CN108690931B

CN108690931B - Method for producing wear-resistant iron-based sintered alloy

Info

Publication number: CN108690931B
Application number: CN201810290965.7A
Authority: CN
Inventors: 筱原伸幸; 鸭雄贵; 植田义久; 米田贵则; 中村竹志
Original assignee: Fine Sinter Co Ltd; Toyota Motor Corp
Current assignee: Fine Sinter Co Ltd; Toyota Motor Corp
Priority date: 2017-04-04
Filing date: 2018-04-03
Publication date: 2020-10-30
Anticipated expiration: 2038-04-03
Also published as: US20180282844A1; JP2018178143A; CN108690931A; BR102018006453A2; JP6842345B2

Abstract

A wear-resistant iron-based sintered alloy is manufactured from a mixed powder including first hard particles, second hard particles, graphite particles, and iron particles. The first hard particles are Fe-Mo-Ni-Co-Mn-Si-C alloy particles. The second hard particles are Fe-Mo-Si alloy particles. The mixed powder includes 5 to 50 mass% of the first hard particles, 1 to 8 mass% of the second hard particles, and 0.5 to 1.5 mass% of the graphite particles, when the total amount of the above particles is set to 100 mass%. In the sintering process, sintering is performed so that the hardness of the first hard particles becomes 400 to 600Hv and the hardness of the second hard particles exceeds 600 Hv. Then, oxidation treatment was performed so that the difference in density between the sintered bodies before and after the oxidation treatment became 0.05g/cm³Or larger.

Description

Method for producing wear-resistant iron-based sintered alloy

Background

1. Field of the invention

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

2. Description of the prior art

The iron-based sintered alloy can be applied to valve seats and the like. Hard particles may be included in the sintered alloy to further improve wear resistance. When the hard particles are included, graphite particles and iron particles are mixed into the hard particles to form a powder, and the mixed powder is compression-molded into a compact for sintered alloy. The shaped body for a sintered alloy is then generally heated and is thus sintered and becomes a sintered alloy.

As a method for producing such a sintered alloy, there has been proposed a method for producing a wear-resistant iron-based sintered alloy in which a mixed powder in which hard particles, graphite particles and iron particles are mixed is compression-molded into a molded body for sintered alloy, and the molded body for sintered alloy is sintered and C of the graphite particles in the molded body for sintered alloy is diffused into the hard particles and the iron particles (refer to, for example, japanese unexamined patent application publication No. 2004-156101(JP 2004-156101A)).

Here, the hard particles include Mo: 20 to 70 mass%, C: 0.2 to 3 mass% and Mn: 1 to 15 mass%, the balance including inevitable impurities and Co. The mixed powder includes 10 to 60 mass% of the hard particles and 0.2 to 2 mass% of the graphite particles when the total amount of the hard particles, the graphite particles, and the iron particles is set to 100 mass%. Since the hard particles are dispersed in such a sintered alloy, abrasion can be prevented.

Disclosure of Invention

However, the matrix material to which the hard particles are bonded in the wear-resistant iron-based sintered alloy manufactured in the manufacturing method described in JP 2004-156101A is soft because the matrix material is an Fe — C material in which C of the graphite particles has diffused into the iron particles. Therefore, when the wear-resistant iron-based sintered alloy and the metallic material of the sliding partner member in contact therewith are in metallic contact with each other, the contact surface of the wear-resistant iron-based sintered alloy may be plastically deformed, and adhesive wear is liable to occur on the contact surface. In order to prevent such a problem, it is necessary to increase the hardness of the wear-resistant iron-based sintered alloy. However, there is a risk that the machinability of the wear-resistant iron-based sintered alloy is thus deteriorated, and it is difficult to achieve both the adhesive wear resistance and the machinability.

The present invention provides a method for producing a wear-resistant iron-based sintered alloy, by which machinability can be ensured while preventing adhesive wear.

The present inventors expect that adhesive wear of the contact surfaces will be accelerated when the iron matrix of the wear-resistant iron-based sintered alloy is plastically deformed as described above. In this regard, the inventors studied to add other hard particles by which plastic deformation of the iron matrix can be prevented, in addition to the hard particles by which frictional wear has been prevented so far. Accordingly, the inventors focused on molybdenum as a main component of the hard particles, and found that when molybdenum carbide and an iron-molybdenum intermetallic compound precipitated during sintering are dispersed in an iron matrix, plastic deformation of the iron matrix can be controlled. In addition to this, the inventors have obtained new findings: when a part of the iron matrix derived from the iron particles is oxidized to ferroferric oxide, its wear resistance can be improved without deteriorating the machinability of the sintered alloy.

One aspect of the present invention relates to a method of manufacturing a wear-resistant iron-based sintered alloy, comprising: a molding step of compression-molding a mixed powder including hard particles, graphite particles, and iron particles into a sintered alloy compact; and a sintering step of sintering the compact for sintered alloy and diffusing C of graphite particles in the compact for sintered alloy into the hard particles and the iron particles, wherein the hard particles include first hard particles and second hard particles, wherein the first hard particles include Mo: 20 to 70 mass%, Ni: 5 to 40 mass%, Co: 5 to 40 mass%, Mn: 1 to 20 mass%, Si: 0.5 mass% to 4.0 mass% and C: 0.5 to 3.0 mass%, the balance including Fe and inevitable impurities, wherein the second hard particles include Mo: 60 to 70 mass% and Si: 2.0 mass% or less, the balance including Fe and unavoidable impurities, wherein the mixed powder includes 5 to 50 mass% of the first hard particles, 1 to 5 mass% of the second hard particles, and 0.5 to 1.5 mass% of the graphite particles when the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles is set to 100 mass%, and wherein in the sintering process, sintering is performed so that the hardness of the first hard particles becomes 400 to 600Hv and the hardness of the second hard particles exceeds 600Hv, after the sintering process, an oxidation treatment is performed on the sintered body sintered by the sintered alloy compact so that a part of iron contained in an iron matrix derived from the iron particles becomes ferroferric oxide, and the oxidation treatment is performed so that the density of the sintered body before the oxidation treatment and the density of the sintered body after the oxidation treatment are both performed The difference therebetween became 0.05g/cm³Or more.

