JP2015158002A - Sintered machine part and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a machine part which has excellent fatigue strength and is composed of an iron-based sintered metal.SOLUTION: A sintered machine part is formed by mixing a diffusion alloy steel powder containing 1.5-2.2 wt.% of Ni and 0.5-1.1 wt.% of Mo and an addition graphite powder and molding and sintering and has a ratio of carbon of 0.35 wt.% or lower and a density of 7.55 g/cmor greater. The square root √αof the estimated maximum cavity envelope area in the estimated target region set within the surface layer with a specified depth from the surface is 200 μm or smaller.

Description

  The present invention relates to a sintered machine component and a manufacturing method thereof.

  The sintered body is obtained by compressing and molding a mixed powder containing metal powder and graphite powder and then sintering at a predetermined temperature. The sintered body is used for machine parts and the like because a net shape product or a near net shape product can be produced, and the cost can be reduced by reducing the yield and the number of processing steps. Among these, iron-based sintered bodies are widely used in automobile parts and electrical products because of their excellent mechanical properties.

  However, many pores remain inside the sintered body, and these pores act as a stress concentration source and behave like cracks in the molten metal, so tensile, compression, bending strength and impact Various strengths such as strength and fatigue strength are reduced.

  For example, a technique for increasing the density of a sintered body by alternately performing a compression molding process and a sintering process twice on a mixed powder is known (for example, Patent Document 1). However, in this case, there is a problem that the manufacturing cost increases.

  For example, Patent Document 2 uses a metal powder having a coarse particle size distribution to increase the density of the sintered body without using costly processes such as two-stage molding and two-stage sintering.

Furthermore, Patent Document 3 shows that fatigue strength is improved by controlling the dispersion and size of the pores of the sintered body. Specifically, the fatigue strength is improved by setting the cross-sectional void number ratio of the sintered body to 2000 pieces / mm ² or more and the maximum pore diameter to 60 μm or less.

JP-A-4-337001 Special table 2007-537359 gazette Japanese Patent Laid-Open No. 10-317090

  However, even if the size of the vacancies appearing in the cross section of the sintered body is controlled as in Patent Document 3, coarse vacancies may exist in the interior that does not appear in the cross section. Such a sintered body may not have sufficient fatigue strength.

  An object of the present invention is to guarantee excellent fatigue strength in an ultra-high-density machine part made of an iron-based sintered metal.

In order to achieve the above object, the present invention is formed using a mixed powder containing a diffusion alloy steel powder, the proportion of carbon is 0.35 wt% or less, and the density is 7.55 g / cm ³ or more. There is provided a sintered machine part, wherein a square root √α _max of an estimated maximum pore envelope area in an estimation target region set in a surface layer having a predetermined depth from the surface is 200 μm or less.

The sintered machine part as described above includes, for example, a mixing step for obtaining a mixed powder containing diffusion alloy steel powder and 0.35 wt% or less of graphite powder, and a pressure for obtaining a green compact by compression molding the mixed powder. A sintering step of sintering the green compact at a predetermined sintering temperature to obtain a sintered body having a density of 7.55 g / cm ³ or more, and a particle size D90 of the graphite powder. It can be manufactured by a method for manufacturing a sintered machine part having a particle size distribution when the cumulative mass from the small diameter side of the particle size distribution on the mass basis is 90% is 8 μm or less.

  As described above, in the present invention, the diffusion alloy steel powder obtained by diffusion bonding the alloy components to the steel powder is used as the alloy steel powder. Alloy steel powder has low hardness compared to fully alloyed steel powder that is completely alloyed with iron powder and alloy components, so the use of diffusion alloy steel powder can improve formability and increase density. it can.

