MIXED POWDER FOR POWDER METALLURGY AND METHOD OF MANUFACTURING SINTERED PART

Abstract

A mixed powder for powder metallurgy of the present disclosure includes a metal powder and a graphite powder. The graphite powder includes a spherical graphite powder. Particles of the spherical graphite powder each have an aspect ratio of 4 or less, where the aspect ratio is a ratio of a major axis length to a minor axis length. A content ratio of the spherical graphite powder to the graphite powder is 70% or greater. The graphite powder has an apparent density of 0.2 g/cm.sup.3 or greater. The graphite powder has a tap density of 0.5 g/cm.sup.3 or greater.

Claims

1. A mixed powder for powder metallurgy comprising: a metal powder; and a graphite powder, wherein the graphite powder comprises a spherical graphite powder, particles of the spherical graphite powder each have an aspect ratio of 4 or less, where the aspect ratio is a ratio of a major axis length to a minor axis length, a content ratio of the spherical graphite powder to the graphite powder is 70% or greater, the graphite powder has an apparent density of 0.2 g/cm.sup.3 or greater, and the graphite powder has a tap density of 0.5 g/cm.sup.3 or greater.

2. The mixed powder for powder metallurgy according to claim 1, wherein the metal powder comprises an iron-based powder, and particles of the iron-based powder each comprise iron in an amount greater than 50 mass %.

3. The mixed powder for powder metallurgy according to claim 2, wherein the content ratio of the iron-based powder to the mixed powder for powder metallurgy is 90 mass % or greater.

4. The mixed powder for powder metallurgy according to claim 3, wherein the metal powder further comprises a copper powder, the content ratio of the copper powder to the mixed powder for powder metallurgy is greater than 0 mass % and 8.0 mass % or less, and the content ratio of the graphite powder to the mixed powder for powder metallurgy is 0.1 mass % to 2.0 mass %.

5. The mixed powder for powder metallurgy according to claim 1, wherein the graphite powder has an average particle size of 3 m to 40 m.

6. A method of manufacturing a sintered part, the method comprising: providing the mixed powder for powder metallurgy according to claim 1; preparing a compact by pressing a raw material powder comprising the mixed powder for powder metallurgy; and sintering the compact.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 is a schematic diagram illustrating an example of a mixed powder for powder metallurgy according to an embodiment.

[0007] FIG. 2 is a scanning electron microscope (SEM) image taken with a scanning electron microscope, showing a graphite powder present in the mixed powder for powder metallurgy according to the embodiment.

[0008] FIG. 3 is a schematic diagram illustrating a method of manufacturing a sintered part, according to another embodiment.

[0009] FIG. 4 is an SEM image taken with a scanning electron microscope, showing a graphite powder of Sample No. 1-11.

[0010] FIG. 5 is a graph showing a relationship between an average particle size and an apparent density, regarding graphite powders used in Test Example 1.

[0011] FIG. 6 is a graph showing a relationship between the average particle size and a tap density, regarding the graphite powders used in Test Example 1.

[0012] FIG. 7 is a schematic diagram used to illustrate a method of evaluating dustiness, regarding the graphite powders used in Test Example 1.

[0013] FIG. 8 is a graph showing the results of the evaluation of dustiness, regarding the graphite powders used in Test Example 1.

[0014] FIG. 9 is a schematic diagram used to illustrate a method of evaluating dimensional variations, regarding sintered parts prepared in Test Example 2.

[0015] FIG. 10 is a graph showing the results of the evaluation of dimensional variations, regarding the sintered parts prepared in Test Example 2.

DETAILED DESCRIPTION

[0016] There is a need for a mixed powder for powder metallurgy that is unlikely to exhibit uneven distribution of a graphite powder even if the mixed powder does not include any binder. Graphite powders have a lower specific gravity than metal powders, such as iron powders and copper powders, and, therefore, are likely to become airborne. Sufficient studies have not been conducted regarding configurations of a graphite powder that is, by itself, unlikely to become airborne.

[0017] An object of the present disclosure is to provide a mixed powder for powder metallurgy including a graphite powder that is unlikely to become airborne.

[0018] In the mixed powder for powder metallurgy of the present disclosure, the graphite powder includes a spherical graphite powder in a predetermined content ratio, and, consequently, the graphite powder is unlikely to become airborne.

