JP2020139193A - Machine component, and method for producing the same - Google Patents

Abstract

To provide a machine component made of a sintered metal in which the hardness (wear resistance) of a slide face to slide with a mating component is increased.SOLUTION: A gear 1 as a machine component is made of a sintered compact having a composition essentially consisting of Fe, and the balance Ni, Mo and C with inevitable impurities, and has a slide face F to slide with a mating component. The sintered compact has: a first surface hardened layer 6 formed by carbonitriding, quenching, and tempering; and a second surface hardened layer 7 formed by work-hardening the surface layer part of the first surface hardened layer 6, and the surface of the second surface hardened layer 7 is provided with the slide face F.SELECTED DRAWING: Figure 4

Description

The present invention relates to a mechanical part and a method for manufacturing the same, and more particularly to a mechanical part made of a sintered body of an iron alloy and having a sliding surface that slides with a mating part and a method for manufacturing the same.

In recent years, in order to reduce the cost of mechanical products such as automobiles and other vehicles and industrial machines, mechanical parts such as gears and cams, which are the constituent parts, are sintered from molten products made of molten materials to iron-based sintering. Attempts have been made to replace the body with a sintered product as the base material. However, since the sintered body is a porous body having innumerable internal pores, the sintered product has mechanical strength and wear resistance (wear resistance of the sliding surface sliding with the mating part) as compared with the molten product. ) Is inferior. Therefore, various technical means for improving the mechanical strength and abrasion resistance of the sintered body have been proposed.

For example, Patent Document 1 below describes sintering by carburizing, quenching, and quenching a sintered body of an iron-based alloy containing a predetermined amount of nickel, molybdenum, copper, and carbon, and the balance being iron and unavoidable impurities. It is described that a surface hardened layer of about HV 700 to 800 is formed on the body and the internal hardness of the sintered body is adjusted to about HV 450 to 500. According to such technical means, it is possible to obtain a mechanical part made of sintered metal having both desired wear resistance and toughness.

Further, in Patent Document 2 below, a predetermined surface of a sintered body (for example, a surface that becomes a sliding surface that slides with a mating component) is subjected to plastic working such as shot peening or rolling to obtain the predetermined surface. It is described that the surface opening is sealed, and then cold forging is performed on the predetermined surface to which the lubricant is applied. By doing so, it is possible to obtain a sliding surface having high strength and high hardness and a small surface roughness (arithmetic mean roughness) Ra value.

JP-A-2007-138226 Japanese Patent No. 6087042

By the way, mechanical parts such as gears and cams are often used in the presence of a lubricant such as a lubricating oil or a lubricating grease. For example, in a variable valve timing device provided between a crankshaft and a camshaft to adjust the opening / closing timing of an engine intake / exhaust valve, engine oil circulates inside the variable valve timing device, so that the engine oil is formed inside the device. Engine oil is always present in the sliding parts between the mechanical parts. Therefore, it is considered that an exceptionally high wear resistance is not required for the sliding surface of the mechanical part.

However, the engine oil may contain minute hard foreign matter such as soot generated in the engine, and the above device operates with the engine oil containing the hard foreign matter intervening in the sliding parts between the parts. If this is done, wear of the sliding surface of the mechanical component is likely to proceed. In particular, when the sliding surface becomes rough due to various treatments (for example, carburizing, quenching, tempering, etc.) on the sintered body, the sliding surface is more likely to be worn. .. In this regard, the technical means disclosed in Patent Document 2 can realize high strength and high hardness of the sliding surface while suppressing the surface roughness of the sliding surface of the mechanical component to a small value. It can be said that it is suitable as a technical means for improving the wear resistance of the sliding surface. However, as described above, it is not always possible to appropriately suppress the wear of the sliding surface in a mechanical part that is continuously used in a harsh environment in which a lubricant containing a hard foreign substance is present.

In view of the above circumstances, the present invention is harsh because it is made of an iron-based sintered body and has a sliding surface that slides with a mating component, and the hardness of the sliding surface is increased to improve wear resistance. It is an object of the present invention to provide mechanical parts capable of exhibiting a predetermined function for a long period of time even in an environment.

