US20120304821A1

US20120304821A1 - Hard particles for blending in sintered alloy, wear-resistant iron-based sintered alloy containing hard particles, valve seat formed of sintered alloy, and process for manufacturing hard particles

Info

Publication number: US20120304821A1
Application number: US13/201,782
Authority: US
Inventors: Kimihiko Ando; Tadayoshi Kikko; Yusaku Yoshida
Original assignee: Fine Sinter Co Ltd; Toyota Motor Corp
Current assignee: Fine Sinter Co Ltd; Toyota Motor Corp
Priority date: 2010-02-19
Filing date: 2010-10-13
Publication date: 2012-12-06
Also published as: JP2011190526A; CN102782166A; WO2011101706A1; JP4948636B2

Abstract

Hard particles for blending as a starting material in a sintered alloy contain 20 to 40 mass % of molybdenum, 0.5 to 1.0 mass % of carbon, 5 to 30 mass % of nickel, 1 to 10 mass % of manganese, 1 to 10 mass % of chromium, 5 to 30 mass % of cobalt, 0.05 to 2 mass % of yttrium, and the balance being inadvertent impurities and iron.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to hard particles suitable for blending in a sintered alloy, and particularly to hard particles suitable for enhancing the wear resistance of a sintering alloy. The invention further relates to a wear-resistant iron-based sintered alloy containing such hard particles, a valve seat formed of such a sintered alloy, and a process for manufacturing such hard particles.
2. Description of the Related Art
Iron-based sintered alloys are sometimes used in valve seats and the like. Hard particles are sometimes included in a sintered alloy to further enhance the wear resistance of the alloy. When including hard particles, it is customary to mix a powder of the hard particles into a powder composed of low-alloy steel or stainless steel, form the mixed powder into a pressed compact, and sinter the pressed compact to give a sintered alloy.
An example of such hard particles that have been disclosed are hard particles containing 20 to 60 mass % of molybdenum (Mo), 0.2 to 3 mass % of carbon (C), 5 to 40 mass % of nickel (Ni), 1 to 15 mass % of manganese (Mn) and 0.1 to 10 mass % of chromium (Cr), with the balance being inadvertent impurities and iron (see, for example, Japanese Patent Application Publication No. 2001-181807 (JP-A-2001-181807)).
When these bard particles are used to manufacture an iron-based sintered alloy, the adherence between the hard particles and the iron base serving as the base metal can be increased. In addition, by including Mo in the hard particles, better solid lubricating properties can be ensured.
When the valve seats for an internal combustion engine are manufactured of the sintered alloy disclosed in JP-A-2001-181807, the Mo in the hard particles acts as a solid lubricant, enabling the lubricating properties between the valves and the sliding surface of the valve seat to be increased.
However, when a valve is opened and closed, the valve seat and the valve do not merely slide against each other. Exhaust valves in particular operate in a higher temperature environment than intake valves. In this environment, the valve seat and the valve come into intermittent contact with the opening and closing of the valve. Because the valve and the valve seat slide against each other during such contact, there exists a combination of adhesive wear during contact and abrasive wear during sliding (also referred to below as “combined wear”). Rigorous consideration of this form of wear suggests that there will be times where the wear resistance of the valve seat cannot be adequately enhanced by merely increasing the lubricating properties.