By the way, when the compression-molded body (compact compact) of the mixed powder is sintered, the graphite powder blended in the mixed powder is dissolved in the alloy steel powder, so that the place where the graphite powder is present becomes a void. Usually, the graphite powder has finer particles than the alloy steel powder, and therefore the pores associated with the solid solution of the graphite powder as described above are fine. Therefore, in an iron-based sintered body having a density that is not so high, the influence of vacancies due to the solid solution of the graphite powder as described above is small, and the influence is not considered. However, according to the verification by the present inventors, since the number of internal vacancies becomes very small when the sintered body is densified, the vacancies caused by the solid solution of graphite cannot be ignored, and the blending ratio of the graphite powder is It was revealed that the density of the sintered body was greatly affected. Therefore, in the present invention, the density of the sintered body is increased by using the diffusion alloy steel powder, and the density of the graphite powder in the mixed powder is suppressed to a low level, thereby further increasing the density. Specifically, the proportion of carbon contained in the sintered machine component (substantially equivalent to the amount of graphite powder in the mixed powder) was set to 0.35 wt% or less. Thereby, the density of a sintered compact can be raised to the ultra high density of 7.55 g / cm < ³ > or more, without using an expensive method.

Furthermore, in the present invention, as described above, in the estimation target region set in the surface layer of a predetermined depth from the surface of the sintered machine part, the size of the internal void from the size of the void that appeared in the cross section. The square root √α _max of the estimated maximum envelope area for estimating is set to 200 μm or less. As a result, it is assured that almost no coarse pores serving as a stress concentration source are formed inside the surface layer of the sintered body, and excellent fatigue strength of the sintered machine component is assured.

  The sintered machine part preferably includes 1.5 to 2.2 wt% of Ni and 0.5 to 1.1 wt% of Mo, with the balance being composed of Fe, carbon, and inevitable impurities. For example, diffusion alloy steel powder in which Ni is diffused and deposited around the Fe—Mo alloy, Ni is 1.5 to 2.2 wt%, Mo is 0.5 to 1.1 wt%, and the balance is Fe and inevitable impurities. Can be used to obtain a sintered machine component having the above composition.

  In order to suppress the influence of pores accompanying solid solution of graphite powder, it is effective not only to reduce the blending ratio of graphite powder but also to reduce the particle size. Specifically, the particle diameter D90 of the graphite powder is preferably 8 μm or less. The particle size of the powder is measured using a laser diffraction / scattering method. This measurement method utilizes the fact that when the particles are irradiated with light, the amount of scattered light and the pattern to be scattered differ depending on the particle diameter.

According to the sintered machine part described above, it is possible to increase the density to 7.55 g / cm ³ or more without performing a recompression process (for example, a sizing process) after the sintering process.

  When manufacturing the above sintered machine parts, if the carbonitriding process is performed after the sintering process, the fatigue strength can be further increased.

  As described above, according to the present invention, excellent fatigue strength can be ensured in an ultra-high-density machine part made of iron-based sintered metal.

It is the side view and sectional drawing of a test piece used for a ring compression fatigue strength test. It is a perspective view which shows the state which cut | disconnected the test piece in calculation of an estimated largest void | hole envelope area.

  A sintered machine part according to an embodiment of the present invention is manufactured through the following mixing process, compression molding process, sintering process, and heat treatment process.

  In the mixing step, a mixed powder is produced by mixing alloy steel powder, graphite powder, and lubricant at a predetermined ratio.

  The alloy steel powder is such that each particle contains Fe and another metal (alloy component). As the alloy component, for example, one or more of Ni, Mo, Mn, and Cr can be used. In the present embodiment, alloy steel powder containing Ni and Mo as alloy components and the balance being Fe and inevitable impurities is used. Ni has the effect of strengthening the mechanical properties of the sintered body and improving the toughness of the sintered body after heat treatment. Mo has the effect of enhancing the mechanical properties of the sintered body and improving the hardenability during heat treatment. It is desirable to classify the alloy steel powder by passing through a sieve having an opening of 250 μm in advance.

  As the alloy steel powder, diffusion alloy steel powder in which an alloy component is diffused and adhered around the steel powder is used. In this embodiment, a diffusion alloy steel powder in which Ni is diffused and adhered around the Fe—Mo alloy is used. In this way, the hardness of the alloy steel powder before sintering can be suppressed compared to the steel powder in which Fe and Ni are completely alloyed by diffusing and attaching a metal such as Ni to the Fe alloy. Formability during molding is ensured. As a result, a relatively large amount of Ni can be blended. Specifically, the mixing ratio of Ni in the diffusion alloy steel powder of the present embodiment is 1.5 to 2.2 wt%, preferably 1.7 to 2.2 wt%. On the other hand, even if Mo is added in a large amount, the effect is saturated, and on the contrary, it causes the formability to deteriorate. For this reason, the mixture ratio of Mo in the diffusion alloy steel powder is 0.5 to 1.1 wt%, preferably 0.8 to 1.1 wt%, and more preferably 0.9 to 1.1 wt%.