The present invention, which was conceived to achieve the above object, is composed of a sintered body containing Fe as a main component and Ni, Mo, C and unavoidable impurities as the balance, and has a sliding surface that slides with a mating component. A second surface-hardened mechanical component having a first surface-hardened layer formed by carburizing, quenching and tempering, and a second surface-hardened layer formed by work-hardening the surface layer portion of the first surface-hardened layer. It has a layer, and is characterized in that a sliding surface is provided on the surface of the second surface-hardened layer.

The sintered body, which is the base material of the mechanical parts according to the present invention, contains Fe as a main component and Ni and Mo as elements for improving hardenability. Therefore, if the sintered body is carburized and hardened in the process of manufacturing mechanical parts, a first surface hardened layer (carburized and hardened layer) having high strength and high hardness can be formed with high accuracy. Further, the sintered body, which is a base material of mechanical parts, has a second surface-hardened layer formed by work-hardening the surface layer portion of the first surface-hardened layer, and the second surface-hardened layer of the second surface-hardened layer. Since a sliding surface with the mating component is provided on the surface, it is possible to obtain a sliding surface with significantly increased hardness. Therefore, according to the present invention, a machine made of sintered metal, which has high mechanical strength of the entire part, is rich in toughness (fatigue resistance), and has significantly improved wear resistance of the sliding surface with the mating part. You can get the parts.

The rougher the sliding surface, the easier it is for the sliding surface to wear when it slides with the mating component. Therefore, the surface roughness of the sliding surface is preferably Ra 1.0 μm or less. The term "Ra" as used herein means the arithmetic mean roughness specified in JIS B 0601: 2013.

From the viewpoint of suppressing wear on the sliding surface, the hardness of the sliding surface is preferably 900 or more, preferably 1000 or more, based on the Vickers hardness [Hv0.2] measured under the condition of a load of 0.2 kg. More preferred. Further, the thickness (depth from the surface) of the second surface-hardened layer can be 5 to 50 μm. With such a configuration, it is possible to effectively suppress the wear of the sliding surface even when the mechanical parts are used in a harsh environment where the wear of the sliding surface is likely to progress.

In the above configuration, the sintered body contains 1.0 to 3.0% by mass of Ni, 0.5 to 2.0% by mass of Mo, 0.1 to 0.3% by mass of C, and the balance is Fe. And can be an unavoidable impurity.

Further, in order to achieve the above object, the present invention is composed of a sintered body containing Fe as a main component and Ni, Mo, C and unavoidable impurities as the balance, and has a sliding surface that slides with a mating component. A method for manufacturing mechanical parts, in which a first surface-hardened layer is formed on the sintered body by subjecting the sintered body to carburizing and quenching, and then at least the above-mentioned sliding surface of the sintered body. Provided is a method for manufacturing a mechanical part, which comprises forming a second surface-hardened layer on the surface layer portion of the first surface-hardened layer by subjecting the sintered body to a surface treatment for plastically deforming the surface to be formed. To do.

According to such a manufacturing method, it is possible to enjoy the same action and effect as the mechanical parts according to the present invention.

Shot peening can be selected as the above surface treatment. With shot peening, a surface-hardened layer (second surface-hardened layer) can be easily and accurately formed at any location. In particular, as a shot material, the average particle size (number average particle size) is 200 μm.
If the following items are selectively used, the surface roughness of the collision surface on which the shot material collides can be finished to Ra 1.0 μm or less, so that the wear resistance of the sliding surface is improved (the amount of wear on the sliding surface is suppressed). ) It becomes advantageous on.

The sintered body may be obtained by sintering a green compact of a raw material powder containing graphite powder and a partially alloyed powder in which a part of Ni and Mo is alloyed with Fe, and not Cu.

From the above, according to the present invention, the hardness of the sliding surface that slides with the mating part is remarkably increased, so that the machine is used in a harsh environment where a hard foreign substance is present at the sliding portion with the mating part. Even in such a case, it is possible to provide a mechanical component capable of stably exhibiting a desired function for a long period of time.

