JP6842345B2 - Abrasion-resistant iron-based sintered alloy manufacturing method - Google Patents

Description

The present invention relates to a method for producing a wear-resistant iron-based sintered alloy containing hard particles suitable for improving the wear resistance of the sintered alloy.

Conventionally, an iron-based sintered alloy may be applied to valve seats and the like. The sintered alloy may contain hard particles in order to further improve the wear resistance. When hard particles are contained, graphite particles and iron particles are mixed with the hard particles to form a powder, and the mixed powder is compacted into a molded product for a sintered alloy. After that, it is common to heat the molded product for a sintered alloy to obtain a sintered alloy by sintering.

As a method for producing such a sintered alloy, a compact for a sintered alloy is compactly molded from a mixed powder in which hard particles, graphite particles, and iron particles are mixed, and the graphite particles of the molded product for a sintered alloy are formed. A method for producing an abrasion-resistant iron-based sintered alloy that sinters a molded body for a sintered alloy while diffusing C into hard particles and iron particles has been proposed (see, for example, Patent Document 1).

Here, the hard particles are Mo: 20 to 70% by mass, C: 0.2 to 3% by mass, Mn: 1 to 15% by mass, the balance is composed of unavoidable impurities and Co, and the mixed powder is hard particles and graphite particles. , And, when the total amount of iron particles is 100% by mass, hard particles are contained in an amount of 10 to 60% by mass, and graphite particles are contained in an amount of 0.2 to 2% by mass. Since hard particles are dispersed in such a sintered alloy, it is possible to suppress abrasive wear.

Japanese Unexamined Patent Publication No. 2004-156101

However, since the matrix material for connecting the hard particles of the abrasion-resistant iron-based sintered alloy produced by the production method described in Patent Document 1 is an Fe-C-based material in which C of graphite particles is diffused in the iron particles. soft. Therefore, when the wear-resistant iron-based sintered alloy and the metal material of the sliding partner material in contact with the wear-resistant iron-based sintered alloy come into metal contact with each other, the contact surface of the wear-resistant iron-based sintered alloy is easily plastically deformed, and this contact Easy to stick and wear on the surface. To prevent this, it is desirable to increase the hardness of the wear-resistant iron-based sintered alloy, but on the other hand, this may reduce the machinability of the wear-resistant iron-based sintered alloy, resulting in adhesive wear resistance. It is difficult to achieve both property and machinability.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for producing a wear-resistant iron-based sintered alloy capable of ensuring machinability while suppressing adhesive wear. To provide.

As described above, the inventors considered that the plastic deformation of the iron-based matrix of the wear-resistant iron-based sintered alloy promotes the adhesive wear of the contact surface. From this point of view, the inventors have considered adding other hard particles capable of suppressing plastic deformation of the iron-based matrix in addition to the conventional hard particles that suppress absolute wear. Therefore, the inventors focused on molybdenum as the main component of the hard particles, and scattered iron-molybdenum intermetallic compounds and molybdenum carbides precipitated during sintering in the iron-based matrix. It was found that the plastic deformation of molybdenum can be controlled. In addition to this, by converting a part of the iron in the iron-based base derived from iron particles to triiron tetroxide, the wear resistance of the sintered alloy can be improved without impairing the machinability of the sintered alloy. I got a new finding.

The present invention is based on such findings, and the method for producing an abrasion-resistant iron-based sintered alloy according to the present invention is a molding for a sintered alloy from a mixed powder containing hard particles, graphite particles, and iron particles. Sintering in which the molded body for a sintered alloy is sintered while the molding step of compacting the body and the C of the graphite particles of the molded body for a sintered alloy are diffused into the hard particles and the iron particles. A method for producing an wear-resistant iron-based sintered alloy, which comprises a step, wherein the hard particles include a first hard particle and a second hard particle, and the first hard particle is the first hard particle. Mo: 20 to 70% by mass, Ni: 5 to 40% by mass, Co: 5 to 40% by mass, Mn: 1 to 20% by mass, Si: 0.5 to 4.0. Mass%, C: 0.5 to 3.0% by mass, the balance is Fe and unavoidable impurities, and the second hard particles are Mo: 60 to 70 when the second hard particles are 100% by mass. Mass%, Si: 2.0% by mass or less, the balance is Fe and unavoidable impurities, and the mixed powder is the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles. When 100% by mass, the first hard particles are contained in an amount of 5 to 50% by mass, the second hard particles are contained in an amount of 1 to 5% by mass, and the graphite particles are contained in an amount of 0.5 to 1.5% by mass. In the sintering step, the first hard particles are sintered so that the hardness of the first hard particles becomes Hv400 to 600, and the hardness of the second hard particles exceeds Hv600. A sintered body sintered from a molded body for a sintered alloy is subjected to an oxidation treatment so that a part of the iron of the iron-based matrix derived from the iron particles becomes triiron tetroxide, and the oxidation treatment is performed. It is characterized in that the oxidation treatment is performed before and after so that the density difference of the sintered body is 0.05 g / cm ^{3 or more.}

According to the present invention, machinability can be ensured while suppressing adhesive wear.

