US20220105564A1

US20220105564A1 - Sintered material, gear, and method for producing sintered material

Info

Publication number: US20220105564A1
Application number: US17/424,904
Authority: US
Inventors: Shigeki Egashira; Takayuki Tashiro; Tomoyuki Ishimine; Kosuke TOMINAGA
Original assignee: Sumitomo Electric Sintered Alloy Ltd; Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Sintered Alloy Ltd; Sumitomo Electric Industries Ltd
Priority date: 2019-01-30
Filing date: 2020-01-29
Publication date: 2022-04-07
Also published as: DE112020000592T5; JPWO2020158788A1; WO2020158788A1

Abstract

A sintered material with a composition composed of an iron-based alloy and a structure in which the number of compound particles 0.3 μm or more in size is less than 200 per 100 μm×100 μm unit area in a cross section, wherein the sintered material has a relative density of 93% or more.

Description

The present application claims the priority of the international application PCT/JP2019/003261, filed Jan. 30, 2019, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a sintered material, a gear, and a method for producing the sintered material.

BACKGROUND ART

Patent Literature 1 discloses a sintered body with a relative density of 93% or more.

CITATION LIST

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2017-186625

SUMMARY OF INVENTION

A sintered material according to the present disclosure has
a composition composed of an iron-based alloy and
a structure in which the number of compound particles 0.3 μm or more in size is less than 200 per 100 μm×100 μm unit area in a cross section,
wherein the sintered material has a relative density of 93% or more.
A gear according to the present disclosure is formed of a sintered material according to the present disclosure.
A method for producing a sintered material according to the present disclosure includes the steps of
preparing a raw powder containing an iron-based powder,
forming a green compact with a relative density of 93% or more from the raw powder, and
sintering the green compact,
wherein the iron-based powder contains at least one of a pure iron powder and an iron-based alloy powder,
the iron-based powder is subjected to reduction treatment in the step of preparing the raw powder, and
the iron-based powder is heated in a reducing atmosphere to a temperature of 950° C. or more and less than 1100° C. in the reduction treatment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A FIG. 1 is a schematic perspective view of a gear as an example of a sintered material according to an embodiment.

FIG. 1B is an enlarged cross-sectional view of a dash-dotted circle 1B of FIG. 1A.

FIG. 2 is an enlarged schematic cross-sectional view of a cross-sectional structure of a sintered material according to an embodiment.

FIG. 3 is a graph of the relationship between the number of compound particles 0.3 μm or more in size per unit area and contact fatigue strength in a sintered material of each sample prepared in a test example 1.

PROBLEMS TO BE SOLVED BY PRESENT DISCLOSURE

There is a demand for an iron-based sintered material with improved fatigue strength.
In sintered materials, a void generally acts as a starting point of cracking and decreases strength, such as tensile strength or fatigue strength. The present inventors, however, have found that a compound particle, rather than a void, in a dense sintered material with a relative density of 93% or more acts as a starting point of cracking and decreases fatigue strength.
Accordingly, it is an object of the present disclosure to provide a sintered material with high fatigue strength. It is another object of the present disclosure to provide a gear with high fatigue strength. It is still another object of the present disclosure to provide a method for producing a sintered material with high fatigue strength.

Advantageous Effects of Present Disclosure

A sintered material according to the present disclosure and a gear according to the present disclosure have high fatigue strength. A method for producing a sintered material according to the present disclosure can produce a sintered material with high fatigue strength.

Description of Embodiments of Present Disclosure

First, embodiments of the present disclosure are described below.
(1) A sintered material according to an embodiment of the present disclosure has
a composition composed of an iron-based alloy and
a structure in which the number of compound particles 0.3 μm or more in size is less than 200 per 100 μm×100 μm unit area in a cross section,
wherein the sintered material has a relative density of 93% or more.
A sintered material according to the present disclosure has high fatigue strength. One reason for this is that a sintered material according to the present disclosure is a dense sintered material with a relative density of 93% or more. Another reason is that a sintered material according to the present disclosure has fewer compound particles 0.3 μm (300 nm) or more in size at least in a surface layer of the sintered material. The compound particles are composed of a compound such as an oxide, a sulfide, and/or a nitride. In the dense sintered material composed of an iron-based alloy, a compound particle of 0.3 μm or more can act as a starting point of cracking. In the presence of fewer compound particles of 0.3 μm or more at least in a surface layer of the sintered material, however, even when a stress is applied to the sintered material inwardly from the surface of the sintered material, the compound particles rarely act as starting points of cracking. Even when cracking occurs, the compound particles rarely develop a crack. With a lower occurrence or slower development of cracking, a sintered material according to the present disclosure can have improved fatigue strength. Such a sintered material according to the present disclosure is suitable for gears and the like.
The surface layer of the sintered material herein refers to a region up to 200 μm in depth from the surface of the sintered material. The cross section may be taken from the surface layer of the sintered material.
(2) A sintered material according to an embodiment of the present disclosure has a relative density of 97% or more.
This embodiment can more easily improve fatigue strength due to the higher density.
(3) In a sintered material according to an embodiment of the present disclosure,
the ratio of n₂₀to n, (n₂₀/n)×100, is 1% or less, wherein n denotes the number of the compound particles 0.3 μm or more in size per the unit area, and n₂₀denotes the number of compound particles 20 μm or more in size per the unit area.
This embodiment includes fewer coarse compound particles of 20 μm or more. Such an embodiment can reduce the occurrence or development of cracking caused by coarse compound particles. Thus, the embodiment can more easily improve fatigue strength.
(4) In a sintered material according to an embodiment of the present disclosure,
the iron-based alloy contains at least one element selected from the group consisting of C, Ni, Mo, Mn, Cr, B, and Si, the remainder being Fe and impurities.
A steel composed of an iron-based alloy containing the above-mentioned element, for example, an iron-based alloy containing C has high strength such as tensile strength. The embodiment composed of a high-strength iron-based alloy can more easily improve fatigue strength.
(5) A gear according to an embodiment of the present disclosure is composed of the sintered material according to any one of (1) to (4).
A mating material exerts stress on the tooth surface of each tooth of the gear during use. A gear according to the present disclosure, however, can reduce the occurrence or development of cracking caused by the compound particles in the surface layer of the sintered material. Thus, a gear according to the present disclosure can be used as a sintered gear with high fatigue strength for extended periods.
(6) A method for producing a sintered material according to an embodiment of the present disclosure includes the steps of:
preparing a raw powder containing an iron-based powder;
forming a green compact with a relative density of 93% or more from the raw powder; and
sintering the green compact,
wherein the iron-based powder contains at least one of a pure iron powder and an iron-based alloy powder,
the iron-based powder is subjected to reduction treatment in the step of preparing the raw powder, and
the iron-based powder is heated in a reducing atmosphere to a temperature of 950° C. or more and less than 1100° C. in the reduction treatment.
In a method for producing a sintered material according to the present disclosure, the steps of forming a green compact with a relative density of 93% or more and sintering the green compact overlap a method for producing a basic sintered material described in Patent Literature 1. In a method for producing a sintered material according to the present disclosure, however, an iron-based powder heated to the particular temperature and reduced is used as a raw powder. The particular reduced powder is used to form a dense green compact. The use of the particular reduced powder can effectively decrease the number of compound particles of an oxide or the like. Thus, a method for producing a sintered material according to the present disclosure can produce a dense sintered material that has a relative density of 93% or more and has fewer compound particles 0.3 μm or more in size at least in the surface layer of the sintered material. Due to the fewer compound particles, the sintered material thus produced can reduce the occurrence or development of cracking caused by the compound particles. Thus, the sintered material has high fatigue strength. Thus, a method for producing a sintered material according to the present disclosure can produce a sintered material, typically a sintered material according to the present disclosure, with high fatigue strength.
(7) In a method for producing a sintered material according to an embodiment of the present disclosure, the temperature is maintained for 5 hours or more in the reduction treatment.
In this embodiment, the iron-based powder is appropriately reduced, and therefore a sintered material with a small number of compound particles 0.3 μm or more in size can be produced.

