Abstract:
There is provided an iron-based mixture powder for sintering, as well as an iron-based sintered alloy using same, that are capable of reducing the cutting resistance of the iron-based sintered alloy and of mitigating the shortening of cutting tool life even when a metal fluoride powder is used. The iron-based mixture powder for sintering comprises an iron-based powder, a graphite powder, a hard powder that is harder than the iron-based powder, and a metalfluoride powder. With respect to particle asperity as expressed by the following equation, 
       particle asperity=(perimeter of a section of a particle) 2 /(sectional area of the section×4Pi),
 
     the particle asperity of the metal fluoride powder is within the range of 2 to 5.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to iron-based mixture powders for sintering comprising at least an iron-based powder, a graphite powder, and a hard powder that is harder than the iron-based powder, and more particularly to iron-based mixture powders for sintering from which an iron-based sintered alloy with superior machinability may be sintered. 
       BACKGROUND ART  
       [0002]    Conventionally, iron-based mixture powders for sintering in which an iron-based powder, a graphite powder and a hard powder that is harder than the iron-based powder are mixed have occasionally been used. After filling a mold with such an iron-based mixture powder for sintering, a compact is produced through pressure molding. By sintering the compact, an iron-based sintered alloy may be obtained. During sintering, the carbon in the graphite powder dissolves in (forms a solid solution with) the iron-based powder, thereby hardening the iron-based powder. Then, with the iron of this hardened iron-based powder as a base, the hard powderis dispersed in the iron-based sintered alloy. 
         [0003]    The iron-based sintered alloy thus sintered undergoes cutting as required, and a finished product is obtained. However, the iron-based sintered alloy thus obtained is porous inside, and has higher cutting resistance as compared to metal alloys obtained through melting. Therefore, it has been conventional practice to further add to the mixture powder a powder containing S, such as BaS, CaS, MnS, etc., as a free-machining component. 
         [0004]    However, when a powder containing S is used, a gas such as H 2 S, SO x , etc., may be generated during sintering, the yield of S may drop, and sufficient improvements in machinability may consequently be unattainable in some cases. 
         [0005]    As such, in view of such circumstances, there has been proposed an iron-based mixture powderfor sintering in which, for example, instead of a powder containing S, a metal fluoride powder such as an alkaline earth metal fluoride powder, etc., is mixed (see, for example, PatentLiterature 1). An iron-based sintered alloy sintered from this iron-based mixture powder for sintering allows for improved machinability as a result of the metal fluoride powder&#39;s adhesion to recessed parts in the iron-based powder. 
       CITATION LIST  
     Patent Literature  
       [0006]    {PTL 1} 
         [0007]    Japanese Patent Publication (Kokai) No. 2002-155301 A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    However, even in cases where the metal fluoride powder disclosed in Patent Literature 1 is used, the particles of the metal fluoride powder would sometimes flocculate when mixed with the other powders of the iron-based mixture powder for sintering. Thus, when an iron-based sintered alloy in which the particles of the metal fluoride powder have flocculated is machined, this metal fluoride powder does not function sufficiently as a free-machining component and cutting resistance with respect to cutting tools rises, thereby potentially shortening cutting tool life. 
         [0009]    The present invention is made in view of such circumstances, and an aspect thereof lies in providing an iron-based mixture powder for sintering that is capable of, even when a metal fluoride powder is used, reducing the cutting resistance of the iron-based sintered alloy sintered therefrom and of mitigating the shortening of cutting tool life, as well as in providing an iron-based sintered alloy using same. 
       SOLUTION TO PROBLEM  
       [0010]    With a view to solving the problems above, the present inventors have, through diligent research, come to believe that, in mixing a metal fluoride powder in an iron-based mixture powder for sintering, it is preferable to disperse the metal fluoride particles within the iron-based mixture powder for sintering. The present inventors have further found that the shape of the particles is important for favorable dispersion of the metal fluoride particles, thatthe particles become more susceptible to flocculation the closer their shape becomes to being spherical, and that, conversely, the particles are more readily dispersed within the iron-based mixture powder for sintering the further their shape is from being spherical (i.e., the more recesses and projections they have). 
         [0011]    The present invention is based on these novel findings by the present inventors, and an iron-based mixture powder for sintering according to an embodiment of the present invention comprises: an iron-based powder; a graphite powder; a hard powder that is harder than the iron-based powder; and a metal fluoride powder, wherein, with respect to particle asperity as expressed by the follow equation, 
         [0000]      particle asperity=(perimeter of a section of a particle) 2 /(sectional area of the section×4Pi),
 