Schematic conceptual diagram of the wear test used in Examples and Comparative Examples. Schematic conceptual diagram of the machinability test used in Examples and Comparative Examples. (A) A graph showing the result of the wear test wear amount ratio to the addition amount of the first hard particles in Examples 1 to 3 and Comparative Examples 1 and 9, and (b) Examples 1 to 3 and Comparative Examples 1 and 9 The graph which showed the result of the tool wear amount ratio with respect to the addition amount of the 1st hard particle. (A) A graph showing the results of the wear test wear amount ratio to the addition amount of the second hard particles in Examples 1, 4 and 5 and Comparative Examples 3, 4 and 9, and (b) Examples 1, 4 and 5 and The graph which showed the result of the tool wear amount ratio with respect to the addition amount of the 2nd hard particle in Comparative Examples 3, 4 and 9. (A) Graph showing the result of the wear test wear amount ratio to the addition amount of graphite particles in Examples 1, 6 and 7 and Comparative Examples 5, 6 and 9, (b) Examples 1, 6 and 7 and Comparative Example. The graph which showed the result of the tool wear amount ratio with respect to the addition amount of graphite particles in 5, 6 and 9. (A) A graph showing the results of the wear test wear amount ratio to the hardness of the first hard particles in Examples 1, 3, 5, 8 and Comparative Examples 8, 9, and (b) Examples 1, 3, 5, The graph which showed the result of the tool wear amount ratio with respect to the hardness of the 1st hard particle in 8 and Comparative Examples 8 and 9. (A) A graph showing the results of the wear test wear amount ratio to the density difference of the sintered body in Examples 1 to 8 and Comparative Examples 7 and 9, and (b) Burning in Examples 1 to 8 and Comparative Examples 7 and 9. The graph which showed the result of the tool wear amount ratio with respect to the density difference of a body. (A) Photograph of the surface of the test piece according to Example 1 after the wear test, (b) Photograph of the surface of the test piece according to Comparative Example 7 after the wear test. (A) Tissue photograph of the test piece according to Example 1, (b) Tissue photograph of the test piece according to Comparative Example 5, and (c) Tissue photograph of the test piece according to Comparative Example 6. (A) A graph showing the results of the wear test wear ratio in Examples 1 and 9 and Comparative Example 10, and (b) a graph showing the results of the tool wear ratio in Examples 1 and 9 and Comparative Example 10.

Hereinafter, embodiments of the present invention will be described in detail.
The molded product for a sintered alloy (hereinafter referred to as a molded product) according to the present embodiment is a compact powder molded product containing a mixed powder containing first and second hard particles, graphite particles, and iron particles, which will be described later. The wear-resistant iron-based sintered alloy (hereinafter referred to as a sintered alloy) is obtained by sintering a molded product while diffusing C of graphite particles into hard particles and iron particles. The following hard particles, a compact formed by compacting a mixed powder obtained by mixing the same, and a sintered alloy obtained by sintering the compact will be described.

1. 1. About the first hard particles The first hard particles are particles that are blended as a raw material in a sintered alloy and have a high hardness with respect to the iron particles and the iron-based matrix of the sintered alloy, thereby causing the sintered alloy to wear aggressively. It is a particle intended to be suppressed.

The first hard particles are particles made of a Co-Mo-Ni-Fe-Mn-Si-C alloy. Specifically, the first hard particles are Mo: 20 to 70% by mass, Ni: 5 to 40% by mass, Co: 5 to 40% by mass, Mn: when the first hard particles are 100% by mass. It is composed of 1 to 20% by mass, Si: 0.5 to 4.0% by mass, C: 0.5 to 3.0% by mass, and the balance is Fe and unavoidable impurities. Further, Cr may be added to the first hard particles in a range of 10% by mass or less, if necessary. The hardness of the first hard particles before sintering is preferably in the range of Hv400 to 600.

The first hard particles can be produced by an atomizing treatment in which a molten metal containing the above-mentioned composition in the above-mentioned ratio is prepared and the molten metal is sprayed. Alternatively, as another method, the coagulated body obtained by coagulating the molten metal may be pulverized by mechanical pulverization. The atomizing treatment may be either a gas atomizing treatment or a water atomizing treatment, but a gas atomizing treatment in which rounded particles can be obtained is more preferable in consideration of sinterability and the like.

Here, the lower limit value and the upper limit value of the composition of the above-mentioned hard particles are limited to the reasons described later, and within the range, the hardness, solid lubricity, adhesion, cost, and the like are taken into consideration. It can be appropriately changed according to the degree of importance of each characteristic of the applied member.

1-1. Mo: 20-70% by mass
Of the composition of the first hard particles, Mo can generate C and Mo carbides of carbon powder at the time of sintering to improve the hardness and wear resistance of the first hard particles. Further, Mo can oxidize solid-solved Mo and Mo carbides to form a Mo oxide film in a high-temperature use environment, and can obtain good solid lubricity in a sintered alloy.

Here, when the Mo content is less than 20% by mass, not only the amount of Mo carbide produced is small, but also the oxidation start temperature of the first hard particles becomes high, and the formation of Mo oxide in a high temperature use environment is suppressed. Will be done. As a result, the solid lubricity of the obtained sintered alloy becomes insufficient, and the abrasive wear resistance thereof is lowered. On the other hand, if the Mo content exceeds 70% by mass, not only is it difficult to produce by the atomizing method, but also the adhesion between the hard particles and the iron-based matrix may decrease. A more preferable Mo content is 30 to 50% by mass.

1-2. Ni: 5-40% by mass
Of the composition of the first hard particles, Ni can increase the austenite structure of the matrix of the first hard particles and improve its toughness. Further, Ni can increase the solid solution amount of Mo of the first hard particles to improve the wear resistance of the first hard particles.

Further, Ni diffuses into the iron-based matrix of the sintered alloy during sintering, can increase the austenite structure of the iron-based matrix, increase the toughness of the sintered alloy, and dissolve Mo in the iron-based matrix. The amount can be increased and the abrasion resistance can be improved.

Here, if the Ni content is less than 5% by mass, it is difficult to expect the effect of Ni described above. On the other hand, if the Ni content exceeds 40% by mass, the effect of Ni described above is saturated, and the cost of the first hard particles increases. A more preferable Ni content is 20 to 40% by mass.

1-3. Co: 5-40% by mass
Of the composition of the first hard particles, Co can increase the austenite structure in the matrix of the first hard particles and the iron-based matrix of the sintered alloy and improve the hardness of the first hard particles, similar to Ni. Can be made to.