DETAILS OF EMBODIMENTS OF PRESENT DISCLOSURE

A sintered material according to an embodiment of the present disclosure, a gear according to an embodiment of the present disclosure, and a method for producing a sintered material according to an embodiment of the present disclosure are described below with reference to the accompanying drawings.

[Sintered Material]

A sintered material 1 according to an embodiment is described below with reference mainly to FIG. 1.
FIG. 1A illustrates an external gear as an example of the sintered material 1 according to the embodiment. FIG. 1A illustrates a cross section in which a plurality of teeth 3 are partly cut away.

(Outline)

The sintered material 1 according to the embodiment is a dense sintered material of an iron-based alloy composed mainly of Fe (iron). The sintered material 1 according to the embodiment contains fewer compound particles 2 with a size of 0.3 μm or more (FIG. 2). More specifically, the sintered material 1 according to the embodiment has a composition composed of an iron-based alloy and a structure described below and has a relative density of 93% or more.
In the structure, the number of compound particles 2 with a size of 0.3 μm or more per unit area in a cross section of the sintered material 1 is less than 200. The unit area is 100 μm×100 μm. “The number of compound particles 0.3 μm or more in size per 100 μm×100 μm unit area in a cross section” is hereinafter also referred to as a “number density”.
Further details are described below.

(Composition)

An iron-based alloy is an alloy containing an additive element and the remainder composed of Fe and impurities. The additive element may be at least one element selected from the group consisting of carbon (C), nickel (Ni), molybdenum (Mo), manganese (Mn), chromium (Cr), boron (B), and silicon (Si). Iron-based alloys containing the above-mentioned element in addition to Fe have higher strength than pure iron. The sintered material 1 composed of an iron-based alloy with high strength has high fatigue strength.
The amount of the above-mentioned element based on 100% by mass of an iron-based alloy may be as described below. The strength of an iron-based alloy tends to increase with the element content. The sintered material 1 composed of a high-strength iron-based alloy can easily improve fatigue strength.
<C> 0.1% or more by mass and 2.0% or less by mass
<Ni> 0.0% or more by mass and 5.0% or less by mass
<Total amount of Mo, Mn, Cr, B, and Si> 0.1% or more by mass and 5.0% or less by mass
Mo, Mn, Cr, B, and Si may be hereinafter collectively referred to as “elements such as Mo”.
Iron-based alloys containing C, typically carbon steel, have high strength. A C content of 0.1% or more by mass can result in improved strength and hardenability compared with a C content of less than 0.1% by mass, for example, pure iron. A C content of 2.0% or less by mass results in a smaller reduction in ductility or toughness while high strength is maintained. The C content may be 0.1% or more by mass and 1.5% or less by mass, 0.1% or more by mass and 1.0% or less by mass, or 0.1% or more by mass and 0.8% or less by mass.
Iron-based alloys containing Ni can have improved toughness as well as improved strength. The hardenability as well as the strength tends to increase with the Ni content. When quenching and tempering are performed after sintering, a Ni content of 5.0% or less by mass tends to result in a low retained austenite content of a sintered material after tempering. This can prevent softening caused by the formation of a large amount of retained austenite. Thus, the sintered material 1 after quenching and tempering tends to have a tempered martensite phase as a main structure and have increased hardness. The Ni content may be 0.1% or more by mass and 4.0% or less by mass or 0.25% or more by mass and 3.0% or less by mass.
When the total amount of elements such as Mo is 0.1% or more by mass, the strength can be further improved. When the total amount of elements such as Mo is 5.0% or less by mass, a reduction in toughness or embrittlement is suppressed while high strength is maintained. The total amount of elements such as Mo may be 0.2% or more by mass and 4.5% or less by mass or 0.4% or more by mass and 4.0% or less by mass. The element content may be as described below.
<Mo> 0.0% or more by mass and 2.0% or less by mass or 0.1% or more by mass and 1.5% or less by mass
<Mn> 0.0% or more by mass and 2.0% or less by mass or 0.1% or more by mass and 1.5% or less by mass
<Cr> 0.0% or more by mass and 4.0% or less by mass or 0.1% or more by mass and 3.0% or less by mass
<B> 0.0% or more by mass and 0.1% or less by mass or 0.001% or more by mass and 0.003% or less by mass
<Si> 0.0% or more by mass and 1.0% or less by mass or 0.1% or more by mass and 0.5% or less by mass
Iron-based alloys particularly containing Mo and Mn have higher strength. Mn contributes to improved hardenability and strength. Mo contributes to improved high-temperature strength and reduced temper embrittlement. Each of the Mo and Mn contents is preferably in the above range.
The total composition of the sintered material 1 can be measured by energy dispersive X-ray analysis (EDX or EDS) or high-frequency inductively coupled plasma spectroscopy (ICP-OES), for example.

(Structure)

The sintered material 1 according to the embodiment contains the compound particles 2 (FIG. 2). The compound particles 2 may be composed of an oxide, sulfide, carbide, and/or nitride containing at least one of the constituent elements of the sintered material 1 and impurity elements. For the constituent elements of the sintered material 1, see the above section “Composition”. The impurity elements may be incidental impurities and an element added as a deoxidizer. The compound particles 2 may be inevitably formed during the production process.

<<Number>>

The sintered material 1 according to the embodiment contains a small number of compound particles 2 with a size of 0.3 μm or more among the compound particles 2 present at least in the surface layer of the sintered material 1 in a cross section. Quantitatively, when a 100 μm×100 μm square region in a cross section of the sintered material 1 is defined as a region with a unit area, the number of compound particles 2 of 0.3 μm or more per the unit area, that is, the number density is less than 200. When the number density is less than 200, for example, even if a stress is applied to the sintered material 1 inwardly from a surface 11 of the sintered material 1, the compound particles 2 in the surface layer, such as the surface 11 and the vicinity thereof, rarely act as starting points of cracking. Furthermore, a crack rarely develops along the compound particles 2 into the sintered material 1. Thus, a large crack is less likely to develop. Thus, the sintered material 1 according to the embodiment has high fatigue strength.
A smaller number density results in a lower occurrence or slower development of cracking caused by the compound particles 2. Thus, the sintered material 1 can have improved fatigue strength. Thus, the number density is preferably 190 or less, more preferably 185 or less, 170 or less, or 150 or less. The number density is preferably 100 or less, more preferably 80 or less. As described above, the sintered material 1 according to the embodiment is composed of the iron-based alloy rather than pure iron. Pure iron has lower hardness and strength than iron-based alloys. In typical sintered materials made of pure iron, therefore, an oxide does not act as a starting point of cracking. In contrast, in iron-based alloys, a compound particle of an oxide or the like acts as a starting point of cracking. Furthermore, an oxide is more easily formed in iron-based alloys than in pure iron. Thus, a small number of compound particles 2 in the sintered material 1 according to the embodiment is effective for the improvement of fatigue strength.
The number of compound particles 2 is decreased, for example, by reducing an iron-based powder used as a raw material in the production process to decrease the amount of oxide, as described later. The production method is described in detail later.
The number density is ideally 0. In consideration of the productivity of the dense sintered material 1, however, the number density may be 10 or more or 20 or more.

