SE1850423A1 - Iron-based sintered body and method of manufacturing the same - Google Patents

Abstract

Provided is an iron-based sintered body having excellent mechanical properties. In the sintered body, the area fraction of pores is 15 % or less and the area-based median size D50 of the pores is 20 pm or less.

Description

IRON-BASED SINTERED BODY AND METHOD OF MANUFACTURINGTHE SAME TECHNICAL FIELD id="p-1"

[0001] This disclosure relates to an iron-based sintered body, and relates inparticular to an iron-based sintered body suitable for manufacturing highstrength sintered parts for automobiles, the sintered body having high sintereddensity and having reliably improved tensile strength and toughness (impactenergy value) after performing the processes of carburizing, quenching, andtempering on the sintered body. Further, this disclosure relates to a methodof manufacturing the iron-based sintered body.[0002] Powder metallurgical techniques enable producing parts withcomplicated shapes in shapes that are extremely close to product shapes(so-called near net shapes) with high dimensional accuracy, and consequentlysignificantly reducing machining costs. For this reason, powdermetallurgical products are used for various machines and parts in many fields.[0003] In recent years, there is a strong demand for powder metallurgicalproducts to have improved toughness in terms of improving the strength forminiaturizing parts and reducing the weight thereof and safety. In particular,for powder metallurgical products (iron-based sintered bodies) which are veryoften used for gears and the like, in addition to higher strength and highertoughness, there is also a strong demand for higher hardness in terms of wearresistance. In order to meet the above-mentioned demands, iron-basedsintered bodies of which components, structures, density and the like arecontrolled suitably are required to be developed, since the strength andtoughness of an iron-based sintered body varies widely depending on thoseproperties.[0004] Typically, a green compact before being subjected to sintering isproduced by mixing iron-based powder with alloying powders such as copperpowder and graphite powder and a lubricant such as stearic acid or lithiumstearate to obtain mixed powder; filling a mold with the mixed powder; andcompacting the powder.

The density of a green compact obtained through a typical powder metallurgical process is usually around 6.6 Mg/m3 to 7.l Mg/m3. The green Ref. NO. Po1629s4-PcT (1/35) compact is then sintered to form a sintered body Which in turn is furthersubjected to optional sizing or cutting work, thereby obtaining a powdermetallurgical product. Further, when even higher strength is required,carburizing heat treatment or bright heat treatment may be performed aftersintering. id="p-5"

[0005] Based on the components, iron-based powders used here arecategorized into iron powder (e.g. iron-based powder and the like) and alloysteel powder. Further, when categorized by production method, iron-basedpowders are categorized into atomized iron powder and reduced iron powder.Within these categories specified by production methods, the term "ironpowder" is used with a broad meaning encompassing alloy steel powder aswell as iron-based powder. id="p-6"

[0006] In terms of obtaining a sintered body with high strength and hightoughness, it is advantageous that iron-based powder being a main componentin particular allows alloying of the powder to be promoted and highcompressibility ofthe powder to be maintained.

First, known iron-based powders obtained by alloying include: (l) mixed powder obtained by adding alloying element powders to iron-basedpowder, (2) pre-alloyed steel powder obtained by completely alloying alloyingelements, (3) partially diffusion alloyed steel powder (also referred to as compositealloy steel powder) obtained by partially adding alloying element powders in adiffused manner to the surface of particles of iron-based powder orpre-alloyed steel powder. id="p-7"

[0007] The mixed powder (l) mentioned above advantageously has highcompressibility equivalent to that of pure iron powder. However, insintering, the alloying elements are not sufficiently diffused in Fe and form anon-uniform microstructure, which would result in poor strength of theresulting sintered body. Further, since Mn, Cr, V, Si, and the like are moreeasily oxidized than Fe, when these elements are used as the alloying elements,they get oxidized in sintering, which would reduce the strength of theresulting sintered body. In order to suppress the oxidation and reduce the amount of oxygen in the sintered body, it is necessary that the atmosphere for Ref. NO. Po1629s4-PcT (2/35) sintering, and the C02 concentration and the dew point in the carburizingatmosphere are strictly controlled in the case of performing carburizing aftersintering. Accordingly, the mixed powder (l) mentioned above cannot meetthe demands for higher strength in recent years and has become unused.[0008] On the other hand, when the pre-alloyed steel powder obtained bycompletely alloying the elements of (2) mentioned above is used, the alloyingelements can be completely prevented from being segregated, so that themicrostructure of the sintered body is made uniform, leading to stablemechanical properties. In addition, also in the case where Mn, Cr, V, Si, andthe like are used as the alloying elements, the amount of oxygen in thesintered body can be advantageously reduced by limiting the kind and theamount of the alloying elements. However, when the pre-alloyed steelpowder is produced by atomization from molten steel, oxidation in theatomization of the molten steel and solid solution hardening of steel powderdue to complete alloying would be caused, which makes it difficult to increasethe density of the green compact after compaction (forming by pressing).When the density of the green compact is low, the toughness of the sinteredbody obtained by sintering the green compact is low. Therefore, also whenthe pre-alloyed steel powder is used, demands for higher strength and highertoughness cannot be met. id="p-9"

[0009] The partially diffusion alloyed steel powder (3) mentioned above isproduced by adding alloying elements to iron-based powder or pre-alloyedsteel powder, followed by heating under a non-oxidizing or reducingatmosphere, thereby partially diffusion bonding the alloying element powdersto the surface of particles of iron-based powder or pre-alloyed steel powder.Accordingly, advantages ofthe iron-based mixed powder of (l) above and thepre-alloyed steel powder of (2) above can be obtained. id="p-10"

[0010] Thus, when the partially diffusion pre-alloyed steel powder is used,oxygen in the sintered body can be reduced and the green compact can have ahigh compressibility equivalent to the case of using pure iron powder.Therefore, the sintered body has a multi-phase structure consisting of acompletely alloyed phase and a partially concentrated phase, increasing the strength ofthe sintered body.

Ref. NO. Po1629s4-PcT (3/35) id="p-11"

[0011] As basic alloy components used in the partially diffusion alloyed steelpowder, Ni and Mo are used heavily.

Ni has the effect of improving the toughness of a sintered body.Adding Ni stabilizes austenite, which allows more austenite to remain asretained austenite without transforming to martensite after quenching.Further, Ni serves to strengthen the matrix of a sintered body by solid solutionstrengthening. id="p-12"

[0012] Meanwhile, Mo has the effect of improving hardenability.Accordingly, Mo suppresses the formation of ferrite during quenching,allowing bainite or martensite to be easily formed, thereby strengthening thematrix ofthe sintered body. Further, Mo is contained as a solid solution in amatrix to solid solution strengthen the matrix, and forms fine carbides tostrengthen the matrix by precipitation. id="p-13"

[0013] As an example of the mixed powder for high strength sintered partsusing the above-described partially diffusion alloyed steel powder, JP3663929 B2 (PTL 1) discloses mixed powder for high strength sintered partsobtained by mixing Ni: 1 mass% to 5 mass%, Cu: 0.5 mass% to 4 mass%, andgraphite powder: 0.2 mass% to 0.9 mass% to alloy steel powder in which Ni:0.5 mass% to 4 mass% and Mo: 0.5 mass% to 5 mass% are partially alloyed.The sintered material described in PTL 1 contains 1.5 mass% of Ni atminimum, and substantially contains 3 mass% or more of Ni according toExamples of PTL 1. This means that a large amount of Ni as much as 3mass% or more is required to obtain a sintered body having a high strength of800 MPa or more. Further, obtaining a material having a high strength of1000 MPa or more by subjecting a sintered body to carburizing, quenching,and tempering also requires a large amount of Ni as much as for example 3mass% or 4 mass%. id="p-14"