Here, if the Co content is less than 5% by mass, it is difficult to expect the effect of Ni described above. On the other hand, if the Co content exceeds 40% by mass, the effect of Co described above is saturated, and the cost of the first hard particles increases. A more preferable Co content is 10 to 30% by mass.

1-4. Mn: 1 to 20% by mass
Of the composition of the first hard particles, Mn efficiently diffuses from the first hard particles to the iron-based matrix of the sintered alloy during sintering, so that the adhesion between the first hard particles and the iron-based matrix can be improved. it can. Furthermore, Mn can increase the austenite structure in the matrix of the first hard particles and the iron-based matrix of the sintered alloy.

Here, when the Mn content is less than 1% by mass, the amount of Mn diffused into the iron-based matrix is small, so that the adhesion between the hard particles and the iron-based matrix is lowered. As a result, the mechanical strength of the obtained sintered alloy is lowered. On the other hand, if the Mn content exceeds 20% by mass, the above-mentioned effect of Mn is saturated. A more preferable Mn content is 2 to 8% by mass.

1-5. Si: 0.5 to 4.0% by mass
Of the composition of the first hard particles, Si can improve the adhesion of the Mo oxide film of the first hard particles. Here, if the Si content is less than 0.5% by mass, it is difficult to expect the effect of Si described above. On the other hand, if the Si content exceeds 4.0% by mass, the moldability to the molded product is hindered and the density of the sintered alloy decreases. A more preferable Si content is 0.5 to 2% by mass.

1-6. C: 0.5 to 3.0% by mass
Of the composition of the first hard particles, C can be combined with Mo to form Mo carbides, and the hardness and wear resistance of the first hard particles can be improved. Here, if the C content is less than 0.5% by mass, the effect of wear resistance is not sufficient, while if the C content exceeds 3.0% by mass, the moldability to the molded product is hindered. This will reduce the density of the sintered alloy. A more preferable content of C is 0.5 to 2% by mass.

1-7. Cr: 10% by mass or less Of the composition of the first hard particles, Cr can suppress excessive oxidation of Mo during use. For example, the addition of Cr is effective when the operating environment temperature of the sintered alloy is high, the formation of a Mo oxide film on the first hard particles increases, and the Mo oxide film on the first hard particles peels off.

Here, if the Cr content exceeds 10% by mass, the formation of a Mo oxide film in the first hard particles is suppressed too much. In the case of a corrosive environment such as alcohol fuel, it is desirable to add Cr in order to improve corrosion resistance. On the other hand, in an environment where adhesive wear is likely to occur, it is desirable to suppress the Cr content in order to promote oxidation.

1-8. Particle size of the first hard particles The particle size of the first hard particles can be appropriately selected depending on the use and type of the sintered alloy, but the particle size of the first hard particles is in the range of 44 to 250 μm. Is preferable, and more preferably, it is in the range of 44 to 105 μm.

Here, when the first hard particles contain hard particles having a particle size of less than 44 μm, the wear resistance of the wear-resistant iron-based sintered alloy may be impaired because the particle size is too small. On the other hand, when the first hard particles contain hard particles having a particle size of more than 250 μm, the machinability of the wear-resistant iron-based sintered alloy may decrease because the particle size is too large.

2. Regarding the second hard particles The second hard particles, like the first hard particles, are blended as a raw material in the sintered alloy and have high hardness with respect to the iron particles and the iron-based matrix of the sintered alloy. The second hard particles dramatically increase the hardness of the sintered alloy with a small amount of addition to suppress the plastic deformation of the iron-based matrix of the sintered alloy, and as a result, the adhesive wear of the sintered alloy is prevented. It is a particle intended to be reduced.

The second hard particles are particles made of an Fe—Mo alloy, and when the second hard particles are 100% by mass, Mo: 60 to 70% by mass, Si: 2.0% by mass or less, and the balance is Fe. And unavoidable impurities. The hardness of the second hard particles before sintering is preferably in the range of Hv600 to 1600.

The second hard particles are produced by pulverizing a solidified body obtained by solidifying a molten metal by mechanical pulverization. Further, like the first hard particles, it may be produced by gas atomization treatment, water atomization treatment or the like.

2-1. Mo: 60-70% by mass
Of the composition of the second hard particles, Mo can generate C and Mo carbides of carbon powder at the time of sintering to improve the hardness and wear resistance of the second hard particles. Further, Mo can oxidize solid-solved Mo and Mo carbides to form a Mo oxide film in a high-temperature use environment, and can obtain good solid lubricity in a sintered alloy. Further, by precipitating molybdenum carbides at the grain boundaries of the iron-based matrix during sintering, it is possible to suppress plastic deformation of the iron-based matrix during use and suppress adhesion wear.

Here, if the Mo content is less than 60% by mass, it is difficult to suppress the plastic deformation of the iron-based matrix due to the molybdenum carbide described above, and the adhesive wear resistance is lowered. On the other hand, if the Mo content exceeds 70% by mass, it is difficult to produce by the pulverization method, and the yield is lowered.

2-2. Si: 2.0% by mass or less When Si is contained in the composition of the second hard particles, the second hard particles can be easily produced by the pulverization method. Here, if the Si content exceeds 2.0% by mass, the hardness of the second hard particles becomes high, the moldability to the molded product is hindered, and the density of the sintered alloy is only lowered. Not only that, the machinability of the sintered alloy is also reduced.

2-3. Particle size of the second hard particles The particle size of the second hard particles can be appropriately selected depending on the use and type of the sintered alloy, but the particle size (maximum particle size) of the second hard particles is 100 μm or less. It is preferably in the range, more preferably 75 μm or less. As a result, the second hard particles can be uniformly dispersed by the matrix, and the hardness of the sintered alloy can be increased. Here, when the second hard particles contain hard particles having a particle size of more than 100 μm, the machinability of the sintered alloy may decrease because the particle size is too large. The particle size of the second hard particles is preferably 1 μm or more from the viewpoint of production.