<<Method for Measuring Compound Particle Number Density>>

The number density in a cross section of the sintered material 1 may be measured as described below. A more specific measurement method is described later in the test example 1.
(1) A cross section of the sintered material 1 is taken. As shown in FIG. 1B, it is desirable that a cross section of the sintered material 1 be on the surface 11 of the sintered material 1 or in the vicinity thereof, that is, in the surface layer. This is because when a stress is applied to the sintered material 1 inwardly from the surface 11 during the use of the sintered material 1, the compound particles 2 with a size of 0.3 μm or more in the surface layer tend to act as starting points of cracking. In the following description, the measurement point of the compound particles 2 is in the surface layer.
A cross section of the sintered material 1 is taken such that a region up to 200 μm in depth from the surface 11 of the sintered material 1 can be observed. For example, when the sintered material 1 is an annular gear illustrated in FIG. 1A, the surface 11 may be the surface of a tooth tip 30 of each tooth 3, the surface of a tooth surface 31, the surface of a tooth bottom 32, an end face 40 located at an axial end of a through-hole 41, or the inner peripheral surface of the through-hole 41. In particular, a cross section of the surface layer of the tooth surface 31 or the tooth bottom 32, to which a stress is easily applied, may be taken. The cross section may be a plane perpendicular to the axial direction of the gear (FIG. 1B) or a plane parallel to the axial direction. More specifically, the cross section may be a plane perpendicular to the thickness direction of the gear (FIG. 1B) or a plane parallel to the thickness direction of the gear.
When the sintered material 1 is an annular gear as illustrated in FIG. 1A, the cross section may be a curved surface instead of a flat surface. For example, the cross section may be a curved surface along a cylindrical surface that is coaxial with the axis of the gear. The axis of the gear is the axis of the through-hole 41. The cylindrical surface may be the inner peripheral surface of the through-hole 41. Alternatively, the cross section may be a curved surface parallel to part of the cylindrical surface, such as a curved surface along the surface of the tooth tip 30 or the surface of the tooth bottom 32.
The outermost surface of the sintered material 1 and the vicinity thereof are preferably removed. This is because measurement may be inappropriate in the outermost surface of the sintered material 1 and the vicinity thereof due to the presence of impurities. The removal thickness may range from approximately 10 to 30 μm. The surface 11 of the sintered material 1 is the surface after the outermost surface and the vicinity thereof are removed.
(2) A cross section of the sintered material 1 is observed with a scanning electron microscope (SEM), and a rectangular region 200 μm in length is extracted inwardly from the surface 11 as a measurement region, that is, as a visual field. The rectangular region may have a width of 50 μm. The observation magnification is selected from the range of 3,000 to 10,000 times, for example. The number of measurement regions is one or more.
(3) One extracted measurement region is divided into a plurality of subregions. The number of divisions k is 50 or more or 80 or more. Particles 0.3 μm or more in size in each subregion are extracted using a commercial automatic particle analysis system, commercial software, and the like. The term “particles with 0.3 μm or more in size”, as used herein, refers to particles 0.3 μm or more in diameter.
The diameter of each particle is determined as described below. The area, the cross-sectional area in this embodiment, of an extracted particle is determined. The diameter of a circle with the same area as the particle is determined. The diameter of the particle is considered to be the diameter of the circle.
Extracted particles may include voids in addition to particles composed of a compound such as an oxide described above. Thus, compound particles and voids in the particles are distinguished by a component analysis with a SEM-EDS or the like.
Only the compound particles are extracted from each subregion. The number of compound particles n_kis then determined. The total number N of compound particles in one measurement region is determined by totaling the number n_kof each subregion. The total number N thus determined and the area S (μm²) of the measurement region are used to determine the number of compound particles n in 100 μm×100 μm. The number n in one measurement region is determined using (N×100×100)/S. The number n is considered to be the number density of the sintered material 1.

<<Size>>

The size of the compound particles 2, the diameter of the particles in this embodiment, is preferably minimized. In particular, the number of coarse compound particles 2 of 20 μm or more is preferably minimized. When many of the compound particles 2 are small and when the number of coarse compound particles 2 is small, together with a small number of compound particles 2 of 0.3 μm or more, the development of cracking can be easily prevented. Quantitatively, the ratio (n₂₀/n)×100 may be 1% or less.
The n is the number of compound particles 2 with a size of 0.3 μm or more per unit area.
The n₂₀is the number of compound particles 2 with a size of 20 μm or more per unit area.
The unit area is 100 μm×100 μm.
The ratio (n₂₀/n)×100 is the ratio of the number n₂₀to the number n.
A ratio of 1% or less indicates a small number of coarse compound particles 2. A ratio of 1% or less indicates that more than 99% in the number n of the compound particles 2 is less than 20 μm in size. In other words, many of the compound particles 2 are small. The number n₂₀decreases with decreasing ratio. Thus, the coarse compound particles 2 rarely act as starting points of cracking. The ratio is preferably 0.8% or less, more preferably 0.7% or less, ideally 0%.
The size of the coarse compound particles 2 is preferably 150 μm or less, more preferably 100 μm or less or 50 μm or less, for example.
As the size of 99% or more in the number n of the compound particles 2 decreases, the compound particles 2 are less likely to act as starting points of cracking. Furthermore, the compound particles 2 are less likely to develop a crack. For example, the size of the compound particles 2 is preferably less than 20 μm, more preferably 10 μm or less, 5 μm or less, or 3 μm or less. More preferably, all the compound particles 2 in the unit area are 20 μm or less in size.
<<Structure after Heat Treatment>>
The sintered material 1 according to the embodiment may be an as-sintered material. Alternatively, the sintered material 1 according to the embodiment may be subjected to at least one of a carburizing treatment and a quenching and tempering treatment after sintering. In particular, the sintered material 1 subjected to both the carburizing treatment and the quenching and tempering treatment has better mechanical characteristics and is preferred.
The sintered material 1 subjected to the carburizing treatment has a carburized layer (not shown) in a region up to approximately 1 mm from the surface 11. A region near the surface 11 of the sintered material 1 with the carburized layer is harder than the interior of the sintered material 1. Thus, the sintered material 1 with the carburized layer can have improved wear resistance.
The sintered material 1 subjected to quenching and tempering has a structure composed of martensite. The martensite is mainly tempered martensite. The sintered material 1 with the martensite structure is hard and tough and easily has high strength. When the sintered material 1 is composed substantially entirely of martensite and has a structure in which the retained austenite content is somewhat small, the sintered material 1 has higher hardness and toughness. Thus, the sintered material 1 has high fatigue strength.