  For example, artificial graphite is used as the graphite powder. The graphite powder having a particle size D90 of 8 μm or less is used, preferably 6 μm or less, more preferably 4 μm or less. The graphite powder having a particle diameter D90 of 2 μm or more, preferably 3 μm or more is used. The blending ratio of the graphite powder is 0.35 wt% or less, preferably 0.3 wt% or less, more preferably 0.25 wt% or less with respect to the entire mixed powder. Further, the blending ratio of the graphite powder is 0.05 wt% or more, preferably 0.1 wt% or more, more preferably 0.15 wt% or more with respect to the whole mixed powder.

  The lubricant is added for the purpose of reducing the friction between the mold and the powder or between the powders when the mixed powder is compression-molded. As the lubricant, metal soap, amide wax or the like is used, for example, ethylene bisstearylamide (EBS).

  In the compression molding step, the above-mentioned mixed powder is put into a mold cavity and compression molded to form a green compact having a predetermined shape. At this time, it is preferable that the molding temperature is not less than room temperature and not more than the melting point of the lubricant. In particular, when the molding is performed at a temperature lower by 10 to 20 ° C. than the melting point of the lubricant, the yield strength of the powder is reduced and the compressibility is enhanced, so that the molding density can be increased. If necessary, the surface of the mold may be coated with a film for reducing friction (such as a DLC film).

When the molding pressure is increased, the density of the green compact can be increased. On the other hand, when the molding pressure is too high, lamination (laminar peeling) due to density unevenness or damage to the mold occurs in the green compact. In the present embodiment, the compression molding process is performed at a molding pressure of about 1150 to 1350 MPa, and the density of the green compact is set to 7.4 g / cm ³ or more.

Next, in the sintering step, the green compact is sintered at a predetermined sintering temperature. The sintering temperature is set, for example, within a range of 1100 to 1350 ° C. The sintering process is performed in an inert atmosphere, for example, in an atmosphere of a mixed gas of nitrogen and hydrogen, argon gas, or the like. By sintering the green compact, the graphite powder in the green compact dissolves in the alloy steel powder, and the portion where the graphite powder is present becomes a void. At the same time, the whole green compact shrinks due to the sintered bonding of the alloy steel powder. As a result, the density increase effect due to the shrinkage of the green compact exceeds the density decrease due to the solid solution of the graphite powder, and the density of the sintered body becomes higher than the density of the green compact. Density of the sintered body, 7.55 g / cm ³ or more, and preferably from 7.6 g / cm ³ or more.

  After the sintering step, the sintered body is subjected to surface treatment without performing a recompression step. In the present embodiment, the sintered body is subjected to a carburizing quenching and tempering process. As a result, the hardness of the surface is increased and the internal toughness is secured, so that the propagation of cracks is suppressed. The surface treatment is not limited to the above carburizing quenching and tempering, but various heat treatments such as submerged quenching and tempering, induction quenching and tempering, carbonitriding, vacuum carburizing, nitriding, soft nitriding, sulfurizing, diamond-like carbon Various surface modifications such as formation of hard coatings such as (DLC) and resin coatings, various plating, rust prevention treatment including blackening and steam treatment, etc. are applicable. Is also possible. When carbonitriding is performed, the nitrided layer depth is 5% or more, preferably 20% or more of the surface layer having a predetermined depth from the surface described later. Thus, the sintered machine part according to the embodiment of the present invention is completed.

  The above sintered machine parts can be used as gears or cams, for example. This sintered machine part contains 1.5 to 2.2 wt% of Ni, 0.5 to 1.1 wt% of Mo, 0.05 to 0.35 wt% of carbon, and the balance is composed of Fe and inevitable impurities. This sintered machine part has an internal hardness of 300 to 500 HV (preferably 400 to 500 HV), a crushing strength of 1600 MPa or more (preferably 1750 MPa or more, more preferably 1900 MPa or more), and a ring compression fatigue strength of 290 MPa or more (preferably). 315 MPa or more, more preferably 340 MPa or more).