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

Brief description of the drawings

Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, wherein like reference numerals represent like elements, and wherein:

FIG. 1 is a schematic conceptual view of a wear test used in examples and comparative examples;

FIG. 2 is a schematic conceptual view of a machinability test used in examples and comparative examples;

fig. 3A is a graph showing the results of the wear test wear amount ratio with respect to the addition amount of the first hard particles in examples 1 to 3 and comparative examples 1 and 9;

fig. 3B is a graph showing the results of the tool wear amount ratio with respect to the addition amount of the first hard particles in examples 1 to 3 and comparative examples 1 and 9;

fig. 4A is a graph showing the results of the abrasion test abrasion amount ratio with respect to the addition amount of the second hard particles in examples 1, 4, and 5 and comparative examples 3, 4, and 9;

fig. 4B is a graph showing the results of the tool wear amount ratio with respect to the addition amount of the second hard particles in examples 1, 4, and 5 and comparative examples 3, 4, and 9;

FIG. 5A is a graph showing the results of the abrasion test abrasion loss ratio with respect to the addition amount of graphite particles in examples 1, 6 and 7 and comparative examples 5, 6 and 9;

fig. 5B is a graph showing the results of the tool wear amount ratio with respect to the addition amount of graphite particles in examples 1, 6, and 7 and comparative examples 5, 6, and 9;

fig. 6A is a graph showing the results of the abrasion test abrasion amount ratio with respect to the hardness of the first hard particles in examples 1, 3, 5, and 8 and comparative examples 8 and 9;

fig. 6B is a graph showing the results of the tool wear amount ratio with respect to the hardness of the first hard particles in examples 1, 3, 5, and 8 and comparative examples 8 and 9;

FIG. 7A is a graph showing the results of the wear test wear amount ratio with respect to the density difference of sintered bodies in examples 1 to 8 and comparative examples 7 and 9;

FIG. 7B is a graph showing the results of the tool wear amount ratio with respect to the density difference of sintered bodies in examples 1 to 8 and comparative examples 7 and 9;

FIG. 8A is a surface photograph of a test piece according to example 1 after a wear test;

FIG. 8B is a surface photograph of a test piece according to comparative example 7 after a wear test;

FIG. 9A is a photograph of the structure of a test piece according to example 1;

FIG. 9B is a photograph of the structure of a test piece according to comparative example 5;

FIG. 9C is a photograph of the structure of a test piece according to comparative example 6;

FIG. 10A is a graph showing the results of the abrasion amount ratio in the abrasion test in examples 1 and 9 and comparative example 10; and

fig. 10B is a graph showing the results of the tool wear amount ratio in examples 1 and 9 and comparative example 10.

Detailed description of the embodiments

Embodiments of the present invention will be described in detail below. The compact for sintered alloy according to the present embodiment (hereinafter referred to as compact) is obtained by compression molding a mixed powder including first and second hard particles, graphite particles, and iron particles, which will be described below. A wear-resistant iron-based sintered alloy (hereinafter referred to as a sintered alloy) is obtained by sintering the molded body and diffusing C of the graphite particles into the hard particles and the iron particles. Hard particles, a molded body obtained by compression molding of mixed powder in which hard particles are mixed, and a sintered alloy obtained by sintering the molded body will be described below.

1. First hard particles

The first hard particles are particles that are mixed as a raw material into the sintered alloy and have high hardness with respect to the iron particles and the iron matrix of the sintered alloy, and thus prevent frictional wear of the sintered alloy.

The first hard particles are particles made of a Co-Mo-Ni-Fe-Mn-Si-C alloy. Specifically, when the amount of the first hard particles is set to 100 mass%, the first hard particles include Mo: 20 to 70 mass%, Ni: 5 to 40 mass%, Co: 5 to 40 mass%, Mn: 1 to 20 mass%, Si: 0.5 to 4.0 mass%, and C: 0.5 to 3.0 mass%, the balance including Fe and inevitable impurities. In addition, Cr may be added to the first hard particles in a range of 10 mass% or less, if necessary. The hardness of the first hard particles before sintering is preferably in the range of 400 to 600 Hv.

The first hard particles may be produced by preparing a molten metal in which the above compositions are mixed together in the above ratio and subjecting the molten metal to an atomization process. In addition, as another method, a solidified body in which the molten metal has solidified may be formed into a powder by mechanical grinding. As the atomization treatment, gas atomization treatment or water atomization treatment may be performed. However, the gas atomization treatment is more preferable in view of sintering property and the like, because round particles are obtained.

Here, the lower and upper limit values of the above hard particle composition may be appropriately changed in consideration of the reasons for limitation to be described below and hardness, solid lubricity, adhesiveness, and cost within such ranges, according to the degree of importance of the characteristics of the application part.

1-1. Mo: 20 to 70% by mass

In the composition of the first hard particles, Mo may generate Mo carbide together with C of the carbon powder during sintering and improve hardness and wear resistance of the first hard particles. In addition, as for Mo, since Mo and Mo carbide in a solid solution state are oxidized to form a Mo oxide film under a high-temperature use environment, favorable solid lubricity for the sintered alloy can be obtained.

Here, when the Mo content is less than 20 mass%, not only the amount of Mo carbide generated is reduced, but also the oxidation initiation temperature of the first hard particles is increased, and the generation of Mo oxide under a high-temperature use environment is prevented. Therefore, the solid lubricity of the obtained sintered alloy is insufficient, and the frictional wear resistance thereof is reduced. On the other hand, when the Mo content exceeds 70 mass%, not only it is difficult to produce the first hard particles using the atomization method, but also the adhesion between the hard particles and the iron matrix is reduced. More preferably, the Mo content is 30 to 50 mass%.

