EP2436463A2 - Gesinterte Materialien für Ventilführungen und Herstellungsverfahren dafür - Google Patents

Gesinterte Materialien für Ventilführungen und Herstellungsverfahren dafür Download PDF

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Publication number
EP2436463A2
EP2436463A2 EP11007891A EP11007891A EP2436463A2 EP 2436463 A2 EP2436463 A2 EP 2436463A2 EP 11007891 A EP11007891 A EP 11007891A EP 11007891 A EP11007891 A EP 11007891A EP 2436463 A2 EP2436463 A2 EP 2436463A2
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EP
European Patent Office
Prior art keywords
powder
phase
copper
amount
iron
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EP11007891A
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English (en)
French (fr)
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EP2436463B1 (de
EP2436463A3 (de
Inventor
Hiroki Fujitsuka
Hideaki Kawata
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Resonac Corp
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Hitachi Powdered Metals Co Ltd
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Publication of EP2436463A3 publication Critical patent/EP2436463A3/de
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    • 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
    • C22C33/0264—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements the maximum content of each alloying element not exceeding 5%
    • 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
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/10—Sintering only
    • B22F3/1017—Multiple heating or additional steps
    • B22F3/1028—Controlled cooling
    • 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
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/10—Sintering only
    • B22F3/11—Making porous workpieces or articles
    • 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
    • B22F5/00—Manufacture of workpieces or articles from metallic powder characterised by the special shape of the product
    • B22F5/008—Manufacture of workpieces or articles from metallic powder characterised by the special shape of the product of engine cylinder parts or of piston parts other than piston rings
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C13/00—Alloys based on tin
    • 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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C9/00—Alloys based on copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C9/00—Alloys based on copper
    • C22C9/02—Alloys based on copper with tin as the next major constituent
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01L—CYCLICALLY OPERATING VALVES FOR MACHINES OR ENGINES
    • F01L3/00—Lift-valve, i.e. cut-off apparatus with closure members having at least a component of their opening and closing motion perpendicular to the closing faces; Parts or accessories thereof
    • F01L3/02—Selecting particular materials for valve-members or valve-seats; Valve-members or valve-seats composed of two or more materials

Definitions

  • the present invention relates to sintered materials for valve guides that may be used in an internal combustion engine, and also relates to production methods for sintered materials for valve guides. Specifically, the present invention relates to a technique for further improving wear resistance of the sintered materials for valve guides while production cost is not greatly increased.
  • a valve guide used in an internal combustion engine is a tubular component having an inner circumferential surface for guiding valve stems of an intake valve and an exhaust valve.
  • the intake valve may be driven so as to take fuel mixed gas into a combustion chamber of the internal combustion engine
  • the exhaust valve may be driven so as to exhaust combustion gas from the combustion chamber.
  • the valve guide is required to have wear resistance and is also required to maintain smooth sliding conditions so as not to cause wear of the valve stems for long periods.
  • Valve guides made of a cast iron are generally used, but valve guides made of a sintered alloy have recently come into wide use.
  • sintered alloys can have a specific metallic structure, which cannot be obtained from ingot materials, and therefore the sintered alloys can have wear resistance. Moreover, once a die assembly has been made, products having the same shape can be mass-produced, and therefore the sintered alloys are suitable for commercial production. Furthermore, a sintered alloy can be formed into a shape similar to that of a product, and thereby material yield can be high in machining. Valve guides made of a sintered alloy are disclosed in, for example, Japanese Examined Patent Publication No. 55-034858 , and Japanese Patents Nos. 2680927 , 4323069 , and 4323467 .
  • the sintered material for valve guides disclosed in Japanese Examined Patent Publication No. 55-034858 is made of an iron-based sintered alloy consisting of, by weight, 1.5 to 4 % of C, 1 to 5 % of Cu, 0.1 to 2 % of Sn, not less than 0.1 % and less than 0.3 % of P, and the balance of Fe.
  • a photograph and a schematic view of a metallic structure of this sintered material are shown in Figs. 3A and 3B , respectively.
  • an iron-phosphorus-carbon compound phase is precipitated in a pearlite matrix which is strengthened by adding copper and tin.
  • the iron-phosphorus-carbon compound absorbs C from the surrounding matrix and grows into a plate shape, whereby a ferrite phase is dispersed at a portion surrounding the iron-phosphorus-carbon compound phase. Moreover, a copper alloy phase is dispersed in the matrix.
  • the copper alloy phase is formed such that Cu is solved in the matrix during sintering at high temperature in an amount greater than the solid solubility limit at room temperature and is precipitated in the matrix by cooling.
  • the sintered material for valve guides disclosed in Japanese Patent No. 2680927 is an improved material of the sintered material disclosed in Japanese Examined Patent Publication No. 55-034858 .
  • this material in order to improve machinability, magnesium metasilicate minerals and magnesium orthosilicate minerals are dispersed as intergranular inclusions in the metallic matrix of the sintered material disclosed in Japanese Examined Patent Publication No. 55-034858 .
  • this sintered material has been commercially used by domestic and international automobile manufacturers.
  • the sintered materials for valve guides disclosed in Japanese Patent Nos. 4323069 and 4323467 have further improved machinability.
  • the machinabilities thereof are improved by decreasing amount of phosphorus. That is, the dispersion amount of the hard iron-phosphorus-carbon compound phase is decreased to only the amount that is required for maintaining wear resistance of a valve guide.
  • These sintered materials have started to be commercially used by domestic and international automobile manufacturers.
  • valve guides have been subjected to higher temperatures and higher pressures while internal combustion engines are running. Moreover, in view of recent environmental issues, amounts of lubricant supplied to an interface between a valve guide and a valve stem have been decreased. Therefore, valve guides must withstand more severe sliding conditions. In view of these circumstances, a sintered material for valve guides is required to have high wear resistance equivalent to those of the sintered materials disclosed in Japanese Examined Patent Publication No. 55-034858 and Japanese Patent No. 2680927 .
  • an object of the present invention is to provide valve guide materials and to provide production methods therefor, and the sintered materials for valve guides have high wear resistance but the production cost is reduced.
  • the sintered materials for valve guides have wear resistance equivalent to those of the conventional sintered materials for valve guides, that is, the sintered materials for valve guides disclosed in Japanese Examined Patent Publication No. 55-034858 and Japanese Patent No. 2680927 .