[0019] In the related art, it has been believed that when a mixed powder for powder metallurgy prior to press molding is one in which a graphite powder is uniformly mixed with a metal powder, the graphite powder will be uniformly dispersed in the resulting compact. However, diligent studies conducted by the present inventors found that in instances where the graphite powder primarily includes a flaky graphite powder, the graphite powder becomes airborne when the mixed powder for powder metallurgy is supplied to the press mold, and that the graphite powder that has become airborne is deposited on the powder that has been placed into the mold cavity. That is, it was found that in the instance where the graphite powder has become airborne, the graphite powder becomes unevenly distributed in the mixed powder for powder metallurgy within the cavity. The expression unevenly distributed refers to instances in which the distribution of the graphite powder is uneven in the mixed powder for powder metallurgy. The expression unevenly distributed not only refers to instances in which the graphite powder is non-uniformly distributed in the mixed powder for powder metallurgy because of poor dispersion, but also refers to instances in which the graphite powder is non-uniformly distributed in a compact produced with the mixed powder for powder metallurgy. If pressing is performed in a state in which the graphite powder is unevenly distributed, the graphite powder becomes non-uniformly distributed in the resulting compact. To address this, a compact was produced by using a spherical graphite powder, and it was found that the graphite powder was uniformly dispersed in the resulting compact. Now, embodiments of the present disclosure will be enumerated and described. [0020] (1) According to an embodiment of the present disclosure, a mixed powder for powder metallurgy includes a metal powder and a graphite powder. The graphite powder includes a spherical graphite powder. Particles of the spherical graphite powder each have an aspect ratio of 4 or less, where the aspect ratio is a ratio of a major axis length to a minor axis length. A content ratio of the spherical graphite powder to the graphite powder is 70% or greater. The graphite powder has an apparent density of 0.2 g/cm.sup.3 or greater. The graphite powder has a tap density of 0.5 g/cm.sup.3 or greater.

[0021] The inclusion of a spherical graphite powder in the graphite powder makes it likely that the apparent density and the tap density of the graphite powder increase. The content ratio of the spherical graphite powder to the graphite powder of 70% or greater makes it likely that the graphite powder has an apparent density of 0.2 g/cm.sup.3 or greater and that the graphite powder has a tap density of 0.5 g/cm.sup.3 or greater. The apparent density of 0.2 g/cm.sup.3 or greater and the tap density of 0.5 g/cm.sup.3 or greater make it unlikely that the graphite powder becomes airborne. Accordingly, when the mixed powder for powder metallurgy is supplied to a press mold, the graphite powder is unlikely to become airborne, and, therefore, it is unlikely that the graphite powder is unevenly distributed in a compact that is produced with the mixed powder for powder metallurgy. When it is unlikely that the graphite powder is unevenly distributed in a compact, a sintered part manufactured with the mixed powder for powder metallurgy is unlikely to have differences in carbon concentration. Sintered parts that are unlikely to have differences in carbon concentration are unlikely to have variations in dimensional accuracy.

[0022] Since the graphite powder itself is unlikely to become airborne, it is not necessary to add a binder for preventing the graphite powder from becoming airborne. In other words, since the graphite powder itself is unlikely to become airborne, it is sufficient to simply mix the metal powder with the graphite powder.

[0023] Generally, as stated above, when a mixed powder for powder metallurgy including a graphite powder is supplied to a press mold, the graphite powder can become airborne. If the graphite powder becomes airborne, pieces of equipment around the mold may be contaminated. Mixed powders for powder metallurgy including a graphite powder that is unlikely to become airborne are unlikely to be deposited on, for example, pieces of equipment other than the apparatus to which the mixed powder for powder metallurgy is supplied. Accordingly, such a mixed powder for powder metallurgy facilitates the prevention of contamination of the equipment. [0024] (2) In the mixed powder for powder metallurgy according to (1), the metal powder may include an iron-based powder. Particles of the iron-based powder each may contain iron in an amount greater than 50 mass %.

[0025] Mixed powders for powder metallurgy including an iron-based powder are suitable as a raw material powder for sintered parts. Sintered parts that are manufactured using the mixed powder for powder metallurgy as a raw material powder have high strength. [0026] (3) In the mixed powder for powder metallurgy according to (2), the content ratio of the iron-based powder to the mixed powder for powder metallurgy may be 90 mass % or greater.

[0027] When the content ratio of the iron-based powder to the mixed powder for powder metallurgy is 90 mass % or greater, sintered parts that are manufactured using the mixed powder for powder metallurgy as a raw material powder are likely to have further improved strength. [0028] (4) In the mixed powder for powder metallurgy according to (3), the metal powder may further include a copper powder. The content ratio of the copper powder to the mixed powder for powder metallurgy may be greater than 0 mass % and 8.0 mass % or less. The content ratio of the graphite powder to the mixed powder for powder metallurgy may be 0.1 mass % to 2.0 mass %.

[0029] The copper powder and the graphite powder contribute to improved strength of sintered parts that are manufactured using the mixed powder for powder metallurgy as a raw material powder. When the copper powder and the graphite powder are included in amounts within the ranges mentioned above, the strength of sintered parts is likely to be further improved. [0030] (5) In the mixed powder for powder metallurgy according to any one of (1) to (4), the graphite powder may have an average particle size of 3 m to 40 m.

[0031] When the average particle size of the graphite powder is 3 m or greater, the graphite powder is unlikely to aggregate. When the average particle size of the graphite powder is 40 m or less, the graphite powder is likely to diffuse into the metal powder, particularly, the iron-based powder, in the solid phase, at the sintering temperature when a sintered part is manufactured using the mixed powder for powder metallurgy as a raw material powder. [0032] (6) According to another embodiment of the present disclosure, a method of manufacturing a sintered part includes providing the mixed powder for powder metallurgy according to any one of (1) to (5); preparing a compact by pressing a raw material powder including the mixed powder for powder metallurgy; and sintering the compact.