SUMMARY OF THE INVENTION

The present invention provides hard particles for blending in a sintered alloy, which hard particles are able to enhance the wear resistance even when combined wear emerges in a high-temperature environment. Further the present invention provides a wear-resistant iron-based sintered alloy containing the hard particles, a valve seat formed of the sintered alloy, and a process for manufacturing the hard particles.
As a result of extensive investigations, the inventors learned that further increasing the hardness of hard particles while maintaining the existing lubricating properties of hard particles would be effective for the wear resistance in such an environment. They also discovered that a method that increases the hardness of the hard particles while ensuring the lubricating properties of the particles by adding yttrium (Y), which has a high oxidizability compared with other elements, to the hard particles would be the most effective method for achieving the above goal.
The invention is based on this discovery. A first aspect of the present invention relates to hard particles for blending as a starting material in a sintered alloy.
The hard particles are composed of 20 to 40 mass % of Mo, 0.5 to 1.0 mass % of C, 5 to 30 mass % of Ni, 1 to 10 mass % of Mn, 1 to 10 mass % of Cr, 5 to 30 mass % of cobalt Co, 0.05 to 2 mass % of Y, and the balance being inadvertent impurities and iron. In this specification, unless noted otherwise, “%” means percent by mass.
According to the invention, because Y oxidizes very easily in air compared within other elements, by adding Y to the ingredients making up the hard particles, yttrium oxide (Y₂O₃) forms in the hard particles and disperses within the hard particles. As a result, the hard particles are dispersion-strengthened, enabling the hardness of the hard particles to be increased. In addition, Y oxide can suppress adhesive wear, and thus is able to further improve the wear resistance. Moreover, when a sintered alloy containing these hard particles is machined, the sintered alloy does not readily adhere to the cutting tool, enabling the machinability of the sintered alloy to be enhanced by the Y oxide.
If Y is added in an amount of less than 0.05% of the mass of the hard particles, even should Y₂O₃form within the hard particles, a hardness capable of providing the sintered alloy in which the hard particles have been used with the desired wear resistance cannot be fully achieved.
On the other hand, the more Y is added to the hard particles, the greater the degree to which the hardness of the hard particles can be increased. However, when the amount of Y added is more than 2% of the mass of the hard particles, the hard particles become brittle, decreasing the wear resistance of sintered alloy manufactured using such hard particles. The technical significance of the other ingredients in the hard particles, and the contents thereof, are described in detail in the embodiments and examples of the invention provided below.
In preferred embodiment, the hard particles may contain, respectively, from 21 to 39 mass % of Mo, from 0.7 to 0.9 mass % of C, from 6 to 28 mass % of Ni, from 1 to 9 mass % of Mn, from 2 to 9 mass % of Cr, or from 7 to 29 mass % of Co.
A second aspect of the present invention related to a wear-resistant iron-based sintered alloy obtained by using the foregoing hard particles for blending in a sintered alloy is described. This wear-resistant iron-based sintered alloy is obtained by mixing a powder composed of the above hard particles with a ferrous powder so as to disperse the hard particles, then sintering the mixed powders, wherein the hard particles account for 10 to 60 mass % of the wear-resistant iron-based sintered alloy.
In this wear-resistant iron-based sintered alloy, during sintering, the ferrous metal becomes the base material which connects together the hard particles. When the hard particles are included in an amount of less than 10 mass % of the sintered alloy, the wear resistance effects by the hard particles may not be fully emerge. On the other hand, when the hard particles are included in an amount of more than 60 mass % of the sintered alloy, the proportion of the iron base decreases, as a result of which the hard particles may not be held with sufficient bonding strength within the sintered alloy. Therefore, in an environment where wear arises, such as a contacting and sliding environment, the sintered alloy may end up shedding hard particles, allowing wear of the sintered alloy to proceed.
In the above wear-resistant iron-based sintered alloy, the ferrous powder may be formed into the base of the wear-resistant iron-based sintered alloy by sintering. Also, in the above wear-resistant iron-based sintered alloy, an oxide of yttrium may be present at a surface of the wear-resistant iron-based sintered alloy.
A third aspect of the present invention relates to a valve seat which is formed of the foregoing wear-resistant iron-based sintered alloy. Through this invention, even when a form of wear wherein the above-described adhesive wear during contact and abrasive wear during sliding are both present arises in a high-temperature environment, the hardness of the hard particles can be increased without a loss in the existing solid lubricating properties of these particles. This enables the wear resistance of the valve seat to be even further enhanced compared with the wear resistance of valve seats in the related art.
A fourth aspect of the present invention relates to a process for manufacturing hard particles which includes; preparing a melt containing 20 to 40 mass % of molybdenum, 0.5 to 1.0 mass % of carbon, 5 to 30 mass % of nickel, 1 to 10 mass % of manganese, 1 to 10 mass % of chromium, 5 to 30 mass % of cobalt, 0.05 to 2 mass % of yttrium, and the balance being inadvertent impurities and iron; powderizing the melt; and oxidizing the yttrium. In this process, the meat may be powdering by gas atomization.
Even when a form of wear that is a combination of abrasive wear and adhesive wear arises in a high-temperature environment, the above-described sintered alloy which the hard particles according to this aspect of the invention have been blended has increased solid lubricating properties and increased hardness, thus enabling the wear resistance to be further enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a graph showing the valve seat wear test results in Examples 1 to 4 according to the invention and in Comparative Examples 1 to 3;