1-2. Ni: 5 to 40% by mass

In the composition of the first hard particles, Ni may enlarge the austenite structure of the matrix of the first hard particles and improve the toughness thereof. In addition, Ni may increase the solid solution amount of Mo of the first hard particles and improve the wear resistance of the first hard particles.

In addition, Ni diffuses into the iron matrix of the sintered alloy during sintering, which can expand the austenite structure of the iron matrix, improve the toughness of the sintered alloy, increase the solid solution amount of Mo in the iron matrix, and improve wear resistance.

Here, when the Ni content is less than 5 mass%, it is difficult to expect the above effect of Ni. On the other hand, when the Ni content exceeds 40 mass%, although the above effect of Ni is maximized, the cost of the first hard particles increases. More preferably, the Ni content is 20 to 40 mass%.

1-3. Co: 5 to 40% by mass

In the composition of the first hard particles, Co may enlarge the austenite structure in the iron matrix of the sintered alloy and the matrix of the first hard particles, and improve the hardness of the first hard particles, similar to Ni.

Here, when the Co content is less than 5 mass%, it is difficult to expect the above effect of Ni. On the other hand, when the Co content exceeds 40 mass%, although the above effect of Co is maximized, the cost of the first hard particles increases. More preferably, the Co content is 10 to 30 mass%.

1-4. Mn: 1 to 20% by mass

In the composition of the first hard particles, since Mn is effectively diffused from the first hard particles into the iron matrix of the sintered alloy during sintering, the adhesion between the first hard particles and the iron matrix can be improved. In addition, Mn may enlarge the austenite structure in the iron matrix of the sintered alloy and the matrix of the first hard particles.

Here, when the Mn content is less than 1 mass%, since the amount of Mn diffused into the iron matrix is small, the adhesion between the hard particles and the iron matrix is reduced. The mechanical strength of the sintered alloy thus obtained is reduced. On the other hand, when the Mn content exceeds 20 mass%, the above effect of Mn is maximized. More preferably, the Mn content is 2 to 8 mass%.

1-5. Si: 0.5 to 4.0 mass%

In the composition of the first hard particles, Si may improve adhesion between the first hard particles and the Mo oxide film. Here, when the Si content is less than 0.5 mass%, it is difficult to expect the above effect of Si. On the other hand, when the Si content exceeds 4.0 mass%, the formability of the molded body deteriorates and the density of the sintered alloy decreases. More preferably, the Si content is 0.5 to 2 mass%.

1-6. C: 0.5 to 3.0 mass%

In the composition of the first hard particles, C combines with Mo to form Mo carbide, and may improve hardness and wear resistance of the first hard particles. Here, when the C content is less than 0.5 mass%, the wear-resistant effect is insufficient. On the other hand, when the C content exceeds 3.0 mass%, the formability of the formed body is deteriorated and the density of the sintered alloy is decreased. More preferably, the C content is 0.5 to 2 mass%.

1-7. Cr: 10% by mass

Hereinafter, in the composition of the first hard particles, Cr may prevent excessive oxidation of Mo during use. For example, when the use environment temperature of the sintered alloy is high, the amount of the Mo oxide film generated in the first hard particles is increased, and the Mo oxide film is peeled off from the first hard particles, the addition of Cr is effective.

Here, when the Cr content exceeds 10 mass%, the formation of the Mo oxide film in the first hard particles is prevented too much. Here, in a corrosive environment such as an alcohol fuel environment, Cr needs to be added in order to improve corrosion resistance. On the other hand, in an environment where adhesive wear may occur, in order to accelerate oxidation, the Cr content needs to be reduced.

1-8 particle size of first hard particles

The particle size of the first hard particles may be appropriately selected according to the application, type, and the like of the sintered alloy. However, the particle diameter of the first hard particles is preferably in the range of 44 μm to 250 μm, and more preferably in the range of 44 μm to 105 μm.

Here, when hard particles having a particle diameter of less than 44 μm are included as the first hard particles, the wear resistance of the wear-resistant iron-based sintered alloy may be reduced because the particle diameter is too small. On the other hand, when hard particles having a particle diameter of more than 250 μm are included as the first hard particles, the machinability of the wear-resistant iron-based sintered alloy may be deteriorated because the particle diameter is too large.

2. Second hard particles

Like the first hard particles, the second hard particles are particles that are mixed as a raw material into the sintered alloy and have high hardness with respect to the iron particles and the iron matrix of the sintered alloy. The second hard particles are particles that, when added in a small amount, significantly increase the hardness of the sintered alloy, prevent plastic deformation of the iron matrix of the sintered alloy, and thus reduce adhesive wear of the sintered alloy.

The second hard particles are particles made of an Fe — Mo alloy, and include Mo when the amount of the second hard particles is set to 100 mass%: 60 to 70 mass% and Si: 2.0 mass% or less, and the balance including Fe and inevitable impurities. The hardness of the second hard particles before sintering is preferably in the range of 600 to 1600 Hv.

The second hard particles are produced by forming a solidified body in which the molten metal has solidified into a powder by mechanical grinding. In addition, the second hard particles can be produced by a gas atomization treatment, a water atomization treatment, or the like, as in the first hard particles.

2-1. Mo: 60 to 70% by mass

In the composition of the second hard particles, Mo may generate Mo carbide together with C of the carbon powder during sintering and improve hardness and wear resistance of the second hard particles. In addition, regarding Mo, since Mo in a solid solution state and Mo carbide are oxidized to form a Mo oxide film under a high-temperature use environment, favorable solid lubricity for the sintered alloy can be obtained. In addition, when molybdenum carbide is precipitated at grain boundaries of the iron matrix during sintering, adhesive wear and plastic deformation of the iron matrix during use can be prevented.