[0033] Sintered parts that are manufactured using the mixed powder for powder metallurgy according to the embodiment of the present disclosure, as a raw material powder, have excellent dimensional accuracy.

Details of Embodiments of Present Disclosure

[0034] Specific examples of the mixed powder for powder metallurgy and the method of manufacturing a sintered part of the present disclosure will be described with reference to the drawings. In the drawings, the same reference signs represent the same or corresponding parts. In the drawings, features may be represented in a partially exaggerated or simplified manner for convenience of description. In the drawings, ratios of dimensions of components may be different from actual ones. Note that the present invention is not limited by the illustrative embodiments but is defined by the claims, and it is intended that all modifications that correspond to the meaning of the claims and are within the scope thereof be embraced herein.

<Mixed Powder for Powder Metallurgy>

[0035] A mixed powder 1 for powder metallurgy according to an embodiment will be described with reference to FIGS. 1 and 2. As illustrated in FIG. 1, the mixed powder 1 for powder metallurgy includes a metal powder 2 and a graphite powder 5. One feature of the mixed powder 1 for powder metallurgy is that the graphite powder 5 includes a spherical graphite powder 6, as shown in FIG. 2. In FIG. 1, particles of the metal powder 2 and particles of the graphite powder 5 are illustrated as being circular for ease of understanding.

<<Metal Powder>>

The metal powder 2 is a main powder of the mixed powder 1 for powder metallurgy. The metal powder 2 includes, for example, an iron-based powder 3. The metal powder 2 of the present example includes the iron-based powder 3 and a copper powder 4.

[Iron-Based Powder]

The iron-based powder 3 is a main powder of the metal powder 2. That is, the iron-based powder 3 is a main powder of the mixed powder 1 for powder metallurgy. The mixed powder 1 for powder metallurgy, which includes the iron-based powder 3, is suitable as a raw material powder for a sintered part, which will be described below.

[0036] Particles of the iron-based powder 3 may each contain iron in an amount greater than 50 mass %. The content ratio of an element included in the particles is based on a total mass of the elements included in the particle, with the total mass being taken as 100 mass %. The particles of the iron-based powder 3 are each made of pure iron or an iron alloy. The pure iron is made of 99.9 mass % or more iron and incidental impurities. The iron alloy contains one or more additive elements, with the balance being iron and incidental impurities. The content ratio of the iron in the iron alloy may be 80 mass % or greater or 90 mass % or greater. Examples of the additive elements include chromium. The iron-based powder 3 may be a pure iron powder made of pure iron, an iron alloy powder made of an iron alloy, or a mixed powder of a pure iron powder and an iron alloy powder. The iron alloy powder may be a powder formed from an alloy that has already been made and in which one or more additive elements have been dissolved in iron or may be a powder alloyed by partially diffusing one or more additive elements into a pure iron powder. The iron alloy powder may be a powder prepared by forming a powder from an alloy that has already been made and in which one or more additive elements have been dissolved in iron and, further, partially diffusing one or more additive elements into the powder. The iron-based powder 3 can be prepared by any method. For example, the iron-based powder 3 can be prepared by a water atomization method, a gas atomization method, or a reduction method.

[0037] The content ratio of the iron-based powder 3 to the mixed powder 1 for powder metallurgy is, for example, 90 mass % or greater. When the content ratio of the iron-based powder 3 is 90 mass % or greater, sintered parts that are manufactured using the mixed powder 1 for powder metallurgy as a raw material powder have high strength. The content ratio of the iron-based powder 3 to the mixed powder 1 for powder metallurgy may be 93 mass % or greater or 95 mass % or greater. The content ratio of the iron-based powder 3 to the mixed powder 1 for powder metallurgy is, for example, 99 mass % or less or 98 mass % or less. The content ratio of the iron-based powder 3 to the mixed powder 1 for powder metallurgy is, for example, 90 mass % to 99 mass %, 90 mass % to 98 mass %, 93 mass % to 98 mass %, or 95 mass % to 98 mass %.

[0038] The iron-based powder 3 has an average particle size of, for example, 20 m to 250 m. When the average particle size of the iron-based powder 3 is 20 m or greater, the iron-based powder 3 has good flowability, which facilitates pressing. When the average particle size of the iron-based powder 3 is 250 m or less, sintered parts that are manufactured using, as a raw material powder, the mixed powder 1 for powder metallurgy including the metal powder 2 have a dense structure. The average particle size of the iron-based powder 3 may be 30 m to 200 m, 50 m to 200 m, or 50 m to 150 m. The average particle size of the iron-based powder 3 is a median diameter D50, which is a diameter at a cumulative 50%, starting from the small-diameter-side, in a number-based particle size distribution measured by a laser diffraction particle size distribution analyzer.