(Relative Density)

The sintered material 1 according to the embodiment has a relative density of 93% or more. Such a sintered material 1 is dense and has fewer voids. Thus, in the sintered material 1, cracking or breaking caused by voids rarely occurs or substantially does not occur. Thus, the sintered material 1 has high fatigue strength. As the relative density increases, the compound particles 2, rather than voids, tend to act as starting points of cracking. The sintered material 1 according to the embodiment, however, contains a small number of compound particles 2 of 0.3 μm or more at least in the surface layer, as described above. Thus, the occurrence or development of cracking caused by not only voids but also the compound particles 2 can be suppressed in the sintered material 1. A relative density of 95% or more or 97% or more results in the sintered material 1 with higher fatigue strength and is preferred. The relative density may be 98% or more or 99% or more. The relative density is ideally 100% but may be 99.6% or less in consideration of productivity.
The relative density (%) of the sintered material 1 is determined by taking a plurality of cross sections from the sintered material 1, observing each cross section with a microscope, and analyzing the observed image. The microscope is a SEM or an optical microscope, for example.
When the sintered material 1 is, for example, columnar or tubular, a cross section is taken from each end face region of the sintered material 1 and a region near the center of the sintered material 1 in the axial direction.
The end face region may be a region up to 3 mm in depth from the surface of the sintered material 1, depending on the length of the sintered material 1 in the axial direction.
The area near the center may be an area up to 1 mm from the center of the length toward each end face, that is, an area of 2 mm in total, depending on the length of the sintered material 1 in the axial direction.
The cross section may be a plane crossing the axial direction, typically a plane perpendicular to the axial direction.
A plurality of observation fields are taken from each cross section. For example, 10 or more observation fields are taken. The area of one observation field is 500 μm×600 μm=300,000 μm², for example.
When a plurality of observation fields are taken from one cross section, it is preferable to equally divide the cross section and take observation fields from each divided region.
An observed image of each observation field is subjected to image processing. The image processing is binarization, for example A region of metal is extracted from the processed image. The area of the region of metal thus extracted is determined. Furthermore, the ratio of the area of the region of metal to the area of the observation field is determined. This area ratio is considered to be the relative density of each observation field. The relative densities of the observation fields are averaged. The average is considered to be the relative density (%) of the sintered material 1.

(Mechanical Characteristics)

Depending on the composition and the mating material, the sintered material 1 according to the embodiment has a high contact fatigue strength of 2.3 GPa (2300 MPa) or more, for example. The test example 1 described later may be referred to with respect to this point.

(Applications)

The sintered material 1 according to the embodiment can be used for various general structural parts, for example, mechanical parts. Examples of the mechanical parts include various gears including sprockets, rotors, rings, flanges, pulleys, and bearings. In particular, a mating material exerts stress on the tooth surface of each tooth of gears during use. The mating material is a mating gear or a chain, for example. Due to stress loading, gears are parts in which a lower occurrence of cracking in the surface layer is desired. As described above, the sintered material 1 according to the embodiment is dense and contains a small number of compound particles 2 with a size of 0.3 μm or more at least in the surface layer. Thus, cracking rarely occurs in the surface layer. Thus, the sintered material 1 is suitable for gears. The sintered material 1 with high contact fatigue strength as described above can be more suitable for gears.

[Gear]

A gear according to an embodiment is composed of the sintered material 1 according to the embodiment. Thus, the gear according to the embodiment substantially has the composition and structure of the sintered material 1 according to the embodiment. The gear according to the embodiment may be a helical gear illustrated in FIG. 1A, a spur gear, a bevel gear, or a screw gear. The gear according to the embodiment may be an external gear illustrated in FIGS. 1A and 1B or an internal gear.

(Main Advantageous Effects)

The sintered material 1 according to the embodiment and the gear according to the embodiment have a high relative density, are dense, and contain a small number of compound particles 2 with a size of 0.3 μm or more. The sintered material 1 according to the embodiment and the gear according to the embodiment have high fatigue strength. These advantageous effects are more specifically described below in the test example.

[Method for Producing Sintered Material]

The sintered material 1 according to the embodiment can be produced, for example, by a method for producing a sintered material according to an embodiment including the following steps.
(First step) A raw powder containing an iron-based powder is prepared.
(Second step) A green compact with a relative density of 93% or more is formed from the raw powder.
(Third step) The green compact is sintered.
The iron-based powder contains at least one of a pure iron powder and an iron-based alloy powder.
In the first step, the iron-based powder is subjected to reduction treatment. The iron-based powder is heated in a reducing atmosphere to a temperature of 950° C. or more and less than 1100° C. in the reduction treatment.
Each step is described below.

(First Step: Preparation of Raw Powder)

The composition of the raw powder may be adjusted to the composition of the iron-based alloy constituting the sintered material. The raw powder includes an iron-based powder. The iron-based powder herein refers to a powder of a metal with a composition containing Fe. The iron-based powder is an alloy powder of an iron-based alloy with the same composition as the iron-based alloy constituting the sintered material, an alloy powder of an iron-based alloy with a different composition from the iron-based alloy constituting the sintered material, or a pure iron powder, for example. The iron-based powder can be produced by a water atomization process or a gas atomization process. Specific examples of the raw powder include the following.
(a) The raw powder contains an alloy powder of an iron-based alloy with the same composition as the iron-based alloy constituting the sintered material.
(b) The raw powder contains an alloy powder of the following iron-based alloy and a carbon powder. The iron-based alloy contains at least one element selected from the group consisting of Ni, Mo, Mn, Cr, B, and Si, the remainder being Fe and impurities.
(c) The raw powder contains a pure iron powder, a powder of at least one element selected from the group consisting of Ni, Mo, Mn, Cr, B, and Si, and a carbon powder.
When the raw powder contains an alloy powder, as described in (a) or (b), a sintered material uniformly containing Ni and/or elements such as Mo is easily produced. The raw powder may contain the alloy powder described in one of (a) and (b) and a powder of at least one element described in (c).
The size of the raw powder can be appropriately selected. For example, the alloy powder and the pure iron powder, which are iron-based powders, may have an average particle size of 20 μm or more and 200 μm or less or 50 μm or more and 150 μm or less. When the iron-based powder, such as an alloy powder, serving as a main component of the raw powder has an average particle size in the above range, the raw powder is easily compressed by pressurization. Thus, a dense green compact with a relative density of 93% or more can be easily produced. In particular, the iron-based powder with an average particle size of 50 μm or more is more reliably compressed.
A powder composed of Ni and/or elements such as Mo has an average particle size of approximately 1 μm or more and 200 μm or less, for example. The carbon powder has an average particle size of approximately 1 μm or more and 30 μm or less, for example. The carbon powder may be smaller than the alloy powder or the pure iron powder.
The average particle size herein refers to the particle size at which the cumulative volume is 50% in a volumetric particle size distribution measured with a laser diffraction particle size distribution analyzer (D50).
The raw powder may contain at least one of a lubricant and an organic binder. When the total amount of the lubricant and the organic binder is 0.1% or less by mass based on 100% by mass of the raw powder, for example, a dense green compact can be easily produced. When the raw powder does not contain a lubricant or an organic binder, a dense green compact can be more easily produced. Furthermore, it is not necessary to remove the lubricant and the organic binder from the green compact in a subsequent step. From these perspectives, the omission of a lubricant and an organic binder contributes to improved mass productivity of the sintered material 1.