  • the present invention provides a sintered material for valve guides, consisting of, by mass %, 1.3 to 3 % of C, 1 to 4 % of Cu, and the balance of Fe and inevitable impurities.
  • the sintered material exhibits a metallic structure made of pores and a matrix.
  • the matrix is a mixed structure of a pearlite phase, a ferrite phase, an iron carbide phase, and a copper phase, and a part of the pores includes graphite that is dispersed therein.
  • the iron carbide phase is dispersed at 3 to 25 % by area ratio and the copper phase is dispersed at 0.5 to 3.5 % by area ratio with respect to a cross section of the metallic structure, respectively.
  • the present invention provides a sintered material for valve guides, consisting of, by mass %, 1.3 to 3 % of C, 1 to 4 % of Cu, 0.05 to 0.5 % of Sn, and the balance of Fe and inevitable impurities.
  • the sintered material exhibits a metallic structure made of pores and a matrix.
  • the matrix is a mixed structure of a pearlite phase, a ferrite phase, an iron carbide phase, and at least one of a copper phase and a copper-tin alloy phase, and a part of the pores includes graphite that is dispersed therein.
  • the iron carbide phase is dispersed at 3 to 25 % by area ratio and the copper phase and the copper-tin alloy phase are dispersed at 0.5 to 3.5 % by area ratio with respect to a cross section of the metallic structure, respectively.
  • the iron carbide phase can be observed as a plate-shaped iron carbide having an area of not less than 0.05 % in a visual field in a cross-sectional structure at 200-power magnification.
  • a total area of the plate-shaped iron carbides having an area of not less than 0.15 % in the above visual field is 3 to 50 % with respect to a total area of the plate-shaped iron carbides, wear resistance is improved.
  • At least one kind selected from the group consisting of manganese sulfide particles, magnesium silicate mineral particles, and calcium fluoride particles are preferably dispersed in particle boundaries of the matrix and in the pores at not more than 2 mass %.
  • the present invention provides a production method for the sintered material for valve guides according to the first aspect of the present invention.
  • the production method includes preparing an iron powder, a copper powder, and a graphite powder, and mixing the copper powder and the graphite powder with the iron powder into a raw powder consisting of, by mass %, 1.3 to 3% of C, 1 to 4 % of Cu, and the balance of Fe and inevitable impurities.
  • the production method also includes filling a tube-shaped cavity of a die assembly with the raw powder, and compacting the raw powder into a green compact having a tube shape.
  • the production method further includes sintering the green compact at a heating temperature of 970 to 1070 °C in a nonoxidizing atmosphere so as to obtain a sintered compact.
  • the present invention provides a production method for the sintered material for valve guides according to the second aspect of the present invention.
  • This production method includes preparing an iron powder, a graphite powder, and one selected from the group consisting of a combination of a copper powder and a tin powder, a copper-tin alloy powder, and a combination of a copper powder and a copper-tin alloy powder.
  • This production method also includes mixing the graphite powder and the one selected from the group with the iron powder into a raw powder consisting of, by mass %, 1.3 to 3% of C, 1 to 4 % of Cu, 0.05 to 0.5 % of Sn, and the balance of Fe and inevitable impurities.
  • the production method also includes filling a tube-shaped cavity of a die assembly with the raw powder, and compacting the raw powder into a green compact having a tube shape.
  • the production method further includes sintering the green compact at a heating temperature of 950 to 1050 °C in a nonoxidizing atmosphere so as to obtain a sintered compact.
  • the green compact is desirably held at the heating temperature for 10 to 90 minutes in the sintering. Moreover, the sintered compact is cooled from the heating temperature to room temperature after the sintering, and the cooling rate is desirably 5 to 20 °C per minute while the sintered compact is cooled from 850 to 600 °C. In addition, when the sintered compact is cooled from the heating temperature to room temperature, the sintered compact is desirably isothermally held in a temperature range of 850 to 600 °C for 10 to 90 minutes and is then cooled.
  • At least one kind selected from the group consisting of a manganese sulfide powder, a magnesium silicate mineral powder, and a calcium fluoride powder is desirably added to the raw powder at not more than 2 mass %.
  • the sintered materials for valve guides of the present invention since phosphorus is not used in the entire composition, the production cost can be low. Moreover, the iron carbide phase is dispersed in a similar shape and in similar amount as in the case of a conventional sintered material for valve guides, whereby degree of wear resistance is maintained. Therefore, the sintered materials for valve guides of the present invention can be obtained at low production cost but have superior wear resistance. According to the production methods for the sintered materials for valve guides of the present invention, the sintered materials for valve guides of the present invention can be produced as easily as in a conventional manner.
  • iron carbides which improve wear resistance, are not dispersed in the shape of plates in a matrix.
  • a conventional sintered material for valve guides which includes P (for example, Japanese Examined Patent Publication No. 55-034858 )
  • iron-phosphorus-carbon eutectic compounds are dispersed in a matrix, and the compounds absorb C from the surrounding matrix and grow into a plate shape.
  • P is expected to be essential to generate the iron-phosphorus-carbon eutectic compounds.
  • the inventors of the present invention have researched the reason that the plate-shaped iron carbides are not generated in the iron-copper-carbon sintered material.
  • a copper powder and a graphite powder may be added to an iron powder so as to obtain a raw powder, and the raw powder may be compacted and sintered, whereby an iron-copper-carbon sintered material is obtained.
  • Some of the iron-copper-carbon sintered materials may be used as a material for general structure, and some may be used as a material for sliding such as a bearing.
  • sintering is performed at a heating temperature (sintering temperature) of not less than the melting point of Cu (1084.5 °C). Therefore, the copper powder that was added to raw powder is melted under such temperature and generates a liquid phase. The liquid phase fills spaces among the raw powder particles due to capillary force and wets and covers the surface of the iron powder particles, and Cu is diffused from this liquid phase into the iron powder. Therefore, Cu is uniformly diffused and is solid solved in the iron matrix.
  • C added in the form of the graphite powder starts to be diffused to the iron matrix at approximately 800 °C in the sintering.