[Copper Powder]

The copper powder 4 contributes to improved strength of sintered parts that are manufactured using the mixed powder 1 for powder metallurgy as a raw material powder. At least a portion of the copper powder 4 melts during sintering in the process of manufacturing a sintered part.

[0039] Particles of the copper powder 4 may each contain copper in an amount greater than 50 mass %. The particles of the copper powder 4 are each made primarily of pure copper or a copper alloy. The pure copper is made of 99.5 mass % or more copper and incidental impurities. The copper alloy contains one or more additive elements, with the balance being copper and incidental impurities. The content ratio of the copper in the copper alloy may be 80 mass % or greater or 90 mass % or greater. Examples of the additive elements include phosphorus. The copper powder 4 may be a pure copper powder made of pure copper, a copper alloy powder made of a copper alloy, or a mixed powder of a pure copper powder and a copper alloy powder.

[0040] The content ratio of the copper powder 4 to the mixed powder 1 for powder metallurgy is, for example, greater than 0 mass % and 8.0 mass % or less. Although the copper powder 4 contributes to improved strength of sintered parts, the copper powder 4 can also contribute to a reduction in the dimensional accuracy of sintered parts. When the content ratio of the copper powder 4 to the mixed powder 1 for powder metallurgy is 8.0 mass % or less, it is unlikely that the dimensional accuracy of sintered parts is reduced. The content ratio of the copper powder 4 to the mixed powder 1 for powder metallurgy may be 0.1 mass % to 6.0 mass %, 0.3 mass % to 5.0 mass %, or 0.5 mass % to 3.0 mass %.

[0041] The copper powder 4 has an average particle size of, for example, 5 m to 90 m. When the average particle size of the copper powder 4 is 5 m or greater, the copper powder 4 has good flowability, which facilitates pressing. When the average particle size of the copper powder 4 is 90 m or less, sintered parts that are manufactured using, as a raw material powder, the mixed powder 1 for powder metallurgy including the metal powder 2 have a dense structure. The average particle size of the copper powder 4 may be m to 80 m, 10 m to 60 m, or 20 m to 50 m. The average particle size of the copper powder 4 is, for example, smaller than the average particle size of the iron-based powder 3. The average particle size of the copper powder 4 is a median diameter D50 in a number-based particle size distribution, as with the average particle size of the iron-based powder 3.

[Others]

Although not illustrated, the mixed powder 1 for powder metallurgy may further include one or more powders selected from the group consisting of a nickel powder, a molybdenum powder, and a manganese sulfide powder. The content ratio of the nickel powder to the mixed powder 1 for powder metallurgy is, for example, greater than 0 mass % and 10 mass %, or less or 0.1 mass % to 4 mass %. The content ratio of the molybdenum powder to the mixed powder 1 for powder metallurgy is, for example, greater than 0 mass % and 10 mass %, or less or 0.1 mass % to 1 mass %. The content ratio of the manganese sulfide powder to the mixed powder 1 for powder metallurgy is, for example, greater than 0 mass % and 5 mass % or less, or 0.1 mass % to 2.0 mass %.

[0042] The metal powder that is primarily included in the metal powder 2 may be a metal powder other than the iron-based powder 3, as long as the mixed powder 1 for powder metallurgy can be sintered. The metal powder 2 does not have to include the copper powder 4.

<<Graphite Powder>>

The graphite powder 5 contributes to improved strength of sintered parts that are manufactured using the mixed powder 1 for powder metallurgy as a raw material powder. At least a portion of the graphite powder 5 diffuses into the metal powder 2, particularly, the iron-based powder 3, in the solid phase, during sintering in the process of manufacturing a sintered part.

[0043] As shown in FIG. 2, the graphite powder 5 includes the spherical graphite powder 6. Particles of the spherical graphite powder 6 each have an aspect ratio of 4 or less, where the aspect ratio is a ratio of a major axis length to a minor axis length. When a particle has an aspect ratio of 4 or less, the particle has a shape close to a spherical shape. The lower the aspect ratio, the closer the shape of the particle to a spherical shape. The aspect ratio of the particles of the spherical graphite powder 6 can be measured by observing the graphite powder 5 with a scanning electron microscope (SEM). An SEM image is to be taken such that 50 or more particles are included therein. The SEM image is a two-dimensional image. In the SEM image, the major axis and the minor axis of each of the particles are to be extracted by image processing. The major axis is the longest line segment among straight lines passing through an areal center of gravity of the particle and crossing a contour of the particle. The minor axis is the shortest line segment among straight lines that are orthogonal to the major axis. The aspect ratio of each of the particles is the value calculated using the major axis and the minor axis of each of the particles, by dividing the length of the major axis by the length of the minor axis, that is, the value calculated as (length of the major axis/length of the minor axis). The aspect ratio of the particles of the spherical graphite powder 6 may be 3.5 or less, 3.0 or less, 2.5 or less, or 2.0 or less. The aspect ratio of the particles of the spherical graphite powder 6 is, for example, 1 or greater.

