Given the excellent role of rare earths in steel, Since 1970s, powder metallurgy workers in China have been trying to add rare earths to iron-based powder metallurgy materials. After decades of research, many units and individuals in China have published reports on rare earth iron-based powder metallurgy

The research paper on materials has achieved significant results. This article discusses the research on iron based powder metallurgy materials containing rare earth elements in China, including the forms and quantities of rare earth additions, the effects of rare earth elements on alloys, the distribution and existing forms of rare earth elements in alloys, and the effects of rare earth elements on other phases in alloys.

1、Morphology of rare earth addition

The rare earth elements added to iron-based powder metallurgy materials are mainly in the form of mixed rare earth, pure Ce, pure La, Ce oxide, La oxide, mixed rare earth oxide, and rare earth ferrosilicon alloy, which are directly added during wet grinding.

Due to the active nature of rare earth metal powders, they are prone to oxidation during the production process. Adding rare earth elements in the form of oxides makes it difficult to mix them evenly. However, due to the stable nature of the oxides and poor purification effect, the alloy quality is unstable. After adding rare earth oxide, the mechanical properties of the sintered sample began to decrease, and with the increase of the addition amount, the mechanical properties of the sintered sample showed a downward trend. So the added rare earth elements are generally in the form of pure rare earth and rare earth intermediate alloys, but the experimental operation of adding pure rare earth is difficult, so rare earth is more often added in the form of rare earth and iron intermediate alloys.

From the RE2Fe phase diagram, it can be seen that there are various intermetallic compounds between rare earth and iron within a large composition range. These compounds have high hardness, and their brittleness increases with the increase of rare earth content. But rare earth ferrosilicon

The higher the rare earth content in gold, the less rare earth ferrosilicon alloy is added, which affects the uniformity of rare earth dispersion in the alloy. In order to obtain a uniform composition of RE2Fe alloy, it is necessary to melt RE2Fe alloy at least twice. It is worth mentioning that the finer the RE2Fe intermediate alloy powder is ground, which is beneficial for the uniform dispersion of rare earths in the sintered alloy. However, the oxidation resistance of the powder deteriorates. When adding RE2Fe alloy directly during wet grinding, it is important to choose intermediate alloy powder with slightly better oxidation resistance.

2、Quantity of rare earth added

At room temperature, there is an optimal range for the addition of rare earths. The optimal addition amount of rare earth elements varies for different components of iron-based powder metallurgy and production processes. Adding 2% to 3% (mass fraction) of rare earth ferrosilicon alloy to Fe2016C alloy results in a rare earth element content of 0145% to 0168% (mass fraction). After sintering, its performance is optimal. The suitable addition amount of mixed rare earth metals in sintered carbon steel is 017%~1% (mass fraction). In alloy Fe2214C2

The appropriate amount of Ce in 17Cr2118Mo2110Cu is 014% (mass fraction). The appropriate addition amount of non oxidized rare earth alloy in the basic formula FMPS is one time the unit amount (equivalent to about 0104% Ce+0103% La (mass fraction)). In the study of wear resistance of powder metallurgy iron based materials, the optimal amount of rare earth added to Fe2111C alloy is 012% (mass fraction), and in Fe2Cu2Mn2Sn2

The optimal amount of addition in S2P alloy is 0113% (mass fraction). Wu Qingding et al. conducted research on different powder metallurgy iron-based alloys, and found that in Fe2Cu2RE iron-based friction reducing materials, the addition of Cu+RE was 4% -6% (mass fraction), Cu/RE ratio of 7:3, sintering temperature of 900 ℃~At 950 ℃, it has good performance; In Fe2Mo2Mn2 Sn2 Ce2 C

The optimal amount of Ce added in the sintered alloy system is 012% (mass fraction); The optimal amount of mixed rare earth added to F0212J sintered copper molybdenum steel is 016% (mass fraction). The appropriate amount of mixed rare earth elements in Fe22C iron-based powder metallurgy friction materials is 011% (mass fraction). The addition of 0102% to 0115% (mass fraction) in Fe2Mn powder hot forged steel is beneficial for performance.Therefore, in powder metallurgy iron-based materials, it is necessary to choose the optimal amount of rare earth addition based on different steel grades and production processes, and maximize the utilization of rare earth elements Beneficial effect, preparing rare earth iron based powder metallurgy materials with excellent performance.