Here, when the Mo content is less than 60 mass%, it is difficult to prevent plastic deformation of the iron matrix according to the above molybdenum carbide, and the adhesive wear resistance is lowered. On the other hand, when the Mo content exceeds 70 mass%, it is difficult to produce the second hard particles using the milling method, and the yield thereof is reduced.

2-2. Si: 2.0 mass% or less

When Si is included in the composition of the second hard particles, the second hard particles are easily manufactured using a grinding method. Here, when the Si content exceeds 2.0 mass%, the hardness of the second hard particles increases, the formability of the formed body deteriorates, the density of the sintered alloy decreases, and the machinability of the sintered alloy also deteriorates.

2-3. particle size of second hard particles

The particle size of the second hard particles may be appropriately selected according to the application, type, and the like of the sintered alloy. However, the particle diameter (maximum particle diameter) of the second hard particles is preferably in the range of 100 μm or less, and more preferably 75 μm or less. Therefore, the second hard particles can be uniformly dispersed into the matrix, and the hardness of the sintered alloy can be increased. Here, when hard particles having a particle diameter of more than 100 μm are included as the second hard particles, since the particle diameter is too large, machinability of the sintered alloy may be deteriorated. Here, the particle diameter of the second hard particles is preferably 1 μm or more in consideration of the production.

3. Graphite particles

The graphite particles may be natural graphite particles or artificial graphite particles or a mixture thereof as long as C of the graphite particles can be solid-solution diffused into the iron matrix and the hard particles during sintering. The particle size of the graphite particles is preferably in the range of 1 μm to 45 μm. As the powder including the preferred graphite particles, graphite powder (CPB-S commercially available from Nippon Kokuen Group) can be exemplified.

4. Iron particles

The iron particles serving as the sintered gold matrix are iron particles containing Fe as a main component. As the powder including iron particles, a pure iron powder is preferable. However, as long as moldability is not deteriorated during compression molding and diffusion of elements (e.g., Mn of the above first hard particles) is not reduced, a low alloy steel powder may be used. Fe-C powder may be used as the low alloy steel powder. For example, a powder having the following composition, which includes C when the amount of the low alloy steel powder is set to 100 mass%, may be used: 0.2 to 5 mass%, the balance including inevitable impurities and Fe. In addition, such powder may be a gaseous atomized powder, a water atomized powder, or a reduced powder. The particle size of the iron particles is preferably in the range of 150 μm or less.

5. Mixing ratio of mixed powder

A mixed powder including the first hard particles, the second hard particles, graphite particles, and iron particles is prepared. When the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles is set to 100 mass%, the mixed powder includes 5 to 50 mass% of the first hard particles, 1 to 5 mass% of the second hard particles, and 0.5 to 1.5 mass% of the graphite particles.

The mixed powder may include only the first hard particles, the second hard particles, the graphite particles, and the iron particles, but may include about several mass% of other particles as long as the mechanical strength and wear resistance of the obtained sintered alloy are not reduced. In this case, when the total amount of the first and second hard particles, the graphite particles, and the iron particles is 95 mass% or more with respect to the mixed powder, a sufficient effect can be expected. For example, at least one type of particle that improves machinability selected from the group consisting of: sulfides (e.g. MnS), oxides (e.g. CaCO)₃) Fluorides (e.g., CaF), nitrides (e.g., BN), and oxysulfides.

Since the first hard particles are included by 5 to 50 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 and the frictional wear resistance of the sintered alloy can be improved at the same time.

Here, when the first hard particles are included in an amount of less than 5 mass% with respect to the total amount, it is clearly understood from experiments by the inventors to be described below that a sufficient effect of the frictional wear resistance according to the first hard particles cannot be exhibited.

On the other hand, when the amount of the first hard particles exceeds 50 mass% with respect to the total amount, since the amount of the first hard particles is too large, when the molded body is molded from the mixed powder, it is difficult to mold the molded body. In addition, since there is much contact between the first hard particles and the portion where the iron particles are sintered becomes smaller, the frictional wear resistance of the sintered alloy is reduced.

Since the second hard particles are included by 1 to 5 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, plastic deformation of the iron matrix during use can be prevented and adhesive wear of the sintered alloy can be reduced.

Here, when the content of the second hard particles is less than 1 mass% with respect to the total amount, it is clearly understood from experiments by the inventors to be described below that the adhesive wear resistance of the sintered alloy is reduced. On the other hand, when the content of the second hard particles exceeds 5 mass% with respect to the total amount, the machinability of the sintered alloy deteriorates.

Since 0.5 to 1.5 mass% of the graphite particles is included with respect to the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles, C of the graphite particles can be diffused in a solid solution state into the first and second hard particles without melting the first and second hard particles after sintering, and furthermore, a pearlite structure can be secured in the iron matrix. Therefore, the mechanical strength and wear resistance of the sintered alloy can be improved at the same time.

Here, when the content of the graphite particles is less than 0.5 mass% with respect to the total amount, since the ferrite structure of the iron matrix tends to increase, the strength of the iron matrix itself of the sintered alloy decreases. On the other hand, when the content of the graphite particles exceeds 1.5 mass% with respect to the total amount, a cementite structure precipitates and the machinability of the sintered alloy deteriorates.

6. Method for producing wear-resistant iron-based sintered alloy

In this way, the obtained mixed powder is compression-molded into a sintered alloy compact (molding step). The sintered alloy compact includes first hard particles, second hard particles, graphite particles, and iron particles in the same ratio as in the mixed powder.

The compression-molded compact for sintered alloy is sintered to produce a sintered body, and C of the graphite particles in the compact for sintered alloy is diffused into the first and second hard particles and the iron particles (sintering step). In this case, not only is there more diffusion of iron from the iron matrix (iron particles) into the first and second hard particles, but also the second hard particles do not contain carbon. Therefore, carbon of the graphite particles easily diffuses into the second hard particles, Mo carbides are generated at grain boundaries between the second hard particles, and the hardness of the sintered alloy can be increased.