[0044] A content ratio of the spherical graphite powder 6 to the graphite powder 5 is 70% or greater. The content ratio of the spherical graphite powder 6 to the graphite powder 5 is a ratio of the number of particles of the spherical graphite powder 6 to the number of particles of the graphite powder 5. The content ratio of the spherical graphite powder 6 to the graphite powder 5 of 70% or greater makes it likely that the graphite powder 5 has a high apparent density and a high tap density. The apparent density and the tap density will be described below. The content ratio of the spherical graphite powder 6 to the graphite powder 5 can be determined from the SEM image described above. In the SEM image, 50 or more particles are randomly selected, and the number of particles having an aspect ratio of 4 or less, among the randomly selected particles, is divided by the number of the randomly selected particles. The percentage of the resulting value is the content ratio of the spherical graphite powder 6 to the graphite powder 5. The content ratio of the spherical graphite powder 6 to the graphite powder 5 may be 75% or greater, 80% or greater, 85% or greater, 90% or greater, or 95% or greater. The content ratio of the spherical graphite powder 6 to the graphite powder 5 may be 100%. When the content ratio of the spherical graphite powder 6 to the graphite powder 5 is 100%, the aspect ratio of all the particles of the graphite powder 5 is 4 or less. When the content ratio of the spherical graphite powder 6 to the graphite powder 5 is less than 100%, the graphite powder 5 includes particles having an aspect ratio greater than 4, which may be flaky particles 6s, for example.

[0045] The apparent density of the graphite powder 5 is 0.2 g/cm.sup.3 or greater. The graphite powder 5, which has the apparent density of 0.2 g/cm.sup.3 or greater, is unlikely to become airborne. The apparent density of the graphite powder 5 is a density measured using a procedure described in JIS Z 2504:2020, Metallic powders-Determination of apparent density. The funnel that is used for the measurement has an orifice diameter of 5.50.1 mm and a funnel angle of 60. The apparent density of the graphite powder 5 may be greater than 0.2 g/cm.sup.3, 0.3 g/cm.sup.3 or greater, or 0.4 g/cm.sup.3 or greater.

[0046] The tap density of the graphite powder 5 is 0.5 g/cm.sup.3 or greater. The graphite powder 5, which has the tap density of 0.5 g/cm.sup.3 or greater, is unlikely to become airborne. The tap density of the graphite powder 5 is a density measured using a procedure described in JIS Z 2512:2012, Metallic powders-Determination of tap density. The tap density of the graphite powder 5 may be 0.6 g/cm.sup.3 or greater, 0.7 g/cm.sup.3 or greater, or 0.8 g/cm.sup.3 or greater.

[0047] The graphite powder 5 has an average particle size of, for example, 3 m to 40 m. When the average particle size of the graphite powder 5 is 3 m or greater, the graphite powder 5 is unlikely to aggregate. When the average particle size of the graphite powder 5 is 40 m or less, the graphite powder 5 is likely to diffuse into the metal powder 2, particularly, the iron-based powder 3, in the solid phase, at the sintering temperature when a sintered part is manufactured using the mixed powder 1 for powder metallurgy as a raw material powder. The average particle size of the graphite powder 5 may be 5 m to 35 m, 5 m to 30 m, or 10 m to 30 m. The average particle size of the graphite powder 5 is, for example, smaller than the average particle size of the metal powder 2. The average particle size of the graphite powder 5 is a median diameter D50 in a number-based particle size distribution, as with the average particle size of the iron-based powder 3.

[0048] The content ratio of the graphite powder 5 to the mixed powder 1 for powder metallurgy is, for example, 0.1 mass % to 2.0 mass %. When the content ratio of the graphite powder 5 to the mixed powder 1 for powder metallurgy is 0.1 mass % or greater, the strength of sintered parts is likely to be improved. When the content ratio of the graphite powder 5 to the mixed powder 1 for powder metallurgy is 2.0 mass % or less, the dimensional accuracy of sintered parts is likely to be maintained or improved. The content ratio of the graphite powder 5 to the mixed powder 1 for powder metallurgy may be 0.5 mass % to 2.0 mass %, 0.5 mass % to 1.5 mass %, 0.6 mass % to 1.3 mass %, or 0.8 mass % to 1.1 mass %.

<Method of Manufacturing Sintered Part>

A method of manufacturing a sintered part, according to another embodiment, will be described with reference to FIG. 3. The method of manufacturing a sintered part includes step A for providing a mixed powder for powder metallurgy; step B for preparing a compact by pressing a raw material powder including the mixed powder for powder metallurgy; and step C for sintering the compact. One feature of the method of manufacturing a sintered part is to use the mixed powder 1 for powder metallurgy of the above embodiment as a mixed powder for powder metallurgy.

<<Step A>>

In step A, the mixed powder 1 for powder metallurgy of the above embodiment is provided. The mixed powder 1 for powder metallurgy is included in a raw material powder 7, which forms the sintered part. The raw material powder 7 may include a lubricant (not illustrated). The lubricant serves to increase lubricity against a mold 91 that is exhibited during press molding, which will be described below. The lubricant improves moldability, thereby making it likely that a compact 8 has high density. The lubricant may be a lubricant known in the art that is used for press molding. Examples of lubricants that can be used include metal soaps, such as lithium stearate and zinc stearate, and amides, such as ethylene bis-stearamide. The lubricant may be coated onto the mold 91. In the instance where a lubricant is included, a content ratio of the lubricant to the raw material powder 7 is, for example, 0.1 mass % to 1.2 mass % or 0.4 mass % to 1.0 mass %.