In the above sintered machine part, the square root √α _max of the estimated maximum pore envelope area in the estimation target region set in the surface layer at a predetermined depth from the surface is 200 μm or less, preferably 150 μm or less, more preferably 100 μm or less. (A method for calculating √α _max will be described later). For example, when the depth from the surface of the region where the tensile stress is applied when a load is applied to the surface of the sintered machine part is defined as 100%, the surface layer has a depth of 30% from the surface. For example, if the sintered machine part is a gear, or if the sintered machine part is a cam, or if the sintered machine part is a cam, the tooth surface or cam in the region where the tensile stress extends from the cam surface (contact surface with the cam follower) in the depth direction. The depth from the surface is calculated, and a region having a depth of 30% from the tooth surface or cam surface when the depth is defined as 100% is defined as the surface layer. As a specific example in the case of a gear, for example, a region from a tooth surface to a depth having a value of 10% of the pitch circle radius is defined as a surface layer. Further, as a specific example in the case of a cam, for example, a region from the cam surface to a depth having a value of 10% of the cam effective radius is defined as a surface layer. The square root √α _max of the estimated maximum hole envelope area in the estimation target region set in the surface layer is set as the above range.

  The present invention is not limited to the above embodiment, and for example, a recompression process (for example, a sizing process) may be performed after the sintering process.

  In order to confirm the effect of the present invention, the following evaluation test was performed. In the following tests, Sigmaloy 2010 manufactured by JFE Steel Co., Ltd. was used as the diffusion alloy steel powder. As a lubricant, 0.5 wt% of ACRAWAX C manufactured by Lonza Japan Co., Ltd. was added. Artificial graphite was used as the graphite powder. A test piece was produced through a compression molding process, a sintering process, and a heat treatment process using the mixed powder obtained by mixing these. The test piece had a cylindrical shape with an outer diameter of 23.2 mm, an inner diameter of 16.4 mm, and an axial dimension of 7 mm. The compression molding process was performed at room temperature. The sintering process was performed at 1250 ° C. for 150 minutes in a tray pusher furnace in a nitrogen and hydrogen atmosphere. In the heat treatment step, carburizing treatment was performed under the condition of 880 ° C. × 40 min, followed by quenching at 840 ° C. and tempering under the condition of 180 ° C. × 60 min. In the following description, the sintered body before heat treatment is referred to as “as-sinter product”, and the sintered body after heat treatment is referred to as “carburized product”.

  In each of the following tests, the method for measuring the sintered density was in accordance with JIS Z2501, and the method for measuring the crushing strength was in accordance with JIS Z2507. The crushing strength test conditions were 0.5 mm / min stroke control.

The ring compression fatigue strength was measured by the following method. As shown in FIG. 1, the radius (radius to the center of thickness) of the cylindrical test piece is R, the thickness is h, the axial dimension is d, and the diametrical repeated load W is applied to the test piece. Add until the specimen breaks. The ratio between the maximum value and the minimum value of the repeated load W is set to 0.1. The maximum tensile stress σ _max when no damage occurs even when the repeated load W is continuously applied 1 × 10 ⁷ times is the ring compression fatigue strength of the test piece. The maximum tensile stress σ _max is defined by the following formula 1. In Equation 1, A is the cross-sectional area of the test piece and is represented by A = d · h. The maximum bending moment M is represented by M = 0.318WR. The section modulus κ is expressed by the following formula 2.

The estimated maximum hole envelope area is calculated by the following method. First, it is assumed that the extreme value distribution of pores in the sintered body follows a double exponential distribution. Thereby, the maximum value of the hole envelope area is estimated using extreme value statistics. Specifically, the square root √α _{max of} the estimated maximum hole envelope area is calculated through the following procedure.

First, as shown in FIG. 2, the cylindrical test piece 1 is cut along a plane including an axis, and the cut surface 2 is subjected to mirror polishing. Then, the cut surface 2 of the test piece subjected to mirror polishing is observed with a microscope, and an image of a region of the reference area S ₀ (mm ² ) determined in the estimation target region in the surface layer 3 is acquired. The obtained image is binarized using image analysis software, and the envelope area of the holes is analyzed. Obtained is the largest pore envelope area most large in reference area S ₀ of the of the envelope area, to the square root and √Arufa. The above measurement is repeated n times while changing the inspection region within the estimation target region.