3、The Effect of Rare Earth Elements on Iron Based Alloys

The Effect of 311 on Alloy Properties

After the addition of rare earth, the mechanical properties of the sintered sample showed a consistent trend with the amount of rare earth material added. When the amount of rare earth is added

When the optimal amount is not exceeded, the microhardness, impact toughness, tensile strength, bending strength, and wear resistance of the sintered sample significantly improve with the increase of rare earth addition. When the rare earth addition exceeds the optimal amount

After measuring, the mechanical properties of the sintered sample began to decrease again. However, rare earths have little effect on the density of the alloy, and have a significant impact on the hardness and compressive strength.There are still differences in the impact.

Zhao Yu et al. found that adding an appropriate amount of rare earth to Fe21C alloy increases hardness by 517% and wear resistance by three times. Research has also shown that the addition of rare earths to the alloy increases its hardness by 29%, compressive strength by 44%, and tensile strength by 46%, indicating that rare earths can significantly improve the material's properties. A systematic study was conducted on the effect of rare earth elements on the hardness of alloys. It was found that the carbon content of alloys increases with the increase of rare earth content within a certain range. When the rare earth content exceeds a certain value, the carbon content actually decreases; After sintering, the tensile strength of the alloy is 3115% higher than that without the addition of rare earth, the impact toughness is 2719% higher than that without the addition of rare earth, and the apparent hardness is 3617% higher than that without the addition of rare earth. However, there are also literature reports indicating that rare earths have no significant effect on the hardness of the alloy. Wu Yansheng et al. also showed that the addition of rare earth elements increased the bending strength by 11%, compressive strength by 33%, and wear resistance by 316 times compared to the sample without rare earth elements. The effect on apparent hardness was not significant, but rare earth elements can significantly improve the micro hardness of the alloy.

When discussing the influence of rare earth elements on compressive strength, the compressive strength and density of sintered parts are closely related. As the rare earth content increases, the density and compressive strength first decrease and then increase, reaching the optimal value at 1% rare earth content (mass fraction). The density and compressive strength

From the initial 6146g/cm3, 340N/mm to 6160g/cm3

418N/mm;: With the increase of rare earth content, the initial shrinkage temperature of the material during sintering increases; Before significant shrinkage during sintering, the maximum expansion of the material increases linearly with the rare earth content;

312 Refine grain size

Adding trace amounts of rare earths to iron-based powder metallurgy materials can make the pearlite structure fine and dense, bend grain boundaries, reduce interlayer spacing, and refine carbides. The mechanism of its action is as follows: (1) Rare earth has a decrease

The effect of metastable eutectic transformation temperature increases the degree of undercooling during the cooling process, thereby increasing the nucleation rate of austenite and refining the grains;

(2) The free energy of rare earth oxygen (sulfur) compounds is very low, with a NaCl type lattice structure, and their atomic spacing is very close to that of austenite atoms,

The mismatch between the two is very small and can be used as heterogeneous nuclei to refine grains,Moreover, rare earth oxides as the nucleation substrate of carbides are easier to form independently than carbides; (3) The radius of rare earth atoms is larger than that of iron atoms, making it easy to fill the surface defects of new phases in the growing iron or alloy grains, generating thin films that can hinder the continued growth of grains, helping to refine the grains; (4) Rare earth is a surface active element that increases surface tension when added Reducing the driving force for grain growth; (5) The carbon concentration in sintered iron austenite is uneven, and there are many high carbon and low carbon regions, which also prepare favorable conditions for the nucleation of cementite and ferrite during pearlite transformation.

313 Promote sintering

Due to the addition of rare earth elements, the grain size is significantly refined and the number of grain boundaries is increased, providing more diffusion channels for alloy elements, which is conducive to obtaining a uniform and dense sintered body. At the same time, the rare earth elements have the function of modifying inclusions and reducing surface active oxygen of particles, purifying the surface of iron, copper, and chromium, which is beneficial for improving the quality of iron, copper, and chromium

The bonding strength, on the other hand, reduces the resistance of atomic diffusion during the sintering process, thereby accelerating the sintering process of the compact and reducing the sintering temperature. Data shows that the initial densification temperature of alloys with rare earth elements is reduced by 50 ℃ to 150 ℃ compared to alloys without rare earth elements.

