JP2016000854A - Sintering machine part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sintering machine part which is reduced in coarse pores and is high-strength and low-cost.SOLUTION: A gear 1 is formed with an iron-based sintered body obtained by molding and sintering a raw material powder comprising an iron-based coarse powder of an average particle size of 60 μm or greater and an iron-based fine powder of particle sizes smaller than the square root √areaof the estimated maximum pore enveloping area of a sintering sample formed with the coarse powder. The blend ratio of the fine powder in the raw material powder is 5-20 wt.%, and the density of the iron-based sintered body is 7.6 g/cmor higher.

Description

  The present invention relates to sintered machine parts.

  The sintered body can be obtained by filling a mold with a mixed powder containing metal powder and graphite powder, compression molding, and sintering at a predetermined temperature. Therefore, net shape molding or near net shape molding that obtains a state closer to the final product is possible. Further, as compared with the case of cutting the melted material, cost reduction can be achieved by improving the material yield and reducing the number of processing steps. Among sintered bodies, iron-based sintered bodies are particularly widely used as machine parts used in various fields such as automobile parts and industrial machines because of their excellent mechanical properties.

  By the way, many voids remain inside the sintered body. These vacancies act as a stress concentration source and behave like a crack in the molten metal, which causes a reduction in various strengths such as tension, compression, bending strength, impact strength, and fatigue strength. In order to solve this problem, it is effective to increase the density of the sintered body to reduce the porosity. From this viewpoint, various attempts have been made.

  For example, in Patent Document 1, a sintered body is obtained by using a metal powder having a coarse powder and performing a surface densification process such as shot peening without using a costly process such as two-stage molding and two-stage sintering. It is described to increase the density of.

  Patent Document 2 describes that by using a mixed powder containing a coarse powder and a fine powder, the coarse pores of the sintered body are reduced to improve the density and strength.

Special table 2007-537359 gazette Japanese Patent No. 5113555

  However, in patent document 1, the surface densification process is performed after sintering. For this reason, an increase in cost due to an increase in the number of processes becomes a problem, and net shape molding, which is a merit of sintered body parts, cannot be utilized.

  In Patent Document 2, the average particle size of the coarse powder is 50 μm or less, and the average particle size of the fine powder is 25 μm or less, both of which are more than the powder particle size normally used in the field of powder metallurgy (about 100 μm is more). Is also a fine powder. Therefore, the types of powder that can be used are limited, and there is a concern that the material cost will rise.

  In view of the above, an object of the present invention is to provide a low-cost sintered machine component in which the rough atmospheric holes that are the starting points of destruction are made as small as possible.

The present invention relates to a sintered machine part having a load-loading surface to which a load is applied, an iron-based coarse powder having an average particle size of 60 μm or more, and an estimated maximum void of a sintered sample formed from the coarse powder. It is formed of an iron-based sintered body obtained by molding and sintering a raw material powder containing iron-based fine powder having a particle size less than the square root √area _{max of the} envelope area, and the blending amount of the fine powder in the raw material powder is 5 ˜20 wt%, the sintered density of the iron-based sintered body is 7.6 g / cm ³ or more, and the √area _max is set to a depth at which the stress due to the load is applied from the portion corresponding to the load loading surface of the sintered sample. A region up to a depth of 30% when the thickness is 100% is obtained as a predicted volume.

Thus, by using coarse powder and fine powder as iron-based powder, it becomes easy to fill fine powder between the particles of coarse powder. For this reason, the pores remaining in the sintered iron-based sintered body can be reduced to increase the density of sintered machine parts, and the crack propagation starting from the rough atmospheric pores can be further promoted. It becomes possible to suppress destruction and damage of machine parts. In addition, since the particle size of the iron-based fine powder to be added is smaller than the √area _max value of the sintered sample formed of the coarse powder, it is theoretically estimated that it exists in the iron-based sintered body. The particle size of the fine powder is smaller than the large number of coarse pores. Therefore, it is possible to fill all of the rough air holes with fine powder. Therefore, it is possible to reliably prevent the formation of rough atmospheric holes after sintering and increase the strength of the sintered machine part. In addition, it is possible to easily determine the particle size of the fine powder suitable for eliminating the rough atmospheric pores, and it becomes easy to select the powder to be prepared in the raw material powder preparation process.