In the present embodiment, sintering is performed by adjusting the sintering temperature and sintering time so that the hardness of the first hard particles becomes 400 to 600Hv and the hardness of the second hard particles exceeds 600 Hv. As for the hardness of the first and second hard particles in the obtained sintered alloy, these hardnesses are values measured using a micro vickers hardness tester under a measurement load of 0.1 kgf. When the hardness of the first hard particles is set within 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 400Hv, the difference in hardness from the iron matrix in which carbon is in a solid solution state is small, and the wear resistance of the sintered alloy is reduced. On the other hand, when the hardness of the sintered alloy exceeds 600Hv, the machinability of the sintered alloy may be deteriorated.

In addition, when the hardness of the second cemented carbide is set within such a range, the wear resistance of the soft iron matrix can be improved. Here, when the hardness of the second hard particles is less than 600Hv, the wear resistance of the sintered alloy may be reduced.

The hardness of the first and second hard particles can be adjusted by appropriately setting the component ratio, the content of the graphite particles, the sintering temperature, and the sintering time within the above content range. The sintering temperature may be about 1050 ℃ to 1250 ℃, and particularly about 1100 ℃ to 1150 ℃. The sintering time at the above sintering temperature may be 30 minutes to 120 minutes, and more preferably 45 minutes to 90 minutes. The sintering atmosphere may be an inert atmosphere such as an inert gas atmosphere. As the non-oxidizing atmosphere, a nitrogen atmosphere, an argon atmosphere, a vacuum atmosphere, or the like can be used.

The matrix of the iron-based sintered alloy obtained by sintering preferably includes a pearlite-containing structure in order to ensure its hardness. The pearlite-containing structure may be a pearlite structure, a mixed pearlite-austenite structure, or a mixed pearlite-ferrite structure. In order to secure wear resistance, it is preferable to contain ferrite having low hardness in a small amount.

After the sintered body is prepared, the sintered body is subjected to oxidation treatment so that a part of iron contained in the iron matrix derived from the iron particles is changed into ferroferric oxide (Fe)₃O₄). Oxidation treatment was performed so that the difference in density between the sintered body before and after oxidation treatment became 0.05g/cm³Or more. In the oxidation treatment, an oxide mainly comprising magnetite is produced. Therefore, the quality of the sintered body after the oxidation treatment is improved. Thus, a higher density difference indicates a greater amount of magnetite produced.

When the density difference before and after the oxidation treatment in the sintered body was set to 0.05g/cm³Or more, the wear resistance of the sintered alloy can be improved. Here, when the difference in density before and after the oxidation treatment in the sintered body is less than 0.05g/cm³In this case, since the ratio of magnetite in the sintered alloy is small, adhesive wear is accelerated due to metal contact with the counterpart. The result is a reduction in the wear resistance of the sintered alloy.

In such an oxidation treatment, the sintered body is heated under a temperature condition of 500 to 600 ℃ for 30 to 90 minutes, for example, in a water vapor atmosphere. Therefore, in the density difference in the above range, iron (Fe) as the sintered body base may be oxidized into iron tetraoxide (Fe)₃O₄)。

7. Use of wear-resistant iron-based sintered alloy

The sintered alloy obtained in the above manufacturing method has higher mechanical strength and wear resistance under high-temperature use environment than those of the prior art. For example, it is applicable to a wastegate valve and a valve system (e.g., a valve seat and a valve guide) of a turbocharger for an internal combustion engine that uses compressed natural gas or liquefied petroleum gas as fuel in a high-temperature use environment.

For example, when a valve seat of an exhaust valve of an internal combustion engine is made of a sintered alloy, even if a wear pattern combining adhesive wear when the valve seat and the valve are in contact with each other and frictional wear when the two slide over each other develops, the wear resistance of such a valve seat is still improved as compared with the prior art. In particular, under the use environment where compressed natural gas or liquefied petroleum gas is used as fuel, although it is difficult to form a Mo oxide film under such environment, adhesive wear can be reduced.

Embodiments in which the present invention is implemented in practice will be described below in conjunction with comparative examples.

Example 1: optimum addition amount of first hard particles ]

The sintered alloy according to example 1 was prepared according to the following preparation method. As the first hard particles, hard particles made of an alloy (commercially available from Daido Steel co., Ltd) including Mo: 40 mass%, Ni: 30 mass%, Co: 20 mass%, Mn: 5% by mass; si: 0.8 mass% and C: 1.2 mass%, the balance comprising Fe and inevitable impurities (i.e., Fe-40Mo-30Ni-20Co-5Mn-0.8 Si-1.2C). The first hard particles were classified into a range of 44 μm to 250 μm using a sieve according to JIS standard Z8801. Here, the "particle size of particles" in the present specification is a value obtained by classifying according to this method.

As the second hard particles, second hard particles made of an Fe-65 alloy (commercially available from Kinsay materc co., Ltd) including Mo: 65% by mass, and the balance including Fe and inevitable impurities. The second hard particles are classified to a range of 75 μm or less.

Next, graphite powder including graphite particles (CPB-S, commercially available from Nippon Kokuen Group) and reduced iron powder including pure iron particles (jis p255M-90, commercially available from JFE Steel Corporation) were prepared. The above first hard particles, second hard particles and graphite particles in proportions of 40 mass%, 3 mass% and 1.1 mass%, respectively, were mixed together with the remaining iron particles (specifically 55.9 mass%) for 30 minutes using a V-type mixer. Thereby obtaining a mixed powder.