         [0000]    the particle asperity of the metal fluoride powder is within a range of 2 to 5. 
         [0012]    According to an embodiment of the present invention, since the particle asperity of the metal fluoride powder is within the range of 2 to 5, the shape of the particles of the metal fluoride powder is one that readily adheres to (engages with) the iron-based particles and the hard particles. The particles of the metal fluoride powder to be mixed in the iron-based mixture powder for sintering thus become more readily dispersible (i.e., less likely to flocculate). Further, since a metal fluoride, which serves as a free-machining component, is dispersed among the iron-based particles and the hard particles in an iron-based sintered alloy sintered from such an iron-based mixture powder for sintering, it is possible to reduce the cutting resistance of the iron-based sintered alloy and to mitigate the shortening of cutting tool life. 
         [0013]    In other words, when the particle asperity of the metal fluoride powder is less than 2, the metal fluoride powder particles are prone to flocculation during mixing. Metal fluoride powder particles with an asperity exceeding 5 are difficult to produce. 
         [0014]    The term “particle asperity” as used in connection with the present invention is an index indicating the recess/projection shape of particles, where the asperity of a particle approaches 1 as the shape of the particle becomes closer to being spherical (i.e., as the section ofthe particle becomes closer to being a true circle). Further, the term “perimeter of a section of a particle” refers to the perimeter of a given section of a particle (e.g., alkaline earth metal fluoride powder particle), and the term “sectional area of the section” refers to the sectional area of the section as measured with respect to the aforementioned perimeter. Further, the term “iron-based” is used in connection with the present invention to refer to a material whose principal component (base) is iron. 
         [0015]    Further, examples of the metal fluoride powder may include potassium fluoride, sodium fluoride, calcium fluoride, magnesium fluoride, strontium fluoride, barium fluoride, lead fluoride, selenium fluoride, tellurium fluoride, etc. The metal fluoride powder may preferably be an alkaline earth metal fluoride powder, and the alkaline earth metal fluoride powder may further preferably be a strontium fluoride powder. 
         [0016]    According to an embodiment of the present invention, the alkaline earth metal fluoride powder is dispersed without causing the sintered body to become brittle, and without dissolving in the iron base or disappearing due to gasification. Further, of the above, strontium fluoride powders, as compared to other metal fluoride powders (e.g., barium fluoride, lead fluoride, etc.), have a lower density, namely 4.24, and it is speculated that when the same amount(the same mass %) of a fluoride is added, its volume would be greatest as compared to others. Therefore, it is speculated that the amount of strontium fluoride that comes into contactwith the other powders, such as the iron-based powder, the hard powder, etc., would increase, thereby improving the engagement with these powders. It is speculated that the machinability of the resultant iron-based sintered alloy would consequently improve as compared to those in which other metal fluorides are used. 0.5 to 3 mass % of the alkaline earth metal fluoride powder relative to the total amount of the iron-based mixture powder for sintering may preferably be contained. According to an embodiment of the present invention, by adding an alkaline earth metal fluoride powder, it is possible to reduce cutting resistance, improve cutting tool life, and, further, reduce surface damage to the sintered alloy. In other words, when the alkaline earth metal fluoride powder content is less than 0.5 mass %, it may not, in some cases, be possible to improve the machinability of the resultant iron-based sintered alloy. When it exceeds 3 mass %, however,the alkaline earth metal fluoride powder becomes excessive, thereby increasing the likelihood of surface damage to the sintered alloy. 