  Moreover, the particle size of both the coarse powder and fine powder to be used can be made larger than the coarse powder and fine powder used in Patent Document 2. Accordingly, the fluidity of the iron-based powder is improved, and the filling property into the cavity in the molding and compression process is improved. In addition, an increase in material costs can be suppressed.

  Partially diffused alloy steel powder can be used as the coarse powder. As this partial diffusion alloy steel powder, it is preferable to use, for example, an Fe—Ni—Mo system.

  As the fine powder, the same iron-based powder as the coarse powder or an iron-based powder different from the coarse powder can be used.

  According to this invention, generation | occurrence | production of the rough air hole in an iron-type sintered compact can be suppressed at low cost. Thus, since the rough atmospheric holes that can become a stress concentration source and can be a starting point of a crack are reduced, it is possible to provide a sintered machine component having high strength at low cost.

It is a front view which shows the gear which is an example of a machine component. It is a table | surface which shows the test result which verified the change of (root) area _max when changing the particle size and addition amount of a fine powder. It is a table | surface which shows the evaluation criteria of a sintered density. It is a table | surface which shows the evaluation criteria of (square) area _max value. 2 is a photomicrograph of each test piece of Example 3 and Comparative Example 1.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1A shows a gear as an example of a sintered machine part. The gear 1 has a plurality of tooth surfaces 1a as load-loading surfaces that transmit torque. A shaft is fixed to the inner peripheral surface 1b of the gear 1, or the shaft is rotatably fitted.

  The gear 1 of this embodiment is formed of an iron-based sintered body. The gear 1 includes a raw material powder preparation step for preparing a raw material powder, a molding and compression step for compressing and molding the raw material powder to form a green compact, and a sintering for sintering the green compact by heating it at a sintering temperature or higher. Manufactured through a kneading step and a surface treatment step.

[Raw material powder preparation process]
In the raw material powder preparation step, a raw material powder containing iron-based powder, carbon powder as a carbon solid solution source, and a molding lubricant responsible for lubrication during molding is manufactured.

  As a typical example of the iron-based powder mentioned here, a low alloy steel powder containing Fe and another metal (alloy component) alloyed with Fe can be given. As an alloy component of the low alloy steel powder, one or more kinds of metals among Ni, Mo, Mn, and Cr can be used. For example, Ni and Mo are contained as an alloy component, and the balance is Fe and inevitable impurities. The low-alloy steel powder can be 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. As the iron-based powder, pure iron powder, stainless steel powder, high-speed steel powder and the like can be used in addition to the low alloy steel powder.

  As a specific example of the low alloy steel powder, it is preferable to use a Fe—Ni—Mo based partial diffusion alloy steel powder containing Ni and Mo, the balance being Fe and inevitable impurities. This partially diffused alloy steel powder is obtained by diffusion-bonding Ni around the Fe—Mo alloy. Thus, the hardness of the alloy steel powder before sintering is higher than that of steel powder (pre-alloyed steel powder) in which Fe and Ni are completely alloyed by diffusing and attaching a metal such as Ni to the Fe alloy. Therefore, the moldability at the time of compression molding is ensured. As a result, a relatively large amount of Ni can be blended. Specifically, the mixing ratio of Ni in the partial diffusion alloy steel powder of the present embodiment is 0.5 to 5.0 wt%, preferably 1.5 to 2.2 wt%, more 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 compounding ratio of Mo in the partial diffusion alloy steel powder is 0.5 to 3.0 wt%, preferably 0.8 to 1.1 wt%, and more preferably 0.9 to 1.1 wt%.