The iron-based powder is subjected to reduction treatment. The reduction treatment reduces an oxide film that may exist on the surface of particles constituting the iron-based powder and oxygen on the surface. This decreases the oxygen concentration of the iron-based powder. A raw powder containing an iron-based powder with a low oxygen concentration is used to produce a green compact with a low oxygen concentration. In a green compact with a low oxygen concentration, oxygen in the green compact and an element in the green compact are less likely to bind together and form an oxide while sintering. Consequently, the sintered material 1 thus produced contains a small number of compound particles 2 composed of an oxide.
The reduction treatment is performed by heating the iron-based powder in a reducing atmosphere. At a heating temperature of 950° C. or more, oxygen is satisfactorily decreased in the iron-based powder. For example, the oxygen concentration of the iron-based powder tends to be as low as 800 ppm or less, 750 ppm or less, or 600 ppm or less on a volume basis. The oxygen concentration of the iron-based powder tends to decrease with increasing heating temperature. This effectively decreases the number of compound particles 2 composed of an oxide. Thus, the heating temperature may be 960° C. or more, 980° C. or more, or 1000° C. or more.
At a heating temperature of less than 1100° C., the iron-based powder can be prevented from sintering. The number of compound particles 2 decreases with increasing heating temperature. For example, the oxygen concentration of the iron-based powder tends to be very low at a heating temperature of 1100° C. or more or more than 1100° C. This greatly decreases the number of compound particles 2 composed of an oxide. At a heating temperature of 1100° C. or more, however, the iron-based powder is sintered, and powder particles are bonded to each other. It is therefore necessary to break the bonded powder particles. The breakage introduces strain into the powder particles. Residual strain in the powder particles impairs plastic deformability. Thus, when the iron-based powder with residual strain is used as a raw powder, it is conceivable that the iron-based powder undergoes poor plastic deformation, and a green compact with a relative density of 93% or more cannot be formed. Consequently, it is difficult or substantially impossible to produce the sintered material 1 with a relative density of 93% or more.
In contrast, at a heating temperature of less than 1100° C., the iron-based powder is prevented from sintering. Thus, the breakage can be omitted. Alternatively, the degree of breakage may be decreased. Thus, a dense green compact with a relative density of 93% or more can be more reliably formed. As the heating temperature decreases, the iron-based powder is more reliably prevented from sintering. Consequently, a dense green compact can be satisfactorily formed. Thus, the heating temperature may be 1080° C. or less, 1050° C. or less, or 1030° C. or less.
The holding time at the heating temperature in the reduction treatment may be selected in the range of 0.1 hours or more and 10 hours or less. Heating is stopped when the holding time has elapsed.
At the same heating temperature, the oxygen concentration of the iron-based powder tends to decrease with increasing holding time. Thus, the number of compound particles 2 composed of an oxide tends to decrease. From this perspective, the holding time is preferably 0.5 hours or more, more than 1 hour, more than 3 hours, particularly 5 hours or more.
A shorter holding time tends to result in a shorter processing time and a shorter production time of the sintered material. Consequently, this results in improved productivity of the sintered material. From this perspective, the holding time may be 9 hours or less or 8 hours or less.
To decrease the amount of oxide and the production time of the sintered material, the holding time may be selected in the range of more than 3 hours and 10 hours or less or 5 hours or more and 8 hours or less.
The reducing atmosphere is an atmosphere containing a reducing gas or a vacuum atmosphere, for example. The reducing gas may be hydrogen gas or carbon monoxide gas. In particular, hydrogen has high reducibility, and a hydrogen atmosphere is preferred. The pressure of the vacuum atmosphere is 10 Pa or less, for example.

(Second Step: Forming)

In this step, the raw powder containing the reduced iron-based powder is compressed to form a green compact with a relative density of 93% or more. In a method for producing a sintered material according to an embodiment, a green compact with a relative density of 93% or more can be used to produce a sintered material with a relative density of 93% or more. This is because typically the sintered material substantially retains the relative density of the green compact. The relative density of the sintered material increases with the relative density of the green compact. Thus, the green compact may have a relative density of 95% or more, 97% or more, or 98% or more. As described above, in consideration of productivity, the green compact may have a relative density of 99.6% or less.
The relative density of the green compact may be determined in the same manner as the relative density of the sintered material 1. In particular, when the green compact is formed by uniaxial pressing, a cross section of the green compact may be taken from a region near the center of the length of the green compact in the pressing direction or from an end face region at both ends in the pressing direction. The cross section may be a plane crossing the pressing direction, typically a plane perpendicular to the pressing direction.
The green compact can typically be produced with a press machine equipped with a uniaxial press mold. The mold may typically include a die with a through-hole and upper and lower punches fitted in upper and lower openings of the through-hole, respectively. The inner peripheral surface of the die and an end face of the lower punch form a cavity. The cavity is filled with the raw powder. The raw powder in the cavity can be compressed with the upper and lower punches at a predetermined forming pressure to produce the green compact.
The shape of the green compact may conform with the final shape of the sintered material or may be different from the final shape of the sintered material. The green compact in a shape different from the final shape of the sintered material may be subjected to processing, such as cutting, in a step after forming. As described later, the processing after forming is preferably performed on the green compact before sintering in terms of efficiency. In this case, for example, the shape of the green compact may be a simple shape, such as a column or a cylinder. The powder compact in a simple shape is easy to form with high precision and has high productivity.
A lubricant may be applied to the inner peripheral surface of the mold. In such a case, even the raw powder not containing a lubricant is prevented from sticking to the mold. When the raw powder does not contain a lubricant, a dense green compact can be easily formed, as described above. The lubricant is a higher fatty acid, metallic soap, fatty acid amide, or higher fatty acid amide, for example.
The relative density of the green compact tends to increase with the forming pressure. Thus, a dense green compact can be easily produced. Consequently, a dense sintered material can be easily produced. The forming pressure is 1560 MPa or more, for example. The forming pressure may be 1660 MPa or more, 1760 MPa or more, 1860 MPa or more, or 1960 MPa or more.

(Third Step: Sintering)

In this step, the green compact is sintered to produce a sintered material with a relative density of 93% or more.
The sintering temperature and the sintering time may be appropriately selected according to the composition of the raw powder.
The sintering temperature is 1100° C. or more and 1400° C. or less, for example. The sintering temperature may be 1110° C. or more and 1300° C. or less or 1120° C. or more and less than 1250° C. The method for producing a sintered material according to the embodiment uses a dense green compact, as described above. This eliminates the need for densification by high-temperature sintering at 1250° C. or more. The dense sintered material as described above is produced by sintering at a relatively low temperature of less than 1250° C.
The sintering time is 10 minutes or more and 150 minutes or less, for example.

The sintering atmosphere is a nitrogen atmosphere or a vacuum atmosphere, for example. The nitrogen atmosphere or the vacuum atmosphere has a low oxygen concentration. This suppresses the formation of an oxide. The oxygen concentration is 1 ppm or less on a volume basis, for example. The pressure of the vacuum atmosphere is 10 Pa or less, for example.

(Other Steps)

The method for producing a sintered material according to the embodiment may include at least one of a first processing step, a heat treatment step, and a second processing step described below.

In this step, the green compact is subjected to cutting after the second step or the forming step and before the third step or the sintering step. The cutting may be rotating or turning. Specific processing may be gear cutting or drilling. The green compact before sintering has better cutting properties than sintered materials after sintering or molten materials. From this perspective, cutting before the sintering step contributes to improved mass productivity of the sintered material.

The heat treatment in this step may be a carburizing treatment or quenching and tempering. Alternatively, the heat treatment in this step may be carburizing and quenching.
The carburizing conditions may be as described below.
The carbon potential (C.P.) is 0.6% or more by mass and 1.8% or less by mass.
The processing temperature is 910° C. or more and 950° C. or less.
The processing time is 60 minutes or more and 560 minutes or less. The optimum carburizing time generally varies with the product size of the sintered material. Thus, the above time is only an example.
The quenching conditions include an austenitizing temperature of 800° C. or more and 1000° C. or less, an austenitizing time of 10 minutes or more and 150 minutes or less, and subsequent quenching by oil cooling or water cooling.
The tempering conditions include a processing temperature of 150° C. or more and 230° C. or less and a processing time of 60 minutes or more and 240 minutes or less.

In this step, the sintered material after sintering is subjected to finish processing. The finish processing is polishing, for example. The finish processing decreases the surface roughness of the sintered material and enables the production of the sintered material with good surface properties. The finish processing also enables the production of the sintered material that has the design dimensions.

(Main Advantageous Effects)

The method for producing a sintered material according to the embodiment can produce a dense sintered material with a high relative density and with a small number of compound particles 0.3 μm or more in size, typically the sintered material 1 according to the embodiment. Thus, the method for producing a sintered material according to the present embodiment can produce the sintered material 1 with high fatigue strength.