         [0017]    Further, with respect to the aforementioned powder content, the average particle size of theparticles of the alkaline earth metal fluoride powder may preferably be within the range of 1 to 20 micrometers. By keeping it within such a range, cutting resistance may be reduced and cutting tool life further improved, thereby making it possible to further reduce surface damage to the sintered alloy. Specifically, when the average particle size of (the particles of) the alkaline earth metal fluoride powder exceeds 20 micrometers, it becomes difficult for the alkaline earth metal fluoride to disperse at the grain boundary in the iron-based sintered alloy. This may result in increased cutting resistance and shortened cutting tool life, thereby increasing the likelihood of surface damage to the resultant sintered alloy. 
         [0018]    A lubricant, a binder, or the like, may further be added to the iron-based mixture powder for sintering. A silicone resin may preferably be further added as the binder. According to an embodiment of the present invention, by adding a silicone resin, it is possible to improve the binding force between the mixture powder and the alkaline earth metal fluoride. In addition, since silicone resins are thermosetting resins, when a compact is produced by pressure molding the iron-based mixture powder for sintering and the compact is further sintered, it is possible to maintain the aforementioned binding force without the silicone resin softening. In addition, it is possible to maintain the aforementioned binding force even when a compact is produced through a warm mold lubrication method, or the like. 
       ADVANTAGEOUS EFFECTS OF INVENTION  
       [0019]    According to an embodiment of the present invention, it is possible to reduce the cutting resistance of an iron-based sintered alloy and to mitigate the shortening of cutting tool lifeeven when a metal fluoride powder is used. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS  
         [0020]    {FIG.  1 }  FIG. 1  is a diagram showing projection views and the particle asperity of fluoride powders (particles of strontium fluoride powders) according to Example 1 and Comparative Example 1. 
           [0021]    {FIG.  2 }  FIG. 2  is a diagram showing the relationship between particle asperity and cutting tool wearwith respect to the strontium fluoride powders according to Example 1 and Comparative Example 1. 
           [0022]    {FIG.  3 }  FIG. 3  is an enlarged photograph of the metallographic structure of a specimen according to Example 2. 
           [0023]    {FIG.  4 A}  FIG. 4A  is a diagram showing test results for Example 3 and Comparative Example 2 with respect to cutting tool wear. 
           [0024]    {FIG.  4 B}  FIG. 4B  is a diagram showing test results for Example 3 and Comparative Example 2 with respect to cutting resistance. 
           [0025]    {FIG.  4 C}  FIG. 4C  is a diagram showing test results for Example 3 and Comparative Example 2 with respect to rattler value. 
           [0026]    {FIG.  5 A}  FIG. 5A  is a diagram showing test results for Example 4 and Comparative Example 2 with respect to cutting tool wear. 
           [0027]    {FIG.  5 B}  FIG. 5B  is a diagram showing test results for Example 4 and Comparative Example 2 with respect to cutting resistance. 
           [0028]    {FIG.  5 C}  FIG. 5C  is a diagram showing test results for Example 4 and Comparative Example 2 with respect to rattler value. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
       [0029]    Embodiments of the present invention are described below. A mixture powder according to an embodiment of the present invention may be an iron-based mixture powder for sintering for producing, through sintering, valve seats for internal combustion engines, etc. By producing a compact by pressure molding this iron-based mixture powder for sintering, and further sintering the compact, it is possible to obtain an iron-based sintered alloy. 
         [0030]    An iron-based mixture powder for sintering according to an embodiment of the present invention may be a mixture powder comprising an iron-based powder, a graphite powder, a hard powderthat is harder than the iron-based powder, and a metal fluoride powder. 
         [0031]    The iron-based powder may be a powder comprising particles whose principal component is iron, and may include a pure iron powder, such as an atomized iron powder, a reduced powder, etc., a steel powder in which an alloying element is pre-alloyed (pre-alloyed steel powder), a steel powder in which an alloying element is partially alloyed (partially alloyed steel powder), etc. Further, it may be a powder in which these powders are mixed. The iron-based powder forms the base of the iron-based sintered alloy. Further, the iron-based powder may preferably have an average particle size of 80 to 100 micrometers, and the iron-based powder content may preferably be 40 to 90 mass % relative to the total amount of the iron-based mixture powder for sintering. 