[0014] However, Ni is an element which is disadvantageous in terms ofaddressing recent environmental problems and recycling, so its use isdesirably avoided as possible. Also in respect of cost, adding several mass%of Ni is significantly disadvantageous. Further, when Ni is used as analloying element, sintering is required to be performed for a long time in orderto sufficiently diffuse Ni in iron powder or steel powder. Moreover, when Ni being an austenite phase stabilizing element is not sufficiently diffused, a Ref. NO. Po1629s4-PcT (4/35) high Ni concentration area is stabilized as the austenite phase (hereinafter alsoreferred to as y phase) and the other area where Ni is hardly contained isstabilized as other phases, resulting in a non-uniform metal structure in thesintered body. id="p-15"

[0015] As a Ni-free technique, JP 3651420 B2 (PTL 2) discloses a techniqueassociated with partially diffusion alloyed steel powder of Mo free of Ni.That is, PTL 2 states that optimization of the Mo content results in a sinteredbody having high ductility and high toughness that can resist repressing aftersintering. id="p-16"

[0016] Further, regarding a high density sintered body free of Ni, JPH04-285141 A (PTL 3) discloses mixing iron-based powder having a meanparticle diameter of 1 um to 18 um with copper powder having a mean particlediameter of 1 um to 18 um at a weight ratio of 100:(0.2 to 5), and compactingthe mixed powder and sintering the green compact. In the techniquedisclosed in PTL 3, iron-based powder having a mean particle diameter that isextremely smaller than that of typical one is used, so that a sintered bodyhaving a density as extremely high as 7.42 g/cm3 or more can be obtained.[0017] WO 2015/045273 A1 (PTL 4) discloses that a sintered body havinghigh strength and high toughness is obtained using powder free of Ni, inwhich Mo is adhered to the surface of iron-based powder particles bydiffusion bonding to achieve a specific surface area of 0.1 mZ/g or more.[0018] Further, JP 2015-014048 A (PTL 5) discloses that a sintered bodyhaving high strength and high toughness is obtained using powder in whichMo is adhered to iron-based powder particles containing reduced iron powderby diffusion bonding. id="p-19"

[0019] JP 2015-004098 A (PTL 6) describes that Fe-Mn-Si powder is addedto iron powder particles of a small particle size and the mixed powder is warmcompacted in a lubricated mold, thereby reducing the maximum pore length ofthe sintered body to obtain a sintered body having high strength and high toughness.

CITATION LISTPatent Literature id="p-20"

[0020] PTL 1: JP 3663929 B2 Ref. NO. Po1629s4-PcT (5/35) PTL 2: JP 3651420 B2 PTL 3: JP H04-285141 APTL 4: WO 2015/045273 A1PTL 5: JP 2015-014048 APTL 6: JP 2015-004098 A SUMMARY (Technical Problem) id="p-21"

[0021] However, the sintered materials obtained in accordance with thedescription of PTL 2, PTL 3, PTL 4, PTL 5, and PTL 6 above have been foundto have the following respective problems. id="p-22"

[0022] The technique disclosed in PTL 2 is designed to achieve high strengthby recompression after sintering. Accordingly, when a sintered material ismanufactured by a typical metallurgical process, both sufficient strength andtoughness are hardly achieved at the same time. id="p-23"

[0023] Further, the iron-based powder used for the sintered materialdescribed in PTL 3 has a mean particle diameter of 1 um to 18 um which issmaller than normal. Such a small particle diameter results in poorflowability of the mixed powder inducing cracking and chipping of the greencompact due to unevenness ofthe powder in filling the mold. Therefore, it isdifficult to obtain a sintered body having sufficient strength and toughness.[0024] Further, since the powder described in PTL 4 has extremely largespecific surface area, use of such powder results in low flowability of thepowder and induces cracking and chipping of the green compact due tounevenness of the powder in filling the mold. Therefore, it is difficult toobtain a sintered body having sufficient strength and toughness. id="p-25"

[0025] Also for the sintered body described in PTL 5, as with the techniquedescribed in PTL 4, reduced iron powder having extremely large specificsurface area is used, which results in low flowability of the powder andinduces cracking and chipping of the green compact due to unevenness of thepowder in filling the mold. Therefore, it is difficult to obtain a sintered bodyhaving sufficient strength and toughness. id="p-26"

[0026] The toughness of the sintered body disclosed in PTL 6 is increased mainly by limiting the maximum pore length; however, high strength and Ref. NO. Po1629s4-PcT (6/35) toughness are hardly achieved by only limiting the maximum pore length, andfurther improvement is required. id="p-27"

[0027] It could be helpful to provide an iron-based sintered body havingexcellent mechanical properties as well as a method of manufacturing thesame.

(Solution to Problem) id="p-28"

[0028] With a view to achieve the above objective, we made various studiesto obtain a sintered body having both high strength and high toughness. As aresult, we discovered the following: for an iron-based sintered body obtained by pressing mixed powdermade of iron-based powder and additives and then sintering, adjusting themean diameter of pores in the sintered body contributes to the improvement inthe impact energy value due to the dispersion of stress concentrations in thestructure. id="p-29"

[0029] This disclosure is based on the aforementioned discoveries and furtherstudies. Specifically, the primary features of this disclosure are describedbelow. 1. An iron-based sintered body, comprising an area fraction of pores inthe iron-based sintered body of 15 % or less, and an area-based median sizeD50 of the pores of 20 um or less. id="p-30"

[0030] 2. The iron-based sintered body according to 1. above, comprising Mo,Cu, and C. id="p-31"

[0031] 3. The iron-based sintered body according to 2. above, comprising Moin an amount of 0.2 mass% to 1.5 mass%, Cu in an amount of 0.5 mass% to4.0 mass%, and C in an amount of 0.1 mass% to 1.0 mass%. id="p-32"

[0032] 4. The iron-based sintered body according to any one of 1. to 3. above,wherein the iron-based sintered body has been carburized, quenched, andtempered. id="p-33"

[0033] 5. A method of manufacturing an iron-based sintered body, the methodcomprising: compacting (i) partially diffusion alloyed steel powder in whichMo is adhered to the surface of particles of iron-based powder by diffusionbonding with (ii) mixed powder for powder metallurgy obtained by mixing at least Cu powder and graphite powder at a pressure of 400 MPa or more to Ref. NO. Po1629s4-PcT (7/35) obtain a compact; and then sintering the obtained compact at 1000 °C orhigher for 10 min or more. id="p-34"

[0034] 6. The method of manufacturing a high strength according to 5. above,the method further comprising carburizing, quenching, and tempering aftersintering the obtained compact. id="p-35"

[0035] 7. The method of manufacturing an iron-based sintered body,according to 5. or 6. above, wherein the mixed powder for powder metallurgycontains Mo in an amount of 0.2 mass% to 1.5 mass% and the balanceconsisting of Fe and incidental impurities. id="p-36"

[0036] 8. The method of manufacturing an iron-based sintered body,according to any one of 5. to 7. above, wherein the partially diffusion alloyedsteel powder has a mean particle diameter of 30 um to 120 um and a specificsurface area of less than 0.10 m2/g, and a circularity of particles of thepartially diffusion alloyed steel powder that have a diameter in a range of 50um to 100 um is 0.65 or less. id="p-37"

[0037] 9. The method of manufacturing an iron-based sintered body,according to any one of 5. to 8. above, wherein the amount of the Cu powdermixed is 0.5 mass% to 4.0 mass% of the mixed powder for powder metallurgy.(Advantageous Effect) id="p-38"

[0038] This disclosure can provide an iron-based sintered body having both high strength and high toughness.