FIG. 2 is a graph showing the valve seat wear test results in Examples 5 to 10 according to the invention and in Comparative Examples 4 to 15;

FIG. 3 is a graph showing the valve seat wear test results in Examples 11 to 15 according to the invention and in Comparative Examples 16 and 17; and

FIG. 4 is diagram illustrating the wear tests carried out in the examples according to the invention and the comparative examples.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the invention are described in detail below. The hard particles according to these embodiments are hard particles for blending as a starting material in a sintered alloy. The hardness of the hard particles is higher than that of the base of the sintered alloy. The hard particles are composed of 20 to 40 mass % of molybdenum (Mo), 0.5 to 1.0 mass % of carbon (C), 5 to 30 mass % of nickel (Ni), 1 to 10 mass % of manganese (Mn), 1 to 10 mass % of chromium (Cr), 5 to 30 mass % of cobalt (Co), 0.05 to 2 mass % of yttrium (Y) and the balance (i.e., the rest included in the hard particles except Mo, C, Ni, Mn, Cr, Co and Y) being inadvertent impurities and iron.
Such hard particles can be manufactured by preparing a melt containing the above constituents blended in the foregoing proportions, then subjecting the melt to atomizing treatment. Another method that may be used is to solidify the melt, and mechanically grind the solid into a powder. Atomization may be carried out by either gas atomization or water atomization, although gas atomization is preferred for its ability to obtain particles having rounded features, which is desirable from the standpoint of sinterability and other considerations. Gas atomization may be carried out in, for example, a non-oxidizing atmosphere (e.g., in an inert gas atmosphere of nitrogen or argon, or in a vacuum), provided the Y can be oxidized up until the sintered alloy is manufactured (sintered).
Here, taking into consideration the subsequently described reasons for limits in composition, and also, within these ranges, such additional factors as hardness, solid lubricating properties, adherence and cost, the lower limit values and upper limit values for the constituents in the above-described hard particles may be suitably varied according to the degree of importance placed on each property of the member to be used.
Of the constituents in the hard particles, Mo forms Mo carbide, which enhances the hardness and wear resistance of the hard particles. In addition, Mo in solid solution and Mo carbide form a Mo oxide film, which is effective for enhancing the good solid lubricating properties.
At an amount of Mo less than the above-indicated lower limit value, the solid lubricating properties in the solid particles are inadequate. At an amount greater than the upper limit value indicated above, adherence with the ferrous base during sintering decreases. The Mo content of the hard particles is more preferably from 21 to 39 mass %.
Of the constituents in the hard particles, C bonds with Mo to form Mo carbide, and is effective for enhancing the hardness and wear resistance of the hard particles.
At an amount of C less than the above-indicated lower limit value, the wear resistance is inadequate. At a C amount greater than the upper limit value indicated above, the density of the sintered alloy decreases. The C content in the hard particles is more preferably from 0.7 to 0.9 mass %.
Of the constituents in the hard particles, Ni increases the austenite in the base of the hard particles, increases the amount of Mo in solid solution, and enhances the wear resistance. Moreover, the Ni in the hard particles diffuses into the base of the sintered alloy, increasing the austenite in the base and increasing the amount of Mo in solid solution, and is thus effective for enhancing the wear resistance.
At an amount of Ni less than the above-indicated lower limit value, the amount of Mo in solid solution decreases, resulting in an inadequate wear resistance. At an amount of Ni greater than the above-indicated upper limit value, the sintered alloy tends to invite seizure, readily giving rise to adhesive wear. The Ni content in the hard particles is more preferably from 6 to 28 mass %.
Of the constituents in the hard particles, Mn efficiently diffuses from the hard particles to the sintered alloy base during sintering, and is thus effective for improving adherence between the hard particles and the sintered alloy base. In addition, Mn can be expected to have an austenite increasing effect in the base of the hard particles and in the base of the sintered alloy.
At an amount of Mn less than the above-indicated lower limit value, the amount that diffuses to the base of the sintered alloy is small, thus lowering adherence between the hard particles and the base. At an amount of Mn greater than the above-indicated upper-limit value, the density of the sintered alloy decreases. The Mn content in the hard particles is more preferably from 1 to 9 mass %.
When the temperature in the service environment is high end formation of an oxide film on the hard particles increases, delamination of the oxide film from the hard particles will arise. Of the constituents in the hard particles, Cr is effective for suppressing oxidation of the hard particles.
At an amount of Cr below the above-indicated lower limit value, the oxide film in the hard particles becomes too thick, facilitating oxidative wear. At an amount of Cr greater than the above-indicated upper limit value, formation of the oxide film that serves as a solid lubricant is suppressed. The Cr content in the hard particles is more preferably horn 2 to 9 mass %.
Of the constituents in the hard particles, Co increases austenite in the base of the hard particles and in the base of the sintered alloy, in addition to which it is effective for enhancing the hardness of the hard particles.