Test Example 1

Iron-based powders with different oxygen concentrations were used as raw powders to produce sintered materials with different relative densities. The structure and contact fatigue strength of the sintered materials were examined.
The sintered materials were prepared as described below.
A raw powder is used to produce a green compact.
The green compact is sintered.
The sintering is followed by carburizing and quenching and then by tempering.
The raw powder is a mixed powder containing an alloy powder of the following iron-based alloy and a carbon powder.
The iron-based alloy contains 2% by mass of Ni, 0.5% by mass of Mo, 0.2% by mass of Mn, and the remainder composed of Fe and impurities.
The carbon powder content is 0.3% by mass based on 100% by mass of the total mass of the mixed powder.
The alloy powder has an average particle size (D50) of 100 μm. The carbon powder has an average particle size (D50) of 5 μm.
The alloy powder thus prepared was subjected to reduction treatment to prepare alloy powders with different oxygen concentrations. At least one of the heating temperature and the holding time in the reduction treatment was changed to prepare the alloy powders with different oxygen concentrations. The heating temperature is selected in the range of 800° C. or more and 1000° C. or less. The holding time is selected in the range of 1 hour or more and 6 hours or less. The atmosphere during the reduction treatment is a hydrogen atmosphere.
After the reduction treatment, the oxygen concentration (mass ppm) of the alloy powder is measured in each sample. Table 1 shows the measurement results. The oxygen concentration is measured by an inert gas fusion infrared absorption method. More specifically, the alloy powder of each sample is heated and melted in an inert gas to extract oxygen. The amount of extracted oxygen is measured. The oxygen concentration (mass ppm) is a mass ratio based on 100% by mass of the alloy powder.
Table 1 shows the heating temperature (° C.) and holding time (h) of the reduction treatment for the alloy powder of each sample. In samples in which the alloy powder has an oxygen concentration of 1210 mass ppm, the heating temperature is 900° C. In samples in which the alloy powder has an oxygen concentration of 1200 mass ppm or less, the heating temperature is 950° C., 980° C., or 1000° C. For these samples, the holding time is 5 hours except for a sample No. 10. For the same holding time, the oxygen concentration of the alloy powder decreases with increasing heating temperature. The heating temperature of samples with an oxygen concentration of 400 mass ppm is 1000° C. From the comparison between samples No. 9 and No. 10, at the same heating temperature, the oxygen concentration of the alloy powder decreases with increasing holding time.
In samples in which the alloy powder has an oxygen concentration of 1600 mass ppm or more, the heating temperature is 800° C. These samples have different oxygen concentrations due to the different holding times. Also in these samples, at the same heating temperature, the oxygen concentration of the alloy powder decreases with increasing holding time. The holding time of samples with an oxygen concentration of 3020 mass ppm is the shortest among these samples.
The alloy powder, which is the iron-based powder subjected to the reduction treatment, is mixed with the carbon powder. The powders are mixed in a V-type mixer for 90 minutes. The mixed powder is used as a raw powder. The raw powder was pressed to form an annular green compact. The green compact has an inner diameter of 16 mm, an outer diameter of 30 mm, and a thickness of 8 mm.
The green compact was formed by selecting the forming pressure in the range of 1560 MPa or more and 1960 MPa or less such that the green compact of each sample had a relative density (%) of 91%, 93%, 95%, or 97%. The relative density of the green compact tends to increase with the forming pressure. Table 1 shows the relative density (%) of the green compact in each sample.
The green compact was sintered under the following conditions. The sintering was followed by carburizing and quenching and then by tempering under the following conditions to form a sintered material of each sample.
(Sintering conditions) sintering temperature: 1130° C., holding time: 30 minutes, atmosphere: nitrogen
(Carburizing and quenching) 930° C.×90 minutes, carbon potential: 1.4% by mass=> 850° C.×30 minutes=>oil cooling
(Tempering) 200° C.×90 minutes
An annular sintered material with an inner diameter of 16 mm, an outer diameter of 30 mm, and a thickness of 8 mm was prepared as described above. The sintered material has a composition of an iron-based alloy containing 2% by mass of Ni, 0.5% by mass of Mo, 0.2% by mass of Mn, 0.3% by mass of C, and the remainder composed of Fe and impurities. The number density (/(100 μm×100 μm)), the contact fatigue strength (GPa), and the relative density (%) of the sintered material of each sample thus prepared are measured as described below. The number density herein refers to the number of compound particles 0.3 μm or more in size per unit area in a cross section of the sintered material. The unit area is 100 μm×100 μm.

(Structure Observation)

A cross section of the sintered material of each sample was subjected to an automatic particle analysis with a SEM to examine the number density. In the cross section of the sintered material, the number of compound particles was measured on the surface of the sintered material and the vicinity thereof, that is, in the surface layer. An automatic particle analysis system used was JSM-7600F, a SEM manufactured by JEOL Ltd. Particle analysis software used was INCA, Oxford Instruments. The specific measurement procedure is described below.
A rectangular parallelepiped test specimen including part of the outermost surface of the sintered material is cut out from the sintered material of each sample. The dimensions of the test specimen are 4 mm×2 mm×3 mm in height. The test specimen is cut out from the sintered material so as to have a height of 3 mm from the outermost surface in the depth direction. In the test specimen, a surface with an area of 4 mm×2 mm consists of part of the outermost surface of the sintered material. A region up to 25 μm from the outermost surface in the height direction is removed from the rectangular parallelepiped test specimen cut out. The surface after the removal is the surface of the specimen. A 4 mm×approximately 3 mm surface of the test specimen is flattened with a cross-section polisher (CP processing) using argon (Ar) ions. The CP processed surface is used as a measurement surface.
A region 50 μm in width on the measurement surface in a region up to 200 μm in depth, that is, in the height direction from the surface of the test specimen is used as a measurement region. Thus, the measurement region is a rectangular region 50 μm in width and 200 μm in length. One measurement region is taken from one test specimen. FIG. 2 is a schematic view of a measurement region 12 of the sintered material 1 of a sample No. 5. In FIG. 2, the circles schematically represent compound particles 2. The region including the compound particles 2 is an iron-based alloy constituting the parent phase of the sintered material 1. As typically illustrated in FIG. 2, the compound particles 2 are dispersed in the parent phase composed of the iron-based alloy. Hatching is omitted in FIG. 2.
The extracted measurement region is further divided into a plurality of subregions. Then, particles in each subregion are extracted. The measurement region is divided into 82. Thus, the number of divisions k=82. The magnification of the SEM is 10,000 times. The particles are extracted on the basis of the difference in contrast in a SEM observation image. A backscattered electron image is used as a SEM observation image. Binarization conditions are determined on the basis of the contrast intensity threshold of a backscattered electron image. Particles are extracted from a binarized image on the basis of the difference in contrast. A binarized image is subjected to hole filling and opening to separate images of adjacent particles. The area of each extracted particle is determined. The diameter of a circle with the determined area is determined. Particles with a circle diameter of 0.3 μm or more are extracted. The extracted particles of 0.3 μm or more are subjected to a component analysis with a SEM-EDS. The results of the component analysis are used to distinguish particles composed of an oxide or the like from voids. Only particles composed of a compound such as an oxide are extracted. The time for the component analysis is 10 seconds.
The number n_kof the particles composed of an oxide or the like is measured in each subregion. The numbers n_kin k subregions are totaled. This total number is the total number N of the particles composed of an oxide or the like in one measurement region. The total number N and the area S of one measurement region are used to determine the number n per 100 μm×100 μm using n=(N×100×100)/S. The area S is 50 μm×200 μm. The number n in the measurement region in each sample is considered to be the number density in each sample. Table 1 shows the number density in each sample.