  • Cu is an element for decreasing the critical cooling rate of a steel and improves hardenability of the steel. That is, Cu shifts the pearlite nose to the later time side (right side) in the continuous cooling transformation diagram. Therefore, when the sintered material is cooled from the heating temperature in a condition that Cu having such effects is uniformly diffused in the iron matrix, the pearlite nose is shifted to the later time side. As a result, the sintered material is cooled at a cooling rate in an ordinary sintering furnace before the iron carbides (Fe 3 C) grow sufficiently. Accordingly, a fine pearlite structure is formed, and the plate-shaped iron carbides are not easily obtained.
  • the iron-copper-carbon sintered materials that may be used for sliding materials are disclosed in, for example, Japanese Patent No. 4380274 and Japanese Patent Application of Laid-Open No. 2008-202123 .
  • sintering is performed at a heating temperature of approximately 750 to 800 °C, in which the graphite powder is not easily dispersed. In this case, diffusion amount of C into the iron matrix is decreased, and the matrix has a hypoeutectoid composition. Therefore, the metallic structure after the sintering is a mixed phase of pearlite and ferrite, and the plate shaped iron carbides (Fe 3 C) are not obtained.
  • the inventors of the present invention came to have an idea that the plate-shaped iron carbides (Fe 3 C) may be precipitated in the cooling after the sintering by controlling the diffusion condition of Cu. Then, the inventors of the present invention have researched the idea and found that iron carbides (Fe 3 C) in a predetermined plate shape can be obtained even without adding P. The present invention was achieved based on this finding.
  • a sintered material for valve guides according to a first embodiment of the present invention based on the above finding, diffusion of Cu in an iron matrix is controlled.
  • the matrix includes portions having high and low concentrations of Cu and not uniformly includes Cu.
  • plate-shaped iron carbides Fe 3 C are precipitated at the portion having low concentration of Cu.
  • FIG. 1A is a photograph of the metallic structure
  • Fig. 1B is a schematic view of the photograph of the metallic structure.
  • the metallic structure of the sintered material of the present invention is made of pores and a matrix, and the pores are dispersed in the matrix. The pores were generated by spaces that remained among raw powder particles when the raw powder was compacted.
  • the matrix (iron matrix) was mainly made of an iron powder in the raw powder.
  • the matrix is a mixed structure of a pearlite phase, a ferrite phase, an iron carbide phase, and a copper phase.
  • a graphite phase was exfoliated when the sample was polished so as to observe the metallic structure, the graphite phase is not observed.
  • graphite remained inside the large pores and is dispersed as a graphite phase.
  • An iron carbide (Fe 3 C) phase is precipitated in the shape of plates, and the shape and the amount of the iron carbide phase are approximately the same as those of the conventional sintered material shown in Figs. 3A and 3B .
  • the copper phase exists in a condition in which a part of the amount of the copper powder is not dispersed and remains in the matrix, and the powder particles of Cu are not completely diffused.
  • plate-shaped iron carbide (Fe 3 C) phase was precipitated at a portion having low concentration of Cu. That is, by controlling diffusion of Cu in an iron matrix and by forming a matrix including portions having high and low concentrations of Cu, plate-shaped iron carbides (Fe 3 C) are obtained at the portion having low concentration of Cu even without adding P.
  • EMPA Electro Probe Micro Analyzer
  • Fig. 2A shows a photograph of the metallic structure of the sintered material used for the EPMA analysis.
  • the sintered material was etched with Murakami's reagent (a solution of 10 mass % of potassium ferricyanide and 10 mass % of potassium hydroxide).
  • Fig. 2B is a schematic view obtained by analyzing the photograph of Fig. 2A .
  • plate-shaped iron carbides (Fe 3 C) were deeply etched (the gray colored portions), and pearlite portions were lightly etched (the white colored portions).
  • the black portions shown in Figs. 2A and 2B are pores. Accordingly, the plate-shaped iron carbide (Fe 3 C) phase can be distinguished from the iron carbides (Fe 3 C) that form the pearlite as described above.
  • Cu is essential for strengthening the sintered material.
  • Cu is essential for forming the copper phase and thereby improving adaptability to a mating material (valve stem).
  • the amount of Cu is set to be not less than 1 mass %.
  • the amount of Cu is more than 4 mass %, the amount of Cu diffused in the iron matrix becomes too great, whereby plate-shaped iron carbides are difficult to obtain in the cooling after the sintering. Accordingly, the amount of Cu in the sintered material is set to be 1 to 4 mass %.
  • C is essential for forming the iron carbide phase and the graphite phase that can be used as a solid lubricant. Therefore, the amount of C is set to be not less than 1.3 mass %. In this case, C is added in the form of a graphite powder. If the amount of the graphite powder is more than 3.0 mass % in the raw powder, flowability, fillability, and compressibility of the raw powder are greatly decreased, and the sintered material is difficult to produce. Accordingly, the amount of C in the sintered material is set to be 1.3 to 3.0 mass %.
  • the amount of the plate-shaped iron carbide phase is small, the wear resistance is decreased. Therefore, the amount of the plate-shaped iron carbide phase is required to be not less than 3 % by area ratio with respect to a metallic structure including pores in cross-sectional observation. In contrast, when the amount of the plate-shaped iron carbide phase is too great, the degree of wear characteristics with respect to a mating material (valve stem) is increased, whereby the mating material may be worn. In addition, strength of a valve guide is decreased, and machinability of a valve guide is decreased. Therefore, the upper limit of the amount of the plate-shaped iron carbide phase is set to be 25 %.
  • the pearlite has a lamellar structure of fine iron carbides and ferrite
  • the plate-shaped iron carbide phase of the present invention does not include the iron carbides of the pearlite.
  • the plate-shaped iron carbide phase of the present invention is identified in a cross-sectional metallic structure as the dark colored portion as shown in Fig. 2B by using image analyzing software, such as "WinROOF" produced by Mitani Corporation.
  • the dark colored portion that is, the iron carbide phase is separately extracted by controlling a threshold. Therefore, the area ratio of the plate-shaped iron carbide phase can be measured by analyzing the area of the dark colored portions.
  • each of the plate-shaped iron carbides is recognized as a portion having an area of not less than 0.05 % in a visual field of a cross-sectional structure at 200-power magnification as described above. Accordingly, the area ratio of the plate-shaped iron carbide phase also can be measured by adding up the areas of the portions having an area of not less than 0.05 %.
  • the area ratio of the plate-shaped iron carbide phase is set to be the above area ratio in cross section.