Then, the measured n √αs are arranged in order from the smallest, and are set as √α _j (j = 1 to n), respectively (see Equation 3 below). For each j (= 1 to n), a cumulative distribution function F _j (%) expressed by the following formula 4 and a standardized variable y _j expressed by the following formula 5 are calculated.

√α is taken on the horizontal axis of the extreme probability paper, and the above result is plotted to obtain an extreme value distribution (the vertical axis of the extreme probability paper takes F or y). An approximate straight line obtained by the least square method is extrapolated to the extreme value distribution to obtain a and b represented by the following formula (6). However, y is the normalization variable represented by the following _equation 7, T is the recursion period represented by the following _equation 8, V is the volume (mm ³ ) of the estimation target region 3, and V ₀ is the following _equation 9. The reference volume (mm ³ ) and h are the average value (mm) of the measured √α _max expressed by the following formula 10. Confirm that the plot points at 10 to 85% of the F scale, which is the vertical axis of the extreme value probability sheet, are on the approximate line. Thereby, it can be confirmed that the obtained extreme value distribution follows a double exponential distribution.

Substituting the volume V of the estimation target region into the above equation 8 and the point where the recurring period T and the obtained extreme value distribution intersect is the square root √α _max of the estimated maximum hole envelope area.

In this embodiment, the reference area S ₀ is 0.39 mm ² , the number of inspections n is 32, and the volume V of the estimation target region is 200 mm ³ . The surface layer 3 was a region having a depth of 0.54 mm from the inner peripheral surface of the test piece 1. As for the reference area, the radial dimension was 0.54 mm from the inner peripheral surface of the test piece 1, and the axial dimension was 0.74 mm. The estimation target area is a cylindrical area of 0.54 mm from the inner peripheral surface of the test piece 1, and the axial dimension is 7 mm.

Evaluation criteria of the sintered density, × when less than 7.55 g / cm ^3, when the ^{7.55~7.60g / cm 3 ○, 7.60g /} cm 3 or more when was ◎. The evaluation standard of the crushing strength was × when it was less than 1600 MPa, Δ when it was 1600 to 1750 MPa, ◯ when it was 1750 to 1900 MPa, and ◎ when it was 1900 MPa or more. The evaluation criteria for the ring compression fatigue strength were x when less than 290 MPa, Δ when 290 to 315 MPa, ◯ when 315 to 340 MPa, and 時 when 340 MPa. The evaluation standard of the square root √α _max of the estimated maximum pore envelope area was ◎ when less than 100 μm, ◯ when 100 to 150 μm, Δ when 150 to 200 μm, and × when exceeding 200 μm.

(1) About the amount of carbon added The amount of carbon added was investigated. Specifically, a diffusion alloy steel powder containing 2.0 wt% Ni and 1.0 wt% Mo and graphite powder having a particle size D90 of 6.0 μm are mixed, and the addition amount of the graphite powder is set to 0 to 0.00. Plural kinds of mixed powders having different amounts in the range of 4 wt% were prepared. Each mixed powder was molded at 1200 MPa, sintered, and then subjected to carburizing heat treatment to produce a plurality of types of test pieces. The sintered density (as-sinter product), crushing strength (carburized product), and ring compression fatigue strength (carburized product) of each test piece thus obtained were measured. The results are shown in Table 1 below.

As shown in Table 1, Examples 1 to 3 had a density of 7.55 g / cm ³ and exhibited excellent crushing strength and fatigue strength. From this, it is clear that the addition amount of graphite powder is desirably 0.05 to 0.35 wt%, preferably 0.1 to 0.3 wt%, more preferably 0.15 to 0.25 wt%. became.