DETAILED DESCRIPTION id="p-39"

[0039] Our methods and products will be described in detail below.

The area fraction of pores in the disclosed sintered body is 15 % orless and the area-based median size D50 ofthe pores is 20 um or less.[0040] Pores are unavoidably formed in the iron-sintered body obtained bysintering a green compact obtained by compacting alloy steel powder forpowder metallurgy, and it is important to control the pores for improving thestrength and toughness of the sintered body. That is, since smaller poreshardly act as starting points of cracks, it is important that the area-based median size D50 of the pores is 20 um or less.

When the median size D50 More preferably, thearea-based median size D50 is 15 um or less. exceeds 20 um, the toughness is significantly reduced.

Ref. NO. Po1629s4-PcT (8/35) id="p-41"

[0041] Here, the median size D50 of the pores can be measured in thefollowing manner.

First, a sintered body is embedded in a thermosetting resin. A crosssection is then mirror-polished and the cross section is imaged using anoptical microscope at 100>< magnification over a field of view of 843 um ><629 um. The cross-sectional area A of all the pores in 20 fields randomlyselected from the resulting micrograph ofthe cross section is measured. Theequivalent circle diameter dc that is the diameter of a circle having an areaequal to the measured cross-sectional area is determined in accordance withthe following equation (I). Next, the areas of the pores are integrated inascending order of the circle equivalent diameter and a circle equivalentdiameter at which the integrated value is 50 % of the total area of the pores isdefined as an area-based median size D50. tffz A/:f ' ° '(1) id="p-42"

[0042] As described above, the median size D50 of the pores of the sinteredbody is controlled to 20 um or less, since a median size D50 exceeding 20 umincreases pores having an indefinite shape and such pores become stressconcentrations when deformation occurs, which reduces strength andtoughness.

Here, in order to control the area fraction of the pores in the sinteredbody to 15 % or less and the median size D50 of the pores to 20 um or less,partially diffusion alloyed steel powder of mixed powder for powdermetallurgy which is a material of the sintered body is used. The partiallydiffusion alloyed steel powder is obtained by adhering Mo powder particles tothe surface of iron-based powder particles, the steel powder particles having amean particle diameter of 30 um to 120 um a and specific surface area of lessthan 0.10 m2/g, and particles of the steel powder that have a diameter in arange of 50 um to 100 um has a circularity of 0.65. Thus, sintering ispromoted in manufacturing a sintered body to be described, so that a desiredsintered body can be obtained. id="p-43"

[0043] Since the number of pores is preferably smaller, the area fraction ofthe pores in the sintered body is controlled to 15 % or less. This is becausesince an area fraction of the pores exceeding 15 % reduces the content of metal in the sintered body, even if the pore diameter is reduced, sufficient Ref. NO. Po1629s4-PcT (9/35) _10- strength and toughness cannot be obtained. Note that making the pores inThe pores in the sintered body obtained by the following method is at least the sintered body be 0 % requires significant effort and is not realistic. approximately 5 %.[0044] Here, the area fraction of the pores in the sintered body can becalculated by the following method.

In a manner similar to the above, the cross-sectional area A of all thepores in 20 fields is measured and summed to find the total pore area At of allthe observed fields. Dividing At by the total of the areas of all the observedfields gives the area fraction ofthe pores. id="p-45"

[0045] Further, the length of the pores in the sintered body is preferablysmaller. The "mean maximum pore length" that is an indicator of the lengthof the pores is calculated as follows. First, the maximum value of thedistance between two points on the circumferential edge of each pore in thefield of the above micrograph of the cross section is found by image analysisand is defined as the "pore length" ofthe pore. The "maximum pore length"is longest among the "pore lengths" of all the pores included in a field of viewof the micrograph of the cross section. Further, the "mean maximum porelength" is the arithmetic mean value of the maximum pore lengths found for20 fields selected randomly. Note that in order to achieve sufficientmechanical properties, the mean maximum pore length is preferably less than100 um. id="p-46"

[0046] Further, the above sintered body preferably contains Mo, Cu, and C.Mo has the effect ofimproving hardenability. Cu has the effect ofimprovingsolid solution strengthening and hardenability of iron-based powder. C hasthe effect of enhancing the strength of iron-based sintered body by beingprecipitated as a solid solution or fine carbide in iron. Preferred contentrange of the respective elements contained in the disclosed iron-based sinteredbody is Mo: 0.2 mass% to 1.5 mass%, Cu: 0.5 mass% to 4.0 mass%, and C: 0.1mass% to 1.0 mass%. When the elements are less than the above range, thestrength cannot sufficiently be increased, whereas when they are added to bemore than the above range, the structure is extremely hardened and the toughness is reduced.

Ref. No. POl62984-PCT (10/35) _11- id="p-47"

[0047] Next, a method of obtaining the above sintered body will be described.The following method is a mere example, and the disclosed iron-basedsintered body may be obtained by a method other than the following method.

In manufacturing a sintered body by sintering a green compactobtained by compacting mixed powder for powder metallurgy, the mixedpowder is made into the green compact by compaction using a punch by atechnique in which the compaction is performed while rotating the punchabout an imaginary axis in the pressing direction. This method can producemore shear strains in the mixed powder than in typical compaction,facilitating plastic deformation of the mixed powder, and the pores in thesintered body can have a finer diameter. id="p-48"

[0048] Next, a method of manufacturing a sintered body, particularly suitablefor manufacturing a sintered body containing Mo, Cu, and C will bedescribed.

In this method, mixed powder for powder metallurgy containingiron-based powder and additives is compacted by a conventional method toform a green compact, and the green compact is then sintered by aconventional method, thereby obtaining an iron-based sintered body. On thisoccasion, with a view to increasing the density of the sintered body, it ispreferable that Mo-concentrated portions are formed in sintered neck partsbetween particles of the iron-based powder in the green compact; iron-basedpowder having particles with low circularity is used to achieve strongerentanglement between particles of the powder during compaction therebypromoting sintering; and the sintering is also promoted with suppressed Cugrowth. When the density of a sintered body is high, both strength andtoughness are improved; however, since a sintered body obtained by thismanufacturing method has a uniform metal structure, the mechanicalproperties of the sintered body are stable with little variation, unlikeconventional sintered bodies, for example, those using Ni. id="p-49"

[0049] In order to obtain such a sintered body, the sintered body is preferablymanufactured using partially diffusion alloyed steel powder described belowas the iron-based powder ofthe above mixed powder for powder metallurgy.

Mixed powder for powder metallurgy preferably used in this disclosure is obtained by mixing partially diffusion alloyed steel powder in Ref. No. POl62984-PCT (11/35) _12- which Mo is adhered by diffusion bonding to the surface of particles ofiron-based powder of which mean particle diameter, circularity, and specificsurface area are appropriate (hereinafter also referred to as partially alloyedsteel powder) with an appropriate amount of Cu powder having a meanparticle diameter in a range described below as well as graphite powder.[0050] Mixed powder for powder metallurgy according to this disclosure willnow be described in detail. Note that "%" herein means "mass%" unlessotherwise specified. Accordingly, the Mo content, the Cu content, and thegraphite powder content each represents the proportion of the element in theentire mixed powder for powder metallurgy (100 mass%). id="p-51"

[0051] (Iron-based powder) As described above, the partially diffusion alloyed steel powder isobtained by adhering Mo to the surface of particles ofthe iron-based powder,and it is preferred that the mean particle diameter is 30 um to 120 um, thespecific surface area is less than 0.10 m2/g, and particles having a diameter ina range of 50 um to 100 um have a circularity (cross-sectional circularity) of0.65 or less. Here, when the iron-based powder is partially alloyed, theparticle diameter and the circularity hardly change. Accordingly, iron-basedpowder having a mean particle diameter and a circularity in the same range asthat ofthe partially diffusion alloyed steel powder is used. id="p-52"