<<Step B>>

In step B, the raw material powder 7 provided in step A is press-molded. The press molding is performed in the mold 91. The raw material powder 7 is placed into a cavity of the mold 91 and pressed. The press molding of the raw material powder 7 produces the compact 8, which conforms to the shape of the cavity.

[0049] Generally, a graphite powder included in a mixed powder for powder metallurgy can become airborne when the raw material powder is supplied to a mold. The graphite powder 5, which includes the spherical graphite powder 6, is unlikely to become airborne, as stated above. Accordingly, when the mixed powder 1 for powder metallurgy is supplied to the mold 91, it is unlikely that the graphite powder 5 becomes airborne, and, therefore, the graphite powder 5 is unlikely to be unevenly distributed in the compact 8, which is prepared with the mixed powder 1 for powder metallurgy. When it is unlikely that the graphite powder 5 is unevenly distributed in the compact 8, it is unlikely that a sintered part that is manufactured with the raw material powder 7, which includes the mixed powder 1 for powder metallurgy, has differences in carbon concentration. Sintered parts that are unlikely to have differences in carbon concentration are unlikely to have variations in dimensional accuracy. Furthermore, the mixed powder 1 for powder metallurgy including the graphite powder 5, which is unlikely to become airborne, is unlikely to be deposited on pieces of equipment near the mold 91. Accordingly, the mixed powder 1 for powder metallurgy facilitates the prevention of contamination of the equipment.

<<Step C>>

In step C, the press-molded compact 8 is sintered. FIG. 3 illustrates an example in which the compact 8 is sintered in a furnace 92. The sintering causes particles of the mixed powder 1 for powder metallurgy to bind to one another. In the instance where the mixed powder 1 for powder metallurgy includes the iron-based powder 3, the copper powder 4, and the graphite powder 5, the sintering causes the carbon in the graphite powder 5 to diffuse into the iron-based powder 3 in the solid phase and causes the copper in the copper powder 4 to melt and then spread into and wet spaces between particles.

[0050] The sintering is carried out by heating the compact 8 at a temperature not exceeding the melting point of the alloy that forms the sintered part that is to be manufactured. For example, the temperature is 900 C. to 1300 C. When the sintering temperature is 900 C. or greater, the ability of the particles of the mixed powder 1 for powder metallurgy to bind to one another can be improved. The sintering temperature may be 1000 C. to 1300 C., 1050 C. to 1300 C., or 1100 C. to 1300 C. The sintering may be a batch process in which a predetermined amount of compacts 8 are heated at a time in the furnace 92. FIG. 3 illustrates an example in which one compact is heated, but a plurality of compacts 8 may be heated at a time, as illustrated in FIG. 9, which will be described below. An atmosphere for the sintering is, for example, a vacuum, a nitrogen atmosphere, a hydrogen atmosphere, or an argon atmosphere. The sintering may be a continuous process in which compacts 8 are continuously heated.

Test Example 1

[0051] In Test Example 1, mixed powders for powder metallurgy including a graphite powder that primarily included a spherical graphite powder were prepared, mixed powders for powder metallurgy including a graphite powder that primarily included a flaky graphite powder were also prepared, and the dustiness of these graphite powders was evaluated. Particles of the flaky graphite powder had an aspect ratio of greater than 4.

<Description of Samples>

Seven samples were prepared: Samples No. 1-1 to No. 1-5, No. 1-11, and No. 1-12. All of the samples included a mixed powder for powder metallurgy including a copper powder in an amount of 2 mass % and a graphite powder in an amount of 1 mass %, with the balance being an iron-based powder. The iron-based powder was JIP301A, manufactured by JFE Steel Corporation, which is a pure iron powder made of pure iron. The iron-based powder had an average particle size of 95 m. The copper powder was CuAtW-250, manufactured by Fukuda Metal Foil & Powder Co., Ltd., which is a pure copper powder made of pure copper. The copper powder had an average particle size of 35 m. All of the samples further included a lubricant. Regarding each of the samples, the raw material powder that forms a sintered part was a raw material powder including the mixed powder for powder metallurgy and the lubricant. The lubricant was included in an amount of 0.8 mass % based on a total mass of the raw material powder. The lubricant was BAP-1, manufactured by Dainichi Chemical Industry Co., Ltd., which is a powder made of ethylene bis-stearamide. The lubricant had an average particle size of 25 m. Samples No. 1-1 to No. 1-5, No. 1-11, and No. 1-12 included different respective graphite powders.