At an amount of Co less than the above-indicated lower limit value, the above-described effects are unlikely to occur. At an amount of Co greater the above-indicated upper limit value, the wear resistance may decrease. The Co content in the hard particles is more preferably from 7 to 29 mass %.
Of the constituents in the hard particles, Y oxidizes much more readily than the other elements in air. Hence, when Y is added to the other constituents of the hard particles, the yttrium oxide (Y₂O₃) forms in the hard particles and this oxide diffuses within the hard particles, strengthening the hard particles. This oxidation of yttrium does not occur only during formation of the powder of hard particles; it can also occur during the formation of a pressed compact and subsequently during use as a sintered alloy in a high-temperature environment. In addition, because the Y₂O₃is present at the surface of the sintered alloy or the hard particles, adhesive wear can be suppressed and the wear resistance can be enhanced.
Here, at an amount of Y less than the above-indicated lower limit value, even when Y₂O₃has formed within the hard particles, a hardness of a degree that would lead one to expect wear resistance in the sintered alloy manufactured wing such hard particles cannot be achieved.
On the other hand, although adding more Y to hard particles enables the hardness of the hard particles to be increased, with the addition of more than 2%, based on the mass of the hard particles, the hard particles become brittle, lowering the wear resistance of the sintered alloy manufactured using such particles. In such a case, when a powder obtained by mixing the hard particles and iron powder particles serving as the base is formed, the hard particles become too hard, lowering the density of the pressed compact that has been formed. As a result, the density of the sintered alloy obtained by sintering the compact decreases, resulting in a decline in the wear resistance of the sintered alloy.
The hard particles have an average particle size which may be suitably selected according to such considerations as the intended use and type of iron-based sintered alloy. The average particle size may be set at from 20 to 250 μm, but is not limited to this range. The hardness of the hard particles depends on the amount of yttrium added, and may be set to a Vickers hardness (Hv) of from about 600 to about 700. However, the hardness is not limited to this range, provided the hardness of the hard particles is higher than that of the object (e.g., the base of a sintered alloy) in which the hard particles are used.
Such hard particles for blending in a sintered alloy are used by mixing a powder composed of the hard particles into a ferrous powder serving as the base so as to disperse the hard particles. It is more preferable at this time for the hard particles to account for 10 to 60 mass % of the overall mixed powder (i.e., the wear-resistant iron-based sintered alloy).
Because the hard particles are dispersed in the sintered alloy base and make up a hard phase which increases the wear resistance of the sintered alloy, at a hard particle content less than 10 mass %, the wear resistance of the sintered alloy is inadequate. On the other hand, at a hard particle content greater than 60 mass %, not only is there an increased tendency for the sintered alloy to attack the mating material (i.e., an increased hardness that induces wear of the member in contact therewith), it becomes difficult to ensure the retention of the hard particles.
In the mixed powder, a ferrous powder (e.g., pure iron powder or low-alloy steel powder) is used as the base for the wear-resistant iron-based sintered alloy. AC powder may also be added thereto. An iron C powder may be used as the low-alloy steel powder. For example, use may be made of a composition containing from 0.2 to 5 mass % of C per 100 mass % of the low-alloy steel powder, with the balance being inadvertent impurities and iron.
The resulting mixed powder is formed into a pressed compact, and this pressed compact is sintered. The sintering temperature may be set to from about 1,050 to about 1,250° C., and especially from about 1,100 to about 1,150° C. The sintering time at this sintering temperature may be set to from 30 to 120 minutes, and preferably from 45 to 90 minutes. The sintering atmosphere may be a non-oxidizing atmosphere such as an inert gas atmosphere. Exemplary non-oxidizing atmospheres include a nitrogen atmosphere, an argon gas atmosphere, and a vacuum atmosphere.
Next, to ensure the hardness of the base of the iron-based sintering alloy obtained by sintering, it is preferable to include a pearlite-containing microstructure. The pearlite-containing microstructure may be a pearlite microstructure, a pearlite-austenite mixed microstructure, a pearlite-ferrite mixed microstructure or a pearlite-cementite mixed microstructure. To ensure wear resistance, it is preferable for the level of ferrite, which has a low hardness, to be small. The Hv of the base, which varies with the composition, is from about 120 to about 300, and may be adjusted in accordance with, for example, the heat treatment conditions and the amount of C powder added. However, insofar as these factors do not lower the wear resistance, such as the adherence between the hard particles and the base, no limitation is imposed on the above composition and hardness.
The valve seats for exhaust valves in an internal combustion engine may be formed using the above-described wear-resistant iron-based sintered alloy. In a high-temperature environment like that experienced by a valve seat for an exhaust valve in an internal combustion engine, even in cases where there emerges a form of wear that is a combination of adhesive wear during contact between the valve seat and the valve and abrasive wear during sliding therebetween, the hardness of the hard particles can be increased without compromising the existing solid lubricating properties of the hard particles. In this way, the wear resistance of the valve seat can be enhanced even further compared with the wear resistance of valve seats in the related art.