[0052] First, the iron-based powder preferably has a mean particle diameterof 30 um to 120 um and particles having a diameter in a range of 50 um to100 um preferably have a circularity (roundness of the cross section) of 0.65or less. For the reasons described below, the partially alloyed steel powderis required to have a mean particle diameter of 30 um to 120 um and particleshaving a diameter in a range of 50 um to 100 um are required to have acircularity of 0.65 or less. Accordingly, the iron-based powder is alsorequired to meet those conditions. id="p-53"

[0053] Here, the mean particle diameter of the iron-based powder and thepartially alloyed steel powder refers to the median size D50 determined fromthe cumulative weight distribution, and is a particle diameter found bydetermining the particle size distribution using a sieve according to JIS Z8801-1, producing the integrated particle size distribution from the resulting particle size distribution, and finding the particle diameter obtained when the Ref. No. PO162984-PCT (12/35) _13- oversized particles and the undersized particles constitute 50 % by Weighteach. id="p-54"

[0054] Further, the circularity of the particles of iron-based powder andpartially alloyed steel powder can be determined as follows. Although a caseof iron-based powder is explained by way of example, the circularity ofpartially alloyed steel powder particles is also determined through the sameprocess.

First, iron-based powder is embedded in a thermosetting resin. Onthis occasion, the iron-based powder is embedded to be uniformly distributedin an area with a thickness of 0.5 mm or more in the thermosetting resin sothat a sufficient number of cross sections of the iron-based powder particlescan be observed in an observation surface exposed by polishing thepowder-embedded resin. After that, the resin is polished to expose a crosssection of the iron-based powder particles; the cross section of the resin ismirror polished; and the cross section is magnified and imaged by an opticalmicroscope. The cross sectional area A and the peripheral length Lp of theiron-based powder particles in the resulting micrograph of the cross sectionare determined by image analysis. Examples of software capable of suchimage analysis include ImageJ (open source, National Institutes of Health).The circle equivalent diameter dc is calculated from the determinedcross-sectional area A. Here, dc is calculated by the same equation (I) as inthe case ofthe pores. 46:: A/fi - ° - (I)[0055] Next, the peripheral length of a circular approximation of each powderparticle Lc is calculated by multiplying the particle diameter dc by the numbern. The circularity C is calculated from the determined Lc and the peripherallength Lp of the cross section of each iron-based powder particle. Here, thecircularity C is a value defined by the following equation (II).When the circularity C is l, the cross-sectional shape ofthe particle is a perfect circle, and a smaller C value results in a more indefinite shape. (an/LI, - - -(l1)[0056] Note that iron-based powder means powder having an Fe content of 50% or more. Examples of iron-based powder include as-atomized powder (atomized iron powder as atomized), atomized iron powder (obtained by Ref. No. POl62984-PCT (13/35) _14- reducing as-atomized powder in a reducing atmosphere), and reduced iron powder. In particular, iron-based powder used in this disclosure is preferably as-atomized powder or atomized iron powder. This is becausesince reduced iron powder contains many pores in the particles, sufficientdensity would not be obtained during compaction. Further, reduced ironpowder contains more inclusions acting as starting points of fracture in theparticles than atomized iron powder, which would reduce the fatigue strengthwhich is one ofthe important mechanical properties of a sintered body. id="p-57"

[0057] Specifically, iron-based powder preferably used in this disclosure isany one of as-atomized powder obtained by atomizing molten steel, drying thesteel, atomized molten and classifying the resulting powder without performing heat treatment for deoxidation (reduction) and e.g.,decarbonization; and atomized iron powder obtained by reducing as-atomizedpowder in a reducing atmosphere.

Iron-based powder satisfying the above-described circularity can beobtained by appropriately adjusting the spraying conditions for atomizationand conditions for additional processes performed after the spraying.Further, iron-based powder having particles of different circularities may bemixed and the circularity ofthe particles ofthe iron-based powder that have aparticle diameter in a range of 50 um to 100 um may be controlled to fallwithin the above-described range. id="p-58"

[0058] (Partially diffusion alloyed steel powder) Partially diffusion alloyed steel powder is obtained by adhering Mo tothe surface of particles ofthe above iron-based powder, and it is required thatthe mean particle diameter is 30 um to 120 um, the specific surface area isless than 0.10 m2/g, and particles having a diameter in a range of 50 um to 100um have a circularity of 0.65 or less. id="p-59"

[0059] Thus, the partially diffusion alloyed steel powder is produced byadhering Mo to the above iron-based powder by diffusion bonding. The Mocontent is set to be 0.2 % to 1.5 % of the entire mixed powder for powdermetallurgy (100 %). %, the hardenability and strength of a sintered body manufactured using the mixed On the other hand, When the Mo content is less than 0.2 powder for powder metallurgy are poorly improved. when the Mo content exceeds 1.5 %, the effect of improving hardenability Ref. No. POl62984-PCT (14/35) _15- reaches a plateau, and the structure of the sintered body becomes rathernon-uniform. Accordingly, high strength and toughness cannot be obtained.Therefore, the content of Mo adhered by diffusion bonding is set to be 0.2 %to 1.5 %. % to 0.8 %.

The Mo content is preferably 0.3 % to 1.0 %, more preferably 0.4 id="p-60"

[0060] Here, Mo-containing powder can be given as an example of a Mosource. Examples of the Mo-containing powder include pure metal powderof Mo, oxidized Mo powder, and Mo alloy powders such as Fe-Mo(ferromolybdenum) powder. Further, Mo compounds such as Mo carbides,Mo sulfides, and Mo nitrides can be used as preferred Mo-containing powders.Theses material powders can be used alone; alternatively, some of thesematerial powders can be used in a mixed form. id="p-61"

[0061] Specifically, the above-described iron-based powder and theMo-containing powder are mixed in the proportions described above (the Mocontent is 0.2 % to 1.5 % of the entire mixed powder for powder metallurgy(100 %)).can be mixed by a conventional method using a Henschel mixer, a cone blender, or the like.

The mixing method is not particularly limited, and the powders id="p-62"

[0062] Next, mixed powder ofthe above-described iron-based powder and theMo-containing powder is heated so that Mo is diffused in the iron-basedpowder through the contact surface between the iron-based powder and theMo-containing powder, thereby joining Mo to the iron-based powder.Partially alloyed steel powder containing Mo can be obtained by this heattreatment.

As the atmosphere for diffusion-bonding heat treatment, a reducingatmosphere or a hydrogen-containing atmosphere is preferable, and ahydrogen-containing atmosphere is particularly suitable. Alternatively, theheat treatment may be performed under vacuum.