         [0032]    Further, the graphite powder may be a powder containing graphite. By including the graphitepowder in the mixture powder, C (carbon) is diffused during sintering and reinforces the iron-based sintered alloy by being dissolved therein. In addition, an iron-based sintered alloy containing a suitable amount of C allows for such heat treatments as quenching and tempering, through which it is possible to improve the mechanical properties of the iron-based sintered alloy. Further, C is not pre-contained in the iron-based powder for such reasons as the moldability of the raw powders, ease of adjusting C content, etc. In addition, the graphite powder may further contain a metal powder such as copper, etc., or an alloy powder. The graphite powder may preferably have an average particle size of 25 micrometers or below, andthe graphite powder content may preferably be 0.2 to 5 mass % relative to the total amount of the iron-based mixture powder for sintering. When the graphite powder content exceeds 5 mass %, ductility may drop significantly, and strength may drop. 
         [0033]    The hard powder may be a powder comprising hard particles that are harder than the iron particles of the iron-based powder. By dispersing the hard particles in the iron-based sinteredalloy, it is possible to improve the wear resistance of the iron-based sintered alloy. Examples of the hard particles constituting the hard powder may include: (1) particles comprising, in mass %, 20-70% Mo, 0.2-3% C, and 1-15% Mn, with the remainder comprising incidental impurities and Co; (2) particles comprising, in mass %, 20-70% Mo, 0.5-3% C, 5-40% Ni, and 1-20% Mn, with the remainder comprising incidental impurities and Fe; (3) particles comprising,in mass %, 20-60% Mo, 0.2-3% C, 5-40% Ni, 1-15% Mn, and 0.1-10% Cr, with the remainder comprising incidental impurities and Fe; (4) particles comprising, in mass %, 20-40% Mo, 0.5-1.0%C, 5-30% Ni, 1-10% Mn, 1-10% Cr, 5-30% Co, and 0.05-2% Y, with the remainder comprising incidental impurities and Fe; etc. There are no particular limitations so long as the hard particles are harder than the iron particles, and Si, etc., by way of example, may further be included in addition to these particles. Further, the hard powder may preferably have an average particle size of 80 to 120 micrometers, and the hard powder content may preferably be 10to 60 mass % relative to the total amount of the iron-based mixture powder for sintering. 
         [0034]    Next, from among metal fluoride powders (powders comprising particles of a metal fluoride), a strontium fluoride powder (a powder comprising particles of strontium fluoride) is prepared as an alkaline earth metal fluoride powder (a powder comprising particles of an alkaline earth metal fluoride). Alkaline earth metal fluoride powders, such as strontium fluoride powders, etc., used in metallurgy, such as sintering, etc., have hitherto been produced generally by precipitating alkaline earth metal fluoride particles from a solution in which an alkaline earth metal is dissolved in hydrogen fluoride, and particles of such powders have shapes that are close to being spherical. However, in the present embodiment, a bulk material inwhich crystals of strontium fluoride (an alkaline earth metal) are consolidated, or a bulk material obtained by melting them, is pulverized with a mill, etc., to produce particles having recesses and projections. 
         [0035]    Then, assuming that a powder&#39;s particle asperity=(perimeter of a section of a particle) 2 /(sectional area of the section×4Pi), pulverizing conditions, such as the shape of the pulverizing part of the mill, the pulverizing load, etc., were so selected that the asperity of the strontium fluoride particles would fall within the range of 2 to 5, and strontium fluoride was thus produced. It is noted that as the asperity of the particles approaches 1, the particles become more spherical. With respect to methods in which strontium fluoride particles are precipitated, particle asperity is extremely close to 1. 