EXAMPLES

Concrete examples of the invention are described below together with comparative examples.

Example 1

Valve seats formed of a sintered alloy containing the hard particles according to Example 1 were fabricated by the method show below. That is, an alloy powder was produced from a melt having the composition shown in Table 1 by atomization using an inert gas (nitrogen gas). The alloy powder was classified in a range of from 44 μm to 180 μm to give a powder of hard particles. This hard particle powder, a graphite powder and a pure iron powder were mixed in a blender, thereby forming a mixed powder as the mixed material. The hard particle powder content in the mixed powder was set to 30 mass % and the graphite powder content was set to 0.6 mass %, with the balance being pure iron powder.
Using a mold, the mixed powder formulated as described above was subjected to an applied pressure of 78.4×10⁷Pa (8 tonf/cm²) so as to compression-mold test pieces having a ring shape, thereby forming pressed compacts. The pressed compacts were fired for 60 minutes at 1,120° C. in an inert atmosphere (nitrogen gas atmosphere), thereby forming sintered alloy (valve seats) as test pieces.

Examples 2 to 4

Valve seats were fabricated in the way as in Example 1. These examples differed from Example 1 in that the compositions of the hard particles were as shown in Table 1. That is, the content of yttrium within the hard particles of the valve seats (sintered alloy) was, in the respective examples, 0.2 mass %, 1.0 mass %, and 2.0 mass %.

Comparative Example 1

Valve seats were fabricated in the way as in Example 1. This example differed from Example 1 in that the composition of the hard particles was as shown in Table 1. That is, the composition of the hard particles in the valve seats (sheered alloy) was set to the composition and content shown in above-described JP-A-2001-181807.

