Good machinability Fe-based sintered alloy and process of manufacture therefor

Machinability is drastically improved while maintaining some degree of hardness in an Fe-based sintered alloy. A good machinability Fe-based sintered alloy has an overall composition of, in percent by weight, at least one element selected from the group consisting of P in an amount of 0.1 to 1.0% and Si in an amount of 2.0 to 3.0%, B in an amount of 0.003 to 0.31%, 0 in an amount of 0.007 to 0.69%, C in an amount of 0.1 to 2.0%, and the balance consisting of Fe and unavoidable impurities, has a matrix hardness ranging from Hv 150 to 250, and has free graphite dispersed therein.

BACKGROUND OF THE INVENTION
 The present invention relates to a good machinability Fe-based sintered
 alloy and a process of manufacture therefor, and more particularly relates
 to a technique which can improve machinability by sintering a boron
 compound powder added to a mixed powder of an Fe-based material.
 An Fe-based sintered alloy can be produced in near-net shape so that
 manufacturing cost for processing can be reduced, and moreover, elements
 may be dispersed therein having specific gravities which differ greatly,
 and in different alloys in which dissolution is difficult, whereby
 properties may be obtained such as wear resistance, etc. For this reason,
 Fe-based sintered alloys are often used in various fields of technology.
 For example, mechanical parts made of Fe-based sintered alloy can be made
 without considerable machining processing, even if the parts are of
 complicated configuration, whereby such parts can be widely employed in
 valve driving systems, bearings, and the like, in automobiles,
 motorcycles, etc. However, most mechanical parts made of Fe-based sintered
 alloys must be machined, therefore poor machinability still present
 problems.
 In order to improve the machinability of Fe-based sintered alloys, many
 attempts have heretofore been made. In one attempt, an Fe powder
 containing sulfur is used as a starting material powder. In another
 attempt, sulfide is added to and mixed with a starting material powder. In
 still another attempt, a sintered compact is sulfurized in an atmosphere
 of hydrogen sulfide gas. However, when sulfur as a cutting facilitating
 component is dispersed in the matrix of a sintered alloy, improvement in
 machinability is limited. Moreover, sulfur is an element which decreases
 strength, particularly toughness, in sintered alloys, and also promotes
 corrosion in sintered alloys; therefore, use of such sintered alloys is
 limited.
 Another technique which fills resin, etc., into pores of a sintered alloy
 is also available. In such a sintered alloy, the resin in the pore serves
 as an initiating point for chip breaking, whereby the chip-breaking
 property is superior. However, in such a technique, using certain types of
 resin may shorten the service life of a cutting tool such as a cutter.
 Moreover, a process for removing the resin from the pores after cutting
 processing may be required, depending on the purpose for which the
 sintered alloy is to be used.
 Therefore, the present applicant proposed an improved method for an
 Fe-based sintered alloy, in which a boron compound powder is added to a
 mixed powder of an Fe-based material including carbon, and is sintered, in
 Japanese Unexamined Patent Application Publication No. 241701/97.
 According to this proposed technique, diffusion of the carbon into the
 matrix is suppressed by the boron, whereby machinability can be improved
 with a decrease in hardness of the Fe-based sintered alloy.
 However, further improvement of machinability has been recently demanded to
 enhance high performance alloys for automobiles.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide a good
 machinability Fe-based sintered alloy which is further improved over the
 above Fe-based sintered alloy and a process of manufacture therefor.
 Generally, it is known that such materials harden when the carbon content
 of an Fe-based sintered part is increased, and that machinability thereof
 is lowered thereby. However, according to research by the inventors, the
 following knowledge was obtained. In the case in which the matrix of the
 Fe-based sintered portion closely resembles pure iron and the hardness
 thereof is too low, the amount of wear on a cutting tool conversely
 increases.
 FIG. 1 is a chart showing the amount of wear on a cutting tool in cutting
 processing with respect to 4 kinds of Fe-1.5Cu--C-based sintered parts
 (A-D) having different hardnesses, which are produced by changing the C
 content, and an Fe-1.5Cu--C-based sintered part (E), which has improved
 machinability by a technique disclosed in the above-mentioned Japanese
 Unexamined Patent Application Publication No. 157706/97.
 Hitherto, it was expected that machinability of a part A having the lowest
 hardness would be most desirable and that the amount of tool wear thereof
 would be minimal. However, as is apparent from FIG. 1, the softest part A
 in which hardness of a surface thereof ranges from Hv 110 to 120 (load=100
 gf) actually has the highest amount of wear, and a part C in which the
 hardness ranges from Hv 200 to 230 has the least amount of wear. It is
 apparent that the amount of wear on the cutting tool is drastically
 reduced in comparison with the amount of wear on the part A, in the case
 in which hardnesses range from Hv 150 to 250. As a reason for this, it is
 believed that adhesive wear is generated on an edge of the cutting tool
 during cutting processing since ferrite, which is a matrix of the Fe-based
 sintered portion, has high viscosity.
 As shown in FIG. 1, an Fe-based sintered alloy having improved
 machinability by a technique shown in the Japanese Unexamined Patent
 Application Publication No. 157706/97 has the smallest amount of wear, and
 remarkable improvement in machinability appears. Moreover, it is believed
 that machinability can be further improved by increasing matrix hardness
 and suppressing generation of adhesive wear.
 Therefore, the inventors found that the amount of wear on a cutting tool is
 remarkably reduced when hardness is increased by alloying ferrite and is
 set within a specific range.
 In consideration of this situation, a good machinability Fe-based sintered
 alloy of this invention has an overall composition consisting of, in
 percent by weight, at least one element selected from the group consisting
 of P in the amount of 0.1 to 1.0% and Si in the amount of 2.0 to 3.0%, B
 in the amount of 0.003 to 0.31%, O in the amount of 0.007 to 0.69%, C in
 the amount of 0.1 to 2.0%, and the balance consisting of Fe and
 unavoidable impurities, has a matrix hardness ranging from Hv 150 to 250,
 and has free graphite dispersed therein. Here, the Hv refers to a Vickers
 hardness at a load of 100 gf.
 In this invention, free graphite is dispersed and functions as a solid
 lubricant, whereby machinability is improved. Boron is contained at 0.003%
 by weight or more in the Fe-based sintered alloy, whereby the boron
 prevents graphite from diffusing as C so as to ensure that the graphite
 remains free and prevents pearite from forming in the matrix. According to
 the research of the inventors, reasons for the improved machinability due
 to the boron are as follows.
 That is to say, boron compound powder (for example, boron oxide (B.sub.2
 O.sub.3)), added to a powder mixture, dissolves at about 500.degree. C.,
 which is lower than the temperature at which C diffuses into the matrix