         [0036]    Thus, since the particle asperity of the strontium fluoride powder is made to fall within the range of 2 to 5, the shape of the particles of the strontium fluoride powder is conducive to adhesion with the iron-based particles and the hard particles. Consequently, with respect to the iron-based mixture powder for sintering, the particles of the strontium fluoride powder to be mixed become readily dispersible (less likely to flocculate). Further, since strontium fluoride, which serves as a free-machining component, is dispersed among the iron-based particles and the hard particles in the iron-based sintered alloy sintered from this iron-based mixture powder for sintering, it is possible to reduce the cutting resistance of the iron-based sintered alloy and to mitigate the shortening of cutting tool life. 
         [0037]    Further, the strontium fluoride powder content may preferably be 0.5 to 3 mass % relative tothe total amount of the iron-based mixture powder for sintering, and the average particle size of the strontium fluoride powder may preferably be within the range of 1 to 20 micrometers. By adopting such a range, it is possible to reduce cutting resistance, further improve cutting tool life, and further reduce surface damage to the sintered alloy. 
         [0038]    A lubricant, a binder, etc., may further be added to the iron-based mixture powder for sintering. By way of example, for the binder, a silicone resin, e.g., one that is methyl-based, may further be added to the iron-based mixture powder for sintering. When adding this silicone resin to the iron-based mixture powder for sintering, this resin may preferably be addedby diluting it with an organic solvent, mixing it with an iron powder and strontium fluoride, and removing any excessive organic solvent through heating and drying. The silicone resincontent may preferably be 1 mass % or less relative to the total amount of the iron-based mixture powder for sintering. 
         [0039]    By adding a silicone resin, it is possible to improve the binding force between the mixture powder and the alkaline earth metal fluoride. Further, since silicone resins are thermosetting resins, in producing a compact by pressure molding the iron-based mixture powder for sintering and then further sintering the compact, it is possible to maintain the aforementionedbinding force without the silicone resin softening. In addition, it is possible to maintainthe aforementioned binding force even when a compact is produced through a warm mold lubrication method, etc., since they are resistant to temperatures of at least 150 degrees C. 
         [0040]    Further, the iron-based mixture powder for sintering may contain, for the lubricant, a thermoplastic resin powder, zinc stearate, lithium stearate, stearic acid, oleic acid amide, stearic acid amide, a molten mixture of stearic acid amide and ethylenebis stearic acid amide, ethylenebis stearic acid amide, polyethylene with a molecular mass of ten thousand or less, or a molten mixture of ethylenebis stearic acid amide and polyethylene with a molecular mass of ten thousand or less. 
       EXAMPLES  
       [0041]    The present invention is described below through examples. 
       Example 1 
       [0042]    An iron-based mixture powder for sintering was produced by mixing an iron-based powder, a graphite powder, a hard powder that is harder than the iron-based powder, a metal fluoride powder, and a lubricant. Specifically, an iron-based powder (Fe)—1.1 mass % graphite powder (Gr)—30 mass % hard powder—1 mass % strontium fluoride powder (SrF 2 )—0.8 mass % lubricant (ZnSt) were prepared. The iron-based powder was a pure iron powder produced through a reductive method, and the iron-based particles forming the iron-based powder were a powder with an average particle size of 100 micrometers. The hard powder was produced through a gas atomization method, and the hard particles forming the hard powder comprised: 0.8 mass % C—1.1 mass % Si—5.1 mass % Mn—21 mass % Ni—6 mass % Cr—39 mass % Mo—22 mass % Co—4.5 mass % Fe—0.2 mass % Y. The hard powder was a powder with an average particle size of 100 micrometers. 
         [0043]    Further, the strontium fluoride powder was produced by pulverizing, with a mill, etc., a bulk material into which strontium fluoride (alkaline earth metal) crystals had been consolidated. The particles of this powder were particles having recesses and projections and an average particle size of 5 micrometers. Further, defining powder particle asperity as being: 
         [0000]      powder particle asperity=(perimeter of a section of a particle) 2 /(sectional area of the section×4Pi),
 
