Effect of secondary reduction on microhardness and compressibility of iron powder

With the increase of the microhardness of iron powder, the compressibility of iron powder decreases.That is, the powder particles with high micro hardness are hard, and it is not easy to deform in the pressing, and the pressing density is low. On the contrary, the powder with low micro hardness is soft, and it is easy to deform in the compression, so that the powder particles can be close to each other, and the powder particle gap can be further reduced, and the compaction density can be higher.

The microhardness of the primary iron powder is generally in the range of 65~98HV. After the secondary reduction at high temperature, the microhardness is further reduced, and the pressing density of the powder can be increased by 0.2~0.5g/cm3.This is because in the process of high temperature secondary reduction, the defects of powder are eliminated;The impurities in the powder surface and powder particles are further removed, so that the content of carbon, oxygen, sulfur and nitrogen in the powder can be reduced. At the same time, the work hardening during the preparation of iron powder is eliminated, and the grain is further grown, which reduces the micro hardness of the powder to a certain extent and improves the compressibility of the powder.

High temperature reduction can reduce the content of hydrogen consumption (HL) and other impurity elements in the powder to a certain extent, mainly due to many solid-gas multiphase reactions such as deoxygenation, decarbonization and desulfurization produced in the process of high temperature reduction of iron powder, which improves the purity of iron powder.

The hydrogen consumption in the powder changes the microhardness of the powder by producing shrinkage hole and solidifying the powder oxidation. The increase of HL in the iron powder is an important reason for the improvement of microhardness and the decrease of compressibility of the powder.Besides hydroxide, HL also contains other impurities, and non-metallic impurities (Si Mn, etc.) in iron powder also have different effects on the microhardness, so that the change rule of microhardness and hydrogen consumption is not complete.