(2) About the particle size of graphite powder The particle size of the graphite powder added to mixed powder was investigated. Specifically, a diffusion alloy steel powder containing 2.0 wt% Ni and 1.0 wt% Mo is mixed with 0.2 wt% graphite powder, and the particle size D90 of the graphite powder is set to 4.0 to 25. A plurality of types of mixed powders having different sizes in the range of 0 μm were prepared. Each mixed powder was molded at 1200 MPa, sintered, and then subjected to a carburizing heat treatment to prepare a plurality of types of test pieces. The sintered density (as-sinter product), crushing strength (carburized product), and ring compression fatigue strength (carburized product) of each test piece thus obtained were measured. The results are shown in Table 2 below.

As shown in Table 2, Examples 4 and 5 have a density of 7.55 g / cm ³ . From this, it became clear that the particle size D90 of the graphite powder is desirably 8 μm or less, preferably 6 μm or less, more preferably 4 μm or less.

(3) Molding pressure during compression molding The molding pressure in the compression molding process was investigated. Specifically, a mixed powder obtained by blending 0.2 wt% of graphite powder having a particle diameter D90 of 6.0 μm with a diffusion alloy steel powder containing 2.0 wt% Ni and 1.0 wt% Mo is formed at a molding pressure of 1000. A plurality of types of green compacts were formed by compression molding in a range of ˜1400 MPa, and a plurality of types of test pieces were produced by subjecting each green compact to sintering and carburizing heat treatment. The sintered density of each specimen thus obtained (as-sinter product), the square root of the estimated maximum pore envelope area √α _max (as-sinter product), crushing strength (carburized product), and ring compression fatigue strength (Carburized product) was measured. The results are shown in Table 3.

As shown in Table 3, Examples 6 and 7 had a density of 7.55 g / cm ³ and exhibited excellent mechanical properties (compression strength and fatigue strength). From this, it became clear that the molding pressure is preferably in the range of 1150 to 1350 MPa. Note that Comparative Example 7 could not be measured because a crack occurred in the test piece during compression molding.

(4) Classification of diffusion alloy steel powder The effect of removing coarse particles in the diffusion alloy steel powder was investigated. Specifically, a diffusion alloy steel powder containing 2.0 wt% Ni and 1.0 wt% Mo is passed through a sieve having openings of 106, 180, and 250 μm to obtain a plurality of types of diffusion alloy powder having different classification degrees. It was. A plurality of types of test pieces are obtained by compressing and molding a mixed powder in which 0.2 wt% of graphite powder having a particle diameter D90 of 6.0 μm is blended with each diffusion alloy steel powder at 1200 MPa, followed by sintering and carburizing heat treatment. Was made. The sintered density of each specimen thus obtained (as-sinter product), the square root of the estimated maximum pore envelope area √α _max (as-sinter product), crushing strength (carburized product), and ring compression fatigue strength (Carburized product) was measured. The results are shown in Table 4.

As shown in Table 4, Examples 8 to 10 had a density of 7.55 g / cm ³ and exhibited mechanical properties (compression strength and fatigue strength) superior to those of the comparative example. From this, it became clear that it is desirable to pass the diffusion alloy steel powder through a sieve having an opening of 250 μm or less, preferably an opening of 180 μm or less, more preferably an opening of 106 μm or less.

As shown in Tables 3 and 4, in Examples 6 to 10 having excellent mechanical properties, the square root √α _{max of the} estimated maximum pore envelope area is 200 μm or less. From this, it has become clear that the square root √α _max of the estimated maximum pore envelope area is desirably 200 μm or less, preferably 150 μm or less, more preferably 100 μm or less.

(5) Addition amount of Ni The addition amount of Ni in the alloy steel powder was investigated. Specifically, a plurality of types of diffusion alloy steel powders having Mo addition amount of 1.0 wt% and varying Ni addition amount are prepared, and artificial graphite having a particle diameter D90 of 6.0 μm is prepared for each diffusion alloy steel powder. A mixed powder containing 0.2 wt% was compression molded at 1200 MPa, subjected to sintering and carburizing heat treatment to produce a plurality of types of test pieces. The sintered density (as-sinter product) and crushing strength (carburized product) of each test piece thus obtained were measured. The results are shown in Table 5 below.

As shown in Table 5, Examples 11 to 13 had a density of 7.55 g / cm ³ and exhibited excellent crushing strength. From this, it became clear that the addition amount of Ni is desirably about 1.5 to 2.2 wt%.