  • the amount of large plate-shaped iron carbides is preferably 3 to 50 % with respect to the entire amount of the plate-shaped iron carbides. In this case, the large plate-shaped iron carbides have an area of not less than 0.15 %, which is measured in a visual field of a cross-sectional structure at 200-power magnification.
  • the amount of the copper phase is set to be not less than 0.5 % by area ratio with respect to a metallic structure including pores in cross-sectional observation.
  • the copper phase is made of the copper powder added to the raw powder. If the amount of the copper phase is too great, that is, if the amount of the copper powder added to the raw powder is too great, the diffusion amount of Cu into the iron matrix is increased, whereby the plate-shaped iron carbide phase is difficult to obtain. Therefore, the amount of the copper phase is set to be not more than 3.5 % by area ratio with respect to a metallic structure including pores in cross-sectional observation.
  • a sintered material for valve guides according to a second embodiment of the present invention is a modification of the sintered material for valve guides of the First Embodiment, in which the strength is improved by adding Sn.
  • the amount of Sn is set to be not less than 0.05 mass %.
  • the amount of Sn is set to be 0.5 mass %.
  • the sintered material for valve guides according to the Second Embodiment since Sn is added, Sn is solid solved into a part or the entire area of the copper phase in the sintered material for valve guides of the first embodiment. Therefore, a combination of a copper phase and a copper-tin alloy phase, or a copper-tin alloy phase is dispersed.
  • the amount of these copper system phases (the copper phase and the copper-tin alloy phase, or the copper-tin alloy phase) is set to be not less than 0.5 % by area ratio with respect to a metallic structure in cross-sectional observation in view of the adaptability to a mating material.
  • the amount of the copper system phases (the copper phase and the copper-tin alloy phase, or the copper-tin alloy phase) is set to be 0.5 to 3.5 % by area ratio with respect to a metallic structure in cross-sectional observation.
  • the sintered material for valve guides diffusion of Cu in the iron matrix is controlled, whereby the matrix includes portions having high and low concentration of Cu and not uniformly includes Cu.
  • the plate-shaped iron carbides (Fe 3 C) are precipitated at the portion having low concentration of Cu in the matrix.
  • a copper powder and a graphite powder are mixed with an iron powder so as to obtain a mixed powder as a raw powder.
  • sintering is performed at a heating temperature (sintering temperature) of less than the melting point of Cu (1085 °C) so as to prevent generation of a Cu liquid phase. Therefore, Cu is diffused into the iron matrix only by solid-phase diffusion.
  • the graphite powder is added to the raw powder at not less than the amount at which C diffused at the heating temperature forms hypereutectoid composition.
  • a part of the amount of C added in the form of the graphite powder is uniformly diffused and is solved in the iron matrix (austenite).
  • the residual amount of C remains as a graphite phase which functions as a solid lubricant.
  • the sintering is performed in a nonoxidizing atmosphere as is conventionally done.
  • the upper limit of the heating temperature is set to be less than the melting point of Cu in the sintering.
  • the upper limit of the heating temperature is set to be 1070 °C.
  • Cu is essential for improving the strength of the sintered material, and if the amount of Cu diffused into the iron matrix is too small, the strength of the sintered material is decreased. From this point of view, the lower limit of the heating temperature in the sintering is set to be 970 °C.
  • the copper powder is added at 1 to 4 mass %.
  • the amount of the copper powder is less than 1 mass %, the strength of the sintered material is decreased.
  • the amount of the copper powder is more than 4 mass %, the amount of Cu diffused in the iron matrix becomes too great, whereby the plate-shaped iron carbides are difficult to obtain in the cooling after the sintering. Therefore, the copper powder is added to the raw powder at 1 to 4 mass %.
  • the amount of the graphite powder is selected so that C diffused in the iron matrix forms an eutectoid composition or a hypereutectoid composition and so that a part of the amount of the graphite powder remains as a solid lubricant. Therefore, the graphite powder is added to the raw powder at not less than 1.3 mass %. On the other hand, when the graphite powder is added to the raw powder at more than 3.0 mass %, the flowability, the fillability, and the compressibility of the raw powder are greatly decreased, and the sintered material is difficult to produce. Therefore, the graphite powder is added to the raw powder at 1.3 to 3.0 mass %.
  • the diffusions of the elements of Cu and C are greatly affected by the heating temperature and are relatively less affected by the holding time at the heating temperature. Nevertheless, because Cu and C may not be sufficiently diffused if the holding time is too short in the sintering, the holding time is preferably set to be not less than 10 minutes. On the other hand, because Cu may be too diffused if the holding time is too long in the sintering, the holding time is preferably set to be not more than 90 minutes.
  • the sintered compact After the sintering, while the sintered compact is cooled from the heating temperature to room temperature, the sintered compact is preferably cooled from 850 to 600 °C at a cooling rate of not more than 20 °C/minute. In this case, the precipitated iron carbides tend to grow in the shape of plates. On the other hand, if the cooling rate is too low, a long time is required for the cooling and thereby the production cost is increased. Therefore, the cooling rate is preferably not less than 5 °C/minute in the temperature range of 850 to 600 °C.
  • the sintered compact may be isothermally held at a temperature during cooling from 850 to 600 °C so as to grow the precipitated iron carbides in the shape of plates.
  • the isothermal holding time is preferably not less than 10 minutes.
  • the isothermal holding time is preferably not more than 90 minutes at the temperature in the range of 850 to 600 °C.
  • an iron powder, a copper powder, and a graphite powder are prepared.
  • the copper powder and the graphite powder are mixed with the iron powder into a raw powder consisting of, by mass %, 1.3 to 3% of C, 1 to 4 % of Cu, and the balance of Fe and inevitable impurities.
  • the obtained raw powder is filled in a tube-shaped cavity of a die assembly, and the raw powder is compacted into a green compact having a tube shape.
  • the compacting is conventionally performed as a process for producing a sintered material for valve guides.
  • the green compact obtained by the compacting is sintered at a heating temperature of 970 to 1070 °C in a nonoxidizing atmosphere.
  • the production method for the sintered material for valve guides according to the First Embodiment in order to control the diffusion amount of Cu, copper powder is used, and the sintering is performed by solid-phase diffusion. In this case, since the diffusion bonding between the iron powder particles is also performed by solid-phase diffusion, the strength is lower than that of an iron-copper-carbon sintered material used as a structural material. Therefore, in the production method for the sintered material for valve guides according to the Second Embodiment, the strength of the sintered material is improved. That is, Sn having low melting point is used for generating liquid-phase sintering in the same manner as Japanese Examined Patent Publication No. 55-034858 .