Further, for example when a Mo compound such as oxidized Mopowder is used as the Mo-containing powder, the temperature of the heattreatment is preferably set to be in a range of 800 °C to 1100 °C. When thetemperature of the heat treatment is lower than 800 °C, the Mo compound isinsufficiently decomposed and Mo is not diffused into the iron-based powder, so that Mo hardly adheres to the iron-based powder. When the heat Ref. NO. Po1629s4-PcT (15/35) _16- treatment temperature exceeds 1100 °C, sintering between iron-based powderparticles is promoted during the heat treatment, and the circularity of theiron-based powder particles exceeds the predetermined range. On the otherhand, when a metal and an alloy, for example, Mo pure metal and an alloysuch as Fe-Mo are used for the Mo-containing powder, a preferred heattreatment temperature is in a range of 600 °C to 1100 °C. When thetemperature of the heat treatment is lower than 600 °C, Mo is not sufficientlydiffused into the iron-based powder, so that Mo hardly adheres to theiron-based powder. On the other hand, when the heat treatment temperatureexceeds 1100 °C, sintering between iron-based powder particles is promotedduring the heat treatment, and the circularity of the partially alloyed steelpowder exceeds the predetermined range. id="p-63"

[0063] When heat treatment, that is, diffusion bonding is performed asdescribed above, since partially alloyed steel powder particles are usuallysintered together and solidified, grinding and classification are performed toobtain particles having a predetermined particle diameter described below.Specifically, in order to achieve the predetermined particle diameter, thegrinding conditions are tightened or coarse powder is removed byclassification using a sieve with openings of a predetermined size, asnecessary. In addition, annealing may optionally be performed. id="p-64"

[0064] Specifically, it is important that the mean particle diameter of thepartially alloyed steel powder is in a range of 30 um to 120 um. The lowerlimit of the mean particle diameter is preferably 40 um, more preferably 50um. Meanwhile, the upper limit of the mean particle diameter is preferably100 um, more preferably 80 um.

As described above, the mean particle diameter ofthe partially alloyedsteel powder refers to the median size D50 determined from the cumulativeweight distribution, and is a particle diameter found by determining theparticle size distribution using a sieve according to JIS Z 8801-1, producingthe integrated particle size distribution from the resulting particle sizedistribution, and finding the particle diameter obtained when the oversizedparticles and the undersized particles constitute 50 % by weight each.

Here when the mean particle diameter of the partially alloyed steel powder particles is smaller than 30 um, the flowability ofthe partially alloyed Ref. No. PO162984-PCT (16/35) _17- steel powder is reduced, and for example the productivity in compaction usinga mold is affected. On the other hand, when the mean particle diameter ofthe partially alloyed steel powder particles exceeds 120 um, the driving forceis weakened during sintering and coarse pores are formed around the coarseiron-based powder particles. This reduces the sintered density and leads toreduction in the strength and toughness of a sintered body and the sinteredbody having been carburized, quenched, and tempered. The maximumparticle diameter of the partially alloyed steel powder particles is preferably180 um or less. id="p-65"

[0065] Further, in terms of compressibility, the specific surface area of thepartially alloyed steel powder particles is set to be less than 0.10 m2/g. Here,the specific surface area of the partially alloyed steel powder refers to thespecific surface area of particles of the partially alloyed steel powder exceptfor additives (Cu powder, graphite powder, lubricant). id="p-66"

[0066] When the specific surface area of the partially alloyed steel powderexceeds 0.10 mz/g, the flowability ofthe mixed powder for powder metallurgyis reduced. Note that the lower limit of the specific surface area is notspecified; however, the lower limit of the specific surface area achievedindustrially is approximately 0.010 m2/g. The specific surface area can becontrolled as desired by adjusting the particle size of coarse particles of morethan 100 um and fine particles of less than 50 um after diffusion bonding bysieving. Specifically, the specific surface area is reduced by reducing theproportion of fine particles or increasing the proportion of coarse particles.[0067] Further, particles of the partially alloyed steel powder that have aThe Reducing diameter of 50 um to 100 um are required to have a circularity of 0.65.circularity is preferably 0.60 or less, more preferably 0.58 or less.the circularity increases the entanglement between particles duringcompaction and improves the compressibility of the mixed powder for powdermetallurgy, so that coarse pores in the green compact and the sintered bodyare reduced. On the other hand, an excessively low circularity reduces thecompressibility of the mixed powder for powder metallurgy. Accordingly,the circularity is preferably 0.40 or more. id="p-68"

[0068] The circularity ofthe partially alloyed steel powder particles having a diameter of 50 um to 100 um can be measured as follows. First, the particle Ref. NO. Po1629s4-PcT (17/35) _18- diameter of the partially alloyed steel powder particles is calculated in thesame manner as that ofthe above-described iron-based powder particles and isexpressed as dc, and the partially alloyed steel powder particles having dc in arange of 50 pm to 100 pm are extracted. Here, optical microscopy imagingperformed is such that at least 150 particles of the partially alloyed steelpowder that have a diameter in a range of 50 pm to 100 pm can be extracted.The circularity of the extracted partially alloyed steel powder particles wascalculated in the same manner as in the case ofthe above-described iron-basedpowder.

Note that the particle diameter of the partially alloyed steel powderparticles is limited to 50 pm to 100 pm because reducing the circularity oftheparticles of this range can most effectively promote sintering. Specifically,since particles of less than 50 pm are fine particles which originally facilitatesintering, reducing the circularity of such particles of less than 50 pm doesnot significantly promote sintering. Further, since particles having a particlediameter exceeding 100 pm are extremely coarse, reducing the circularity ofthose particles does not significantly promote sintering. id="p-69"

[0069] In this disclosure, the remainder components in the partially alloyedsteel powder are iron and inevitable impurities. Here, impurities containedin the partially alloyed steel powder may be C (except for graphite content), O,N, S, and others, the contents of which may be set to C: 0.02 % or less, O: 0.3% or less, N: 0.004 % or less, S: 0.03 %or less, Si: 0.2 % or less, Mn: 0.5 % orless, and P: 0.1 % or less in the partially alloyed steel powder without anyparticular problem. The content of O, however, is preferably 0.25 % or less.It should be noted that when the amount of incidental impurities exceeds theabove range, the compressibility in compaction using the partially alloyedsteel powder decreases, which makes it difficult to obtain a green compacthaving sufficient density by the compaction. id="p-70"

[0070] In this disclosure, a sintered body manufactured using mixed powderfor powder metallurgy is further subjected to carburizing, quenching, andtempering, and Cu powder and graphite powder are then added to the partiallyalloyed steel powder obtained as described above for the purpose of achievinga tensile strength of 1000 MPa. id="p-71"

[0071] (Cu powder) Ref. No. PO162984-PCT (18/35) _19- Cu is an element useful in improving the solid solution strengtheningand the hardenability of iron-based powder thereby increasing the strength ofsintered parts. The amount of Cu added is preferably 0.5 % or more and 4.0or less. When the amount of Cu powder added is less than 0.5 %, theadvantageous effects of adding Cu are hardly obtained. On the other hand,when the Cu content exceeds 4.0 %, not only does the effects improving thestrength of the sintered parts reach a plateau but also the density of thesintered body is reduced. Therefore, the amount of Cu powder added islimited to a range of 0.5 % to 4.0 %.The amount added is preferably in a rangeof 1.0 % to 3.0 %. id="p-72"

[0072] Further, when Cu powder of large particle size is used, in sintering agreen compact of mixed powder for powder metallurgy, molten Cu penetratesbetween particles of the partially alloyed steel powder to expand the volumeof the sintered body after sintering, which would reduce the density of thesintered body. In order to prevent the density of the sintered body fromdecreasing in such a way, the mean particle diameter of the Cu powder ispreferably set to be 50 um or less. More preferably, the mean particlediameter of the Cu powder is 40 um or less, still more preferably 30 um orless. Although the lower limit of the mean particle diameter of the Cupowder is not specified, the lower limit is preferably set to be approximately0.5 um in order not to increase the production cost of the Cu powderunnecessarily. id="p-73"

[0073] The mean particle diameter of the Cu powder can be calculated by thefollowing method.