(6) Addition amount of Mo The addition amount of Mo in the alloy steel powder was investigated. Specifically, a plurality of types of diffusion alloy steel powders with Ni addition amount of 2.0 wt% and Mo addition amount changed are prepared, and graphite powder having a particle diameter D90 of 6.0 μm is prepared for each diffusion alloy steel powder. The mixed powder containing 0.2 wt% was compression-molded at 1200 MPa, subjected to sintering and carburizing heat treatment, and a plurality of types of test pieces were produced. The sintered density (as-sinter product) and crushing strength (carburized product) of each test piece thus obtained were measured. The results are shown in Table 6 below.

As shown in Table 6, Examples 14 to 16 had a density of 7.55 g / cm ³ and exhibited excellent crushing strength. From this, it became clear that the addition amount of Mo is desirably 0.5 to 1.1 wt%, preferably about 0.8 to 1.1 wt%.

(7) About carbonitriding The effect of carbonitriding was investigated. Specifically, after mixing a diffusion alloy steel powder containing 2.0 wt% Ni and 1.0 wt% Mo and a graphite powder having a particle size D90 of 6.8 μm, each mixed powder was molded at 1176 MPa, Sintering was performed, and carbonitriding was further performed to prepare a plurality of types of test pieces in which the depth of the nitrided layer was varied in the range of 0 to 0.5 mm. The ring compression fatigue strength of the test piece thus obtained was measured. The results are shown in Table 7 below. The evaluation criteria for the ring compression fatigue strength in the carbonitriding treatment were ◎ for 340 to 400 MPa, ◎ for 400 to 500 MPa, and ◎ for a pressure of 500 MPa or more.

  As shown in Table 7, Examples 17 and 18 exhibited excellent ring compression fatigue strength. From this, the depth of the nitrided layer containing 0.05 wt% or more of nitrogen is the depth from the surface when the depth from the surface of the region where the tensile stress is applied when a load is applied to the test piece is 100%. It has become clear that it is preferably 5% or more, more preferably 20% or more.

Claims (10)

A sintered machine part formed using a mixed powder containing diffusion alloy steel powder, having a carbon ratio of 0.35 wt% or less and a density of 7.55 g / cm ³ or more,
A sintered machine part having a square root √α _max of an estimated maximum pore envelope area in an estimation target region set in a surface layer of a predetermined depth from the surface of 200 μm or less.   The sintered machine part according to claim 1, comprising 1.5 to 2.2 wt% of Ni, 0.5 to 1.1 wt% of Mo, and the balance consisting of Fe, the carbon, and inevitable impurities.   3. The sintering according to claim 1, wherein the mixed powder contains graphite powder, and the particle diameter D90 when the cumulative mass from the small diameter side of the particle size distribution on the mass basis of the graphite powder becomes 90% is 8 μm or less. Machine parts.   The sintered machine part according to any one of claims 1 to 3, wherein a recompression process is not performed after the sintering process. A mixing step for obtaining a mixed powder containing diffusion alloy steel powder and 0.35 wt% or less graphite powder; a green compacting step for compressing the mixed powder to obtain a green compact; and Sintering at a sintering temperature to obtain a sintered body having a density of 7.55 g / cm ³ or more, and a cumulative mass from the small diameter side of the particle size distribution on the mass basis of the graphite powder is 90% A method for manufacturing a sintered machine part having a particle diameter D90 of 8 μm or less.   The diffusion alloy steel powder is obtained by diffusion-adhering Ni around the Fe—Mo alloy, containing 1.5 to 2.2 wt% of Ni, 0.5 to 1.1 wt% of Mo, and the balance being Fe The method for manufacturing a sintered machine part according to claim 5, comprising: and inevitable impurities.   The method for manufacturing a sintered machine part according to claim 5 or 6, wherein a recompression step is not performed after the sintering step.   The method for producing a sintered machine part according to any one of claims 5 to 7, wherein a molding pressure in the compression molding step is 1150 to 1350 MPa.   The method for producing a sintered machine part according to any one of claims 5 to 8, wherein the diffusion alloy steel powder is passed through a sieve having an opening of 250 µm or less.   The method for manufacturing a sintered machine part according to any one of claims 5 to 9, wherein carbonitriding is performed after the sintering step.

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