  As the iron powder used as the base of the partially diffused alloy steel powder, there are atomized powder, reduced powder, and the like. However, since the reduced powder has a porous particle and is difficult to increase in density, in this embodiment, pores are not formed. Solid atomized powder that does not have, especially water atomized powder is used in consideration of cost. In addition, as an example of the partial diffusion alloy steel powder, the one in which Ni powder is diffusion bonded around the Fe-Mo alloy powder is exemplified, but the alloy powder in which Ni or Mo is diffusion bonded around the pure iron powder is used. You can also

  This partial diffusion alloy steel powder is generally soft and has the same hardness as pure iron powder. As a measure of the hardness of the partially diffused alloy steel powder, a micro Vickers hardness of less than 120HV0.05, desirably less than 100HV0.05, more preferably less than 90HV0.05 is used. This hardness is lower than the particle hardness (approximately 120 HV 0.05 or more) in the Fe—Cr—Mo based fully alloy steel powder (pre-alloy powder). Therefore, compared to this type of complete alloy steel powder, it becomes easier to increase the density even with the same applied pressure.

In the present invention, coarse powder having a large particle size and fine powder having a small particle size are used as the iron-based powder. Among these, as the coarse powder, an iron-based powder having an average particle diameter of 60 μm or more, preferably 70 μm or more and 130 μm or less, more preferably 80 μm or more and 110 μm or less is used. If the average particle size is too small, it is difficult to use a general-purpose iron-based powder, resulting in an increase in cost. On the other hand, if the average particle size is too large, a large amount of coarse powder is contained, so that the filling property in the molding and compression step described later is deteriorated, and coarse air holes are easily generated after sintering. The average particle diameter can be measured based on, for example, a laser diffraction scattering method. In this measurement method, a particle group is irradiated with a laser beam, and a particle size distribution and further an average particle size are obtained by calculation from the intensity distribution pattern of diffracted / scattered light emitted from the particle group. SALD 31000 from the factory can be used. On the other hand, as the fine powder, one having a particle diameter smaller than the square root √area _max of the estimated maximum pore envelope area of the sintered sample formed only with the coarse powder is used. The square root √area _max of the estimated maximum hole envelope area is the square root of the envelope area of the maximum hole estimated to be inherent in the predicted volume, and will be described in detail later.

  As the coarse powder and the fine powder, the same iron-based powder can be used, and different iron-based powders can also be used. “Different” as used herein refers to the case where either one or both of the types of alloying elements and the mixing ratio are different, or the form of powder (whether it is a complete alloy steel powder or a partial diffusion alloy steel powder) Etc.) are also included.

  As the carbon powder, for example, artificial graphite powder is used. As the graphite powder, one having a particle diameter D90 of 10 μm or less is used, preferably 8 μm or less. The graphite powder having a particle diameter D90 of 3 μm or more, preferably 4 μm or more is used. The blending ratio of the graphite powder is 0.3 wt% or less, preferably 0.25 wt% or less with respect to the entire mixed powder. The blending ratio of the graphite powder is 0.05 wt% or more, preferably 0.1 wt% or more with respect to the entire mixed powder. As the carbon powder, carbon black, ketjen black, nanocarbon powder and the like can be used in addition to the graphite powder. Two or more of these powders can be used.

  The molding lubricant is added for the purpose of reducing the friction between the mold and the powder when the mixed powder is compression-molded or between the powders. As the molding lubricant, known lubricant powders such as metal soap (for example, zinc stearate) and amide wax (for example, ethylene bisstearyl amide) can be arbitrarily selected and used. In addition, these lubricants are dispersed in a solvent to form a solution, and this solution is sprayed on the raw material powder, or the raw material powder is immersed in this solution and then the solvent component is volatilized and removed. Good. In fulfilling the object of the present invention, any kind of lubricant powder can be used as long as it does not remain in the raw material after sintering. Two or more types of molding lubricants can be used in combination.