Comparative Example 2

Valve seats were fabricated in the way as in Example 1. This example differed from Example 1 in that the composition of the hard particles was as shown in Table 1. That is, the content of yttrium within the hard particles in the valve seats (sintered alloy) was set to 0 mass % (no yttrium was included).

Comparative Example 3

Valve seats were fabricated in the way as in Example 1. This example differed from Example 1 in that the composition of the hard particles was as shown in Table 1. That is, the content of yttrium within the hard particles in the valve seats (sintered alloy) was set to 5.0 mass %.

TABLE 1

Mo	C	Ni	Mn	Cr	Co	Y	Fe

Example 1	38	0.8	22	5	6	22	0.05	balance
Example 2	38	0.8	22	5	6	22	0.2	balance
Example 3	38	0.8	22	5	6	22	1.0	balance
Example 4	38	0.8	22	5	6	22	2.0	balance
Comp. Ex. 1	34	0.9	10	6	4	31		balance
Comp. Ex. 2	38	0.8	22	5	6	22		balance
Comp. Ex. 3	38	0.8	22	5	6	22	5.0	balance

*Numerical values in the table indicate the mass percent based on the mass of the hard particles.

Wear Test
Next, a wear test on the wear resistance of the sintered alloy was carried out using the test apparatus shown in FIG. 4, and the wear resistance was evaluated. In this wear test, as shown in FIG. 4, using a propane gas burner 10 as the heating source, the sliding portion between the ring-shaped valve seat 12 made of sintered alloy fabricated as described above and the valve face 14 of a valve 13 was placed in a propane gas combustion atmosphere. The valve face 14 is produced by performing nitrocarburizing treatment on SUH11. A wear test was carried out for 8 hours by controlling the temperature of the valve seat 12 at 200° C., applying a load of 18 kgf with a spring 16 at the time of contact between the valve seat 12 and the valve face 14, and bringing the valve seat 12 and the valve face 14 into contact at a rate of 2,000 times per minute.
The amount of wear (wear depth) incurred by the valve scat at this time was measured. The results are shown in FIG. 1. The relative amount of wear shown in FIG. 1 was normalized based on a value of 1 for the amount of wear in the valve seat of Comparative Example 1.
Hardness Test
The hardness of the hard particles according to Example 2 to 4 and Comparative Example 2 was measured using a micro Hv tester at a measurement load of 0.1 kgf. The results are shown in Table 2 below.

	TABLE 2

	Y content	Hardness of hard particles
	(mass %)	(Hv)

Example 2	0.2	617
Example 3	1.0	623
Example 4	2.0	627
Comparative Example 2	0	593

Results 1
As shown in FIG. 1, the relative amount of wear in the valve seats of Examples 1 to 4 was small compared to that in Comparative Examples 1 to 3. As shown in Table 2, the hardness of the hard particles increased as the content (additive amount) of yttrium in the hard particles rose, but the rate of increase in hardness diminished as the content (additive amount) of yttrium in the hard particles rose.
Evaluation 1
It is evident from “Results 1” above, that adding yttrium to the hard particles leads to an increase in the hardness of the hard particles, as a result of which the wear resistance of the valve seat is enhanced. This is apparently because the oxide of yttrium (Y₂O₃) disperses finely in the hard particles, thereby strengthening the hard particles. At an yttrium content less than 0.05 mass %, such strengthening by the Y₂O₃may be inadequate, whereas at an yttrium content above 2 mass %, it appears that sufficient wear resistance cannot be attained due to embrittlement of the hard particles.

Examples 5 to 10

Valve seats were fabricated in the same way as in Example 1. These examples differed from Example 1 in the contents of the respective constituents of the hard particles. That is, as shown in Table 3, the amounts of the respective constituents included in the hard particles were adjusted so as to fall within the following mass percent ranges: Mo, 20 to 40%; C, 0.5 to 1.0%; Ni, 5 to 30%; Mn, 1 to 10%; Cr, 1 to 10%; Co, 5 to 30%; Y, 0.05 to 2%.