3. 3. Graphite particles The graphite particles may be either natural graphite or artificial graphite as long as C of the graphite particles can be dissolved and diffused into the iron-based matrix and the hard particles at the time of sintering. May be a mixture of. The particle size of the graphite particles is preferably in the range of 1 to 45 μm. Examples of the powder composed of preferable graphite particles include graphite powder (manufactured by Nippon Graphite: CPB-S).

4. Iron particles The iron particles that serve as the base of the sintered alloy are composed of iron particles containing Fe as a main component. As the powder composed of iron particles, pure iron powder is preferable, but low alloy steel powder is used as long as the moldability at the time of compaction molding is not hindered and the diffusion of elements such as Mn of the first hard particles is not hindered. It may be. Fe-C powder can be adopted as the low alloy steel powder. For example, when the low alloy steel powder is 100% by mass, C: 0.2 to 5% by mass, and the balance is composed of unavoidable impurities and Fe. It is possible to adopt the one having. Further, these powders may be gas atomizing powder, water atomizing powder or reduced powder. The particle size of the iron particles is preferably in the range of 150 μm or less.

5. About the mixing ratio of the mixed powder A mixed powder is prepared so as to contain the first hard powder, the second hard particles, the graphite particles, and the iron particles. The mixed powder contains 5 to 50% by mass of 1 hard particle and 1 of 2 hard particles when the total amount of the first hard particle, the second hard particle, the graphite particle, and the iron particle is 100% by mass. It contains ~ 5% by mass and contains 0.5 to 1.5% by mass of graphite particles.

The mixed powder may consist only of the first hard particles, the second hard particles, the graphite particles, and the iron particles, and other, on the premise that the mechanical strength and abrasion resistance of the obtained sintered alloy are not impaired. The particles may be contained in an amount of several mass%. In this case, if the total amount of the first and second hard particles, the graphite particles, and the iron particles is 95% by mass or more with respect to the mixed powder, the effect can be sufficiently expected. For example, at least one machinability selected from the group consisting of sulfides (eg MnS), oxides (eg CaCO ₃ ), fluorides (eg CaF), nitrides (eg BN), and acid sulfides in mixed powders. It may contain particles for improvement.

Since the first hard particles contain 5 to 50% by mass with respect to the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles, the mechanical strength of the sintered alloy and the absorptive wear resistance Both sexes can be improved.

Here, when the first hard particles are less than 5% by mass with respect to the total amount thereof, as is clear from the experiments of the inventors described later, the effect of the abrasive wear resistance by the first hard particles can be obtained. It cannot be fully exerted.

On the other hand, when the number of the first hard particles exceeds 50% by mass with respect to the total amount thereof, the amount of the first hard particles is too large, and it is difficult to mold the molded product even if an attempt is made to mold the molded product from the mixed powder. Further, since the contact between the first hard particles increases and the portion where the iron particles are sintered decreases, the abrasive wear resistance of the sintered alloy decreases.

Since the second hard particles contain 1 to 5% by mass with respect to the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles, as described above, the iron-based base at the time of use It is possible to suppress plastic deformation and reduce adhesive wear of the sintered alloy.

Here, when the content of the second hard particles is less than 1% by mass with respect to the total amount thereof, the adhesive wear resistance of the sintered alloy is clear from the experiments of the inventors described later. Decreases. On the other hand, when the content of the second hard particles exceeds 5% by mass with respect to the total amount thereof, the machinability of the sintered alloy is lowered.

The graphite particles are contained in an amount of 0.5 to 1.5% by mass based on the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles. 2 C of graphite particles can be solid-dissolved and diffused in the first and second hard particles without melting the hard particles, and a pearlite structure can be secured in the iron-based matrix. As a result, both the mechanical strength and the wear resistance of the sintered alloy can be improved.

Here, when the graphite particles are less than 0.5% by mass with respect to the total amount thereof, the ferrite structure of the iron-based matrix tends to increase, so that the strength of the iron-based matrix itself of the sintered alloy becomes high. It will drop. On the other hand, when the graphite particles exceed 1.5% by mass with respect to the total amount thereof, a cementite structure is precipitated and the machinability of the sintered alloy is lowered.

6. About the method for producing an abrasion-resistant iron-based sintered alloy The mixed powder thus obtained is compactly molded into a molded product for a sintered alloy (molding step). The molded product for a sintered alloy contains first hard particles, second hard particles, graphite particles, and iron particles in the same proportion as the mixed powder.

While diffusing the C of the graphite particles of the sintered alloy molded body into the first and second hard particles and the iron particles, the compacted sintered alloy molded body is sintered to obtain the sintered body. Manufacture (sintering process). At this time, not only the diffusion of iron from the iron-based matrix (iron particles) to the first and second hard particles increases, but also the second hard particles do not contain carbon, so that the carbon of the graphite particles is the second hard particles. It easily diffuses into, and Mo carbide is generated at the grain boundary of the second hard particles, and the hardness of the sintered alloy can be increased.

In the present embodiment, the sintering temperature and the sintering time are adjusted so that the hardness of the first hard particles becomes Hv400 to 600 and the hardness of the second hard particles exceeds Hv600. It is the hardness of the first and second hard particles in the obtained sintered alloy, and these hardnesses are values measured using a Micro Vickers hardness tester having a measurement load of 0.1 kgf. By setting the hardness of the first hard particles in such a range, the wear resistance and machinability of the sintered alloy can be ensured. Here, when the hardness of the first hard particles is less than Hv400, the difference in hardness from the iron-based matrix in which carbon is dissolved becomes small, and the wear resistance of the sintered alloy is lowered. On the other hand, if the hardness of the sintered alloy exceeds Hv600, the machinability of the sintered alloy is lowered.

Further, by setting the hardness of the second hard particles in such a range, the wear resistance of the soft iron-based base can be improved. Here, when the hardness of the second hard particles is less than Hv600, the wear resistance of the sintered alloy is lowered.