Next, the obtained mixed powder was compression-molded into an annular test piece under a pressurization force of 588MPa using a molding die to form a molded body (compression-molded body) for a sintered alloy. The compression molded body was sintered at 1120 ℃ for 60 minutes in an inert atmosphere (nitrogen atmosphere) to obtain a sintered body. The sintered body was oxidized by heating at 550 ℃ for 50 minutes under a water vapor atmosphere. Thus, a sintered alloy (valve seat) test piece according to example 1 was formed.

Examples 2 and 3: optimum addition amount of first hard particles ]

A sintered alloy test piece 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. Examples 2 and 3 differ from example 1 in that: as shown in table 1, the first hard particles were added at a ratio of 5 mass% and 50 mass%, respectively, with respect to the entire mixed powder.

Examples 4 and 5: optimum addition amount of second hard particles ]

A sintered alloy test piece 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. Examples 4 and 5 differ from example 1 in that: as shown in table 1, the second hard particles were added at a ratio of 1 mass% and 5 mass%, respectively, with respect to the entire mixed powder.

Examples 6 and 7: optimum addition amount of graphite particles ]

A sintered alloy test piece 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. Examples 6 and 7 differ from example 2 in that: as shown in table 1, the graphite particles were added in a ratio of 0.5 mass% and 1.5 mass% with respect to the entire mixed powder.

Example 8: hardness of first hard particles ]

A sintered alloy test piece was prepared in the same manner as in example 1. Example 8 differs from example 1 in that: the sintering temperature was lower than that of example 1, and the hardness of the first hard particles of the sintered body after sintering was decreased (refer to table 1, 545 Hv).

Comparative examples 1 and 2: comparative example of optimum addition amount of first hard particles ]

A sintered alloy test piece 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. Comparative examples 1 and 2 differ from example 1 in that: as shown in table 1, the first hard particles were added at a ratio of 0 mass% (i.e., not added) and 60 mass%, respectively, with respect to the entire mixed powder. Here, in comparative example 2, it was impossible to mold a molded body from the mixed powder.

Comparative examples 3 and 4: comparative example of optimum addition amount of second hard particles ]

A sintered alloy test piece 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. Comparative examples 3 and 4 differ from example 1 in that: as shown in table 1, the second hard particles were added at a ratio of 0 mass% and 10 mass%, respectively, with respect to the entire mixed powder. In comparative example 3, graphite particles were added in an amount of 0.8 mass%.

Comparative examples 5 and 6: comparative example of optimum addition amount of graphite particles

A sintered alloy test piece 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. Comparative examples 5 and 6 differ from example 1 in that: as shown in table 1, the graphite particles were added in a ratio of 0.4 mass% and 1.6 mass%, respectively, with respect to the entire mixed powder.

Comparative example 7: comparative example of density difference of sintered body

A sintered alloy test piece was prepared in the same manner as in example 1. In comparative example 7, the molding pressure in the compression molding process was higher than that in example 1, and the density before the oxidation treatment was high. Therefore, fewer pores exist inside the sintered body, and therefore, oxide generation is prevented and the increase in density of the sintered body after the oxidation treatment is reduced (i.e., the density difference is reduced).

Comparative example 8: comparative example of first hard particle hardness

A sintered alloy test piece was prepared in the same manner as in example 1. Comparative example 8 differs from example 1 in that: the sintering temperature was higher than that of example 1 and the hardness of the first hard particles of the sintered body after sintering was high (refer to table 1, 650 Hv).

Comparative example 9

A sintered alloy test piece was prepared in the same manner as in example 1. Comparative example 9 differs from example 1 in that: particles comprising a Co-40Mo-5Cr-0.9C alloy corresponding to the hard particles described in JP 2004-156101A were used as the first hard particles, the second hard particles were not added, and the sintered body was not subjected to oxidation treatment after sintering.

[ hardness test ]

For the sintered alloy test pieces according to examples 1 to 8 and comparative examples 1 to 9, the hardness of the first hard particles and the second hard particles was measured using a micro vickers hardness tester under a measurement load of 0.1 kgf. The results are shown in table 1.

[ Density measurement test ]

The quality of the sintered alloy test pieces according to examples 1 to 8 and comparative examples 1 and 3 to 8 before and after the oxidation treatment was measured. The measured mass was divided by the volume calculated from the test piece size, and the density of the test piece (sintered body) before and after the oxidation treatment was calculated. In addition, 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 ]

The sintered alloy test pieces according to examples 1 to 8 and comparative examples 1 and 3 to 9 were subjected to wear tests using the tester in fig. 1, and their wear resistance was evaluated. In this test, as shown in fig. 1, a sliding portion between an annular valve seat 12 made of the sintered alloy prepared as described above and a valve face 14 of a valve 13 was placed in a propane gas combustion atmosphere using a propane gas burner 10 as a heating source. The valve face 14 was obtained by carbonitriding according to EV12(SEA standard). The temperature of the valve seat 12 was controlled so that it was 250 ℃, a load of 25kgf was applied by the spring 16 when the valve seat 12 was brought into contact with the valve face 14, the valve seat 12 was brought into contact with the valve face 14 at 3250 times/min, and the wear test was performed for 8 hours.

The total amount of the wear depths in the axial direction of the valve face 14 and the valve seat 12 after the wear test was measured as a wear test wear amount, and a value obtained by dividing the wear test wear amount by the value in comparative example 9 was calculated as a wear test wear amount proportion. The results are shown in table 1.

Fig. 3A, 4A, 5A, 6A, and 7A show the results of plotting the wear amount ratio in the wear test for examples 1 to 8 and comparative examples 1 and 3 to 9, in which the horizontal axis represents the addition amount of the first hard particles, the addition amount of the second hard particles, the addition amount of the graphite particles, the hardness of the first hard particles, and the density difference of the sintered body in the order of the figures.