(Fatigue Characteristics)

A two-cylinder fatigue test with a Nishihara metal abrasion tester was performed to determine the Hertzian stress caused by contact with a rotating mating material. The Hertzian stress was evaluated as the contact fatigue strength. Table 1 shows the contact fatigue strength (GPa).
A known Nishihara metal abrasion tester can be used. The sintered material of each sample is used as a test specimen. The mating material, test conditions, and a method for determining the Hertzian stress are described below.

The composition is SKD11, which is a type of alloy tool steel.
The shape is annular.
The dimensions are 16 mm in inner diameter, 30 mm in maximum outer diameter, 25 mm in minimum outer diameter, and 8 mm in thickness.
The mating material has a long protrusion protruding radially outward from the circumference of a circle with an outer diameter of 25 mm and extending in the circumferential direction. The long protrusion is located at the center of the circumference of the circle with an outer diameter of 25 mm in the thickness direction. The thickness direction is a direction parallel to the axial direction of a through-hole of the mating material. When the long protrusion is cut in a plane in the axial direction, the cross-sectional shape of the long protrusion is a hexagonal shape in which two corners of a rectangle 2.5 mm in height×4 mm in width are cut off. The long protrusion on the circumferential surface side has a width of 4 mm. The outer peripheral surface of the long protrusion has a width of 1.5 mm. The outer peripheral surface of the long protrusion 1.5 mm in width is a contact surface with the test specimen.

The rotation rate is 800 rpm.
The degree of slippage is 30%.
While the mating material is pressed against and applies a load to the sintered material of each sample serving as the test specimen, the test specimen and the mating material are rotated at the rotation rate. Under different loads, they are rotated 10 million times. The Hertzian stress a is determined from the load using the Hertzian stress equation, as described below. The maximum Hertzian stress at which the test specimen is not broken during the rotation of 10 million times is considered to be the contact fatigue strength (GPa). Table 1 shows the contact fatigue strength (GPa).
σ=√[(F/b)×{E/(2πρ)}] (Hertzian stress equation)
In the Hertzian stress equation, F denotes the load (N).
b denotes the width (mm) of the contact surface in each sample. The width b is 1.5 mm.
E denotes the Young's modulus (GPa).
ρ denotes the radius of curvature (mm).
The Young's modulus E and the radius of curvature ρ satisfy the following.
1/E=(½)×{(1−γ₁ ²)/E ₁+(1−γ₂ ²)/E ₂}
1/ρ=(1/ρ₁)+(1/ρ2)
In these equations, E₁denotes the Young's modulus of each sample.
γ₁denotes the Poisson's ratio of each sample.
ρ₁denotes the radius of curvature of each sample. The radius of curvature ρ₁is the outer diameter/2=15 mm.
E₂denotes the Young's modulus of the mating material.
γ2 denotes the Poisson's ratio of the mating material.
ρ₂denotes the radius of curvature of the mating material. The radius of curvature ρ₂is the outer diameter/2=15 mm.
The Young's modulus and Poisson's ratio of each sample and the mating material are intrinsic physical properties. The Young's modulus and the Poisson's ratio can be measured by an ultrasonic method with a general measuring apparatus.

(Relative Density)

The relative density (%) of the sintered material is determined by the image analysis of a microscope image of a cross section of the sintered material, as described above. In the sintered material of each sample, a cross section is taken from an end face region and a region near the center of the length of the through-hole of the sintered material in the axial direction. The end face region is a region within 3 mm from an annular end face of the sintered material. The region near the center is a remaining region after the end face region 3 mm in thickness is removed from each end face of the sintered material, that is, a region 2 mm in length in the axial direction. Each region is cut in a plane perpendicular to the axial direction to take a cross section. Ten or more observation fields are taken from each cross section. The area of the observation field is 500 μm×600 μm=300,000 μm². An observed image of each observation field is subjected to image processing. A region of metal is extracted from the processed image. The area of the region of metal thus extracted is determined. The ratio of the area of the region of metal to the area of the observation field is determined. The ratio is considered to be the relative density. The relative densities of a total of 30 or more observation fields are determined and averaged. The average is considered to be the relative density (%) of the sintered material. Table 1 shows the relative density (%) of the sintered material.

TABLE 1

	Raw powder

Alloy powder

Number density

Contact

Reduction treatment

	Oxygen	Sintered material	of compound	fatigue	Holding	Heating
Sample	concentration	Relative density	particles/	strength	time	temperature
No.	mass ppm	%	(100 × 100) μm²	GPa	h	° C.

101	400	91	45	1.22	5	1000
102	500		99	1.22	5	980
103	800		198	1.22	5	950
104	1210		399	1.21	5	900
105	1620		789	1.21	4	800
106	2000		1311	1.21	3	800
107	2410		2022	1.21	2	800
108	3020		2410	1.20	1	800
1	400	93	48	2.39	5	1000
2	500		109	2.37	5	980
3	800		199	2.34	5	950
111	1210		389	1.95	5	900
112	1620		801	1.60	4	800
113	2000		1309	1.45	3	800
114	2410		1991	1.36	2	800
115	3020		2395	1.33	1	800
4	400	95	48	2.62	5	1000
5	500		102	2.58	5	980
6	800		196	2.54	5	950
116	1210		401	2.08	5	900
117	1620		789	1.81	4	800
118	2000		1295	1.72	3	800
119	2410		2001	1.67	2	800
120	3020		2393	1.65	1	800
7	400	97	49	3.00	5	1000
8	500		101	2.94	5	980
9	800		199	2.82	5	950
10	700		180	2.87	6	950
121	1210		410	2.25	5	900
122	1620		802	2.06	4	800
123	2000		1311	1.94	3	800
124	2410		2009	1.80	2	800
125	3020		2403	1.71	1	800