[Molding compression process]
In the molding and compression step, the raw material powder is charged and filled in the cavity of the molding die and compressed to form a green compact having a shape corresponding to the final shape of the gear 1. The molding at this time is preferably performed by a molding machine suitable for continuous production such as uniaxial and multiaxial pressure molding, CNC press molding, and the like. The molding temperature is preferably room temperature or higher and not higher than the melting point of the lubricant. In particular, when the molding is performed at a temperature 10 to 20 ° lower than the melting point of the molding lubricant, the yield strength of the powder is reduced and the compressibility is increased, so that the molding density can be increased. For further densification, it is possible to employ warm molding in which the mold and powder are heated to 60 ° C. or more for molding. If necessary, a lubricant may be held on the mold surface, or a film (DLC film or the like) for reducing friction may be coated on the mold surface.

When the molding pressure in the molding compression process is increased, the density of the green compact can be increased. On the other hand, if the molding pressure is too high, lamination (laminar peeling) due to density unevenness or damage to the mold occurs inside the green compact. Considering the above, in this embodiment, the molding pressure is set to about 1150 to 1350 MPa. The density (true density) of the green compact thus obtained is 7.4 g / cm ³ or more.

[Sintering process]
Next, after removing the molding lubricant contained in the green compact by degreasing, the green compact is heated above the sintering temperature in the sintering step to form a sintered body. The sintering temperature is set within the range of 1100 ° C. or higher and 1300 ° C. so that a dense sintered body having small pores can be obtained. Further, in order to prevent a decrease in sinterability and strength due to oxidation and decarburization, it is preferable to sinter in an inert or reducing atmosphere mainly containing nitrogen, hydrogen, argon or the like. In addition, it can also be sintered under vacuum. By sintering the green compact, the carbon powder in the green compact dissolves in the iron-based powder, and the portion where the graphite powder is present becomes a void. At the same time, the entire green compact contracts by sintering and bonding the particles of the iron-based 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 carbon powder, and the density of the sintered body becomes higher than the density of the green compact. The sintered body has a true density of 7.6 g / cm ³ or more and a relative density of 90% or more (preferably 95% or more, more preferably 97% or more).

[Surface treatment process]
The sintered body that has undergone the sintering process is transferred to the surface treatment process and subjected to various surface treatments such as quenching and tempering. As an example of the surface treatment, a carburizing quenching and tempering treatment can be given. By carburizing, quenching and tempering, the surface of the gear 1 including the tooth surface 1a is hardened and internal toughness is ensured, which is effective in suppressing crack propagation. In addition to carburizing quenching and tempering, various heat treatments such as full quenching and tempering, induction quenching and tempering, carbonitriding, and vacuum carburizing can also be performed. Besides this, hard coating is formed by nitriding treatment, soft nitriding treatment, sulfuration treatment, diamond-like carbon treatment (DLC), etc., or it is prevented by resin coating formation, various plating treatments, black dyeing treatment treatment, steam treatment, etc. Rust treatment can also be performed. If necessary, a plurality of the surface treatments exemplified above can be combined.

  The gear 1 which consists of an iron-type sintered compact is completed according to the above process. The ratio of each element contained in the gear 1 is the ratio described in the raw material powder preparation step (for example, Ni is 1.5 to 2.2 wt%, Mo is 0.5 to 1.1 wt%, and carbon is 0.05 to 0.35 wt%, and the balance is composed of Fe and inevitable impurities). This process enables net shape molding or near net shape molding, thereby reducing the cost of sintered machine parts. Moreover, since this manufacturing process is 1 time shaping | molding and 1 time sintering, a manufacturing process and manufacturing equipment can be simplified.

  In addition, if necessary, a recompression process (for example, a sizing process) can be performed after the sintering process and before the surface treatment process.