It is a top view of the mechanical parts which concerns on one Embodiment of this invention. FIG. 1 is a cross-sectional view taken along the line AA of FIG. FIG. 2 is a cross-sectional view taken along the line BB of FIG. FIG. 1 is a cross-sectional view taken along the line CC of FIG. It is a block diagram which shows the manufacturing procedure of the mechanical part which concerns on this invention. It is a figure which shows the test result of the confirmation test. It is a figure which shows the hardness measurement result of the test body surface layer part used in the confirmation test.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

1 is a plan view of a mechanical component according to an embodiment of the present invention, FIG. 2 is a cross-sectional view taken along the line AA of FIG. 1, and FIG. 3 is a view taken along the line BB (part) of FIG. The cross-sectional view and FIG. 4 are cross-sectional views taken along the line CC of FIG. The mechanical parts shown in FIGS. 1 to 4 are annular gears 1 used by being incorporated in a variable valve timing device that adjusts the opening / closing timing of intake / exhaust valves of an automobile engine, and more specifically, Fe is mainly used. An external tooth made of an iron-based (iron-based alloy) sintered body containing Ni, Mo, C and unavoidable impurities as components, and having a tooth surface 2 that meshes with the tooth surface of an internal gear on the outer peripheral portion. It is a gear. The gear 1 integrally has an annular portion 3 and a circumferentially endped protruding portion 4 projecting from one side of the annular portion 3 in the axial direction, and the tooth surface 2 has the annular portion 3 and the protruding portion 4. It is provided on the outer peripheral portion of the.

Although detailed illustration is omitted, when the gear 1 is incorporated in the variable valve timing device, the gear 1 is rotatably supported by a rolling bearing having an outer ring fitted and fixed to the inner circumference (center hole) thereof. At the same time, the protruding portion 4 is fitted into a recess provided in the annular metal mating component arranged so as to face the outer side in the axial direction of the gear 1. Then, when the variable valve timing device is driven, both side surfaces (end surfaces on one side and the other side in the circumferential direction) 5 and 5 of the protrusion 4 provided on the gear 1 repeatedly slide with the mating component (the other parts). The sliding direction is along the radial direction of the gear 1). Therefore, in the gear 1 of the illustrated example, both side surfaces 5 and 5 of the protruding portion 4 serve as sliding surfaces F with the mating component.

As shown in FIGS. 2 to 4, the gear 1 has a second surface formed by work hardening (plastic deformation) of the first surface hardened layer 6 and the surface layer portion of the first surface hardened layer 6. It has a hardened layer 7. In the gear 1 of the present embodiment, only the sliding surface F is provided on the surface of the second surface hardened layer 7, and the surfaces other than the sliding surface F are provided on the surface of the first surface hardened layer 6. ing. The details will be described later, but the first surface-hardened layer 6 is formed by carburizing and quenching (carburizing and quenching and tempering) the sintered body, which is the base material of the gear 1, as a heat treatment, and the second surface-hardened layer 7 is formed. Is formed by subjecting the surface to be formed of the sliding surface F to shot carburizing as a surface treatment in the sintered body after the heat treatment. Therefore, although not shown, the sliding surface F is a pear-ground surface in which innumerable indentations (micro dimples having a depth of about 2 μm) formed by the shot material colliding with the sintered body exist. There is.

The surface hardness of the first surface-hardened layer 6 (hardness of surfaces other than the sliding surface F) is about 800 in Vickers hardness [Hv0.2] measured at a load of 0.2 kg, and the second surface-hardened layer The surface hardness of No. 7 (hardness of the sliding surface F) is 900 or more, preferably 1000 or more in the same Vickers hardness. Further, although it depends on the size of the gear 1 and the like, the thickness of the first surface-hardened layer 6 (depth from the surface of the gear 1) is set to, for example, about 300 μm, and the thickness d of the second surface-hardened layer 7 (d). (See FIG. 4) is, for example, about 5 to 50 μm.

As shown in FIG. 5, the gear 1 having the above configuration can be manufactured through the compression molding step S1, the sintering step S2, the shape modification step S3, the heat treatment step S4, and the surface treatment step S5 in this order. Hereinafter, each of the above steps will be described.

[Compression molding step S1]
In this step, among the molding dies having core pins, dies, lower punches and upper punches coaxially arranged so as to be relatively movable up and down, powder compact molding is performed in the cavity defined by the core pins, dies and lower punches. The raw material powder for use is filled, and then the raw material powder filled in the cavity is compression-molded by the lower punch and the upper punch that is moved relatively close to the lower punch. A green compact having a shape corresponding to the above is obtained.