 during heating for sintering, and covers the surfaces of the graphite
 powder. The C of the graphite powder does not diffuse into the ferrite
 matrix and cannot form pearite, and remains as free graphite, and the
 machinability thereof is remarkably improved by functioning as a solid
 lubricant. In this invention, matrix hardness is particularly set as
 described above by containing P and Si, whereby further improvement in
 machinability is achieved.
 P: Action of ferrite strengthening is slight when the P content is under
 0.1% by weight. As a result, a hard matrix is not obtained, thereby
 failing to improve machinability. In contrast, when the P content exceeds
 1.0% by weight, the generation rate of the Fe--P liquid phase increases in
 sintering, whereby a green compact easily loses its shape during
 sintering. Therefore, the P content ranges preferably from 0.1 to 1.0% by
 weight. Moreover, the P can be added in the form of a simple powder;
 however, it is preferably added in the form of an Fe--P alloy powder since
 the simple powder is dangerous.
 Si: Si can be added in the form of a simple powder so that it quickly
 diffuses in the matrix; however, pure Si is expensive, and it is therefore
 preferably added in the economical form of an Fe--Si alloy powder in
 consideration of industrial productivity. Ferrite strengthening effects
 are slight when the Si content is under 2.0% by weight. As a result, a
 hard matrix is not obtained, thereby failing to improve machinability. In
 contrast, when the Si content exceeds 3.0% by weight, the Fe--P sintered
 powder hardens, decreasing compressibility thereof during sintering. As a
 result, the required density in the sintered compact cannot be obtained,
 and the strength thereof is lowered. Therefore, the Si content preferably
 ranges from 2.0 to 3.0% by weight.
 C: C is added in the form of a graphite powder. However, the amount of
 carbon diffused in the matrix is too small when the amount added (i.e.,
 the C content) is less than 0.1% by weight, and the desired strength is
 not obtained, and additionally, the amount of undiffused free graphite is
 small, whereby machinability is not improved. In contrast, when the C
 content is too high and diffusion cannot be suppressed, i.e., when the
 addition amount of the graphite powder exceeds 2.0% by weight, pearite is
 thereby formed.
 B and O: B and O are mainly contained by being added in the form of a boron
 oxide powder. B in the amount of 0.003 to 0.31% by weight and O in the
 amount of 0.007 to 0.69% by weight correspond to B.sub.2 O.sub.3 in the
 amount of 0.01 to 1.0% by weight. Diffusion of C from graphite powder
 cannot be suppressed in sintering when the content of each is less than
 the lower limit, respectively. In contrast, when the upper limit is
 exceeded, not only does the effect of suppression of diffusion of C not
 occur, but also a large amount of boron oxide remains in the matrix,
 whereby material strength is lowered.
 Moreover, by containing Cu in the material of this invention, the strength
 thereof can be improved while maintaining machinability. In this case, the
 Cu content preferably ranges from 1.0 to 5.0% by weight. The Cu also
 strengthens the material by diffusing in the matrix, but the effect
 thereof is slight below 1.0% by weight. In contrast, when the Cu content
 exceeds 5.0% by weight, the strength is lowered by the generating of a
 soft Cu phase. Dimensional contraction caused by generating the Cu liquid
 phase during sintering and the Cu expansion phenomenon caused by the Cu
 which is easily diffused in the Fe matrix by generating the liquid phase,
 are caused by microscopic contractions and expansions in each local area
 of the product. As a result, dimensional changes of the overall product
 vary widely, whereby dimensional accuracy is poor. Moreover, the Cu powder
 is added in the form of a simple powder, and average particle size of the
 Cu powder and the graphite powder range from 1 to 10 .mu.m, which is the
 range usually used.
 In the above-described good machinability Fe-based sintered alloy, the
 machinability can be further improved by dispersing BN in an amount of
 0.06 to 2.25% by weight in the matrix. The BN has chip breaking effects
 and solid lubrication effects, thereby improving machinability. The above
 effects are slight when the BN content is under 0.06% by weight, and the
 strength of the matrix is lowered when the content exceeds 2.25% by
 weight.
 A good machinability Fe-based sintered alloy such as that described above
 can be produced by adding, in percent by weight of the total mixed powder,
 an Fe-based powder consisting of at least one element selected from the
 group consisting of P in the amount of 0.1 to 1.0% and Si in the amount of
 2.0 to 3.0%, the balance consisting of Fe and unavoidable impurities, a
 graphite powder in the amount of 0.1 to 2.0%, and a boron oxide powder in
 the amount of 0.001 to 1.0%. The boron oxide powder is added at 0.1% by
 weight or more. In the case in which the boron oxide powder content is
 less than the above, diffusion of C from the graphite powder cannot be
 suppressed in sintering, whereby pearite is formed. In contrast, even if
 the boron oxide powder is added at 1.0% by weight or more, not only can
 the suppression effects on the diffusion of C not be expected to improve,
 but also a large amount of the boron oxide remains in the matrix, and the
 strength of the material is lowered.
 As an addition method for boron oxide, a method for adding the boron oxide
 in the form of a simple powder or a method for adding boron nitride can be
 employed. BN can be dispersed in the matrix by adding the boron nitride.
 Available powders of boron nitride contain boron oxide as a residue from a
 production process. The available powder of boron nitride in which the
 boron oxide is reduced to 5% by weight or less is used in powder
 metallurgy. However, this available powder of boron nitride is expensive
 since purity is high. Therefore, according to the research of the
 inventors with regard to the boron oxide content included in the boron
 nitride powder, the available powder of boron nitride in which the boron
 oxide content is 10 to 40% by weight is relatively inexpensive, and it was
 found that diffusion of graphite is suppressed by adding this powder in
 amount of 0.1 to 2.5% by weight, instead of the boron oxide powder,
 whereby generation of pearite is suppressed.
 According to this invention, workability and tool life can be improved when
 applied to bearing caps for automobile engines, synchronizer hubs, various
 gears for general-purpose engines, alloys for office equipment, and alloys
 for machine tools, etc., in which cutting processes are conducted on
 surfaces of a sintered alloy and for the sizing thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In the following, preferred embodiments according to the present invention
 will be described in detail.
 A. Manufacture of Sintered Compacts
 Raw material powders were prepared at compounding ratios shown in Table 1
 and were mixed by a V type mixer for 30 minutes. The mixed powders were
 molded at a density of 6.6 g/cm.sup.3 in powder compacting, and five green
 compacts having outer diameters of 32 mm, inner diameters of 15 mm, and
 heights of 10 mm were produced for each mixed powder. Then, each green
 compact was sintered by heating at 1130.degree. C. for 60 minutes in a
 reducing atmosphere (dissociated ammonia gas).