  [Squared root of estimated maximum hole envelope area]

  Of the sintered machine parts, the presence or absence of rough air holes around the load surface (tooth surface 1a in the case of gear 1) on which a large load is applied is considered to have a great influence on the durable life of the machine parts. Therefore, in order to evaluate the durable life of the machine part, it is desirable to quantify the degree of the presence of the rough air hole in some form. As one means for quantification, it is conceivable to define the density (true density or relative density) of the sintered body.

  However, even though the density is an effective measure for evaluating the degree of densification of the entire part, it is not always effective for evaluating the presence or absence of rough atmospheric holes in a region limited to the vicinity of the load surface. For example, even if the density of the entire part exceeds the lower limit value, there may be a small number of coarse air holes around the tooth surface 1a that is the load-loading surface, but this rough air hole may be the starting point of the crack. Although it is conceivable to evaluate the presence or absence of rough atmospheric pores with the density of the region limited to the periphery of the load surface of the sintered machine part, it is not easy to accurately measure the density of such a partial region.

Based on the above verification, the present invention pays attention to the square root √area _max of the envelope area of the largest hole estimated to be inherent in the region of the predicted volume including the load surface, among the sintered machine parts. It was decided to evaluate the degree of coarse air holes present in the predicted volume. Details of the √area _max value estimation method will be described below.

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 √area _{max of} the estimated maximum hole envelope area is calculated through the following procedure.

The specimen subjected to mirror polishing is observed with a microscope, and an image of a y region having a predetermined reference area So (mm ² ) is acquired. The obtained image is binarized using image analysis software, and the envelope area of the holes is analyzed. The largest envelope area among the obtained envelope areas is defined as the maximum hole envelope area in the reference area So, and the square root thereof is defined as √area _max in the region. This measurement is repeated n times while changing the inspection region.

Measured n √area _max are arranged in order from the smallest, and √area _{max, j} (j = 1 to n), respectively. (See Formula 1)

For each j (j = 1 to n), a cumulative distribution function F _j (%) expressed by Expression 2 and a standardized variable y _j expressed by Expression 3 are calculated.

Taking √area _max on the coordinate horizontal axis of the extreme value probability sheet, the above result is plotted to obtain an extreme value distribution. (The vertical axis of the extreme probability sheet takes F or y)
An approximate straight line obtained by the least square method is extrapolated with respect to the extreme value distribution to obtain a and b represented by Expression 4. However, y is the normalization variable represented by Formula 5, T is the recursion period represented by Formula 6, V is the volume of the estimation target region (predicted volume: mm ³ ), and V ₀ is the standard represented by Formula 7. Volume (mm ³ ), h is an average value (mm) of the measured √area _{max, j} represented by Formula 8.

It is confirmed 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 an approximate straight line. Thereby, it can be confirmed that the obtained extreme value distribution follows a double exponential distribution. Substituting the volume V (predicted volume) of the estimation target region into Equation 6, the point where the recurring period T and the obtained extreme value distribution intersect is the square root √area _max of the estimated maximum hole envelope area.

[Fine powder used in the present invention]
As described above, the particle size of the fine powder is determined to be less than the square root √area _max of the estimated maximum pore envelope area of the sintered sample formed only with the coarse powder. Hereinafter, the procedure for determining the particle size will be described in detail.

  First, a sintered sample having the same shape as the gear 1 that is the final product is manufactured using the above-described raw material powder (a powder made of a coarse powder, a carbon powder, and a molding lubricant) that does not contain fine powder. Molding compression and sintering at the time of manufacturing the sintered sample are performed under the same conditions as the molding compression process and sintering process at the time of manufacturing the final product gear 1.

Next, √area _max of this sintered sample is obtained by the above-described procedure. At this time, the reference area So is, for example, a depth of 30% when the depth from the portion corresponding to the tooth surface of the gear-shaped sintered sample to the tensile stress generated by the load accompanying torque transmission is 100%. , The horizontal is 1.33 times the vertical, and the vertical dimension is multiplied by the horizontal dimension. The predicted volume V is 30% when the depth until the tensile stress is applied in the depth direction from the tooth surface equivalent part of the sintered sample is 100%, and the tensile volume is within the tooth surface. The volume is within the range where stress acts (particularly near the tooth root). For example, the number of inspections n is 32.