         [0000]    pulverization conditions, such as the rotation speed of the mill, etc., were varied in such a manner that the asperity would fall within the range of 2 to 5 as shown in  FIG. 3 , and thestrontium fluoride powder was thus produced. 
         [0044]    With respect to asperity, (1) the powder was viewed under magnification (photographed with an electron microscope and imaged), and subsequently (2) that image was put through roundnessmeasurement preprocessing using imaging software (a binarization process where the powder isblack and others are white), (3) using imaging software, the perimeter thereof was taken to be the perimeter of a section of a particle, and the area thereof was measured as the sectional area of that section, and a roundness (asperity) measurement was carried out using the equation mentioned above. Here, N=10 measurements were taken with respect to randomly selected particles. It is noted that the asperities for Example 1 in  FIG. 2  are average values obtained through such a number of measurements (i.e., N=10), and the measured asperities were, in order from the left, 2.2, 2.65, 3.5, and 5.0, which all fall within the range of 2 to 5. An image of a strontium fluoride particle with an asperity of 2.65, which is one of the Examples, is shown in  FIG. 1 . 
         [0045]    These iron-based mixture powders for sintering were pressure molded at 784 MPa and at room temperature, and thereafter sintered at 1120 degrees C. to obtain specimens made of an iron-based sintered alloy and that were of a shape corresponding to a valve seat. 
         [0046]    In the present example, the average particle size was measured through a sieving method. Specifically, using JIS Z8801-1compliant test sieves, the sizes of the particles were determined based on the size of the openings in the mesh that were passed. Specifically, by sieving using several kinds of sieves with varying mesh sizes, particle size distribution, by mass, was determined by computing the weight ratio on each sieve. 
       Comparative Example 1  
       [0047]    A specimen made of an iron-based sintered alloy was produced in a similar fashion to Example1. Differences with respect to the Example lie in the strontium fluoride powder. Specifically, the strontium fluoride powder in Comparative Example 1 was a powder that was obtained by precipitating alkaline earth metal fluoride particles from a solution in which strontium fluoride was dissolved in hydrogen fluoride, and whose particle asperity was 1.0. An image of a strontium fluoride particle of Comparative Example 1 with an asperity of 1.0 is shown inFIG. 1. 
         [0048]    &lt;Cutting Tool Wear Measurement Test&gt; 
         [0049]    300 passes&#39; worth of a cutting process (where one pass corresponds to the cutting length forone valve seat) was performed with respect to specimens of Example 1 and Comparative Example1 at a feed rate of 0.3 mm and a cutting speed of 0.08 mm/rev using a cutting tool (material: carbide). Then, using an optical microscope, the greatest depth of wear of the flank faceof the cutting tool was measured as cutting tool wear Vbmax. The results are shown in  FIG. 2 . 
         [0050]    {Results 1 and Discussion thereof} 
         [0051]    There was less cutting tool wear in Example 1 as compared to Comparative Example 1. It is speculated that this is because strontium fluoride powders comprising particles with an asperity of 2 to 5 are of such shapes that readily adhere to (engage with) the iron-based particles and the hard particles, and the particles of the strontium fluoride powders to be mixed are more readily dispersible (less likely to flocculate) in the iron-based mixture powder forsintering. It is further speculated that, as a result, because the strontium fluoride powder, which serves as a free-machining component, was dispersed among the iron-based particles and the hard particles in the iron-based sintered alloy sintered from this iron-based mixture powder for sintering, the cutting resistance of the iron-based sintered alloy decreased, and there was less cutting tool wear. 
       Example 2 
       [0052]    A specimen made of an iron-based sintered alloy was produced in a similar fashion to Example1. Differences with respect to Example 1 lie in the fact that, by changing the pulverization conditions, there was produced a powder with a particle asperity of 2.75 (where N=10, average value of particle asperity: 2.75, minimum value: 2.18, and maximum value: 3.21). 
         [0053]    &lt;Microscope Observation&gt; 
         [0054]    The specimen of Example 2 was cut out, and a section thereof was observed using an electron microscope. The result is shown in  FIG. 3 . 
         [0055]    &lt;Measurements of Various Components&gt; 
         [0056]    The Sr- and F-contents (the amounts added) of the specimen of Example 2 were measured through X-ray atomic absorption spectroscopy. The results are presented in Table 1 below. It is noted that, of the values presented in Table 1, the theoretical values indicate the respective proportions (in mass %) of Sr and F relative to 1 mass % of the strontium fluoride powderthat has been added, and that the analytical values indicate the respective proportions (in mass %) of Sr and F as measured. The values in parentheses provided with the analytical values indicate values calculated by dividing the analytical values with the respective theoretical values. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                   
                   