[0052] In Samples No. 1-1 to No. 1-5, the graphite powders primarily included a spherical graphite powder. The graphite powders of Samples No. 1-1 to No. 1-5 were graphite powders of the CGB series, manufactured by Nippon Graphite Industries, Co., Ltd.). In Samples No. 1-1 to No. 1-5, the content ratio of the spherical graphite powder to the graphite powder was 70% or greater. The graphite powders of Samples No. 1-1 to No. 1-5 had different respective average particle sizes. The average particle sizes of the graphite powders were each a median diameter D50, which is a diameter at a cumulative 50%, starting from the small-diameter-side, in a number-based particle size distribution measured by a laser diffraction particle size distribution analyzer. The average particle sizes of the graphite powders were 6 m in Sample No. 1-1, 8 m in Sample No. 1-2, 12 m in Sample No. 1-3, 15 m in Sample No. 1-4, and 20 m in Sample No. 1-5. FIG. 2 shows an SEM image of the graphite powder of Sample No. 1-1.

[0053] In Samples No. 1-11 and No. 1-12, the graphite powders primarily included a flaky graphite powder. The graphite powder of Sample No. 1-11 was J-CPB, manufactured by Nippon Graphite Industries, Co., Ltd. The graphite powder of Sample No. 1-12 was CPB, manufactured by Nippon Graphite Industries, Co., Ltd. In Samples No. 1-11 and No. 1-12, the content ratio of the flaky graphite powder to the graphite powder was 70% or greater. The graphite powders of Samples No. 1-11 and No. 1-12 had different respective average particle sizes. The average particle sizes of the graphite powders were 5 m in Sample No. 1-11 and 23 m in Sample No. 1-12. FIG. 4 shows an SEM image of the graphite powder of Sample No. 1-11.

<Apparent Density of Graphite Powder>

The apparent density of the graphite powder of each of the samples was measured. The apparent density of the graphite powder was measured in accordance with JIS Z 2504:2020, Metallic powders-Determination of apparent density. The funnel that was used for the measurement had an orifice diameter of 5.50.1 mm and a funnel angle of 60. The results are shown in FIG. 5. In the graph shown in FIG. 5, the horizontal axis represents the average particle size of the graphite powder, and the vertical axis represents the apparent density of the graphite powder. The solid black circles indicate the results of Samples No. 1-1, No. 1-2, No. 1-3, and No. 1-5. The open white circles indicate the results of Samples No. 1-11 and No. 1-12.

[0054] As shown in FIG. 5, the apparent densities of Samples No. 1-1, No. 1-2, No. 1-3, and No. 1-5 were higher than those of Samples No. 1-11 and No. 1-12. This result indicates that in the instance where the graphite powder primarily includes a spherical graphite powder, the resulting apparent density is higher than in the instance where the graphite powder primarily includes a flaky graphite powder.

<Tap Density of Graphite Powder>

The tap density of the graphite powder of each of the samples was measured. The tap density of the graphite powder was measured in accordance with JIS Z 2512:2012, Metallic powders-Determination of tap density. The measurement apparatus used was a tap denser KYT-5000, manufactured by Seishin Enterprise Co., Ltd. A tapping stroke of 10 mm, a tapping speed of 180 tap/min, and a tap number of 700 or greater were employed. The results are shown in FIG. 6. In the graph shown in FIG. 6, the horizontal axis represents the average particle size of the graphite powder, and the vertical axis represents the tap density of the graphite powder. The solid black circles indicate the results of Samples No. 1-1 to No. 1-5. The open white circles indicate the results of Samples No. 1-11 and No. 1-12.

[0055] As shown in FIG. 6, the tap densities of Samples No. 1-1 to No. 1-5 were higher than those of Samples No. 1-11 and No. 1-12. This result indicates that in the instance where the graphite powder primarily includes a spherical graphite powder, the resulting tap density is higher than in the instance where the graphite powder primarily includes a flaky graphite powder.

<Dustiness of Graphite Powder>

[0056] The dustiness of the graphite powder of each of the samples was investigated. The dustiness of the graphite powder was evaluated by measuring the number of particles of the graphite powder that became airborne, with a testing device 10, illustrated in FIG. 7. The testing device 10 included a case 11, an orifice 12, and a dust meter 13. The case 11 was an acrylic case. The case 11 had a size of 500 mm in width W, 350 mm in depth D, and 400 mm in height H1. The depth D was a length in a direction orthogonal to the plane of FIG. 7. The orifice 12 was attached to a top plate of the case 11. The dust meter 13 was disposed adjacent to a side plate of the case 11. The dust meter 13 was placed on a base 14. The base 14 had a height H2 of 50 mm. The dust meter 13 had a height H3 of 178 mm. A length L was 350 mm, where the length L extended from a center of the orifice 12 to the side plate of the case 11, adjacent to which the dust meter 13 was disposed. The dust meter 13 used was a PM 2.5 dust monitor DC110PRO, manufactured by Sato Shouji Inc. The dust meter 13 measured the number of particles by laser light scattering. The measurement started in an environment in which the number of particles of 0.5 m or greater in size was 500 or fewer.