The raw material powder for compaction molding is a mixed powder obtained by mixing iron powder (alloy steel powder) containing Ni and Mo and graphite powder composed of carbon (C), and is a copper powder as a single powder or an alloy component. Does not contain copper (Cu) as. As the above alloy steel powder, for example, in addition to a complete alloy powder in which Ni and Mo are completely alloyed with Fe, a part of Ni (Ni particles) and Mo (Mo particles) is alloyed with Fe (Fe particles). Partially alloyed powder can be used. However, since the complete alloy powder has a higher hardness than the partially alloyed powder, it is inferior in compression moldability to the partially alloyed powder, and in order to stably obtain a high-density green compact, a large molding pressure is large. It may be necessary to use a press device. Therefore, as the alloy steel powder, it is preferable to use a partially alloyed powder (Fe—Ni—Mo-based alloy steel powder) in which a part of Ni and Mo is alloyed with Fe.

Here, the reason why Cu is not included in the raw material powder will be described. First, when the green compact containing Cu is heated and sintered, there is an advantage that the mechanical strength of the sintered body can be increased by diffusing Cu into Fe during sintering. However, when the green compact containing Cu is heated and sintered, there is a demerit that the density and dimensional accuracy of the sintered body are lowered due to the expansion of Cu. Further, Cu is expensive, and even when a sintered body of a green compact containing Cu is heat-treated (carburized and hardened), the hardenability (surface hardness) of the sintered body is not improved. In consideration of the above, the demerits caused by including Cu in the raw material powder outweigh the advantages enjoyed by including Cu in the raw material powder. Therefore, Cu is not included in the raw material powder.

The molding powder contains 1.0 to 3.0% by mass of Ni, 0.5 to 2.0% by mass of Mo, and 0.1 to 0.3% by mass of C in the sintering step S2 described later. The blending ratio of the alloy steel powder and the graphite powder is adjusted so that a sintered body in which the balance is Fe and unavoidable impurities can be obtained. The reason why the blending ratios of Ni, Mo and C are set within the above numerical range is as follows.

First, Ni and Mo are elements that improve hardenability, and the heat treatment step S4 described later
It is added to improve the hardenability by carburizing and quenching carried out in. The blending ratio of Ni and Mo is within the above range in order to appropriately and at low cost to enjoy the effect of improving hardenability.

Further, C (graphite powder) is mainly blended in order to increase the strength and hardness of the sintered body by reacting iron and carbon at the time of sintering the green compact. Therefore, when the blending amount of the graphite powder is less than 0.1% by mass, it becomes difficult to obtain a sintered body having desired mechanical strength and the like. On the other hand, in the present embodiment, in the heat treatment step S4 described later, the sintered body is subjected to carburizing and quenching as heat treatment to form a first surface hardened layer 6 composed of a carburizing and quenching layer on the surface layer portion of the sintered body. Therefore, when the blending amount of the graphite powder exceeds 0.3% by mass, the first surface-hardened layer 6 is appropriately formed on the sintered body, and the desired toughness (fatigue resistance) of the sintered body is ensured. It becomes difficult to do. Further, since graphite has a small specific gravity, there is a demerit that the density is lowered when the blending amount is large. Therefore, the blending ratio of graphite powder is within the above range.

A solid lubricant such as zinc stearate may be added to the raw material powder for compaction molding in order to improve the moldability and releasability of the green compact and the durability of the molding die. Further, at the time of compression molding of the green compact, either one or both of the molding powder and the molding die may be heated (warm molding).

[Sintering step S2]
In this step, the green compact obtained in the compression molding step S1 is heated and sintered at a predetermined temperature and time to form a sintered body in which the alloy steel powder particles constituting the green compact are neck-bonded to each other. obtain. The heating temperature of the green compact is set higher than the temperature at which iron and carbon start the reaction (900 ° C.), and more specifically, it is set to 1100 to 1300 ° C. Therefore, there is no graphite structure in the structure of the sintered body obtained at the completion of the sintering step S2. The metal structure of the sintered body is 1.0 to 3.0% by mass of Ni and 0.5 to 0.5% of Mo, following the blending ratio of iron powder (alloy steel powder) that constitutes the powder for forming the green compact. It is a structure containing 2.0% by mass and 0.1 to 0.3% by mass of C, and the balance is Fe and unavoidable impurities.