Fine powder can be obtained by sieving the iron-based powder using a sieve having an opening slightly smaller than √area _max determined in this way, and collecting the fine powder that has passed through this sieve. Mesh sieve, so are stepwise standardized by JIS Z8801, less than} area _max, and preferably for collecting the fine powder using a sieve having a nearest mesh to} area _max . Usually, √area _max is in the range of 30 μm to 70 μm, but in this range, the openings 32 μm, 38 μm, 45 μm, 53 μm, 63 μm are standardized. The powder will be collected.

Thus, by using both coarse powder and fine powder as iron-based powder, it becomes easy to fill the fine powder between the particles of the coarse powder. For this reason, the pores remaining in the sintered iron-based sintered body can be reduced to increase the density of the gear 1, crack growth starting from the rough atmospheric pores, and further the destruction of the gear 1 due to it. -It becomes possible to suppress damage. In particular, in the present invention, paying attention to the √area _max of the region including the portion corresponding to the load surface of the sintered sample formed from the coarse powder (particularly the portion corresponding to the maximum load surface), the particle size of the fine powder to be blended is Since it is smaller than this √area _max , theoretically, all fine powders are smaller than the coarse air holes estimated to be present in the iron-based sintered body, so the coarse air holes must be filled with fine powders reliably. Can do. As a result, the number of coarse air holes after sintering can be reduced, and the situation where the rough air holes become a stress concentration source and become the starting point of a crack can be reliably prevented. In addition, by paying attention to the √area _max value, it is possible to easily determine the particle size of the fine powder that is suitable for eliminating the coarse pores. An advantage that is easy is also obtained. In addition, if the √area _max of the iron-based sintered body is obtained afterwards, it is possible to accurately evaluate the superiority or inferiority of various sintered bodies from the magnitude relationship of the √area _max even if the density is the same level. It becomes.

  Further, a coarse powder having an average particle diameter of 60 μm or more (preferably 70 μm or more and 130 μm or less, more preferably 80 μm or more and 110 μm or less) is used, and a fine powder having passed through a sieve having an opening of 32 μm to 68 μm is used. Therefore, it is possible to use a powder having a larger particle size than the coarse powder and fine powder used in Patent Document 2 (the coarse powder has an average particle size of 50 μm or less and the fine powder has an average particle size of 1 to 25 μm). Accordingly, the fluidity of the iron-based powder is improved, and the filling property into the cavity in the molding and compression process is improved. In addition, an increase in material costs can be suppressed.

  By the way, when coarse powder and fine powder are used as iron-based powder as in the present invention, depending on the blending ratio and particle size of the fine powder, the size of coarse air holes generated in the sintered body, and eventually iron-based sintering. Body strength is expected to change. In order to clarify this relationship, the following evaluation test was conducted.

[Test pieces]
As an iron-based powder, partially diffused alloy steel powder (Sigmaloy 2010 manufactured by JFE Steel Co., Ltd.) containing 2 wt% Ni and 1 wt% Mo and the balance being iron and inevitable impurities is used. The partially diffused alloy steel powder is sieved with a sieve having an opening of 150 μm to 250 μm (for example, 180 μm), and the powder passing through the sieve is collected and used as a coarse powder (average particle size of about 90 μm to 100 μm). . Further, the same partially diffused alloy steel powder is sieved with a sieve having an opening of 32 μm, 45 μm, or 63 μm, and powders having a particle size of 32 μm or less, 45 μm or less, and 63 μm or less that have passed through the sieve are collected. Are used as multiple types of fine powders. The fine powder of each particle size described in the table | surface of FIG. 2 is added to a coarse powder in the mixing | blending ratio of the same table | surface, and several types of mixed powder is prepared. Next, ethylene bisstearylamide (ACRAWAX C, manufactured by Lonza Japan Co., Ltd.) is used as a molding lubricant, and this is dispersed in an alcohol solvent (Solmix AP-7 manufactured by Japan Alcohol Sales Co., Ltd.) while applying heat. It mixes with each of a plurality of types of mixed powders, volatilizes the alcohol solvent, and uniformly coats the iron-based powder with the molding lubricant. To this, graphite powder (TIMREX F-10, manufactured by TIMCAL) as a carbon solid solution source was added at a rate of 0.2 wt%, and the resulting mixture was used as the raw material powder.