Since the mean particle diameter of particles having a mean particlediameter of 45 um or less is difficult to be measured by means of sieving, theparticle diameter is measured using a laser diffraction/scattering particle sizedistribution measurement system. Examples of the laserdiffraction/scattering particle size distribution measurement system includeLA-950V2 manufactured by HORIBA, Ltd. Of course, other laserdiffraction/scattering particle size distribution measurement systems may beused; however, for performing accurate measurement, the lower limit and theupper limit of the measurable particle diameter range of the system used are preferably 0.1 um or less and 45 um or more, respectively. Using the system Ref. No. PO162984-PCT (19/35) _20- mentioned above, a solvent in which Cu powder is dispersed is exposed to alaser beam, and the particle size distribution and the mean particle diameter ofthe Cu powder are measured from the diffraction and scattering intensity ofthe laser beam. For the solvent in which the Cu powder is dispersed, ethanolis preferably used, since particles are easily dispersed in ethanol, and ethanolis easy to handle. When a solvent in which the Van der Waals force is strongand particles are hardly dispersed, such as water is used, particles agglomerateduring the measurement, and the measurement result includes a mean particlediameter larger than the real mean particle diameter. Therefore, such asolvent is not preferred. Accordingly, it is preferable that Cu powderintroduced into an ethanol solution is preferably dispersed using ultrasoundbefore the measurement.

Since the appropriate dispersion time varies depending on the targetpowder, the dispersion is performed in 7 stages at 10 min intervals between 0min and 60 min, and the mean particle diameter ofthe Cu powder is measuredafter each dispersion time stage. In order to prevent particle agglomeration,during each measurement, the measurement is performed with the solventbeing stirred. Of the particle diameters obtained through the sevenmeasurements performed by changing the dispersion time by 10 min, thesmallest value is used as the mean particle diameter ofthe Cu powder. id="p-74"

[0074] (Graphite powder) Graphite powder is useful in increasing strength and fatigue strength,and graphite powder is added to the partially alloyed steel powder in anamount in a range of 0.1 % to 1.0 %, and mixing is performed. When theamount of graphite powder added is less than 0.1 %, the above advantageouseffects cannot be obtained. On the other hand, when the amount of graphitepowder added exceeds 1.0 %, the sintered body becomes hypereutectoid, andcementite is precipitated, resulting in reduced strength. Therefore, theamount of graphite powder added is limited to a range of 0.1 % to 1.0 %.The amount of graphite powder added is preferably in a range of 0.2 % to 0.8%. Note that the particle diameter of graphite powder to be added ispreferably in a range of approximately from 1 um to 50 um. id="p-75"

[0075] In this disclosure, the Cu powder and graphite powder described above are mixed with partially diffusion alloyed steel powder to which Mo is Ref. No. PO162984-PCT (20/35) _21- diffusionally adhered to obtain Fe-Mo-Cu-C-based mixed powder for powder metallurgy, and the mixing may be performed in accordance withconventional powder mixing methods. id="p-76"

[0076] Further, in a stage where a sintered body is obtained, if the sinteredbody needs to be further formed into the shape of parts by cutting work or thelike, powder for improving machinability, such as MnS is added to the mixedpowder for powder metallurgy in accordance with conventional methods.[0077] Next, the compacting conditions and sintering conditions preferablefor manufacturing a sintered body using the-above described mixed powderfor powder metallurgy will be described.

In compaction using the above mixed powder for powder metallurgy, alubricant powder may also be mixed in. Further, compaction may beperformed with a lubricant being applied or adhered to a mold. In either case,as the lubricant, any of metal soap such as zinc stearate and lithium stearate,amide-based wax such as ethylenebisstearamide, and other well knownlubricants may suitably be used. When mixing the lubricant, the amountthereof is preferably around from 0.1 parts by mass to 1.2 parts by mass withrespect to 100 parts by mass of the mixed powder for powder metallurgy.[0078] In manufacturing a green compact by compacting the disclosed mixedpowder for powder metallurgy, the compaction is preferably performed at apressure of 400 MPa to 1000 MPa. When the compacting pressure is lessthan 400 MPa, the density of the resulting green compact is reduced, and theproperties of the sintered body are degraded. On the other hand, acompacting pressure exceeding 1000 MPa extremely shortens the life of themold, which is economically disadvantageous. The compacting temperatureis preferably in a range of room temperature (approximately 20 °C) toapproximately 160 °C. id="p-79"

[0079] Further, the green compact is sintered preferably at a temperature in arange of 1100 °C to 1300 °C. When the sintering temperature is lower than1100 °C, sintering stops; accordingly, it is difficult to achieve the desiredtensile strength: 1000 MPa or more. On the other hand, a sinteringtemperature higher than 1300 °C extremely shortens the life of a sinteringfurnace, which is economically disadvantageous. The sintering time is preferably in a range of 10 min to 180 min.

Ref. No. POl62984-PCT (21/35) _22- id="p-80"

[0080] A sintered body obtained using mixed powder for powder metallurgyaccording to this disclosure under the above sintering conditions through sucha procedure can have higher density after sintering than the case of using alloysteel powder which does not fall within the above range even if the greendensity is the same. id="p-81"

[0081] Further, the resulting sintered body may be subjected to strengtheningprocesses such as carburized quenching, bright quenching, inductionhardening, and a carbonitriding process as necessary; however, even whensuch strengthening processes are not performed, the sintered body using themixed powder for powder metallurgy according to this disclosure haveimproved strength and toughness compared with conventional sintered bodieswhich are not subjected to strengthening processes. The strengtheningprocesses may be performed in accordance with conventional methods. id="p-82"

[0082] The disclosed iron-based sintered body obtained as described abovepreferably contains Mo: 0.2 mass% to 1.5 mass%, Cu: 0.5 mass% to 4.0mass%, and C: 0.1 mass% to 1.0 mass%. Specifically, the C content ispreferably in a range of 0.1 % to 1.0 % with which the highest strengtheningeffect and the highest fatigue strength improving effect can be achieved.When the C content is less than 0.1 %, the above advantageous effects cannotbe achieved. On the other hand, a C content exceeding 1.0 % results in ahypereutectoid sintered body, so that cementite is precipitated, resulting inreduced strength. Therefore, the amount of C contained in the sintered bodyis limited to a range of 0.1 % to 1.0 %.0.8 %. for the same reasons as in the case of the above-described mixed powder for Preferably, the C content is 0.2 % to The preferred content of Mo and Cu is determined as described above powder metallurgy. id="p-83"

[0083] Note that when a lubricant and the like are mixed into the above mixedpowder for powder metallurgy in manufacturing a sintered body, the amountof Mo, Cu, and C in the mixed powder for powder metallurgy is controlled sothat the amount of Mo, Cu, and C contained the sintered body fall within theabove range. id="p-84"

[0084] Further, the C content of the sintered body may change from theamount of graphite added depending on the sintering conditions (temperature, time, atmosphere, and others). Accordingly, when the amount of the Ref. No. POl62984-PCT (22/35) _23- graphite powder added is controlled Within the above range depending on thesintering conditions, an iron-based sintered body having a C content preferredin this disclosure (0.1 % to 1.0 %, more preferably 0.2 % to 0.8 %) can be manufactured.

EXAMPLES[0085] A more detailed description of this disclosure will be given belowwith reference to examples; however, the disclosure is not limited solely tothe following examples.

As-atomized powders having particles with different circularities wereused as iron-based powders. The as-atomized powders were subjected togrinding using a high speed mixer (LFS-GS-2J manufactured by Fukae PowtecCorp.) so that the circularities ofthe particles varied.