The hardness of the first and second hard particles shall be adjusted by appropriately setting the ratio of each component in the above-mentioned content range, the graphite particle content, the sintering temperature, and the sintering time. Can be done. As the sintering temperature, about 1050 to 1250 ° C., particularly about 1100 to 1150 ° C. can be adopted. As the sintering time at the above-mentioned sintering temperature, 30 minutes to 120 minutes, more preferably 45 to 90 minutes can be adopted. The sintered atmosphere may be a non-oxidizing atmosphere such as an inert gas atmosphere, and the non-oxidizing atmosphere may be a nitrogen gas atmosphere, an argon gas atmosphere, or a vacuum atmosphere.

The base of the iron-based sintered alloy obtained by sintering preferably contains a structure containing pearlite in order to secure its hardness, and examples of the structure containing pearlite include a pearlite structure and a pearlite-austenite mixed structure. A pearlite-ferrite mixed structure may be used. In order to ensure wear resistance, it is preferable that the amount of ferrite having low hardness is small.

After producing the sintered body, the sintered body is oxidized by oxidizing a part of the iron of the iron-based matrix derived from the iron particles so as to become _{triiron tetroxide (Fe 3} O _4). Before and after the treatment, the oxidation treatment is performed so that the density difference of the sintered body is 0.05 g / cm ^{3 or more.} This oxidation treatment produces an oxide mainly composed of triiron tetroxide, which increases the mass of the sintered body after the oxidation treatment. Therefore, the larger the density difference, the more triiron tetroxide is produced.

The wear resistance of the sintered alloy can be improved by setting the density difference of the sintered body before and after the oxidation treatment to 0.05 g / cm ^{3 or more.} Here, when the density difference between the sintered bodies before and after the oxidation treatment ^{is less than 0.05 g / cm 3} , the proportion of triiron tetroxide in the sintered alloy is small, and the sintered body is hardened by metal contact with the mating member. Wear wear is promoted. As a result, the wear resistance of the sintered alloy is lowered.

As such an oxidation treatment, for example, by heating the sintered body under a temperature condition of 500 to 600 ° C. for 30 to 90 minutes in a steam atmosphere, the sintered body is within the above-mentioned density difference range. Iron (Fe), which is the base of iron (Fe), can be oxidized to triiron tetroxide (Fe ₃ O ₄ ).

7. Application of wear-resistant iron-based sintered alloy The sintered alloy obtained by the above-mentioned manufacturing method has higher mechanical strength and wear resistance in a high-temperature use environment than the conventional ones. For example, it can be suitably used for a valve system (for example, a valve seat, a valve guide) of an internal combustion engine using compressed natural gas or liquefied petroleum gas as a fuel in a high temperature usage environment, and a wastegate valve of a turbocharger.

For example, when a valve seat for an exhaust valve of an internal combustion engine is formed of a sintered alloy, a wear form in which adhesive wear at the time of contact between the valve seat and the valve and absolute wear at the time of sliding of both appears. Even so, the wear resistance of these valve seats can be further improved as compared with the conventional ones. In particular, in a usage environment using compressed natural gas or liquefied petroleum gas as fuel, it is difficult for a Mo oxide film to be formed, but even in such an environment, the adhesive wear can be reduced.

Hereinafter, examples in which the present invention has been specifically carried out will be described together with comparative examples.
[Example 1: Optimal addition amount of first hard particles]
The sintered alloy according to Example 1 was produced by the production method shown below. As the first hard particles, Mo: 40% by mass, Ni: 30% by mass, Co: 20% by mass, Mn: 5% by mass, Si: 0.8% by mass, C: 1.2% by mass, and the balance is Fe. Hard particles (manufactured by Daido Special Steel) produced by the gas atomization method were prepared from an alloy of unavoidable impurities (that is, Fe-40Mo-30Ni-20Co-5Mn-0.8Si-1.2C). The first hard particles were classified in the range of 44 μm to 250 μm using a sieve conforming to JIS standard Z8801. The "particle size" referred to in the present specification is a value classified by this method.

As the second hard particles, second hard particles (manufactured by Kinsei Matek) prepared by a pulverization method were prepared from an Fe-65 alloy in which Mo: 65% by mass and the balance was Fe and unavoidable impurities. The second hard particles were classified to 75 μm or less.

Next, graphite powder made of graphite particles (manufactured by Nippon Graphite Industry: CPB-S) and reduced iron powder made of pure iron particles (JEF steel: JIP255M-90) were prepared. As described above, the ratio of the first hard particles is 40% by mass, the second hard particles is 3% by mass, the graphite particles are 1.1% by mass, and the rest are iron particles (specifically, 55.9% by mass). The mixture was mixed in a V-type mixer for 30 minutes. This gave a mixed powder.

Next, using a molding die, the obtained mixed powder was compactally molded into a ring-shaped test piece with a pressing force of 588 MPa to form a molded product for a sintered alloy (compact molded product). The powder compact was sintered in an inert atmosphere (nitrogen gas atmosphere) at 1120 ° C. for 60 minutes to obtain a sintered body. This sintered body was subjected to an oxidation treatment by heating it in a steam atmosphere at 550 ° C. for 50 minutes to form a test piece of the sintered alloy (valve sheet) according to Example 1.

[Examples 2 and 3: Optimal addition amount of first hard particles]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. Examples 2 and 3 are examples for evaluating the optimum addition amount of the first hard particles. The difference between Examples 2 and 3 from Example 1 is that the first hard particles were sequentially added at a ratio of 5% by mass and 50% by mass with respect to the entire mixed powder, as shown in Table 1. Is.

[Examples 4 and 5: Optimal amount of second hard particles added]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. Examples 4 and 5 are examples for evaluating the optimum addition amount of the second hard particles. The difference between Examples 4 and 5 from Example 1 is that, as shown in Table 1, the second hard particles were sequentially added at a ratio of 1% by mass and 5% by mass with respect to the entire mixed powder. Is.

