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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.

Finish machining of sintered steel is increasingly important for near net shape technology. However, the life of a cutting tool for machining sintered steel is generally much shorter than for carbon steel and thus, finish machining increases the manufacturing cost of sintered products. For this reason, wear characteristics of several grades of cutting tools in turning sintered steel were investigated under different lubrication conditions. As a result, it is found that a P10 grade of cermet and an S01 grade of AlTiN coated carbide are recommended for dry machining. When cutting fluid is necessary for chip control and disposal, air jet assisted wet machining with a K10 uncoated cemented carbide and wet machining with a P10 cermet are recommended. It is also found that a small amount of cutting fluid remained in the workpiece during wet machining caused an intense thermal impact to a P10 uncoated cemented carbide leading to short tool life.

Orthogonal cutting experiment of powder metallurgy steel was performed in cutting speeds ranging from 1 m/s to 150 m/s. High-speed cutting experiment was carried out with a high-speed impact-cutting tester. This study focuses on the change in the effects of free-cutting of manganese sulfide with cutting speed. The principal force and thrust force were measured. The cross sections of the chip and of the machined surface were observed. Color mapping analysis of the tool-chip contact region on the rake face with EPMA was done. Although the serrated type of chip formed in all experiments, the cutting mechanism was analyzed by employing a shear plane model. This paper discusses how the effect that MnS promotes the ductile fracture and the effect that MnS improves the friction property at the tool-chip interface change as the cutting speed increases.

Surface rolling were carried out using P/M spur gears of 0.6Mo-0.2C partially alloyed sintered steel with the density of 7.18 g/cm3. A CNC form rolling machine of axis intersection type was employed, and an unmodified screw-shaped rolling tool was prepared. Then, porosity distributions in the surface layer of rolled gear teeth root were measured and the effects of the surface rolling on the selective surface densification behaviors were evaluated. Moreover, to obtain higher fatigue strength, reverse rolling was introduced to standard gears. As a result, the porosities in the tooth root were sufficiently reduced at 700 μm feed in rolling process. Furthermore, a profile shifted gear was prepared to increase the effect of surface rolling. After case hardening, the single tooth bending fatigue durability reached to about 83 % of the case hardened wrought steel gears of SCM415.

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…

Due to the continual improvements of powder metallurgy techniques, it is possible for sintered materials to compete the wrought materials even in some of the most demanding applications. However, strength of sintered materials was inadequacy, because they have a lot of pores. This paper describes a study on the effect of rolling on the contact fatigue strength of sintered ferrous alloy compacts. The effects of rolling were evaluated by measuring the hardness and porosity distributions at the vicinity of the surfaces of rolled compacts. The effects of the amount of rolling on the contact fatigue strength, and the contact fatigue damage mechanisms were also investigated. Porosity at the vicinity of the surfaces of rolled compacts was decreased with increasing the amount of radial feed, from which the contact fatigue strength of rolled compacts was improved.

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.

Sintered ferrous alloy gears are used in wide engineering fields. However, it is difficult for them to be applied to the parts loading at high stress levels, because their strength is poorer than that of the wrought materials. In this study, sintered ferrous alloy gears were rolled and their tooth root bending fatigue strengths were investigated. The effect of rolling conditions on the porosity distribution and hardness at tooth root, and the tooth root bending fatigue strength were determined. As the amount of stock rolled was increased, the porosity of the vicinity of the surface was decreased and the hardness was increased, and also the tooth root bending fatigue strength was increased to certain level, and then the fatigue strength was decreased. This seemed to be due to the shaving when the gear was rolled.

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.

Fe4N is expected to be used as a superior soft or hard magnetic material mainly because of its high saturation magnetization (186emu/g). The production of its dense compact by using pressureless sintering is said to be difficult, because the diffusivity of Fe atoms is poor even at temperatures just below its decomposition temperature of 940K. Therefore, we investigated whether the dense compact can be obtained by pressureless liquid phase sintering of Fe4N and Zn mixed powder; the melting point of Zn (693K) is lower by 247K than the above decomposition temperature.It was found that a usual simple method, i.e., sintering of Fe4N and Zn mixed powder compact resulted in the expansion of the compact. We, however, succeeded in enhancing the densification of the sintered compact from 54% of theoretical density to 84% by applying the following unique process: Fe4N and 10mass%Zn mixed powder was heat-treated in flowing H2 gas at 703K for 10.8ks to form Fe-Zn compound layer on the surface of Fe4N powder particles, and then the product was mixed with 20mass%Zn powder, compacted and sintered in H2-NH3(2:1) flowing gas at 823K for 3.6ks. The enhanced densification was attributed to good wetting characteristics between Zn liquid phase and the Fe-Zn compound layer.

We think that the dimensional accuracy of sintered machine part can be controlled mainly by regulating the relative density of green compact (Dg) and sintering shrinkage and by adding post-treatment such as repressing (cold forging), grinding or cutting in some cases, even if the compact is designed to considerably be densified by sintering. This opinion is based on the fact that such idea have been adopted since about 15-20 years ago in the fabrication of cemented carbides cutting tools and also in MIM or PIM process, where Dg in both cases is as low as 60% and the relative density of sintered compact (Ds) is 100%; the sintering shrinkage is as large as about 14%. Then, we thought that the promotion of the densification is valuable to increase the mechanical properties of sintered machine parts and thus investigated the conditions in the first step for obtaining high Ds as much as possible for coarse powder (45-150μm) of Fe which is the base metal of the parts.The results obtained were as follows. (1)As for Fe coarse powder compact, Ds was nearly equal to Dg when the compact was sintered in vacuum or H2 gas for 3.6-14.4ks at 1573K. Sintering for 14.4ks at 1743K was necessary for obtaining Ds of 97% even in case of Dg=90%. (2)The addition of a small amount (2-4mass%) of Mo could enhance the densification; the compact with Ds of 96-98% was obtained by solid phase sintering at 1573K for 3.6-14.4ks, under which condition pure Fe compacts hardly densified. This enhancement was attributed to the considerable increase of diffusivity of Fe atoms due to the change in the crystal structure of Fe from fcc to bcc which is caused by the addition.

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.