                 Theoretical 
                   
               
               
                   
                 Analytical Values 
                   
                 Values 
               
             
          
           
               
                   
                 SrF 2  Content 
                 Sr 
                 F 
                 Sr 
                 F 
               
               
                   
                   
               
               
                   
                 1 mass % 
                 0.625 
                 0.27 
                 0.698 
                 0.302 
               
               
                   
                   
                 (89.5%) 
                 (89.4%) 
               
               
                   
                   
               
             
          
         
       
     
         [0057]    {Results 2 and Discussion thereof} 
         [0058]    As shown in  FIG. 3 , strontium fluoride (SrF 2 ) was dispersed at the grain boundary of the iron base and the hard particles. Further, as indicated in Table 1, even after sintering, morethan 85% of the strontium fluoride remained. From such results, it is speculated that the machinability of the iron-based sintered alloy would improve dramatically by virtue of the dispersed strontium fluoride. 
       Example 3  
       [0059]    Specimens made of an iron-based sintered alloy were produced in a similar fashion to Example1. Differences with respect to Example 1 lie in the fact that, by changing the pulverization conditions, there were produced powders whose particle asperity of the strontium fluoride powder was 2.7, and in the fact that 0.5 mass % to 5.0 mass % of the strontium fluoride powder was added as shown in  FIG. 4 . The average particle size was 5 micrometers for all strontium fluoride powders. 
       Comparative Example 2 
       [0060]    A specimen made of an iron-based sintered alloy was produced in a similar fashion to Example1. Differences with respect to Example 1 lie in the fact that no strontium fluoride powder was added to the iron-based mixture powder for sintering. 
         [0061]    &lt;Cutting Tool Wear Test&gt; 
         [0062]    Cutting tool wear tests were performed with respect to the specimens of Example 3 and Comparative Example 2 in a similar fashion to Example 1. To that end, using a cutting resistance measuring method, cutting resistance was measured with a dynamometer attached to a cutting tool fixing part. Cutting tool wear and cutting resistance are respectively presented in  FIG. 4A  and  FIG. 4B . 
         [0063]    &lt;Rattler Value Measurement Test&gt; 
         [0064]    With respect to the specimens of Example 3 and Comparative Example 2, rattler values were measured, as indices indicative of the specimens&#39; susceptibility to surface damage, in accordance with a rattler value measuring method for metal green compacts indicated in Japan PowderMetallurgy Association standard JPMA P 11-1992. The results are shown in  FIG. 4C . It is noted that a greater rattler value indicates that the specimen is more susceptible to surface damage. 
         [0065]    {Results 3 and Discussion thereof} 
         [0066]    As shown in  FIGS. 4A and 4B , by virtue of the fact that it contains strontium fluoride (SrF 2 ), Example 3 exhibited less cutting tool wear and cutting resistance as compared to Comparative Example 2. Further, as shown in  FIG. 4C , of the specimens of Example 3, the specimen whose amount added was 5 mass % exhibited a rattler value exceeding 20%, which was greater than those of the other specimens. From these results, it may be inferred that, in order to improve the machinability of the iron-based sintered alloy by way of strontium fluoride, 0.5 mass % or more of strontium fluoride may preferably be added. Further, in order to reduce the susceptibility to surface damage of the iron-based sintered alloy, 3.0 mass % or less of strontium fluoride may preferably be added, or 1.5 mass % or less may further preferably be added. 