  Each raw material powder is compression-molded at a pressure of 1176 MPa to produce a ring-shaped green compact having an outer diameter of φ23.2 mm, an inner diameter of φ16.4 mm, and an axial dimension of 7 mm. During the compression molding, the mold and the raw material powder are heated to 120 ° C. Further, the outer periphery and the inner periphery of the mold are sprayed with the alcohol-based solvent in which the molding lubricant is dispersed, and a lubricant film is formed on the surface to perform mold lubrication molding. Next, the test pieces of Examples 1 to 6 shown in FIG. 2 are obtained by sintering this ring-shaped green compact in an argon gas atmosphere at a maximum temperature of 1300 ° C. and a maximum temperature holding time of 200 minutes.

  Comparative examples 1 were obtained by sintering a raw material powder consisting of only the same coarse powder as in the example, and a raw material powder consisting of only the same fine powder (particle size of 32 μm or less) as in the example. And it was set as Comparative Example 2. In addition, while using a mixed powder of a coarse powder and a fine powder (particle size of 32 μm or less) as an iron-based powder, a powder with a reduced amount of fine powder (2 wt%) is used as Comparative Example 3, and the fine powder is blended. A larger amount (30 wt%) was used as Comparative Example 4. Further, Comparative Example 5 was prepared by using a mixed powder of coarse powder and fine powder as the iron-based powder and increasing the particle diameter of the fine powder (particle diameter of 63 μm or less). The preparation procedure of raw material powder, compression molding conditions, sintering conditions, etc. in each comparative example are the same as those in Examples 1-6.

Here, Comparative Example 1 corresponds to the sintered sample formed only of the coarse powder without including the fine powder. With respect to this sintered sample, the √area _max value was determined by the procedure described above, and a result of 60 μm was obtained. Therefore, the particle size of the fine powder used in Examples 1 to 5 is lower than the √area _max value of the sintered sample (Comparative Example 1), but the particle size of the fine powder used in Comparative Example 5 is √area _max. It will exceed the value.

After undergoing the above preparation, the sintered density (true density) was measured for each of the sintered test pieces of Examples 1 to 6 and Comparative Examples 2 to 5, and the √area _max value was determined. The sintered density is measured in accordance with JISZ2501. The procedure for obtaining the √area _max value is the same as the procedure already described. At this time, the reference area So was set to 0.39 mm ² , the number of inspections n was set to 32 times, and the predicted volume V was set to 200 mm ³ . The reference area So is a dimension of 0.54 mm from the inner surface of the test piece, which is 30% of the length when the depth at which the tensile stress extends in the depth direction from the surface layer of the test piece is 100%. The reference area So is obtained by multiplying the vertical dimension and the horizontal dimension as a dimension of 0.74 mm which is 1.33 times the vertical dimension. Moreover, the predicted volume V is a 30% region when the depth of the tensile stress in the depth direction from the surface layer of the test piece is 100%, and is an area of a cylindrical region of 0.53 mm from the inner surface of the test piece, It is obtained by multiplying the axial dimension of 7 mm.