[Examples 6 and 7: Optimal addition amount of graphite particles]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. Examples 6 and 7 are examples for evaluating the optimum addition amount of graphite particles. The difference between Examples 6 and 7 from Example 2 is that graphite particles are sequentially added at a ratio of 0.5% by mass and 1.5% by mass with respect to the entire mixed powder, as shown in Table 1. That is the point.

[Example 8: Hardness of first hard particles]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. The difference between Example 8 and Example 1 is that the hardness of the first hard particles of the sintered body after sintering is lowered by lowering the sintering temperature than that of Example 1 (Table 1). See, Hv545).

[Comparative Examples 1 and 2: Comparative Example of Optimal Addition Amount of First Hard Particles]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. Comparative Examples 1 and 2 are comparative examples for evaluating the optimum addition amount of the first hard particles. The difference between Comparative Examples 1 and 2 and Example 1 is that, as shown in Table 1, the first hard particles were sequentially added to 0% by mass (that is, not added) and 60% by mass with respect to the entire mixed powder. It is a point added in the ratio of. In Comparative Example 2, the mixed powder could not be molded into a molded product.

[Comparative Examples 3 and 4: Comparative Example of Optimal Addition Amount of Second Hard Particles]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. Comparative Examples 3 and 4 are comparative examples for evaluating the optimum addition amount of the second hard particles. The difference between Comparative Examples 3 and 4 from that of Example 1 is that the second hard particles were sequentially added at a ratio of 0% by mass and 10% by mass with respect to the entire mixed powder, as shown in Table 1. Further, in Comparative Example 3, graphite particles were added at a ratio of 0.8% by mass.

[Comparative Examples 5 and 6: Comparative Example of Optimal Addition Amount of Graphite Particles]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. Comparative Examples 5 and 6 are comparative examples for evaluating the optimum addition amount of graphite particles. The difference between Comparative Examples 5 and 6 and Example 1 is that graphite particles are sequentially added at a ratio of 0.4% by mass and 1.6% by mass with respect to the entire mixed powder, as shown in Table 1. That is the point.

[Comparative Example 7: Comparative Example of Density Difference of Sintered Body]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. In Comparative Example 7, the molding pressure at the time of powder compaction was increased as compared with the case of Example 1, and the density before the oxidation treatment was increased. As a result, by reducing the pores inside the sintered body, the formation of oxides was suppressed and the increase in the density of the sintered body after the oxidation treatment was reduced (that is, the density difference was reduced).

[Comparative Example 8: Comparative Example of Hardness of First Hard Particle]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. The difference between Comparative Example 8 and Example 1 is that the hardness of the first hard particles of the sintered body after sintering was increased by raising the sintering temperature higher than that of Example 1 (Table 1). See, Hv650).

[Comparative Example 9]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. The difference from Example 1 is that the first hard particles are particles made of a Co-40Mo-5Cr-0.9C alloy corresponding to the hard particles described in JP-A-2004-156101, and the second hard particles are used. The point that is not added and the point that the sintered body is not oxidized after sintering.

<Hardness test>
The test pieces of the sintered alloy according to Examples 1 to 8 and Comparative Examples 1 to 9 were measured using a micro Vickers hardness tester having a measurement load of 0.1 kgf for the first hard particles and the second hard particles. The results are shown in Table 1.

<Density measurement test>
For the test pieces of the sintered alloy according to Examples 1 to 8 and Comparative Examples 1 and 3, each mass before and after the oxidation treatment was measured, and the measured mass was divided by the volume calculated from the dimensions of the test piece. Then, the density of the test piece (sintered body) before and after the oxidation treatment was calculated. Furthermore, the density difference of the test piece (sintered body) before and after the oxidation treatment was calculated. The results are shown in Table 1.

<Abrasion test>
Using the testing machine shown in FIG. 1, wear tests were performed on the test pieces of the sintered alloys according to Examples 1 to 8 and Comparative Examples 1 to 3 to 9, and their wear resistance was evaluated. In this test, as shown in FIG. 1, a propane gas burner 10 is used as a heating source, and a sliding portion between the ring-shaped valve seat 12 made of the sintered alloy produced as described above and the valve face 14 of the valve 13. Was used as a propane gas combustion atmosphere. The valve face 14 is EV12 (SEA standard) subjected to soft nitriding treatment. The temperature of the valve seat 12 is controlled to 250 ° C., a load of 25 kgf is applied at the time of contact between the valve seat 12 and the valve face 14 by the spring 16, and the valve seat 12 and the valve face 14 are subjected to a ratio of 3250 times / minute. Was brought into contact with each other and a wear test was conducted for 8 hours.

The total amount of axial wear depth of the valve seat 12 and the valve face 14 after the wear test was measured as the wear test wear amount, and the value divided by the value of Comparative Example 9 was calculated as the wear test wear amount ratio. The results are shown in Table 1.

In FIGS. 3 to 7 (a), the horizontal axis is, in order, the amount of the first hard particles added, the amount of the second hard particles added, the amount of graphite particles added, the hardness of the first hard particles, and the firing. As the density difference of the solids, the results of the corresponding wear test wear amount ratios of Examples 1 to 8 and Comparative Examples 1 and 3 to 9 were plotted.

Further, the surfaces of the test pieces according to Example 1 and Comparative Example 7 after the wear test after the wear test were observed with a microscope. The results are shown in FIGS. 8 (a) and 8 (b). FIG. 8A is a surface photograph of the test piece according to Example 1 after the wear test, and FIG. 8B is a surface photograph of the test piece according to Comparative Example 7 after the wear test.