314 Strengthening effect

A portion of the solid solution of rare earth elements exists in ferrite, causing lattice distortion of ferrite, thereby strengthening ferrite. Another portion of rare earth elements exist in carbides, promoting carbon dissolution in the matrix and carbides, hindering their dissolution into internal stress zones and crystal defects, and improving the microhardness of the matrix and carbides. Non solid soluble rare earth that can form high melting point oxidation

Granular distribution produces a dispersion strengthening effect.

315 Deteriorating harmful inclusions

In the mechanical ball milling process, a portion of rare earth elements preferentially reduce the active oxygen in the powder system, generating small, uniform, and stable rare earth oxides, which is beneficial for improving the pressing performance of the product.

During the sintering stage, a certain amount of rare earth elements can also interact with low melting point impurities such as phosphorus, arsenic, tin, antimony, bismuth, and lead in the alloy, forming stable compounds with high melting points and inhibiting the segregation of these impurity elements at grain boundaries, playing a role in purifying grain boundaries and improving material strength

316 Regular carbide

Rare earth is a surface active element that can reduce surface tension by 15 when adsorbed on the surface of the cementite crystal nucleus, thereby making the flake like cementite nucleus smooth.

For iron based powder metallurgy materials containing Cr, the lattice constants of rare earths

01485nm16 and (Fe, Cr) 7 C3 carbon in powder metallurgy material matrix

The lattice constant of the compound in the C-direction is close to 01454nm4. According to the principle of minimum energy, the phase interface with a coherent relationship has the lowest energy, so rare earth atoms are prone to enrichment in this direction, hindering crystal growth in this direction and making the shape of the carbide more regular.

317 Reduce powder ball milling particle size

Due to mechanical grinding, alloy powders withstand forces such as impact, shear, friction, and compression, undergoing processes such as deformation, cold welding, hardening, and crushing. Rare earth oxides exist as hard particles in the ball milling system, embedded in the particles to accelerate the deformation, cutting, and crushing of the powder, resulting in the refinement of the powder. Therefore, rare earth elements are beneficial for reducing the powder particle size after mechanical ball milling. And as the amount of rare earth elements added increases, the particle size of the powder gradually decreases after mechanical ball milling. Adding La element is more conducive to the fragmentation of the powder particles than adding Ce element

318 rare earth oxides are beneficial for deoxidation

Adding a certain amount of rare earth oxides is beneficial for the removal of reactive oxygen species during ball milling, and the effect of La2O3 is more pronounced than that of Ce2O3. rare earth Oxides are added as hard particles in the ball milling system, and in mechanical balls

During the grinding process, the embedded particles play a role in cutting and crushing, accelerating the deformation of the powder and the peeling off of surface oxides, exposing active atoms and increasing surface energy. Therefore, it is beneficial to accelerate the rate of oxidation-reduction reaction and increase the degree of reduction of oxides.

4 The Forms and Distribution of Rare Earth Elements in Alloys

Rare earth elements exist in the form of inclusions in iron-based powder metallurgy materials, while others are distributed in carbides and solid solutions, playing an effective alloying role together. Rare earth elements present in carbides and solid solutions have certain rules in phases such as carbides, solid solutions, and inclusions. When the addition amount of rare earth is 012% (mass fraction), its content in carbides accounts for 5915%, in solid solutions accounts for 3712%, and in inclusions accounts for 311%. Early Zhou Zuoping et al. observed the distribution of lanthanum, cerium, and silicon using scanning electron microscopy images and scanning phase analysis: rare earth silicides completely disintegrate and diffuse evenly, and lanthanum and cerium are uniformly distributed in the matrix. Research has shown that after adding trace amounts of La2O3 to iron based powder hot forgings, scanning electron microscopy observation and X-ray diffraction determination show that rare earth elements still exist in the form of La2O3, with a dispersed and uniform distribution.