  • the melting point of Sn is 232 °C, and the liquid-phase generating temperature of a copper-tin alloy varies with the amount of Sn.
  • the amount of Sn is increased in the copper-tin alloy, the liquid-phase generating temperature is decreased. Even when the amount of Sn is approximately 15 mass % in the copper-tin alloy, a liquid phase is generated at 798 °C.
  • Sn is added in the form of at least one of a tin powder and a copper-tin alloy powder. When the tin powder is used, Sn liquid phase is generated while the temperature is rising in the sintering.
  • the Sn liquid phase is filled in the spaces among the raw powder particles by capillary force and covers the copper powder particles, and the Sn liquid phase forms a Cu-Sn eutectic liquid phase on the surface of the copper powder particles.
  • a Cu-Sn eutectic liquid phase is generated in accordance with the temperature while the temperature is increasing in the sintering.
  • the Cu-Sn liquid phase is filled in spaces among the raw powder particles by capillary force and wets and covers the iron powder particles. As a result, growth of necks between the iron powder particles is accelerated, and the diffusion bonding of the iron powder particles is facilitated.
  • the upper limit of the amount of Sn is set to be 0.5 mass %.
  • the lower limit of the heating temperature in the sintering can be low.
  • predetermined diffusion conditions of Cu are obtained at 950 °C, which is lower than that in the production method for the sintered material for valve guides according to the First Embodiment.
  • the upper limit of the heating temperature in the sintering is required to be 1050 °C in order to control the diffusion of Cu into the iron matrix.
  • a copper-tin alloy powder including not less than 8 mass % of Sn (eutectic liquid phase generating temperature: 900 °C) may be used.
  • the preferable production conditions such as the heating time in the sintering, the cooling rate in the cooling, and the isothermal holding time in the cooling, are the same as those in the sintered material for valve guides according to the First Embodiment.
  • an iron powder, a graphite powder, and one selected from the group consisting of a combination of a copper powder and a tin powder, a copper-tin alloy powder, and a combination of a copper powder and a copper-tin alloy powder are prepared.
  • the graphite powder and the one selected from the group are mixed with the iron powder into a raw powder consisting of, by mass %, 1.3 to 3% of C, 1 to 4 % of Cu, 0.05 to 0.5 % of Sn, and the balance of Fe and inevitable impurities.
  • the obtained raw powder is filled in a tube-shaped cavity of a die assembly, and the raw powder is compacted into a green compact having a tube shape.
  • the compacting is conventionally performed as a process for producing a sintered material for valve guides.
  • the green compact obtained by the compacting is sintered at a heating temperature of 950 to 1050 °C in a nonoxidizing atmosphere.
  • the machinability may be improved by conventional methods such as the method disclosed in Japanese Patent No. 2680927 . That is, at least one kind selected from the group consisting of a manganese sulfide powder, a magnesium silicate mineral powder, and a calcium fluoride powder may be added to the raw powder at not more than 2 mass %. Then, by compacting and sintering this raw powder, a sintered material for valve guides is obtained.
  • This sintered material has particle boundaries in the matrix and pores, in which at least one of manganese sulfide particles, magnesium silicate mineral particles, and calcium fluoride particles are dispersed at not more than 2 mass %. Accordingly, the machinability of the sintered material is improved.
  • an iron powder, a copper powder, and a graphite powder were prepared.
  • the copper powder in the amounts shown in Table 1, and 2 mass % of the graphite powder, were added to the iron powder, and they were mixed to form a raw powder.
  • the raw powder was compacted at a compacting pressure of 650 MPa into a green compact with a tube shape.
  • Some of the green compacts had an outer diameter of 11 mm, an inner diameter of 6 mm, and a length of 40 mm (for a wear test).
  • the other green compacts had an outer diameter of 18 mm, an inner diameter of 10 mm, and a length of 10 mm (for a compressive strength test).
  • Another sintered compact sample of sample No. 11 was formed as a conventional example as follows.
  • 5 mass % of the copper-tin alloy powder, 1.4 mass % of the iron-phosphorus alloy powder, and 2 mass % of the graphite powder were added to the iron powder, and they were mixed to form a raw powder.
  • This raw powder was also compacted into two kinds of green compacts having the above shapes and was sintered under the above sintering conditions.
  • This conventional example corresponds to the sintered material disclosed in Japanese Examined Patent Publication No. 55-034858 .
  • the entire compositions of these sintered compact samples are shown in Table 1.
  • the wear test was performed as follows by using a wear testing machine.
  • the sintered compact sample having the tube shape was secured to the wear testing machine, and a valve stem of a valve was inserted into the sintered compact sample.
  • the valve was mounted at a lower end portion of a piston that would be vertically reciprocated. Then, the valve was reciprocated at a stroke speed of 3000 times/minute and at a stroke length of 8 mm at 500 °C in an exhaust gas atmosphere, and at the same time, a lateral load of 5 MPa was applied to the piston.
  • wear amount (in ⁇ m) of the inner circumferential surface of the sintered compact and wear amount (in ⁇ m) of the outer circumferential surface of the valve stem were measured.
  • the compressive strength test was performed as follows according to the method described in Z2507 specified by the Japanese Industrial Standard.
  • a sintered compact sample with a tube shape had an outer diameter of D (mm), a wall thickness of e (mm), and a length of L (mm).
  • the sintered compact sample was radially pressed by increasing the pressing load, and a maximum load F (N) was measured when the sintered compact sample broke.
  • a compressive strength K (N/mm 2 ) was calculated from the following first formula.
  • K F ⁇ D - e / L ⁇ e 2
  • the area ratio of the copper phase was measured as follows. The cross section of the sample was mirror polished and was etched with a nital. This metallic structure was observed by a microscope at 200-power magnification and was analyzed by using image analyzing software "WinROOF" that is produced by Mitani Corporation., whereby the area of the copper phases was measured so as to obtain an area ratio.
  • the area ratio of the iron carbide phase was measured in the same manner as in the case of the area ratio of the copper phase except that Murakami's reagent was used as the etching solution.
  • the area of each phase identified by the image analysis is not less than 0.05 % with respect to the visual field.