Comparative Examples 4 to 15

Valve seats were fabricated in the same way as in Example 1. As shown in Table 3, these examples differed from Example 1 in the amounts of the respective constituents included in the hard particles. That is, in the hard particles of Comparative Examples 4 and 5, only the content of Mo has been set so as to fall outside the range of 20 to 40% for Mo in the composition of the invention. In the hard particles of Comparative Examples 6 and 7, only the content of C has been set so as to fall outside the range of 0.5 to 1.0% for, C in the composition of the invention. In the hard particles of Comparative Examples 8 and 9, only the content of Ni has been set so as to fall outside the range of 5 to 34% for Ni in the composition of the invention. In the hard particles of Comparative Examples 10 and 11, only the content of Mn has been set so as to fall outside the range of 1 to 10% for Mn in the composition of the invention. In the hard particles of Comparative Examples 12 and 13, only the content of Cr has been set so as to fall outside the range of 1 to 10% for Cr in the composition of the invention. In the hard particles of Comparative Examples 14 and 15, only the content of Co has been set so as to fall outside the range of 5 to 30% for Co in the composition of the invention.

TABLE 3

Mo	C	Ni	Mn	Cr	Co	Y	Fe

Example 5	21	0.7	27	5	9	29	0.2	balance
Example 6	30	0.5	18	9	2	19	0.4	balance
Example 7	36	0.8	6	4	5	28	0.5	balance
Example 8	34	0.6	26	1	8	27	0.3	balance
Example 9	30	0.8	18	3	2	27	0.2	balance
Example 10	39	0.6	28	4	2	7	0.3	balance
Comp. Ex. 4	17	0.8	26	4	4	23	0.3	balance
Comp. Ex. 5	46	1.0	21	4	4	23	0.4	balance
Comp. Ex. 6	35	0.0	20	4	6	22	0.5	balance
Comp. Ex. 7	35	1.6	22	4	6	22	0.3	balance
Comp. Ex. 8	35	0.7	2	6	6	21	0.2	balance
Comp. Ex. 9	35	0.8	33	4	6	22	0.2	balance
Comp. Ex. 10	35	0.7	23	0	6	21	0.4	balance
Comp. Ex. 11	35	0.7	20	14	6	21	0.2	balance
Comp. Ex. 12	35	0.7	23	5	0	21	0.5	balance
Comp. Ex. 13	35	0.7	23	5	12	21	0.5	balance
Comp. Ex. 14	35	0.7	23	5	5	3	0.2	balance
Comp. Ex. 15	35	0.7	23	5	5	36	0.2	balance

*Numerical values in the table indicate the mass percent based on the mass of the hard particles.

Wear Test
A wear test was carried out on the valve seats in Examples 5 to 10 and the valve seats in Comparative Examples 4 to 15 in the same way as the wear test carried out on the valve seat in Example 1. The results are shown in FIG. 2. The relative amount of wear shown in FIG. 2 was normalized based on a value of 1 for the amount of wear in the valve seat of Comparative Example 1. The relative amount of wear in the valve seat of Comparative Example 1 is also shown in FIG. 2.
Results 2
As shown in FIG. 2, the relative amount of wear in the valve seats of Examples 5 to 10 was smaller than that in Comparative Examples 4 to 15.
Evaluation 2
It appears from above “Results 2” that the amount of wear in the valve seat can be reduced by adjusting the amounts of the respective constituents included in the hard particles so as to fall within the following mass percent ranges: Mo, 20 to 40%; C, 0.5 to 1.0%; Ni, 5 to 30%; Mn, 1 to 10%; Cr, 1 to 10%; Co, 5 to 30%; Y, 0.05 to 2%.

Examples 11 to 15

Valve seats were fabricated in the same way as in Example 2. These examples differed from Example 2 in that the content of hard particle powder in the mixed powder was set to, as shown in Table 4, respectively 10 mass %, 20 mass %, 30 mass %, 50 mass % and 60 mass %. The graphite powder was added in the same amount as in Example 2.