Oxidized Mo powder (mean particle diameter: l0 um) was added tothe iron-based powders at a predetermined ratio, and the resultant powderswere mixed for 15 minutes in a V blender, then subjected to heat treatment ina hydrogen atmosphere with a dew point of 30 °C (holding temperature: 880°C, holding time: l h). was then adhered to the surface of the particles of the iron-based powders by Mo of a predetermined amount presented in Table l diffusion bonding to produce partially alloyed steel powders for powdermetallurgy. Note that the Mo content was varied as in Samples Nos. l to 8presented in Table l. id="p-86"

[0086] The produced partially alloyed steel powders were each embedded intoa resin and polishing was performed to expose a cross section of the partiallyalloyed steel powder particles. Specifically, the partially alloyed steelpowders were each embedded to be uniformly distributed in an area with athickness of 0.5 mm or more in a thermosetting resin so that a cross section ofa sufficient number of partially alloyed steel powder particles can be observedin the polished surface, that is, the observation surface. After the polishing,the polished surface was magnified and imaged by an optical microscope, andthe circularity of the particles was calculated by image analysis as described above.

Ref. No. POl62984-PCT (23/35) _24- Further, the specific surface area of the partially alloyed steel powder particles was measured through BET theory. The particles of each partiallyalloyed steel powder were confirmed to have a specific surface area of less:han 0.10 mZ/g.[0087] Subsequently, Cu powder of the mean particle diameter and theamount presented in Table l was added to these partially alloyed steelpowders, and graphite powder (mean particle diameter: 5 um) of the amountalso presented in Table l was added thereto. Ethylenebisstearamide wasthen added in an amount of 0.6 parts by mass to the resulting mixed powderfor powder metallurgy: 100 parts by mass, and the powder was then mixed in aV blender for l5 minutes.

Samples Nos. 9 to 25 used partially alloyed steel powder equivalent tothose used in Sample No. 5, yet the amounts of Cu powders and graphitepowders varied. Samples Nos. 26 to 3l used basically the same partiallyalloyed steel powder as that of Sample No. 5, of which mean particle diameterwas adjusted by sieving. Further, Samples Nos. 32 to 38 used partiallyalloyed steel powders having circularities that varied.

After that, each mixed powder was compacted at a density of 7.0g/cm3, thereby manufacturing bar-shaped green compacts having length: 55mm, width: l0 mm, and thickness: 10 mm and ring-shaped green compactshaving outer diameter: 38 mm, inner diameter: 25 mm, and thickness: l0 mm(ten pieces each). The compacting pressure was 400 MPa in each case.[0088] The bar-shaped green compacts and the ring-shaped green compactswere sintered thereby obtaining sintered bodies. The sintering wasperformed under a set of conditions including sintered temperature: ll30 °Cand sintering time: 20 min in a propane converted gas atmosphere.

The measurement of outer diameter, inner diameter, and thickness andmass measurement were performed on the ring-shaped sintered bodies,thereby calculating the sintered body density (Mg/m3). Further, the mediansize, area fraction, and mean maximum pore length of pores in the sinteredbodies were measured in accordance with the above-described methods.

For the bar-shaped sintered bodies, five of them were worked intoround bar tensile test pieces (JIS No. 2), each having a parallel portion with a diameter of 5 mm, to be subjected to the tensile test according to JIS Z224l, Ref. No. POl62984-PCT (24/35) _25- and the other five were bar shaped (unnotched) as sintered and had a sizeaccording to JIS Z2242 to be subjected to the Charpy impact test according toJIS Z2242. Each of these test pieces was subjected to gas carburizing atcarbon potential: 0.8 mass% (holding temperature: 870 °C, holding time: 60min) followed by quenching (60 °C, oil quenching) and tempering (holdingtemperature: 180 °C, holding time: 60 min).

The round bar tensile test pieces and bar-shaped test pieces for theCharpy impact test subjected to carburizing, quenching, and tempering weresubjected to the tensile test according to JIS Z2241 and the Charpy impact testaccording to JIS Z2242; thus, the tensile strength (MPa) and the impactenergy value (J/cmz) were measured and the mean values were calculated withthe number of samples n= 5. id="p-89"

[0089] The measurement results are also presented in Table 1. Theevaluation criteria are as follows.(1) Flowability of mixed powder Mixed powders for powder metallurgy: 100 g were introduced into anozzle having diameter: 2.5 mmcl). When the total amount of powder wascompletely flown within 80 s without stopping, the powder was judged tohave passed (passed). When the powder required a longer time to be flownor the total amount or part of the amount of powder stopped and failed to beflown, the powder was judged to have failed (failed). (2) Sintered body density A sintered body density of 6.95 Mg/m3 or more, that is equal to orhigher than that of a conventional 4Ni material (4Ni-l.5Cu-0.5Mo, maximumparticle diameter of material powder: 180 um) was judged to have passed. (3) Tensile strength When the round bar tensile test pieces having been subjected tocarburizing, quenching, and tempering had a tensile strength of 1000 MPa ormore, the test pieces were judged to have passed. (4) Impact energy valueWhen the bar-shaped test pieces for the Charpy impact test having been subjected to carburizing, quenching, and tempering had an impact energy value of 14.5 J/cmz or more, the test pieces were judged to have passed.

Ref. NO. Po1629s4-PcT (25/35) _26- Note that the test of the impact energy Value Was also performed on the sintered body before carburizing, quenching, and ternpering.

Ref. No. PO162984-PCT (26/35) (SS/LZ) .LOcPV86Z9IOd 'ON 3921 [Table 1] Pmny auflyßd "m1powdßf Aflßfcafbm-img,qußmhang, :empel-ingMo Cu (iraphitecontent conæn: cont/en: Flnwabflízy M0 Cu am Median Mean mp" D _ Tensüe Imïm' EvalualionV 'V enär ens! ÖflêfWu* cimulamy (mflssWfl) (massfm (msm) ( ) content conwm "mf" fiacaiun pflfesizß "Ämw gy :y safengm gydßmewf vm (mass /°) pm m value (Mg/m ) "am(mm/n) (mmm m) (pm) 2 (ma) 2(um) (um) (J/cm ) (mm ) Símered body sflmPle Mean pamcleNO. ' 14 1613 1416 2214 1814 1614 1514 1414 13 3333333'äääï33ß' 10600] -Lz_ _28- id="p-91"

[0091] For each of Samples Nos. 1, 8, 9, 14, 19, 26, 38, and 39*, the mediansize D50 ofthe pores in the sintered body exceeded 20 um, resulting in a lowimpact energy value, lack of toughness, and reduced tensile strength.

Further, for comparing the effects of the components in the sinteredbodies, the Mo content in Sample Nos. 1 to 8, the Cu content in Nos. 9 to 14,and the graphite content in Nos. 15 to 19 were contrasted. Similarly,Samples Nos. 20 to 25 were designed for evaluating the effect of the Cuparticle diameter, Nos. 26 to 31 for evaluating the effect of the alloyedparticle diameter, and Nos. 32 to 38 for evaluating the effect ofthe circularityand the mean particle diameter of the partially alloyed steel powders. Table1 also presents the results of a 4Ni material (4Ni-l.5Cu-0.5Mo, maximumparticle diameter of material powder: 180 um) as the conventional material.The table demonstrates that our examples exhibited better properties over theconventional 4Ni material.