[Shape retouching process S3]
In this step, the sintered body obtained in the sintering step S2 is appropriately subjected to sizing processing or machining (for example, cutting processing) to finish the sintered body into a finished product shape. However, this shape modification step S3 is performed only when the sintered body obtained in the sintering step S2 does not have a predetermined shape and dimensional accuracy, and is not necessarily carried out. When the shape of the sintered body that finally becomes the annular gear (external gear) 1 is modified as in the present embodiment, the sizing process is performed, for example, in order to correct the axial dimension of the sintered body. Will be implemented.

[Heat treatment step S4]
In this step, the sintered body is carburized and hardened to form a first surface hardened layer 6 composed of a carburized and hardened layer over the entire surface layer portion of the sintered body, and then the sintered body is subjected to carburizing and quenching. By performing tempering, retained austenite generated in the Fe structure of the sintered body due to quenching is removed. As a result, a sintered body having increased surface hardness (wear resistance) and toughness (fatigue resistance) can be obtained. For reference, the surface hardness of the sintered body before carburizing and quenching is about 250 in Vickers hardness [Hv0.2] measured under the condition of a load of 0.2 kg, and the surface hardness of the sintered body after carburizing and quenching and quenching. (The surface hardness of the first surface hardening layer 6) and the hardness of the core portion are about 800 and 550, respectively, at the same Vickers hardness. The thickness of the first surface-hardened layer 6 (depth from the surface of the sintered body) can be adjusted by the quenching time or the like, but can be, for example, about 0.3 mm.

[Surface treatment step S5]
In this step, of the heat-treated sintered body (sintered body in which the first surface-hardened layer 6 is formed over the entire surface layer portion), at least a shot as a surface treatment on the planned formation surface of the sliding surface F. Apply peening. Shot peening is cold working in which a hard shot material injected from a powder injection device collides with a work piece (sintered body) at high speed. Of the sintered body, the collision surface where the shot material collides and its vicinity. Plastic deformation occurs in the region. In the portion where the plastic deformation occurs, work hardening occurs due to so-called work-induced martensitic transformation, and the porous metal structure becomes dense (the porosity of the metal structure decreases). Therefore, in the surface layer portion of the sintered body including the surface to be formed of the sliding surface F subjected to shot peening, the porous metal structure is denser and has higher hardness than the non-treated region of shot peening. The surface hardened layer 7 (see FIG. 4) is formed. As described above, the second surface-hardened layer 7 is formed by processing and hardening the surface layer portion of the first surface-hardened layer 6, so that the surface hardness of the second surface-hardened layer 7 is the first surface-hardening. It becomes higher than the surface hardness of the layer 6. As a result, a sintered body having the highest surface hardness of the sliding surface F can be obtained.

The surface hardness of the second surface-hardened layer 7 formed by shot peening depends on the particle size of the shot material used, and the smaller the particle size of the shot material used, the higher the effect of improving the surface hardness. There was found. Specifically, when shot peening is performed using a shot material having an average particle size of more than 200 μm, a second surface-hardened layer 7 having a surface hardness of about 900 Hv0.2 is obtained, and the particle size is 200 μm or less. When shot peening was performed using the shot material of No. 1, a second surface-hardened layer 7 having a surface hardness of about 1100 Hv0.2 was obtained.

In addition, the collision surface with which the shot material collides is a pear ground with innumerable indentations (micro dimples) formed by the collision of the shot material, so compared to the non-collision surface of the shot material. It was found that the value of the arithmetic mean roughness Ra increases, but the smaller the particle size of the shot material used, the more the increase in the Ra value can be suppressed. In particular, when a shot material having an average particle size of 200 μm or less is used, the surface roughness of the collision surface (sliding surface F) of the shot material can be suppressed to Ra 1.0 μm or less. The smaller the value of the surface roughness Ra of the sliding surface F, the more advantageous it is in suppressing the wear of the sliding surface F. Therefore, in consideration of this point, it is preferable that the shot peening to be applied to the surface to be formed of the sliding surface F among the sintered bodies is carried out using a shot material having a particle size of 200 μm or less.