In addition, the surfaces of the test pieces after the wear test according to example 1 and comparative example 7 after the wear test were observed under a microscope. The results are shown in fig. 8A and 8B. Fig. 8A is a picture of the surface of the test piece according to example 1 after the wear test, and fig. 8B is a picture of the surface 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 using a naptal etching solution (nital), and the structure of the sintered alloy was observed under a microscope. The results are shown in fig. 9A to 9C. Fig. 9A is a picture of the structure of the test piece according to example 1, fig. 9B is a picture of the structure of the test piece according to comparative example 5, and fig. 9C is a picture of the structure of the test piece according to comparative example 6.

[ machinability test ]

The sintered alloy test pieces according to examples 1 to 8 and comparative examples 1 and 3 to 9 were subjected to machinability tests using a tester shown in fig. 2, and their machinability was evaluated. In this test, six test pieces 20 having an outer diameter of 30mm, an inner diameter of 22mm and a total length of 9mm were prepared for each of examples 1 to 8 and comparative examples 1 and 3 to 9. A test piece 20 rotating at a rotational speed of 970rpm was wet-cut transversely over a cutting distance of 320m using an NC lathe using a titanium aluminum nitride coated cemented carbide tool (cutting tool) 30 fed at a cutting depth of 0.3mm, 0.08 mm/rev. Then, the maximum wear depth of the relief surface of the tool 30 was measured as a tool wear amount using an optical microscope, and a value obtained by dividing the tool wear amount by the value in comparative example 9 was calculated as a tool wear amount ratio. The results are shown in table 1.

Fig. 3B, 4B, 5B, 6B, and 7B show the results of plotting the proportions of the amounts of wear of the corresponding tools in examples 1 and 3 to 8 and comparative examples 1 to 9, in which the horizontal axis represents the addition amount of the first hard particles, the addition amount of the second hard particles, the addition amount of the graphite particles, the hardness of the first hard particles, and the difference in density of the sintered body in the order of the figures.

[ Table 1]

(result 1: optimum addition amount of first hard particles)

As shown in fig. 3A, examples 1 to 3 were lower in wear test wear amount ratio than comparative examples 1 to 9. Abrasion test the abrasion loss ratio was decreased in the order of example 2, example 1 and example 3. Therefore, when the first hard particles are added, it is considered that the frictional wear resistance of the sintered alloy is improved. However, in comparative example 2, it can be said that the moldability of the molded article is deteriorated because too many first hard particles are added. Based on the above, the optimum addition amount of the first hard particles is 5 to 50 mass% with respect to the entire mixed powder.

Here, as shown in fig. 3B, the tool wear amount ratios of examples 1 to 3 were smaller than that of comparative example 9. The tool wear amount ratio was increased in the order of example 2, example 1 and example 3. However, it is considered that when more first hard particles than in example 3 were added, the machinability of the sintered alloy was deteriorated and the tool wear amount ratio was increased.

(result 2: optimum addition amount of second hard particles)

As shown in fig. 4A, the wear test wear amount ratios of examples 1, 4, and 5 and comparative example 4 were lower than those of comparative examples 3 and 9. However, as shown in fig. 4B, the tool wear amount ratio of comparative example 4 was higher than those of examples 1, 4 and 5. Here, when the surface of the test piece after the wear test was observed, comparative example 3 caused more scratches due to adhesive wear than other examples.

Thus, it is believed that the second hard particles improve the hardness of the sintered alloy after sintering, prevent plastic deformation of the iron matrix of the sintered alloy during use, and reduce adhesive wear of the sintered alloy. Specifically, it is considered that since the second hard particles do not contain Ni, Co, or the like unlike the first hard particles, the iron matrix around the second hard particles can be hardened as compared with in the first hard particles, molybdenum carbides are precipitated at grain boundaries of the iron matrix during sintering, and thus the hardness of the iron matrix after sintering is improved.

Based on the above, when the addition amount of the second hard particles is too small, the surface of the sintered alloy is easily scraped off after the wear test. On the other hand, it is considered that when the addition amount of the second hard particles is too large as in comparative example 4, the sintered alloy is too hard after sintering and the machinability deteriorates. Based on the above results, the optimum addition amount of the second hard particles is 1 to 5 mass% with respect to the entire mixed powder.

(result 3: optimum addition amount of graphite particles)

As shown in fig. 5A, the wear test wear amount ratios of examples 1, 6, and 7 and comparative example 6 are lower than those of comparative examples 5 and 9. However, as shown in fig. 5B, the tool wear amount ratio of comparative example 6 is higher than examples 1, 6, and 7.

As shown in fig. 9A, a pearlite structure is formed in the structure of the sintered alloy shown in example 1. However, as shown in fig. 9C, a cementite structure was formed in the structure of the sintered alloy shown in comparative example 6 due to the increase in the amount of graphite particles. Therefore, it is considered that the tool wear amount ratio of comparative example 6 is higher than examples 1, 6 and 7.

On the other hand, as shown in fig. 9B, it is considered that in the structure of the sintered alloy shown in comparative example 5, since the structure has ferrite as its main portion, the wear test wear amount ratio of comparative example 5 is higher than those of examples 1, 6 and 7 and comparative example 6. Therefore, the optimum addition amount of the graphite particles of the pearlite structure in the iron matrix after sintering can be secured to 0.5 to 1.5 mass% with respect to the entire mixed powder.

(result 4: optimum hardness of first hard particles)

As shown in fig. 6A, the wear test wear amount ratios of examples 1, 3, 5, and 8 and comparative example 8 were lower than comparative example 9. However, as shown in fig. 6B, the tool wear amount ratio of comparative example 8 is higher than examples 1, 3, 5, and 8.

It is considered that since the hardness of the first hard particles in comparative example 9 is higher than those of examples 1, 3, 5 and 8 and comparative example 8, the corresponding parts were more worn and the wear test wear amount ratio of example 9 was higher than those of the other examples. On the other hand, it is considered that in examples 1, 3, 5 and 8, since the hardness (Hv) of the first hard particles was lower than that of comparative example 8 and 600 or less, the tool wear amount ratio of examples 1, 3, 5 and 8 was lower than that of comparative example 8. Here, in examples 1, 3, 5 and 8, it can be said that wear resistance is ensured because the first hard particles having a hardness of 400Hv or more are ensured.