As presented in Table 1, all of Examples of this disclosure weresintered bodies having high tensile strength and toughness. id="p-92"

[0092] Three atomized iron powders having particles of different specificsurface areas and circularities were prepared. The specific surface area andthe circularity were adjusted by grinding each atomized iron powder using ahigh speed mixer (LFS-GS-2J manufactured by Fukae Powtec Corp.) andadjusting the mixing ratio of coarse powder having a particle size of 100 umor more and fine powder having a particle size of 45 um or less. id="p-93"

[0093] Oxidized Mo powder (mean particle diameter: 10 um) was added tothe iron-based powders at a predetermined ratio, and the resultant powderswere mixed for 15 minutes in a V blender, then subjected to heat treatment ina hydrogen atmosphere with a dew point of 30 °C (holding temperature: 880°C, holding time: 1 h). was then adhered to the surface of the particles of the iron-based powders by Mo of a predetermined amount presented in Table 2 diffusion bonding to produce partially alloyed steel powders for powdermetallurgy. These partially alloyed steel powders were each embedded intoa resin and polishing was performed to expose a cross section of the partiallySubsequently, the cross section was alloyed steel powder particles. magnified and imaged by an optical microscope, and the circularity of the Ref. No. POl62984-PCT (28/35) _29- particles was calculated by image analysis. Further, the specific surface areaof the partially alloyed steel powder particles was measured through BETtheory. id="p-94"

[0094] Next, 2 mass% of Cu powder having a mean particle diameter of 35um was added to these partially alloyed steel powders, and 0.3 mass% ofgraphite powder (mean particle diameter: 5 um) was added thereto.Ethylenebisstearamide was then added in an amount of 0.6 parts by mass tothe resulting mixed powder for powder metallurgy: 100 parts by mass, and thepowder was then mixed in a V blender for 15 minutes. Each of the mixedpowders was compacted at a compacting pressure of 686 MPa, therebymanufacturing bar-shaped green compacts having length: 55 mm, width: 10mm, and thickness: 10 mm and ring-shaped green compacts having outerdiameter: 38 mm, inner diameter: 25 mm, and thickness: 10 mm (ten pieceseach). id="p-95"

[0095] The bar-shaped green compacts and ring-shape green compacts weresintered to obtain sintered bodies. The sintering was performed under a setof conditions including sintered temperature: 1130 °C and sintering time: 20min in a propane converted gas atmosphere.

The measurement of outer diameter, inner diameter, and thickness andmass measurement were performed on the ring-shaped sintered bodies,thereby calculating the sintered body density (Mg/mg). Further, the mediansize, area fraction, and mean maximum pore length of pores in the sinteredbodies were measured in accordance with the above-described methods.[0096] For the bar-shaped sintered bodies, five of them were worked intoround bar tensile test pieces (JIS No. 2) having diameter: 5 mm to besubjected to the tensile test according to JIS Z2241, and the other five werebar shaped (unnotched) as sintered to be subjected to the Charpy impact testaccording to JIS Z2242. Each of these test pieces was subjected to gascarburizing at carbon potential: 0.8 mass% (holding temperature: 870 °C,holding time: 60 min) followed by quenching (60 °C, oil quenching) andtempering (holding temperature: 180 °C, holding time: 60 min).

The round bar tensile test pieces and bar-shaped test pieces for theCharpy impact test subjected to carburizing, quenching, and tempering were subjected to the tensile test according to JIS Z2241 and the Charpy impact test Ref. No. POl62984-PCT (29/35) _30- according to JIS Z2242; thus, the tensile strength (MPa) and the impactenergy value (J/cmz) Were measured and the mean values Were calculated Withthe number of samples n= 5.

The measurement results are also presented in Table 2. Theacceptance criteria for the values of the properties Were the same as those in Example l.

Ref. No. POl62984-PCT (30/35) (SS/1ÜlOcPV86Z9IOd 'ON 'PH [Table2]Partialy aloyed steel powder Sintered body AfieâcarbmmgfCu quenchng tempenng. Mo Cu Graphite ,1 M S lf 1 I t I lsme šan peclc content content content Pamce Flowability Mo Cu Porearea Median Maximum "w ., Tensile mpac Evaluation NoteNo. particle , . 51413396 . diameter C content . . energy Demity energy. Cnculanty (massllfo) (massl/l) (maas%) content content fraction poresiæ pore length . strengthdamm area (m) (masw) (WW) (magsllly) (m) t ) ( ) value (Mg/Ina) (wa) value(um) (man ° m "m "m (leaf) (uenl) 40 78 0.55 0.07 0.4 2.0 0,3 35 passed 0,4 2.0 0.3 14 16 85.0 35.0 7.01 1175 15.1 passed Example 41 76 0.52 0.08 0.8 2.0 0,3 35 passed 0,8 2.0 0.3 15 14 73.0 42.0 6.97 1194 15.7 passed Example 42 76 0.59 0.13 0.4 2.0 0,3 35 0,4 2.0 0.3 failed ComparativeExample43 77 0.52 0.15 0.8 2.0 0,3 35 0,8 2.0 0.3 failed ComparativeExample44 76 0.67 0,12 0.4 2.0 0.3 35 0.4 2.0 0.3 failed ComparatiueExample45 77 0.66 0,14 0.8 2,0 0.3 35 - - - - - - - - - - failed ComparatiueExample46 75 0.68 0.06 0.4 2.0 0,3 35 passed 0,4 2.0 0.3 12 25 110,0 21.0 7.10 1060 12.1 failed CoinparativeExample47 77 0.69 0.08 0.8 2.0 0,3 35 passed 0,8 2.0 0.3 13 25 100.0 20.0 7.06 1075 12.3 failed ComparativeExample [L600] -Ig_ _32- id="p-98"

[0098] As can be seen from Table 2, all the sintered bodies having a niedianpore size D50 of 20 uni or less had a high impact energy Value, excellenttoughness, and high tensile strength. Further, When partially alloyed steelpowders having particles of a circularity and a specific surface area Within thedisclosed range Were used, the target values of the sintered body density, the tensile strength, and the impact energy Value Were achieved.

Ref. No. POl62984-PCT (32/35)

Claims (9)

1. An iron-based sintered body, comprising an area fraction of pores inthe iron-based sintered body of 15 % or 1ess, and an area-based median size D50 of the pores of 20 um or 1ess.

2. The iron-based sintered body according to C1aim 1, comprising Mo,Cu, and C.

3. The iron-based sintered body according to C1aim 2, comprising Mo in an amount of 0.2 mass% to 1.5 mass%, Cu in an amount of 0.5 mass% to 4.0 mass%, and C in an amount of 0.1 mass% to 1.0 mass%.

4. The iron-based sintered body according to any one of C1aims 1 to 3,wherein the iron-based sintered body has been carburized, quenched, and tempered.

5. A method of manufacturing an iron-based sintered body, the methodcomprising: compacting (i) partia11y diffusion a11oyed stee1 powder in which Mo isadhered to the surface of partic1es of iron-based powder by diffusion bondingwith (ii) mixed powder for powder meta11urgy obtained by mixing at 1east Cupowder and graphite powder at a pressure of 400 MPa or more to obtain acompact; and then sintering the obtained compact at 1000 °C or higher for 10 min or more.

6. The method of manufacturing a high strength according to C1aim 5,the method further comprising carburizing, quenching, and tempering after sintering the obtained compact.

7. The method of manufacturing an iron-based sintered body, according to C1aim 5 or 6, wherein the mixed powder for powder meta11urgy contains Ref. No. PO162984-PCT (33/35) _34- Mo in an amount of 0.2 mass% to 1.5 mass% and the balance consisting of Fe and incidenta1 impurities.

8. The method of manufacturing an iron-based sintered body, accordingto any one of Claims 5 to 7, wherein the partia11y diffusion a11oyed stee1powder has a mean partic1e diameter of 30 um to 120 um and a specificsurface area of less than 0.10 m2/g, and a circu1arity of partic1es of thepartia11y diffusion a11oyed stee1 powder that have a diameter in a range of 50 um to 100 um is 0.65 or 1ess.

9. The method of manufacturing an iron-based sintered body, according to any one ofC1aims 5 to 8, wherein the amount ofthe Cu powder mixed is 0.5 mass% to 4.0 mass% ofthe mixed powder for powder meta11urgy. Ref. No. PO162984-PCT (34/35)