Dimensional

Tool Wear
 B. Cutting Test
 A cutting test was conducted on each sintered compact, and the flank wear
 width at a tool edge was evaluated as the amount of tool wear. The cutting
 test was performed by cutting over a distance of 7000 m using water-
 soluble cutting oil and an NC lathe which provides slow chipping away of
 cubic boron nitride (CBN) at a cutting speed of 180 mm/min, a feed rate of
 0.04 mm/rev, and a cutting depth of 0.15 mm. Then, the sintered compact
 was polished and the micro-Vickers hardness was measured at random points,
 and the mean values thereof are listed in Table 1 with the amount of tool
 wear.
 C. Evaluation
 1 Effect of P Content
 Samples of differing P content were selected from Table 1 and are described
 in Table 2. The P content, matrix hardness, and amount of tool wear
 described in Table 2 are shown in FIG. 2. As is apparent from FIG. 2, the
 matrix hardness greatly increases until the P content increases to 0.1% by
 weight and the matrix hardness increases with the increase in the P
 content thereafter. In contrast, the amount of tool wear rapidly decreases
 until the P content increases to 0.1% by weight. In addition, in sample
 No. 6 in which the P content exceeds 1.0% by weight, many Fe--P liquid
 phases were generated during sintering, whereby the shape of the green
 compact was lost, and a sintered compact could not be formed. Therefore,
 the reason for the numerical limitation according to this invention in
 which the P content ranges from 0.1 to 1.0% by weight was confirmed.