  • the effects of the amount of Cu in the entire composition of the sintered material and the effects of the amount of the copper powder in the raw powder are shown.
  • the area ratio of the plate-shaped iron carbide phase was approximately constant in the cross-sectional metallic structure and was approximately the same as that of the conventional example (sample No. 11).
  • the amount of Cu (the copper powder) was more than 2.5 mass %, the area ratio of the plate-shaped iron carbide phase was decreased. That is, in the sample of the sample No.
  • the area ratio of the plate-shaped iron carbide phase was decreased to approximately 3 %. Moreover, in the sample of the sample No. 10 including more than 4.0 mass % of Cu, the area ratio of the plate-shaped iron carbide phase was decreased to 1 %.
  • the copper phase was increased in proportion to the amount of Cu (the copper powder).
  • the area ratio of the copper phase was 0.5 % in the cross-sectional metallic structure.
  • the area ratio of the copper phase was increased to 3.5 %.
  • the area ratio of the copper phase was increased to approximately 4 %.
  • the compressive strength was increased in proportion to the amount of Cu (the copper powder).
  • the compressive strength was low, whereby these samples cannot be used as a valve guide.
  • the compressive strength was not less than 500 MPa, and the strength was at an acceptable level sufficient to use as a valve guide.
  • the wear amounts of the valve guides were approximately the same as that of the conventional example (sample No. 11) and were approximately constant and low. As a result, the total wear amounts were also approximately the same as that of the conventional example (sample No. 11) and were approximately constant and low.
  • the samples of the samples Nos. 07 to 09 including 3.0 to 4.0 mass % of Cu the copper powder
  • the influence of the decrease in the amount of the plate-shaped iron carbides was greater than the effect of Cu for strengthening the matrix. Therefore, the wear resistances were decreased, and the wear amounts of the valve guides were slightly increased.
  • the wear resistance was greatly decreased due to the decrease in the amount of the plate-shaped iron carbides.
  • the wear amount of the valve guide was increased, and the total wear amount was greatly increased.
  • the wear resistances of the sintered compacts were approximately equal to that of the sintered material disclosed in Japanese Examined Patent Publication No. 55-034858 .
  • the sintered compacts had strength at an acceptable level to use as a valve guide.
  • the area ratio of the copper phase was 0.5 to 3.5 % in the cross-sectional metallic structure when the amount of Cu was in this range. In this case, the area ratio of the plate-shaped iron carbide phase was required to be approximately not less than 3 % in the cross-sectional metallic structure.
  • the raw powder was compacted and was sintered in the same conditions as in the First Example, whereby samples of samples Nos. 12 to 17 were formed. The entire compositions of these samples are shown in Table 2.
  • the wear test and the compressive strength test were performed under the same conditions as those in the First Example. Moreover, the area ratio of the iron carbide phase and the area ratio of the copper phase were measured. These results are also shown in Table 2. It should be noted that the values of the sample of the sample No.
  • Table 2 Sample No. Mixing ratio mass % Composition mass % Area ratio % Compressive strength MPa Wear amount ⁇ m Notes Iron powder Copper powder Graphite powder Fe Cu C Iron carbide phase Copper phase VG VS Total 12 Bal. 2.00 1.00 Bal. 2.00 1.00 0.0 1.3 889 87 4 91 Exceeds lower limit of amount of C 13 Bal. 2.00 1.30 Bal. 2.00 1.30 3.0 1.2 837 73 2 75 Lower limit of amount of C 14 Bal. 2.00 1.50 Bal. 2.00 1.50 9.8 1.2 664 68 2 70 05 Bal. 2.00 2.00 Bal.
  • the effects of the amount of C in the entire composition of the sintered material and the effects of the amount of the graphite powder in the raw powder are shown.
  • the amount of C diffused in the matrix was small, whereby the plate-shaped iron carbide phase was not precipitated.
  • the amount of C diffused in the matrix was sufficient, and the area ratio of the plate-shaped iron carbide phase was approximately 3 % in the cross-sectional metallic structure.
  • the area ratio of the plate-shaped iron carbide phase was increased in the cross-sectional metallic structure. That is, in the sample of the sample No. 16 including 3 mass % of C (the graphite powder), the area ratio of the plate-shaped iron carbide phase was approximately 25 %. Moreover, in the sample of the sample No. 17 including more than 3 mass % of C (the graphite powder), the area ratio of the plate-shaped iron carbide phase was increased to approximately 28 %.
  • the area ratio of the copper phase was constant in the cross-sectional metallic structure regardless of the amount of C (the graphite powder). This was because the amount of Cu (the copper powder) was constant and the sintering conditions were the same.
  • the plate-shaped iron carbide phase was not precipitated in the matrix, and the compressive strength was the highest.
  • the amount of C (the graphite powder) was increased, the iron carbide phase precipitated in the matrix was increased, whereby the compressive strength was decreased.
  • the compressive strength was approximately 500 MPa. Therefore, when the amount of C (the graphite powder) was not more than 3 mass %, the strength of the sintered compact was at an acceptable level sufficient to use as a valve guide.
  • the wear amount of the valve guide was slightly increased.
  • the wear amount of the valve guide was greatly increased. Since the amount of the hard plate-shaped iron carbide phase precipitated in the matrix was increased with the increase of C (the graphite powder), the wear amount of the valve stem was increased with the increase of C (the graphite powder). According to these wear conditions, the total wear amount was decreased when the amount of C (the graphite powder) was in the range of 1.3 to 3 mass %.
  • the wear resistances of the sintered compacts were approximately equal to that of the sintered material disclosed in Japanese Examined Patent Publication No. 55-034858 .
  • the sintered compacts had strength at an acceptable level to use as a valve guide.
  • the area ratio of the plate-shaped iron carbide phase was 3 to 25 % in the cross-sectional metallic structure when the amount of C was in this range.
  • the precipitation amount of the plate-shaped iron carbide phase was decreased, and the area ratio of the plate-shaped iron carbide phase was decreased in the cross-sectional metallic structure. That is, in the sample of the sample No. 24 in which the heating temperature was 1100 °C and was more than the melting point of Cu (1085 °C), Cu was uniformly diffused into the matrix. As a result, C was not precipitated as a large plate-shaped iron carbide phase and was precipitated in the shape of a pearlite. Therefore, the area ratio of the plate-shaped iron carbide phase was extremely small in the cross-sectional metallic structure.
