Dimensional Change of Iron and Iron-Copper Powder Compacts during Sintering (Part I)

Abstract

The densification during sintering of iron powder compacts with a small addition of phosphorus, antimony, boron, sulphur and copper respectively, was discussed. Specimens were prepared by mixing iron powder with additive powder, followed by compacting and sintering for 1 hr at 1150-1200°C in hydrogen.It has been shown from experimental results that the sintering of iron is accelerated by phosphorus or antimony addition, but not accelerated by boron, sulphur or copper addition. In the case of phosphorus or antimony addition, the crystal structure of iron alloy equilibrated at sintering temperature is α phase (bcc lattice) and in the other case that is γ phase (fcc lattice). The diffusion velocity in γ phase is much smaller than in α phase. Examining on the basis of the Kingery's equation, the ratio of the diffusion coefficient in α phase to that in γ phase was in the range of 140 to 250. These values are comparable with the ratio of the self-diffusion coefficient of α-iron to that of γ-iron at the sintering temperature (1150-1200°C). After all, whether or not the sintering of iron powder compact is accelerated by a additive powder depends upon the crystal structure of the sintering iron body equilibrated at the sintering temperature.According to Meechan's experimental results, the rate process of the sintering of α-iron might be the ring diffusion rather than the vacancy diffusion. However, when the α phase of iron of the sintering body is stabilized by the additives, the sintering process maybe proceed by the vacancy diffusion.