By going through the steps S1 to S5 described above (the shape retouching step S3 is optional), the sintered metal gear 1 described with reference to FIGS. 1 to 4 can be obtained.

When driving a variable valve timing device that incorporates gears (external gears) as described above, engine oil constantly circulates in its internal space, so the sliding surface of the gears is made of metal via an oil film of engine oil. It slides repeatedly with the mating part of. Therefore, it is considered that the wear of the sliding surface can be suppressed as much as possible. However, engine oil often contains minute hard foreign substances such as soot that cannot be completely captured by the oil filter. If the variable valve timing device is driven with engine oil containing hard foreign matter intervening between the gear and the sliding part of the mating part, the sliding surface of the gear tends to wear, which adversely affects the durable life of the device. Extends.

On the other hand, the gear 1 according to the embodiment of the present invention having the configuration described above uses Fe as a main component and a sintered body containing Ni and Mo as elements for improving hardenability as a base material. Therefore, when this sintered body is carburized and hardened (carburized and hardened and tempered), a carburized and hardened layer (first surface hardened layer) having high strength and high hardness is formed with high accuracy. Further, the sintered body which is the base material of the gear 1 has a second surface hardened layer 7 formed by work hardening of the surface layer portion of the first surface hardened layer 6, and the second surface hardened layer 7. Since the sliding surface F with the mating component is provided on the surface of the layer 7, the hardness (wear resistance) of the sliding surface F is 900Hv0.2 or more, preferably 1100Hv0.
.. It is possible to obtain the gear 1 which is raised to 2 or more. Therefore, as described above, even when the variable valve timing device is driven in a harsh environment where engine oil containing hard foreign matter such as soot intervenes between the gear 1 and the sliding portion of the mating component, the sliding of the gear 1 Wear of the moving surface F can be effectively suppressed.

Further, since the gear 1 of the present embodiment has the first surface hardened layer 6 formed by carburizing and quenching the sintered body which is the base material thereof over the entire surface layer portion, it is mechanical as a whole part. High strength. Further, since the sintered body after carburizing and quenching is tempered to increase the toughness of the sintered body, the gear 1 of the present embodiment also has excellent fatigue resistance. Therefore, by using the gear 1 of the present embodiment, it is possible to realize a variable valve timing device capable of stably exhibiting a predetermined function for a long period of time.

Although the sintered metal gear 1 and the manufacturing method thereof according to the embodiment of the present invention have been described above, the present invention is limited to the gear (external gear) 1 as described above. Not that. That is, the present invention can also be applied to other sintered metal mechanical parts (for example, internal gears, cams, bearings, etc.) having a sliding surface with the mating part.

In order to demonstrate the usefulness of the present invention, what kind of surface treatment is applied to the sintered body that has been carburized, quenched and tempered in order to increase the surface hardness of the sintered body and also to roughen the surface of the sintered body. It was confirmed whether it was effective in suppressing the deterioration of the roughness. Specifically, the following four types of test bodies (first to fourth test bodies) are prepared, and the Vickers hardness of the surface of each test body that is the sliding surface with the mating part is loaded with 0. In addition to the measurement under the condition of .2 kg, the arithmetic average roughness Ra of the surface to be the sliding surface was measured using a surface roughness measuring machine (SE600) manufactured by Kosaka Research Institute.
[First test piece]
A gear made of a sintered body containing Fe as a main component and Ni, Mo, C and unavoidable impurities as the balance (a gear having a shape and composition similar to that of gear 1 described with reference to FIG. 1 and the like). The above sintered body is carburized, quenched and tempered.
[Second test piece]
A gas soft nitriding treatment as a surface treatment (surface modification treatment) is applied to a first test piece (a sintered body that has been carburized, quenched, and tempered. The same shall apply hereinafter).
[Third specimen]
Of the first test piece, the surface to be the sliding surface with the mating part is shot peened as a surface treatment. However, shot peening is carried out using a shot material having an average particle size of more than 200 μm.
[Fourth specimen]
Of the first test piece, the surface to be the sliding surface with the mating part is shot peened as a surface modification treatment. However, shot peening is carried out using a shot material having an average particle size of 200 μm or less.