	TABLE 4

	Amount of hard particles added to
	mixed powder (sintered alloy)

	Example 11	10 mass %
	Example 12	20 mass %
	Example 13	30 mass %
	Example 14	50 mass %
	Example 15	60 mass %
	Comparative Example 16	5 mass %
	Comparative Example 17	70 mass %

Comparative Examples 16 and 17

Valve seats were fabricated in the same way as in Example 2. These examples differed from Example 2 in that the content of hard particle powder in the mixed powder was set to, as shown in Table 4, respectively 5 mass % and 70 mass %. The graphite powder was added in the same amount as in Example 2.
Wear Test
Wear tests were carried out on the valve seats in Examples 11 to 15 and the valve seats in Comparative Examples 16 and 17 in the same way as the wear test carried out on the valve seat in Example 1. The results are shown in FIG. 3. The relative amount of wear shown in FIG. 3 was normalized based on a value of 1 for the amount of wear in the valve seat of Comparative Example 1. The relative amount of wear in the valve seat of Comparative Example 1 is also shown in FIG. 3.
Results 2
As shown in FIG. 3, the relative amount of wear in the valve seats of Examples 11 to 15 was smaller than that in Comparative Examples 16 and 17.
Evaluation 2.
From the above “Results 2,” it appears to be preferable for the hard particles for blending in a sintered alloy to account for 10 to 60 mass % of the above wear-resistant iron-based sintered alloy. At an amount of hard particles smaller than this range, the wear resistance effect by the hard particles cannot be fully manifested. At an amount of hard particles larger than this range, the proportion of the iron base decreases, as a result of which the hard particles may be unable to fully adhere to the sintered alloy.
Embodiments of the invention have been described above in detail. However, the invention is not limited to these embodiments and various design modifications may be carried out insofar as they do not depart from the spirit of the invention as set forth in the claims.
The hard particles according to the embodiments of this invention can be advantageously used in valve systems (e.g., valve seats, valve guides) for engines fueled by compressed natural gas, liquefied petroleum gas or gasoline in a high-temperature service environment.

Claims

1. Hard particles for blending as a starting material in a sintered alloy, the hard particles consist of:

20 to 40 mass % of molybdenum;

0.5 to 1.0 mass % of carbon,

5 to 30 mass % of nickel,

1 to 10 mass % of manganese,

1 to 10 mass % of chromium,

5 to 30 mass % of cobalt,

0.05 to 2 mass % of yttrium, and

the balance being inadvertent impurities and iron.

2. The hard particles according to claim 1, wherein the hard particles contain from 21 to 39 mass % of molybdenum.

3. The hard particles according to claim 1, wherein the hard particles contain from 0.7 to 0.9 mass % of carbon.

4. The hard particles according to claim 1, wherein the hard particles contain from 6 to 28 mass % of nickel.

5. The hard particles according to claim 1, wherein the hard particles contain from 1 to 9 mass % of manganese.

6. The hard particles according to claim 1, wherein the hard particles contain from 2 to 9 mass % of chromium.

7. The hard particles according to claim 1, wherein the hard particles contain from 7 to 29 mass % of cobalt.

8. A wear-resistant iron-based sintered alloy, obtained by mixing a powder composed of the hard particles according to claim 1 with a ferrous powder so as to disperse the hard particles, then sintering the mixed powders, wherein

the hard particles account for 10 to 60 mass % of the wear-resistant iron-based sintered alloy.

9. The wear-resistant iron-based sintered alloy according to claim 8, wherein the ferrous powder is formed into the base of the wear-resistant iron-based sintered alloy by sintering.

10. The wear-resistant iron-based sintered alloy according to claim 8, wherein an oxide of yttrium is present at a surface of the wear-resistant iron-based sintered alloy.

11. A valve seat, formed of the wear-resistant iron-based sintered alloy according to claim 8.

12. A process for manufacturing hard particles,

the process comprising:

preparing a melt containing 20 to 40 mass % of molybdenum, 0.5 to 1.0 mass % of carbon, 5 to 30 mass % of nickel, 1 to 10 mass % of manganese, 1 to 10 mass % of chromium, 5 to 30 mass % of cobalt, 0.05 to 2 mass % of yttrium, and the balance being inadvertent impurities and iron;

powderizing the melt; and

oxidizing the yttrium.

13. The process according to claim 12, wherein the melt is powderized by gas atomization.