Table 1 shows that the contact fatigue strength increases with the relative density of the sintered material. More specifically, the sintered materials of samples No. 1 to No. 10 and No. 111 to No. 125 with a relative density of 93% or more have higher contact fatigue strength than samples No. 101 to No. 108 with a relative density of less than 93%. In the samples No. 1 to No. 10, the contact fatigue strength is 2.3 GPa or more at a relative density of 93% or more. The contact fatigue strength is 2.5 GPa or more at a relative density of 95% or more. The contact fatigue strength is 2.8 GPa or more at a relative density of 97% or more. One reason for such results is that a higher relative density results in a smaller number of voids and a lower occurrence of cracking caused by the voids.
Next, in the dense samples No. 1 to No. 10 and No. 111 to No. 125, samples with the same relative density have different contact fatigue strengths. All the sintered materials of the samples No. 1 to No. 10 have higher contact fatigue strength than the samples No. 111 to No. 125. The samples No. 1 to No. 10 are hereinafter referred to as a specific sample group. The specific sample group has a contact fatigue strength of more than 2.25 GPa or 2.3 GPa or more. The sintered materials of the specific sample group satisfy the required characteristics of gears and are expected to be suitable for various gears.
One reason for the different contact fatigue strengths of the samples may be a difference in the number of compound particles 0.3 μm or more in size per unit area, that is, a difference in number density, in the cross section of the sintered material. The number density in the specific sample group is as low as less than 200. Thus, in the specific sample group, the compound particles are unlikely to be starting points of cracking and to develop cracking, and the contact fatigue strength is high. Furthermore, it has been confirmed that the compound particles are present on a fracture surface of a fractured sample. It is considered from this that compound particles 0.3 μm or more in size in the sintered material, particularly compound particles in the surface layer of the sintered material, act as starting points of cracking.
In addition, it has been confirmed that the specific sample group contains a small number of coarse compound particles and a large number of small compound particles. More specifically, in the specific sample group, the ratio (n₂₀/n)×100 is 1% or less. The n is the number of compound particles of 0.3 μm or more per the unit area. The n₂₀is the number of compound particles of 20 μm or more per the unit area. It is also considered from this that the specific sample group could prevent the occurrence or development of cracking caused by the compound particles.
In contrast, the samples No. 111 to No. 125 have a number density of 200 or more and in this embodiment 385 or more. In the sintered materials of the samples No. 111 to No. 125 with high number densities, a large number of compound particles exist from the surface of the sintered material toward the inside thereof. Thus, in these sintered materials, it is considered that each compound particle is likely to act as a starting point of cracking and develop cracking.
One reason for the difference in the number density of compound particles between the specific sample group and the samples No. 111 to No. 125 may be a difference in the oxygen concentration of the raw powder. The oxygen concentration of the alloy powder in the specific sample group is 800 mass ppm or less, which is lower than the oxygen concentration of the alloy powder in the samples No. 111 to No. 125, which is more than 1200 mass ppm. It is considered that in the specific sample group containing the alloy powder with a low oxygen concentration as a main component of the raw powder, an element and oxygen in the green compact are less likely to bind together and form an oxide while sintering. Consequently, the specific sample group may contain a decreased number of oxide particles, and the total number of compound particles may be effectively decreased. In contrast, the samples No. 111 to No. 125 containing the powder with a high oxygen concentration may consequently contain an increased number of oxide particles and an increased total number of compound particles.
This test also shows the following.
(1) The effects of the number of compound particles on the contact fatigue strength increase with the relative density. This is described below with reference to FIG. 3. FIG. 3 is a graph of the relationship between the number density (/(100 μm×100 μm)) and the contact fatigue strength (GPa) in the sintered material of each sample. The horizontal axis of the graph indicates the number density of each sample (/(100 μm×100 μm)). The vertical axis of the graph indicates the contact fatigue strength (GPa) of each sample. The legends 91, 93, 95, and 97 in the graph represent the relative density of each sample.
FIG. 3 shows that the contact fatigue strength changes little at a relative density of 91% even when the number density increases or decreases. At a relative density of less than 93%, the contact fatigue strength of the sintered material is substantially independent of the number of compound particles 0.3 μm or more in size.
At a relative density of 93% or more, attention is paid to a range in which the number density is approximately more than 400. Even with a large number of compound particles 0.3 μm or more in size, the sintered material has higher contact fatigue strength in this range than at a relative density of 91%. The contact fatigue strength does not change significantly in this range. By contrast, the contact fatigue strength changes greatly at a number density of 400 or less. It is understood that the contact fatigue strength tends to be improved with a decrease in number density. In particular, at a relative density of 97% or more, the contact fatigue strength is improved at a number density of less than 200. This supports that at a relative density of 93% or more or 97% or more the compound particles of 0.3 μm or more are more likely to act as starting points of cracking than voids. Thus, to improve the contact fatigue strength of a dense sintered material with a relative density of 93% or more, it is desirable to decrease the number of compound particles.
(2) At the same relative density, the contact fatigue strength of the sintered material increases with decreasing number density. The samples of the specific sample group are compared in terms of this point. For example, at a relative density of 97% or more in the test, the fatigue strength is 2.8 GPa or more at a number density of less than 200. The fatigue strength is 2.9 GPa or more at a number density of 150 or less or 120 or less. The fatigue strength is 3.0 GPa or more at a number density of 100 or less or 50 or less.
(3) In the reduction treatment of an iron-based powder, an alloy powder in this embodiment, in a raw powder in the temperature range of 950° C. or more and less than 1100° C., the number of compound particles 0.3 μm or more in size decreases with increasing heating temperature. At a heating temperature in the above range, a holding time of 5 hours or more results in a small number of the compound particles. As shown in the sample No. 10, the number of the compound particles tends to decrease with increasing holding time. The contact fatigue strength of the sintered material increases with the decreasing number of the compound particles.
These results show that sintered materials with a relative density of 93% or more and with a small number of compound particles 0.3 μm or more in size in a cross section have high fatigue strength. It has also been shown that such a sintered material can be produced by forming a green compact with a relative density of 93% or more using an iron-based powder subjected to reduction treatment under specific conditions as a raw material and by sintering the green compact.
The present invention is defined by the appended claims rather than by these embodiments. All modifications that fall within the scope of the claims and the equivalents thereof are intended to be embraced by the claims.
For example, in the test example 1, the composition and the production conditions of the sintered material may be changed. Modifiable parameters of the production conditions include the heating temperature and holding time in reduction treatment, sintering temperature, sintering time, and sintering atmosphere, for example.

REFERENCE SIGNS LIST

- 1 sintered material
- 11 surface, 12 measurement region
- 2 compound particle
- 3 tooth, 30 tooth tip, 31 tooth surface, 32 tooth bottom
- 40 end face, 41 through-hole

Claims

1. A sintered material comprising:

a composition composed of an iron-based alloy; and

a structure in which the number of compound particles 0.3 μm or more in size is less than 200 per 100 μm×100 μm unit area in a cross section,

wherein the sintered material has a relative density of 93% or more.

2. The sintered material according to claim 1, wherein the relative density is 97% or more.

3. The sintered material according to claim 1, wherein a ratio of n₂₀to n, (n₂₀/n)×100, is 1% or less, wherein n denotes the number of the compound particles 0.3 μm or more in size per the unit area, and n₂₀denotes the number of compound particles 20 μm or more in size per the unit area.

4. The sintered material according to claim 1, wherein the iron-based alloy contains at least one element selected from the group consisting of C, Ni, Mo, Mn, Cr, B, and Si, a remainder being Fe and impurities.

5. A gear comprising the sintered material according to claim 1.

6. A method for producing a sintered material, comprising the steps of:

preparing a raw powder containing an iron-based powder;

forming a green compact with a relative density of 93% or more from the raw powder; and

sintering the green compact,

wherein the iron-based powder contains at least one of a pure iron powder and an iron-based alloy powder,

the iron-based powder is subjected to reduction treatment in the step of preparing the raw powder, and

the iron-based powder is heated in a reducing atmosphere to a temperature of 950° C. or more and less than 1100° C. in the reduction treatment.

7. The method for producing a sintered material according to claim 6, wherein the temperature is maintained for 5 hours or more in the reduction treatment.

8. The sintered material according to claim 2, wherein a ratio of n₂₀to n, (n₂₀/n)×100, is 1% or less, wherein n denotes the number of the compound particles 0.3 μm or more in size per the unit area, and n₂₀denotes the number of compound particles 20 μm or more in size per the unit area.

9. The sintered material according to claim 2, wherein the iron-based alloy contains at least one element selected from the group consisting of C, Ni, Mo, Mn, Cr, B, and Si, a remainder being Fe and impurities.

10. The sintered material according to claim 3, wherein the iron-based alloy contains at least one element selected from the group consisting of C, Ni, Mo, Mn, Cr, B, and Si, a remainder being Fe and impurities.

11. The sintered material according to claim 8, wherein the iron-based alloy contains at least one element selected from the group consisting of C, Ni, Mo, Mn, Cr, B, and Si, a remainder being Fe and impurities.

12. A gear comprising the sintered material according to claim 2.

13. A gear comprising the sintered material according to claim 3.

14. A gear comprising the sintered material according to claim 4.

15. A gear comprising the sintered material according to claim 8.

16. A gear comprising the sintered material according to claim 9.

17. A gear comprising the sintered material according to claim 10.

18. A gear comprising the sintered material according to claim 11.