[0057] The raw material powder 7 of each of the samples was allowed to free-fall from the orifice 12 into the case 11, and the maximum number of counts of particles of the raw material powder 7 that became airborne was measured with the dust meter 13. In the present example, the measurement was also performed, for reference, on Sample No. 100, which was made exclusively of an iron-based powder. The results are shown in FIG. 8. In the graph shown in FIG. 8, the horizontal axis represents the sample number, and the vertical axis represents the maximum number of counts measured by the dust meter 13. In FIG. 7, an image of the particles of the powder that became airborne is represented by the dashed lines for ease of understanding.

[0058] In Sample No. 100, the number of particles that became airborne was 600 or fewer, as shown in FIG. 8. Sample No. 100 did not include any graphite powder and included only an iron-based powder, and, consequently, Sample No. 100 was unlikely to become airborne. A comparison between Sample No. 1-1 and Sample No. 1-11, which had similar average particle sizes, revealed that in Sample No. 1-1, the number of particles that became airborne was reduced by approximately 60%, compared to Sample No. 1-11. It is believed that in Sample No. 1-1, since the graphite powder primarily included a spherical graphite powder, the graphite powder was unlikely to become airborne, and that, consequently, the powder as a whole was unlikely to become airborne. Likewise, a comparison between Sample No. 1-5 and Sample No. 1-12, which had similar average particle sizes, revealed that in Sample No. 1-5, the number of particles that became airborne was reduced by approximately 50%, compared to Sample No. 1-12. It is believed that in Sample No. 1-5, since the graphite powder primarily included a spherical graphite powder, the graphite powder was unlikely to become airborne, and that, consequently, the powder as a whole was unlikely to become airborne. These results indicate that when the graphite powder primarily includes a spherical graphite powder, the powder as a whole is unlikely to become airborne.

[0059] A comparison between Samples No. 1-1, No. 1-2, and No. 1-5, in each of which the graphite powder primarily included a spherical graphite powder, revealed that as the average particle size of the graphite powder increased, the number of particles that became airborne decreased, as shown in FIG. 8. In other words, as the average particle size of the graphite powder decreased, the number of particles that became airborne increased.

[0060] Apparently, in the instance of the graphite powder primarily including a spherical graphite powder, even if the number of particles that become airborne increases, the likelihood of the graphite powder becoming airborne is reduced, and, consequently, the likelihood of the powder as a whole becoming airborne is reduced, compared to the instance in which the graphite powder primarily includes a flaky graphite powder, provided that the average particle sizes are similar.

Test Example 2

In Test Example 2, a sintered part was prepared with Sample No. 1-3, another sintered part was prepared with Sample No. 1-12, and the dimensional variations of the sintered parts were evaluated.

[0061] The raw material powder of Sample No. 1-3 was press-molded to form a compact, and the compact was sintered to form the sintered part. Likewise, the raw material powder of Sample No. 1-12 was press-molded to form a compact, and the compact was sintered to form the sintered part. The conditions for the press molding and the conditions for the sintering were the same between the samples. The compacts had a cylindrical shape with an outside diameter of 70 mm and an inside diameter of 45 mm. Groups of the compacts were prepared, with each of the groups including vertically stacked three compacts. The groups were placed on a bench in a 55 arrangement and subjected to sintering, which was performed in a continuous sintering furnace. The sintering temperature was 1100 C. or greater, and the holding time associated with the sintering temperature was 15 minutes or greater. The atmosphere for the sintering was a nitrogen atmosphere.

[0062] In FIG. 9, multiple sintered parts 20 resulting from the sintering, which were placed on a bench 21, are illustrated. In FIG. 9, each of the sintered parts 20 is illustrated as having a solid cylindrical shape, for ease of understanding. For the evaluation of the dimensional variations in the sintered parts 20, two sintered part groups 20A and two sintered part groups 20B, among the multiple sintered parts 20, were used. The sintered part groups 20A were located at or near a center of the bench 21, and the sintered part groups 20B were located at or near a periphery of the bench 21. Dimensions of each of the sintered parts 20 of the two sintered part groups 20A and each of the sintered parts 20 of the two sintered part groups 20B were measured, and the dimensional variations in the total of 12 sintered parts 20 were determined. The results are shown in FIG. 10. In the graph shown in FIG. 10, the horizontal axis represents the sample number, and the vertical axis represents the range of dimensional variations of the sintered part 20.

[0063] As shown in FIG. 10, Sample No. 1-3, in which the graphite powder primarily included a spherical graphite powder, had smaller dimensional variations than in the instance where the graphite powder primarily included a flaky graphite powder. In the instance of Sample No. 1-3, since the graphite powder primarily included a spherical graphite powder, it was unlikely that the graphite powder became airborne when the mixed powder for powder metallurgy was supplied to the press mold. Thus, it is believed that the graphite powder was also unlikely to be unevenly distributed in the compact, and that, as a result, the sintered parts were unlikely to have differences in carbon concentration. It is believed that the unlikelihood of the occurrence of differences in carbon concentration resulted in the unlikelihood of the occurrence of variations in dimensional accuracy.