Evaluated Item
 Mixing Ratio wt %
 Tool
 Boron Boron
 Matrix Wear
 Sample Fe Cu Fe-20P Fe-40P Graphite Oxide Nitride
 Overall Constituent Composition wt % Hardness
 Amount
 No. Powder Powder Powder Powder Powder Powder Powder Fe
 Cu P Si C B O BN (Hv)
 (.mu.m) Note
 Effect of P content
 1 Balance 1.5 -- -- 0.6 -- 1.0 Balance 1.5 --
 -- 0.6 0.062 0.138 0.80 121 80 Substantial

Degradation
 2 Effect of Si Content
 Samples of differing Si content were selected from Table 1 and are
 described in Table 2. The Si content, matrix hardness, and amount of tool
 wear described in Table 2 are shown in FIG. 3. As is apparent from FIG. 3,
 the matrix hardness greatly increases until the Si content increases to
 2.0% by weight, and the matrix hardness increases with the increase in the
 Si content thereafter. In contrast, the amount of tool wear rapidly
 decreases until the Si content increases to 2.0% by weight. In addition,
 in sample No. 10 in which the Si content exceeds 3.0% by weight,
 compressibility of the powder was decreased, whereby strength of the
 sintered compact was decreased. Therefore, the reason for the numerical
 limitation according to this invention in which the Si content ranges from
 2.0 to 3.0% by weight was confirmed.
 3 Effect of Addition Amount of Boron Oxide Powder
 Samples of differing boron oxide powder content were selected from Table 1
 and are described in Table 2. Addition amount of boron oxide powder,
 matrix hardness, and amount of tool wear described in Table 2 are shown in
 FIG. 4. As is apparent from FIG. 4, the matrix hardness rapidly decreases
 by adding the boron oxide powder at 0.01% by weight, and the amount of
 tool wear also rapidly decreases therewith. In contrast, in sample 17 in
 which the addition amount of boron oxide powder exceeds 1.0% by weight,
 machinability was good; however, strength degradation of the matrix was
 confirmed. Therefore, the reason for the numerical limitation according to
 this invention in which the addition amount of boron oxide powder ranges
 from 0.01 to 1.0% by weight was confirmed.
 4 Effect of Cu Content
 Samples of differing addition amounts of Cu powder (Cu content) were
 selected from Table 1 and are described in Table 3. The addition amount of
 Cu powder, matrix hardness, and amount of tool wear described in Table 3
 are shown in FIG. 5. As is apparent from FIG. 5, there was no remarkable
 change with respect to the matrix hardness and the amount of tool wear by
 adding the Cu powder. In contrast, the strength of the sintered compact is
 improved by adding the Cu powder and increases as the addition amount
 thereof increases. However, dimensional accuracy was lowered by increased
 generation of the Cu liquid phase and the Cu expansion phenomenon in
 sample No. 22. Therefore, the effects of this invention could also be
 confirmed in an Fe--C type alloy (sample No. 18), and in addition,
 improvement in strength was confirmed for a Cu content ranging from 1.0 to
 5.0% by weight without lowering machinability, and the reason for the
 numerical limitation according to this invention was confirmed.

Dimensional

Tool Wear
 5 Effect of C Content
 Samples of differing addition amounts of graphite powder (C content) were
 selected from Table 1 and are described in Table 3. The addition amount of
 graphite powder, matrix hardness, and amount of tool wear described in
 Table 3 are shown in FIG. 6. As is apparent from FIG. 6, in the case in
 which the addition amount of graphite powder is 0.1% by weight, the amount
 of tool wear rapidly decreases. However, pearite was formed in sample No.
 28 in which the addition amount of graphite powder exceeded 2.0% by
 weight, whereby the amount of tool wear increased. Therefore, the reason
 for the numerical limitation according to this invention in which the C
 content ranges from 0.1 to 2.0% by weight was confirmed.
 As explained above, according to the present invention, boron is contained
 in an Fe-based sintered alloy, and the matrix hardness is made to be Hv
 150 to 250, whereby diffusion of C from graphite is prevented and free
 graphite remained, so that machinability can be rapidly improved while
 maintaining a degree of hardness.