  • the compressive strength was increased.
  • the compressive strength was less than 500 MPa and was not at a level that is required in a case of using the sintered compact as a valve guide.
  • the diffusion amount of Cu into the matrix was increased.
  • the compressive strengths were not less than 500 MPa and were at acceptable levels to use as valve guides.
  • the wear amount of the valve guide was decreased.
  • the wear amount of the valve guide was more decreased by the above effects.
  • the diffusion amount of Cu into the matrix was increased. Therefore, in the samples of the samples Nos. 22 and 23 in which the heating temperature was 1050 to 1070 °C, the precipitation amount of the plate-shaped iron carbide phase was decreased with the increase of the heating temperature. Accordingly, the wear amounts of the valve guides were slightly increased.
  • the wear resistance was decreased, and the wear amount of the valve guide was greatly increased.
  • the wear amount of the valve stem was approximately constant regardless of the heating temperature. Therefore, the total wear amount was decreased when the heating temperature was in the range of 970 to 1070 °C.
  • the wear resistance was superior.
  • the sintered compacts had strength at an acceptable level to use as a valve guide.
  • the copper-tin alloy powder used for forming the conventional example (sample No. 11), and a tin powder were prepared.
  • the copper-tin alloy powder consisted of 10 mass % of Sn and the balance of Cu and inevitable impurities.
  • 3 mass % of the copper powder, 2 mass % of the graphite powder, and the tin powder at the amount shown in Table 4 were added to the iron powder, and they were mixed to form a raw powder.
  • the raw powder was compacted and was sintered in the same conditions as in the First Example, whereby samples of samples Nos. 25 to 34 were formed. The entire compositions of these samples are shown in Table 4.
  • the area ratio of the plate-shaped iron carbide phase and the area ratio of the copper alloy phase were decreased in the cross-sectional metallic structure.
  • the decrease amounts of the area ratio of the iron carbide phase and the area ratio of the copper alloy phase were increased with the increase of the amount of Sn. This was because a greater amount of the Cu-Sn liquid phase was generated in the sintering according to the increase of the amount of Sn, whereby the diffusion amount of Cu into the matrix was increased.
  • the area ratio of the plate-shaped iron carbide phase was approximately 5 % and the area ratio of the copper alloy phase was approximately 0.5 % in the cross-sectional metallic structure.
  • the area ratio of the plate-shaped iron carbide phase was decreased to less than 5 % and the area ratio of the copper alloy phase was decreased to less than 0.5 % in the cross-sectional metallic structure.
  • the compressive strength was increased compared with the sample of the sample No. 07 which did not include Sn.
  • the compressive strength was increased with the increase of the amount of Sn. This was because a greater amount of the Cu-Sn liquid phase was generated in the sintering according to the increase of the amount of Sn. In this case, the diffusion amount of Cu into the matrix was increased, and the Cu-Sn liquid phase wetted and covered the surface of the iron powder particles and thereby accelerating neck growth between the iron powder particles.
  • the effect for improving the compressive strength was small.
  • the effect for improving the compressive strength was great.
  • the wear amount of the valve guide was approximately equal to that of the sample of the sample No. 07 which did not include Sn.
  • the wear amount of the valve guide was slightly increased.
  • the plate-shaped iron carbide phase was decreased with the increase of the amount of Sn as described above, the wear amount of the valve guide was not greatly increased. This was because the neck between the iron powder particles grew and thereby the strength was improved.
  • the wear resistance was greatly decreased due to the decrease of the plate-shaped iron carbide phase.
  • the wear amount of the valve guide was suddenly increased.
  • the wear amount of the valve stem was approximately constant regardless of the amount of Sn. Accordingly, when the amount of Sn was in the range of not more than 0.5 mass %, the total wear amount was small, and superior wear resistance was obtained.
  • the strength of the sintered material was improved.
  • the amount of Sn was more than 0.5 mass %, the wear resistance was decreased. Therefore, when Sn is added, it is required that the amount of Sn be 0.05 to 0.5 mass %.
  • the iron powder and the graphite powder used in the First Example, and the copper-tin alloy powder used in the Fourth Example were prepared. Then, 2 mass % of the copper-tin alloy powder and 2 mass % of the graphite powder were added to the iron powder, and they were mixed to form a raw powder.
  • the raw powder was compacted in the same conditions as in the First Example so as to obtain a green compact.
  • the green compact was sintered in the same conditions as in the First Example except that the heating temperature was changed to the temperature shown in Table 5 in the sintering, whereby samples of samples Nos. 35 to 42 were formed.
  • Heating temperature °C Area ratio % Iron Copper Compressive strength MPa Wear amount ⁇ m Notes carbide phase alloy phase VG VS Total 35 900 0.4 1.4 463 84 4 88 Exceeds lower limit of heating temerature 36 950 11.0 1.0 505 66 2 68 Lower limit of heating temerature 37 970 14.7 0.8 588 63 2 65 38 1000 16.0 0.7 667 61 1 62 39 1020 16.7 0.6 696 59 1 60 40 1050 11.3 0.6 719 64 2 66 Upper limit of heating temerature 41 1070 2.7 0.4 751 86 2 88 Exceeds upper limit of heating temerature 42 1100 1.6 0.3 787 90 4 94 Exceeds upper limit of heating temerature 11 1000 17.7 3.2 680 61 2 63 Conventional example
  • the heating temperature was more than 1050 °C
  • the amount of Cu diffused in the matrix was increased, whereby the plate-shaped iron carbide phase was difficult to be formed. Therefore, the precipitation amounts of the iron carbide phases were decreased, and the area ratios of the plate-shaped iron carbide phases were decreased in the cross-sectional the metallic structures.
  • the compressive strength was increased.
  • the compressive strength was less than 500 MPa and was not at a level that is required in a case of using the sintered compact as a valve guide.
  • the samples of the samples Nos. 36 to 42 in which the heating temperature was not less than 950 °C the diffusion amount of Cu into the matrix was increased.
  • the compressive strengths were not less than 500 MPa and were at acceptable levels to use for valve guides.