The vast majority of residual rare earth elements in steel exist in the form of rare earth compounds, while another form of rare earth elements in steel is in an atomic solid solution state. The rule for forming solid solutions is that in a complete crystal, if the difference in atomic size between the solute and solvent does not exceed 15%, there will be a greater solubility, otherwise the solubility will decrease; When it exceeds 30%, it is difficult to form a solid solution. The difference in diameter from rare earth atoms is 41% to 47%, far exceeding the theoretical value of easy formation of solid solutions, making it difficult to form the measured solid solubility. The significant difference in negative charge between rare earth and iron is not conducive to the formation of solid solution 17. Therefore, the solid solubility of rare earths in steel is very low, and the microstructure of steel based on solid solution is uneven, with defects such as grain boundaries, phase boundaries, dislocations, and voids. Therefore, even if trace rare earths can be solidly dissolved in steel, they cannot be uniformly distributed. Most rare earth atoms will preferentially aggregate at defects such as grain boundaries and phase boundaries. The research results using scanning electron microscopy and electron probe detection indicate that non solid solution rare earth elements are mainly concentrated in the pores and the boundaries of the matrix grains in iron-based powder metallurgy friction materials, and their distribution is in the form of aggregated particles, often coexisting with heavy metals such as phosphorus, arsenic, tin, antimony, bismuth, and lead.

5 Effect on other phases of the alloy

511 Impact on chromium

In powder metallurgy iron chromium alloys, the addition of rare earth elements affects the distribution of alloying element chromium. With the increase of rare earth element addition, the chromium content in alloy carbides (Cr, Fe) 7C3 increases, resulting in a significant increase in the microhardness of the carbides, thereby improving the material's wear resistance. Energy spectrum scanning of the carbides showed that the chromium content in the carbides of the sample without the addition of rare earth element cerium was 38108%. However, when 014% Ce (mass fraction) was added, the chromium content in the carbide phase increased to 50121%. When the cerium content increased to 016% (mass fraction), the chromium content of the alloy element increased to 86114%. Moreover, the oxygen content in the carbide decreases, which will greatly improve the bonding strength between the carbide and the matrix.

512 Effect on MoS2

Non solid soluble rare earth elements have a strong promoting effect on the decomposition of MoS2. After organizing and analyzing the results of micro region energy spectrum analysis at high Mo positions of each sample, it was found that the atomic percentages of S and Mo elements were not fixed at around 2:1, but showed a decreasing trend with the increase of rare earth content. When the rare earth content reached a certain value, the added 4% (mass fraction) MoS2 could be completely decomposed. The sintering atmosphere of the experiment is in the reducing state

Pressure sintering is carried out in an atmosphere, with a sintering temperature of 1000 ℃± 10 ℃ and a sintering pressure of 100kgf ± 9kgf.

Under H2 atmosphere, sintered at 1000 ℃, and held for 3 hours, MoS2 will automatically decompose, and the complete layered structure no longer exists. The decomposed Mo is solidly soluble in Fe, which can strengthen the Fe matrix. S and Fe generate FeS, and can still play a lubricating role. X-ray diffraction and energy spectrum analysis of the sintered sample confirmed that MoS2 decomposed into FeS and compounds such as (Fe, Mo) 3C and Mo2C. Further research is needed to confirm whether adding rare earth elements or sintering atmosphere will lead to the decomposition of MoS2.

6、epilogue

Rare earth elements have a significant improvement effect on the properties of iron based powder metallurgy alloys. Numerous studies have shown that trace amounts of rare earth can significantly improve the microhardness, strength, and toughness of alloys. Rare earth can activate sintering, which is beneficial for the fragmentation of alloy powder and the removal of active oxygen during ball milling. After adding rare earths, the microstructure is refined and uniform, the matrix is strengthened, and the presence of inclusions is improved. The mechanism of the action of rare earth elements on sintered iron based materials, the interaction between rare earth elements and other components, and the specific forms and distribution of rare earth elements in alloys are not yet unified, and further research is needed.

With the continuous development of the powder metallurgy industry, iron based powder metallurgy materials have been widely used in the automotive industry and friction braking industry. At present, powder metallurgy parts used in the automotive industry worldwide account for 70% to 80% of its total production, with over 160 types of parts available; The most suitable friction braking material for high-speed trains in China, especially those running at speeds above 160km · h-1, is iron based powder metallurgy material. As a cheap and efficient alloying element, rare earth iron based powder metallurgy materials will have broad application prospects.