Therefore, the hardness of the first hard particles after sintering is preferably in the range of 400 to 600 Hv. Here, considering the second hard particles for improving the wear resistance of the iron matrix, it is necessary that the hardness of the second hard particles is higher than that of the first hard particles and exceeds at least 600Hv under the condition of the above range of addition amount.

(result 5: difference in optimum Density of sintered body)

As shown in fig. 7A, examples 1 to 8 were lower in wear test wear amount ratio than comparative examples 7 and 9. As shown in fig. 7B, the tool wear amount ratio of comparative example 9 is higher than those of examples 1 to 8 and comparative example 7.

In comparative example 7, the difference in density between the sintered body and the sintered body before and after the oxidation treatment was less than 0.05g/cm³Therefore, the amount of oxide mainly including ferroferric oxide in the sintered body was smaller than in the sintered bodies of examples 1 to 8. Therefore, metal contact with the counterpart member is promoted, and as shown in fig. 8B, in the test piece (sintered body) of comparative example 7, it is considered that adhesive wear with the counterpart member is accelerated. On the other hand, it is considered that the wear resistance of the sintered alloys in examples 1 to 8 was already higher than that of comparative example 7 because such adhesive wear was substantially absent (for example, refer to example 1, fig. 8A). Therefore, it is necessary to perform an oxidation treatment so that the difference in density between the sintered body before and after the oxidation treatment becomes 0.05g/cm³Or more.

Example 9: optimum particle diameter of second hard particles ]

A sintered alloy test piece was prepared in the same manner as in example 1. Example 9 is an example for evaluating the optimum particle diameter of the second hard particles. Example 9 differs from example 1 in that: as the second hard particles, second hard particles classified to have a particle diameter (particle size) in a range of more than 75 μm and 100 μm or less are used.

Comparative example 10: comparative example of optimum particle diameter of second hard particles ]

A sintered alloy test piece was prepared in the same manner as in example 1. Comparative example 10 is a comparative example for evaluating the optimum particle diameter of the second hard particles. Comparative example 10 differs from example 1 in that: as the second hard particles, second hard particles classified to have a particle diameter in a range of more than 100 μm and 150 μm or less are used. Here, the test piece according to comparative example 10 is a sintered alloy included in the scope of the present invention, and is set as comparative example 10 for comparison with examples 1 and 9.

In the same manner as in example 1, the test pieces of example 9 and comparative example 10 were subjected to a wear test and a machinability test, and a wear test wear amount and a tool wear amount were measured. The results are shown in fig. 10A and 10B together with the above results of example 1.

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 a graph showing the results of the tool wear amount ratio in examples 1 and 9 and comparative example 10.

(result 6: optimum particle diameter of second hard particles)

As shown in fig. 10A, the abrasion test abrasion amount ratios of examples 1 and 9 and comparative example 10 are similar. However, as shown in fig. 10B, the tool wear amount ratios of examples 1 and 9 were lower than that of comparative example 10, and the tool wear amount ratio of example 1 was the lowest in these examples. This is because in comparative example 10, since the particle diameter of the second hard particles was too large, the machinability of the test piece (sintered body) was deteriorated in some cases. Based on this result, the particle diameter (maximum particle diameter) of the second hard particles is preferably in the range of 100 μm or less, and more preferably the particle diameter (maximum particle diameter) of the second hard particles is in the range of 75 μm or less.

Although the embodiments of the present invention are described above in detail, the present invention is not limited to these embodiments, and various design changes may be made.

Claims

1. A method of making a wear resistant iron-based sintered alloy comprising:

a molding step of compression-molding a mixed powder including hard particles, graphite particles, and iron particles into a sintered alloy compact; and

a sintering step of sintering the compact for sintered alloy, and causing C of graphite particles in the compact for sintered alloy to diffuse into the hard particles and the iron particles and molybdenum carbide to precipitate at grain boundaries of an iron matrix,

the method for producing the wear-resistant iron-based sintered alloy is characterized in that

The hard particles include first hard particles and second hard particles,

wherein the first hard particles include, when the amount of the first hard particles is set to 100 mass%, Mo: 20 to 70 mass%, Ni: 5 to 40 mass%, Co: 5 to 40 mass%, Mn: 1 to 20 mass%, Si: 0.5 to 4.0 mass% and C: 0.5 to 3.0 mass%, the balance including Fe and inevitable impurities,

wherein the second hard particles include, when the amount of the second hard particles is set to 100 mass%, Mo: 60 to 70 mass% and Si: 2.0 mass% or less, the balance including Fe and inevitable impurities,

wherein the mixed powder includes 5 to 50 mass% of the first hard particles, 1 to 5 mass% of the second hard particles, and 0.5 to 1.5 mass% of the graphite particles, when the total amount of the first hard particles, the second hard particles, the graphite particles, and the iron particles is set to 100 mass%, and

wherein in the sintering step, sintering is performed so that the hardness of the first hard particles becomes 400 to 600HV and the hardness of the second hard particles exceeds 600HV, after the sintering step, the sintered body sintered from the compact for cemented alloy is subjected to oxidation treatment so that a part of iron contained in an iron matrix derived from the iron particles becomes ferroferric oxide, and the oxidation treatment is performed so that the oxidation treatment is performed before the oxidation treatmentThe difference between the density of the sintered body and the density of the sintered body after the oxidation treatment became 0.05g/cm³Or larger.

2. The method according to claim 1, characterized in that when the amount of the first hard particles is set to 100 mass%, 10 mass% or less of Cr is further added to the first hard particles.

3. The method according to claim 1 or 2, characterized in that the particle size of the secondary hard particles is in the range of 100 μm or less.