The test pieces of Example 1, Comparative Example 5, and Comparative Example 6 before the abrasion test were etched with nital, and the structure of the sintered alloy was observed with a microscope. The results are shown in FIGS. 9 (a) to 9 (c). 9 (a) is a tissue photograph of the test piece according to Example 1, FIG. 9 (b) is a tissue photograph of the test piece according to Comparative Example 5, and FIG. 9 (c) is a tissue photograph of the test piece according to Comparative Example 6. It is a tissue photograph of the said test piece.

<Machinability test>
Using the testing machine shown in FIG. 2, a machinability test was performed on the test pieces of the sintered alloy according to Examples 1 to 8 and Comparative Examples 1 to 3 to 9, and the machinability of these test pieces was evaluated. In this test, six test pieces 20 having an outer diameter of 30 mm, an inner diameter of 22 mm, and a total length of 9 mm were prepared for each of Examples 1 to 8 and Comparative Examples 1 and 3 to 9. A test piece 20 rotated at a rotation speed of 970 rpm with a carbide tool (cutting tool) 30 coated with titanium nitride aluminum using an NC lathe, with a depth of cut of 0.3 mm, a feed of 0.08 mm / rev, and a cutting distance of 320 m. , Wet traverse cutting. Then, the maximum wear depth of the flank of the tool 30 was measured as the tool wear amount by an optical microscope, and the value divided by the value of Comparative Example 9 was calculated as the tool wear amount ratio. The results are shown in Table 1.

In FIGS. 3 to 7 (b), the horizontal axis is, in order, the amount of the first hard particles added, the amount of the second hard particles added, the amount of graphite particles added, the hardness of the first hard particles, and the firing. As the density difference of the particles, the results of the corresponding tool wear ratios of Examples 1, 3 to 8 and Comparative Examples 1 to 9 were plotted.

(Result 1: Optimal amount of first hard particles added)
As shown in FIG. 3A, the wear test wear ratio of Examples 1 to 3 was smaller than that of Comparative Examples 1 and 9. The wear test wear amount ratio decreased in the order of Example 2, Example 1, and Example 3. From this, it is considered that the abstract wear resistance of the sintered alloy is improved by adding the first hard particles. However, in Comparative Example 2, it can be said that the moldability of the molded product was hindered because the first hard particles were added too much. From the above points, the optimum amount of the first hard particles added is 5 to 50% by mass with respect to the entire mixed powder.

As shown in FIG. 3B, the tool wear ratio of Examples 1 to 3 is smaller than that of Comparative Example 9, and the tool wear is in the order of Example 2, Example 1, and Example 3. The quantity ratio increased. However, it is considered that when the first hard particles are further added as compared with Example 3, the machinability of the sintered alloy is lowered and the tool wear ratio is increased.

(Result 2: Optimal amount of second hard particles added)
As shown in FIG. 4A, the wear test wear ratios of Examples 1, 4 and 5 and Comparative Example 4 were smaller than those of Comparative Examples 3 and 9. However, as shown in FIG. 4B, the tool wear ratio of Comparative Example 4 was larger than that of Examples 1, 4 and 5. When observing the surface of the test piece after the wear test, in Comparative Example 3, there were more scratches due to adhesive wear than in the others.

From this, the second hard particles improve the hardness of the sintered alloy after sintering, thereby suppressing the plastic deformation of the iron-based matrix of the sintered alloy during use, and the adhesive wear of the sintered alloy. Is considered to be reduced. Specifically, unlike the first hard particles, the second hard particles do not contain Ni, Co, etc., so that the iron-based matrix around them can be hardened more than the first hard particles, and sintering is possible. Occasionally, by precipitating molybdenum carbides at the grain boundaries of the iron-based matrix, it is considered that the hardness of the iron-based matrix after sintering is improved.

From the above, if the addition of the second hard particles is too small, the surface of the sintered alloy after the abrasion test is easily scraped off. On the other hand, as in Comparative Example 4, if the second hard particles are added too much, the sintered alloy after sintering becomes too hard, and it is considered that the machinability is lowered. From the above results, the optimum amount of the second hard particles added is 1 to 5% by mass with respect to the entire mixed powder.

(Result 3: Optimal addition amount of graphite particles)
As shown in FIG. 5A, the wear test wear ratios of Examples 1, 6 and 7 and Comparative Example 6 were smaller than those of Comparative Examples 5 and 9. However, as shown in FIG. 5B, the tool wear ratio of Comparative Example 6 was larger than that of Examples 1, 6 and 7.

As shown in FIG. 9A, a pearlite structure was formed in the structure of the sintered alloy shown in Example 1, but as shown in FIG. 9C, the sintered alloy shown in Comparative Example 6 was formed. A cementite structure was formed in the structure of the above due to the increase in the amount of graphite particles. Therefore, it is considered that the tool wear ratio of Comparative Example 6 was larger than that of Examples 1, 6 and 7.

On the other hand, as shown in FIG. 9B, the structure of the sintered alloy shown in Comparative Example 5 has a structure centered on ferrite. Therefore, the wear test wear ratio of Comparative Example 5 is the same as that of Example 1. It is considered that the size was larger than that of 6, 7 and Comparative Example 6. From this, the optimum amount of graphite particles that can secure a pearlite structure in the iron-based matrix after sintering is 0.5 to 1.5% by mass with respect to the entire mixed powder.

(Result 4: Optimal hardness of the first hard particle)
As shown in FIG. 6A, the wear test wear ratios of Examples 1, 3, 5, 8 and Comparative Example 8 were smaller than those of Comparative Example 9. However, as shown in FIG. 6B, the tool wear ratio of Comparative Example 8 was larger than that of Examples 1, 3, 5, and 8.