FIG. 6 summarizes the surface treatment method applied to each test piece, the hardness measurement result of the sliding surface provided on each test piece, the measurement result of the surface roughness of the sliding surface, and the like. In FIG. 6, the first to fourth test pieces are displayed as Comparative Example 1, Comparative Example 2, Example 1 and Example 2, respectively. The measurement results of hardness and surface roughness are average values of three test pieces. In addition, FIG. 7 shows the hardness measurement results of the region including the sliding surface in each test piece.

As shown in FIG. 6, there is sliding between Comparative Example 1 composed of a sintered body not subjected to surface treatment after carburizing and quenching tempering and Comparative Example 2 consisting of a sintered body subjected to gas soft nitriding after carburizing and quenching tempering. There is no big difference in surface hardness and surface roughness. However, as shown in FIG. 7, the internal hardness of Comparative Example 2 composed of a sintered body subjected to gas soft nitriding after carburizing, quenching and tempering is significantly lower than that of Comparative Example 1.

On the other hand, in Examples 1 and 2, which consist of a sintered body subjected to shot peening as a surface treatment after carburizing, quenching and tempering, as shown in FIGS. 6 and 7, the hardness of the sliding surface is higher than that of Comparative Example 1. Although the hardness of the surface layer was improved and the sliding surface was roughened. However, in Example 2 in which shot peening was performed using a shot material having an average particle size of 200 μm or less, roughening of the sliding surface could be suppressed and the effect of improving the hardness of the sliding surface was also increased.

From the above, it is better to select shot peening when performing surface treatment to increase the surface hardness of a sintered body that has been carburized, quenched and tempered, and in particular, a shot material having an average particle size of 200 μm or less is used. It turns out that it is better to select the shot peening used.

1 Gear (gear made of sintered body)
2 Tooth surface 3 Ring part 4 Protruding part 5 Side surface (sliding surface)
6 First surface-hardened layer 7 Second surface-hardened layer F Sliding surface S1 Compression molding process S2 Sintering process S3 Shape modification process S4 Heat treatment process S5 Surface treatment process

Claims (7)

It is a mechanical part that is composed of a sintered body containing Fe as a main component and Ni, Mo, C and unavoidable impurities as the balance, and has a sliding surface that slides with the mating part.
The sintered body has a first surface-hardened layer formed by carburizing, quenching and tempering, and a second surface-hardened layer formed by work-hardening the surface layer portion of the first surface-hardened layer. , A mechanical component characterized in that the sliding surface is provided on the surface of the second surface-hardened layer. The mechanical component according to claim 1, wherein the surface roughness of the sliding surface is Ra 1.0 μm or less. The sliding surface has a Vickers hardness [Hv0.2] of 900 or more measured under a load of 0.2 kg, and the thickness of the second surface-hardened layer is 5 to 50 μm according to claim 1 or 2. Described mechanical parts. The sintered body contains 1.0 to 3.0% by mass of Ni, 0.5 to 2.0% by mass of Mo, 0.1 to 0.3% by mass of C, and Fe and unavoidable impurities in the balance. The mechanical part according to any one of claims 1 to 3. It is a method for manufacturing a mechanical part having a sliding surface that slides with a mating part and is composed of a sintered body containing Fe as a main component and the balance being Ni, Mo, C and unavoidable impurities.
By subjecting the sintered body to carburizing, quenching and tempering, a first surface-hardened layer is formed on the sintered body, and then at least the surface of the sintered body to be formed of the sliding surface is plastically deformed. A method for manufacturing a mechanical part, which comprises forming a second surface-hardened layer on the surface layer portion of the first surface-hardened layer by applying the treatment to the sintered body. The method for manufacturing a mechanical part according to claim 5, wherein the surface treatment is shot peening using a shot material having an average particle diameter of 200 μm or less. The sintered body is a sintered body obtained by sintering a green compact of a raw material powder containing graphite powder and partially alloyed powder in which a part of Ni and Mo is alloyed with Fe, and does not contain Cu. Or the method for manufacturing mechanical parts according to 6.

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