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Effects of an addition of Si and P into amorphous Fe85.2B14Cu0.8 based soft magnetic alloys were investigated in boric-borate buffer solution. The addition of P decreased the passive current density effectively. Substituting Si for P in Fe85.2SixB9P5 − xCu0.8 (x = 0, 1, 2 at.%) alloys further improved corrosion resistance. After the removal of the oxide films, the passive current density was higher than those covered by the oxide films. The lowest passive current density was observed at 5 × 10− 6 A/cm2 on the Fe85.2Si1B9P4Cu0.8 alloy. On the other hand, the pitting potential of the alloys before and after removal of the oxide films was as high as about 0.96 V and was independent of chemical composition of the alloys. The addition of Si and P could enhance the corrosion resistance of Fe85.2SixB9P5 − xCu0.8 alloys via modification of the chemical composition of the oxide films. Cyclic voltammetric results indicated that the addition of Si suppressed the formation rate of bivalent Fe species, which slows down reaction rates of this rate-determining step.

A new molybdenum hybrid-alloyed steel powder has been developed. This powder gives sintered parts with improved fatigue strength by a conventional belt furnace sintering. The powder is based on a molybdenum prealloyed steel powder to which molybdenum powder particles have been diffusion bonded. The sintered compact made of the developed powder has a finer pore structure than that of the conventional molybdenum prealloyed steel powder, because the ferritic iron phase (α-iron phase) with a high diffusion coefficient is formed in the sintering necks where molybdenum is concentrated resulting in enhanced sintering.This developed powder has already been applied as a raw material of a high strength part for automotive engines with advantages of sintering cost, productivity improvement and enhanced flexibility in manufacturing process. This powder also gives an advantage over the conventional diffusion-alloyed steel powder in terms of raw material cost reduction.

Adding small amounts of molybdenum powder to molybdenum prealloyed steel can improve fatigue strength, researchers at Japan's JFE Steel Corporation have found. They say that a finer pore structure is one of the contributors to improved properties…

The effect of copper powder addition on properties of sintered and case-hardened compacts made of the 0.45 mass% Mo prealloyed steel powder has been investigated. The tensile strength of sintered and case-hardened compacts made of the powder increased with 1 mass% Cu addition, but leveled off with 2 mass% Cu addition. The rotating bending fatigue strength of sintered and case-hardened compacts made of the powder increased with 1 mass% Cu addition. However, the fatigue strength decreased with 2 mass% Cu addition and was lower than that of Cu free material. The tensile and fatigue strengths of sintered and case-hardened compacts made of the powder with 1 mass% Cu addition are equal to those made of the conventional 4 mass% Ni diffusion-alloyed steel powder. The reduction of fatigue strength with 2 mass% Cu addition should be attributed to an increase in size of the maximum pore due to Cu liquid phase and probably an increase in fatigue crack growth rate due to Cu addition.

The fatigue strength of the sintered and carburized compact made of the Mo hybrid-alloyed steel powder, which is based on a 0.6 mass% Mo prealloyed steel powder to which 0.2 mass% Mo powder particles have been diffusion bonded, is higher than that of the 0.6 mass% Mo prealloyed steel powder. The sintered compact made of the Mo hybrid-alloyed steel powder has a finer pore structure than that of the Mo prealloyed steel powder, because the Mo-rich region near the surface of the Mo hybrid-alloyed steel powder should exist as the α-iron phase with a high diffusion coefficient at high temperature and result in enhanced sintering. Improvement in the fatigue strength of the sintered and carburized compact made of the Mo hybrid-alloyed steel powder should result from not only the fine pore structure but also the strengthening of the sintering neck by Mo concentration around pores.

A new molybdenum hybrid-alloyed steel powder has been developed. The powder is based on a molybdenum prealloyed steel powder to which molybdenum powder particles have been diffusion bonded. The sintered compact made of the developed powder has a finer pore structure than that of the conventional molybdenum prealloyed steel powder, because the ferritic iron phase (α-phase) with a high diffusion coefficient is formed in the sintering necks where molybdenum is concentrated resulting in enhanced sintering. The rolling contact fatigue strength of the sintered and carburized compacts made of this powder improved by a factor of 3.6 compared with that of the conventional molybdenum prealloyed steel powder. The improvement in the rolling contact fatigue strength should result from the fine pore structure without coarse pores acting as stress concentration points.

Effects of graphite addition and sintering temperature on tensile strength of sintered compacts made from 1%Cr-0.7%Mn-0.3%Mo steel powder have been investigated.A sintered compact with 0.9% graphite addition gives the maximum strength of 760MPa at the process condition of compacting pressure of 690MPa and sintering temperature of 1523K. The major factors controlling tensile strength of 1%Cr-0.7%Mn-0.3%Mo sintered compact have been proved to be sintered density, contents of carbon and MnO⋅Cr2O3 spinet inclusions and amount of cementite precipitated in grain boundaries.The decrease of tensile strength of the sintered compact with 1.2% graphite addition is caused by precipitation of cementite in grain boundaries. The tensile strength lowers with the increase of sintering temperature from 1423K to 1473K because of the decrease of carbon content. The tensile strength increases with the increase of sintering temperature from 1473K to 1523K because of the reduction of MnO⋅Cr2O3 spinel oxide.