       Example 4 
       [0067]    Specimens made of an iron-based sintered alloy were produced in a similar fashion to Example1. Differences with respect to Example 1 lie in the fact that, by changing the pulverization conditions, there were produced powders whose particle asperity of the strontium fluoride powder was 2.7, and in the fact that the particle sizes of the particles of these powders were made to be such that their average particle sizes fell within the range of 1 to 100 micrometers as shown in  FIGS. 5A through 5C . Particles with average particles sizes of 50 micrometers to 100 micrometers were granulated using a liquid adhesive (PVP) as a method of varying particle size. For the other particles, predetermined particle sizes were selected through grading using sieves. It is noted that the amount of strontium fluoride powder added was 1.0 mass %. 
         [0068]    &lt;Cutting Tool Wear Test and Rattler Test&gt; 
         [0069]    A cutting tool wear test and a rattler test were conducted with respect to the specimens of Example 4 as in Example 3. Cutting tool wear, cutting resistance, and rattler values are indicated in  FIG. 5A ,  FIG. 5B , and  FIG. 5C , respectively. The results for Comparative Example2 are also indicated in  FIGS. 5A through 5C . 
         [0070]    {Results 4 and Discussion thereof} 
         [0071]    As shown in  FIGS. 5A and 5B , by virtue of the fact that it contains strontium fluoride (SrF 2 ), Example 4 exhibited less cutting tool wear and cutting resistance as compared to Comparative Example 2. Further, as shown in  FIG. 5A , as the average particle sizes of the powders of Example 4 increased, cutting tool wear also increased. As shown in  FIG. 5B , as the average particle size increased to 50 micrometers and 100 micrometers, cutting resistance increased. Further, as shown in  FIG. 5C , as the average particle size increased to 50 micrometers and 100 micrometers, the rattler value also increased. It is speculated that this was due tothe fact that when the average particle size of (the particles of) the alkaline earth metal fluoride powder exceeded 20 micrometers, it became difficult for the alkaline earth metal fluoride to disperse at the grain boundary in the iron-based sintered alloy, thereby increasing cutting resistance. Further, it is speculated that due to the fact that it became more difficult for the alkaline earth metal fluoride to disperse at the grain boundary in the iron-based sintered alloy, the sintered alloy became more susceptible to surface damage. 
         [0072]    Further, it is speculated that when the average particle size of (the particles of) the strontium fluoride powder is 5 micrometers or less, the degree to which strontium fluoride contributes as a solid lubricant during sintering is greater. Further, it is speculated that, asthe average particle size of the strontium fluoride powder increases to a certain level (as it exceeds 20 micrometers), it inhibits engagement among the iron powder particles instead of contributing as a solid lubricant. 
         [0073]    Embodiments and examples of the present invention have been discussed in detail above. However, the present invention is by no means limited to the above-mentioned embodiments. Rather, various design modifications may be made without departing from the spirit and scope of the present invention as specified in the appended claims. 
         [0074]    In the examples above, perimeters of sections (largest sections) of particles, and sectionalareas of such sections were measured using projected images of particles as observed with a microscope. However, the present invention is by no means limited to such a method as long as asperity may be measured by directly measuring perimeters and sections, for example. 
       INDUSTRIAL APPLICABILITY  
       [0075]    The present invention may be suitably used in valve systems (e.g., valve seats, valve guides) of engines that run on compressed natural gas or gasoline and which are placed under high temperature use conditions.