free graphite was formed by incomplete carbon diffusion during sintering due to FeS on the surface of the iron powder with sulfur.