  • the wear amounts of the valve guides were even less. According to the increase of the heating temperature, the diffusion amount of Cu into the matrix was increased. Therefore, in the sample of the sample No. 40 in which the heating temperature was 1050 °C, the area ratio of the precipitated plate-shaped iron carbide phase was decreased to approximately 11 %. Accordingly, the wear amount of the valve guide was slightly increased. Moreover, in the samples of the samples Nos. 41 and 42 in which the heating temperature was more than 1050 °C, the precipitation amount of the plate-shaped iron carbide phase was greatly decreased, and the wear resistance was decreased. As a result, the wear amounts of the valve guides were greatly increased. The wear amount of the valve stem was approximately constant regardless of the heating temperature. Accordingly, the total wear amount was decreased when the heating temperature was in the range of 950 to 1050 °C.
  • the raw powder was compacted and was sintered in the same conditions as in the First Example except for the cooling rate, whereby samples of samples Nos. 43 to 47 were formed.
  • the cooling rate was changed to the cooling rate shown in Table 6 while the sintered compact was cooled from 850 to 600 °C. In these samples, the wear test and the compressive strength test were performed under the same conditions as those in the First Example.
  • Cooling rate °C/minute Area ratio % Compressive strength MPa Wear amount ⁇ m Notes Iron carbide phase Copper phase VG VS Total 43 5 21.7 1.4 542 59 2 61 05 10 18.3 1.4 620 63 2 65 44 15 16.4 1.3 640 65 2 67 45 20 11.5 1.4 663 67 2 69 46 25 5.3 1.4 722 71 4 75 Upper limit of cooling rate 47 30 2.0 1.4 754 85 5 90 Exceeds upper limit of cooling rate 11 10 17.7 3.2 680 61 2 63 Conventional example
  • the area ratio of the iron carbides was increased in the cross-sectional metallic structure. In other words, when the cooling rate was greater, the area ratio of the iron carbides was decreased. That is, C at amount in which C was supersaturated at room temperature, was solved in the austenite in the heating temperature range in the sintering, and supersaturated C in this heating temperature range was precipitated as iron carbides (Fe 3 C). If the sintered compact in this temperature range is cooled at a low cooling rate, the precipitated iron carbides grow, whereby the amount of the iron carbide phase is increased.
  • the sintered compact in this temperature range is cooled at a high cooling rate, the precipitated iron carbides do not grow. Therefore, the ratio of the pearlite, in which fine iron carbides are dispersed, is increased, and the amount of the iron carbide phase is decreased.
  • the cooling rate was increased to 25 °C/minute during the cooling from 850 to 600 °C, the area ratio of the iron carbide phase came to approximately 5 % in the cross-sectional metallic structure.
  • the cooling rate was more than 25 °C/minute, the area ratio of the iron carbide phase was less than 5 %.
  • the copper phase was not formed of supersaturated Cu that was precipitated and was diffused, but was formed of copper powder that was not dispersed and remained as a copper phase. Therefore, the area ratio of the copper phase in the cross-sectional metallic structure was constant regardless of the cooling rate.
  • the cooling rate was greater during the cooling from 850 to 600 °C, the amount of the fine iron carbides were increased, and the amount of the plate-shaped iron carbide phase was decreased. Therefore, the compressive strength was increased with the increase of the cooling rate.
  • the cooling rate was greater during the cooling from 850 to 600 °C, since the amount of the iron carbide phase for improving the wear resistance was decreased, the wear amount of the valve guide was slightly increased.
  • the cooling rate was increased to more than 25 °C/minute during the cooling from 850 to 600 °C, the area ratio of the iron carbide phase was less than 5 %, and the wear amount of the valve guide was suddenly increased.
  • the cooling rate by controlling the cooling rate during the cooling from 850 to 600 °C, the amount of the plate-shaped iron carbide phase was controlled.
  • the cooling rate by setting the cooling rate to be not more than 25 °C/minute during the cooling from 850 to 600 °C, the area ratio of the plate-shaped iron carbide phase was made to be not less than 5 % in the cross-sectional metallic structure, and superior wear resistance was obtained.
  • the cooling rate is preferably set to be not less than 5 °C/minute during the cooling from 850 to 600 °C.
  • the raw powder was compacted and was sintered in the same conditions as in the First Example except for the cooling process, whereby samples of samples Nos. 48 to 51 were formed.
  • the cooling rate was set to be 30 °C/minute during the cooling from 850 to 780 °C.
  • the sintered compact was isothermally held at 780 °C for a holding time shown in Table 7 and was cooled from 780 to 600 °C at a cooling rate of 30 °C/minute.
  • the wear test and the compressive strength test were performed under the same conditions as those in the First Example.
  • the area ratio of the plate-shaped iron carbide phase and the area ratio of the copper phase were measured.
  • the samples of the samples Nos. 48 to 51 were cooled at the cooling rate at which the area ratio of the plate-shaped iron carbide phase was less than 5 % in the cross-sectional metallic structure in the Sixth Example. In this case, these samples were isothermally held at the temperature in the range of 850 to 600 °C during the cooling from the heating temperature to room temperature. Therefore, the area ratio of the plate-shaped iron carbide phase was increased to not less than 5 %. According to the increase of the isothermal holding time, the area ratio of the plate-shaped iron carbide phase was increased.
  • the copper phase was not formed of supersaturated Cu that was precipitated and was diffused, but was formed of copper powder that was not dispersed and remained as a copper phase. Therefore, the area ratio of the copper phase in the cross-sectional metallic structure was constant regardless of the isothermal holding time.
  • the isothermal holding time at the temperature in the range of 850 to 600 °C was shorter, the time required for growing the plate-shaped iron carbides was shorter, and the area ratio of the plate-shaped iron carbide phase was decreased. In other words, when the isothermal holding time was longer, the time required for growing the iron carbides were longer, and the area ratio of the plate-shaped iron carbide phase was increased. Therefore, the compressive strength was decreased with the increase of the isothermal holding time.
  • the isothermal holding time at the temperature in the range of 850 to 600 °C was longer, the amount of the plate-shaped iron carbide phase for improving the wear resistance was increased. Therefore, the wear amount of the valve guide was decreased with the increase of the isothermal holding time.
  • the amount of the plate-shaped iron carbide phase was controlled.
  • the isothermal holding time is preferably set to be not more than 90 minutes.

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JP5783457B2 (ja) 2015-09-24
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CN102443739B (zh) 2014-05-07
KR101365816B1 (ko) 2014-02-20
JP2012092441A (ja) 2012-05-17
EP2436463A3 (de) 2012-07-11
KR20120034052A (ko) 2012-04-09

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