In Comparative Example 9, since the hardness of the first hard particles was higher than that of Examples 1, 3, 5, 8 and Comparative Example 8, the mating material was worn more, and the wear test wear amount of Example 9 was increased. It is considered that the ratio was larger than that of others. On the other hand, in Examples 1, 3, 5, and 8, the hardness of the first hard particles was lower than that of Comparative Example 8 and was Hv600 or less, so that the amount of tool wear in Examples 1, 3, 5, and 8 was reduced. It is considered that the ratio was smaller than that of Comparative Example 8. In Examples 1, 3, 5, and 8, it can be said that the wear resistance is ensured because the hardness of the first hard particles is Hv400 or more.

From this, the hardness of the first hard particles after sintering is preferably in the range of Hv400 to 600. From the viewpoint of improving the wear resistance of the iron-based matrix, the hardness of the second hard particles is higher than that of the first hard particles, assuming the above-mentioned range of the amount of addition. It is necessary that it is at least Hv600 or higher.

(Result 5: Optimal density difference of sintered body)
As shown in FIG. 7A, the wear test wear ratio of Examples 1 to 8 was smaller than that of Comparative Examples 7 and 9. As shown in FIG. 7B, the tool wear ratio of Comparative Example 9 was larger than that of Examples 1 to 8 and Comparative Example 7.

In Comparative Example 7, since the density difference of the sintered body was ^{less than 0.05 g / cm 3} before and after the oxidation treatment, the sintered body was tetraoxidized as compared with the sintered bodies of Examples 1 to 8. The amount of oxide mainly composed of triiron is small. Therefore, it is considered that the metal contact with the mating material was promoted, and as shown in FIG. 8B, the test piece (sintered body) of Comparative Example 7 was promoted to adhere and wear with the mating material. On the other hand, in Examples 1 to 8, there was almost no such adhesive wear (see, for example, FIG. 8A in Example 1), so that the wear resistance of the sintered alloy is higher than that of Comparative Example 7. it is conceivable that. For this reason, it is necessary to perform the oxidation treatment before and after the oxidation treatment so that the density difference of the sintered body is 0.05 g / cm ^{3 or more.}

[Example 9: Optimal particle size of the second hard particle]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. Example 9 is an example for evaluating the optimum particle size of the second hard particles. The difference between Example 9 and Example 1 is that, as the second hard particles, the second hard particles classified so that the particle size (particle size) exceeds 75 μm and is in the range of 100 μm or less are used. ..

[Comparative Example 10: Comparative Example of Optimal Particle Size of Second Hard Particle]
A test piece of a sintered alloy was prepared in the same manner as in Example 1. Comparative Example 10 is a comparative example for evaluating the optimum particle size of the second hard particles. The difference between Comparative Example 10 and Example 1 is that the second hard particles classified into a range of more than 100 μm and 150 μm or less are used as the second hard particles. The test piece according to Comparative Example 10 is a sintered alloy included in the scope of the present invention, and is designated as Comparative Example 10 for convenience in order to compare with Examples 1 and 9.

Similar to Example 1, the test pieces of Example 9 and Comparative Example 10 were subjected to a wear test and a machinability test, and the wear test wear amount and the tool wear amount were measured. This result is shown in FIGS. 10 (a) and 10 (b) together with the result of Example 1 described above.

FIG. 10A is a graph showing the results of the wear test wear amount ratio in Examples 1 and 9 and Comparative Example 10, and FIG. 10B is the tool wear in Examples 1 and 9 and Comparative Example 10. It is a graph which showed the result of the quantity ratio.

(Result 6: Optimal particle size of the second hard particle)
As shown in FIG. 10A, the wear test wear amount ratios of Examples 1 and 9 and Comparative Example 10 were about the same. However, as shown in FIG. 10B, the tool wear ratio of Examples 1 and 9 was smaller than that of Comparative Example 10, and the tool wear ratio of Example 1 was the smallest as compared with the others. This is because in Comparative Example 10, the particle size of the second hard particles is too large, so that the machinability of the test piece (sintered body) may decrease. From this result, the particle size (maximum particle size) of the second hard particles is preferably in the range of 100 μm or less, and more preferably, the particle size (maximum particle size) of the second hard particles is in the range of 75 μm or less. It is in.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various designs are designed without departing from the spirit of the present invention described in the claims. You can make changes.

Claims (2)

A molding process for compact molding a molded product for a sintered alloy from a mixed powder containing hard particles, graphite particles, and iron particles.
Abrasion resistant iron group including a sintering step of sintering a molded product for a sintered alloy while diffusing C of the graphite particles of the molded product for a sintered alloy into the hard particles and the iron particles. It is a manufacturing method of sintered alloy.
The hard particles include first hard particles and second hard particles.
The first hard particles are Mo: 20 to 70% by mass, Ni: 5 to 40% by mass, Co: 5 to 40% by mass, Mn: 1 to 20 when the first hard particles are 100% by mass. Mass%, Si: 0.5 to 4.0 mass%, C: 0.5 to 3.0 mass%, the balance consists of Fe and unavoidable impurities.
The second hard particles are composed of Mo: 60 to 70% by mass, Si: 2.0% by mass or less, and the balance of Fe and unavoidable impurities when the second hard particles are 100% by mass.
The mixed powder contains 5 to 50% by mass of the first hard particles when the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles is 100% by mass. , The second hard particles are contained in an amount of 1 to 5% by mass, and the graphite particles are contained in an amount of 0.5 to 1.5% by mass.
In the sintering step, sintering is performed so that the hardness of the first hard particles becomes Hv400 to 600 and the hardness of the second hard particles exceeds Hv600.
After the sintering step, the sintered body sintered from the sintered alloy molded body is oxidized so that a part of iron in the iron-based matrix derived from the iron particles becomes triiron tetroxide. A method for producing an abrasion-resistant iron-based sintered alloy, which comprises performing the oxidation treatment before and after the oxidation treatment so that the density difference of the sintered body is 0.05 g / cm ^{3 or more.} .. The method for producing a wear-resistant iron-based sintered alloy according to claim 1, wherein the particle size of the second hard particles is in the range of 100 μm or less.