Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/04—Making non-ferrous alloys by powder metallurgy
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/10—Alloys containing non-metals
  • C22C1/1005—Pretreatment of the non-metallic additives
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/10—Alloys containing non-metals
  • C22C1/1084—Alloys containing non-metals by mechanical alloying (blending, milling)
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C27/00—Alloys based on rhenium or a refractory metal not mentioned in groups C22C14/00 or C22C16/00
  • C22C27/04—Alloys based on tungsten or molybdenum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C30/00—Alloys containing less than 50% by weight of each constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/02—Making ferrous alloys by powder metallurgy
  • C22C33/0207—Using a mixture of prealloyed powders or a master alloy
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/02—Making ferrous alloys by powder metallurgy
  • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F2999/00—Aspects linked to processes or compositions used in powder metallurgy

Definitions

• 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.
• 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.
• 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.
• 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 (Ma) 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)).
• Mo molybdenum
• C carbon
• Ni nickel
• Mo manganese
• Cr chromium
• 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.
• 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.
• 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.
• “%" means per cent by mass.
• Y oxidizes very easily in air compared witliin other elements
• Y 2 O3 yttrium oxide
• the hard particles are dispersion-strengthened, enabling the hardness of the hard particles to be increased.
• Y oxide can suppress adhesive wear, and thus is able to further improve the wear resistance.
• 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.
• 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.
• the ferrous metal becomes the base material which connects together the hard particles.
• 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.
• 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.
• 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.
• 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 ' 8
• the melt may be powderized by gas atomization.
• 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.
• FIG. 1 is a graph showing the valve seat wear test results in Examples I 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;
• FIG. 4 is diagram illustrating the wear tests carried out in the examples according to the invention and the comparative examples.
• 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.
• Mo molybdenum
• C carbon
• Ni nickel
• Mn manganese
• Cr chromium
• Co cobalt
• Y yttrium
• 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).
• a non-oxidizing atmosphere e.g., in an inert gas atmosphere of nitrogen or argon, or in a vacuum
• 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.
• Mo forms Mo carbide, which enhances the hardness and wear resistance of the hard particles.
• Mo in solid solution and Mo carbide form a Mo oxide film, which is effective for enhancing the good solid lubricating properties.
• the solid lubricating properties in the solid particles are inadequate.
• the Mo content of the hard particles is more preferably from 21 to 39 mass%.
• the wear resistance is inadequate.
• the density of the sintered alloy decreases.
• the C content in the hard particles is more preferably from 0.7 to 0.9 mass%.
• 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.
• the amount of Mo in solid solution decreases, resulting in an inadequate wear resistance.
• 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%.
• 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.
• 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.
• the amount that diffuses to the base of the sintered alloy is small, thus lowering adherence between the hard particles and the base.
• the density of the sintered alloy decreases.
• the Mn content in the hard particles is more preferably from 1 to 9 mass%.
• the oxide film in the hard particles becomes too thick, facilitating oxidative wear.
• 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 from 2 to 9 mass%.
• 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.
• the Co content in the hard particles is more preferably from 7 to 29 mass%.
• Y oxidizes much more readily than the other elements in air.
• 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.
• the Y2O3 is present at the surface of the sintered alloy or the hard particles, adhesive wear can be suppressed and the wear resistance can be enhanced.
• 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 ⁇ , 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).
• 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.
• 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.
• a ferrous powder e.g., pure iron powder or low-alloy steel powder
• a C powder may also be added thereto.
• An iron-C powder may be used as the low-alloy steel powder.
• 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.
• 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.
• 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.
• 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 02608
• 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 ⁇ to 180 . ⁇ 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.
• the mixed powder formulated as described above was subjected to an applied pressure of 78.4 x 10 7 Pa (8 tonf cm 2 ) 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 5 120°C in an inert atmosphere (nitrogen gas atmosphere), thereby forming sintered alloy (valve seats) as test pieces.
• 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%.
• 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 (sintered alloy) was set to 8
• 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 v/ithin the hard particles in the valve .seats (sintered alloy) was set to 0 mass% (no yttrium was included).
• 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%.
• 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 seat 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.
• 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.
• 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%.
• 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 30% for Ni in the composition of the invention.
• 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.
• 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.
• 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.
• the hard particles for blending in a sintered alloy to account for 10 to 60 mass% of the above wear-resistant iran-based sintered alloy.
• the wear resistance effect by the hard particles cannot be fully manifested.
• 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.
• 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.
• valve systems e.g., valve seats, valve guides

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• 2010
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