The sintered density and √area _max in the sintered test pieces of Examples 1 to 6 and Comparative Examples 1 to 5 are shown in FIG. The evaluation criteria for the sintered density and the √area _max value in FIG. 2 are as shown in FIGS. 3 and 4. As shown in FIG. 4, the √area _max value of the test piece is less than 60 μm, preferably less than 50 μm, more preferably less than 40 μm. Note that the numerical values (32 μm, 45 μm, 63 μm) in the “fine powder particle size” column in FIG. 2 are fine powders obtained by passing through a sieve having an opening of 32 μm, 45 μm, or 63 μm, respectively. It represents.

As is apparent from the table of FIG. 2, a sintered density of 7.60 g / cm ³ or more is obtained in all examples and comparative examples, but in terms of the value of √area _max , Examples 1 to 6 are comparative. It was found that the size of the coarse air holes can be made smaller than those of Comparative Examples 1 to 5 with the compositions of Examples 1 to 6 smaller than Examples 1 to 5. Since it has already been clarified that the strength of the sintered body is improved as the √area _max value is smaller, it is clear that the strength of the sintered machine part can be increased with the compositions of Examples 1 to 5. It became. This result also means that the internal pore diameter is not uniform even if the sintered density is above a certain level.

  When actually taking and observing micrographs of the test pieces of Example 3 and Comparative Example 1 (cross-sectional pictures in the vicinity of the inner peripheral surface in the axial central portion of the sintered test piece), the execution shown in FIG. In the sintered test piece of Example 3, it was confirmed that the rough atmospheric holes P included in the sintered test piece of Comparative Example 1 shown in FIG.

Further, from the comparison of Example 3, Example 6 and Comparative Example 5 in FIG. 2, the fine powder having a particle size smaller than the √area _max value of the sintered sample (Comparative Example 1) is used as the particle size of the fine powder. In Examples 3 and 6 used, it can be understood that the coarse pores are smaller than those in Comparative Example 5 in which fine powder having a particle size larger than this is used. Therefore, it is necessary to use a fine powder having a particle size smaller than √area _{max of the} sintered sample. In this case, if at least the maximum particle size of the fine powder is less than 60 μm, it is considered that a certain effect is observed in reducing the √area _max value of the iron-based sintered body. Of course, if the maximum particle size of the fine powder is made smaller (preferably the maximum particle size is less than 50 μm, more preferably less than 40 μm), the √area _max value of the iron-based sintered body can be made smaller, The strength of the machined machine parts can be further increased. Furthermore, from comparison between Examples 1 to 5 and Comparative Examples 3 and 4, it was found that the blending ratio of the fine powder in the raw material powder is preferably 5 to 20 wt% (preferably 8 to 15 wt%).

As described above, when the present invention is applied to the gear 1, the depth to which the tensile stress (particularly the maximum tensile stress) caused by the torque transmission from the tooth surface 1a in the depth direction is calculated to calculate the reference area So or The predicted volume V is set and the √area _max value is obtained.

  In the above description, the case where the entire mechanical part such as the gear 1 is formed of an iron-based sintered body having the same composition has been exemplified. However, in the present invention, a part of the mechanical part is made of other materials. The same can be applied to such cases. For example, when the gear 1 shown in FIG. 1 is used as an idle gear, in order to improve the slidability with the shaft, the inner diameter side portion is configured with a low friction sleeve from the broken line shown in FIG. In some cases, the sleeve is fixed to and integrated with the gear body on the outer diameter side, but in this case, the present invention can be applied to the gear body excluding the sleeve on the inner diameter side.

Further, the present invention can be applied to various parts such as a cam, a planetary carrier, a sprocket, a clutch member, etc. regardless of the gear 1 as long as it is a mechanical part that requires strength. For any mechanical part, calculate the depth at which the stress (compressive stress in the case of a cam) due to the load extends in the depth direction from the load-bearing surface (for example, the cam surface in the case of a cam) that receives a large load. Thus, the reference area So and the predicted volume V are set, and the √area _max value is evaluated.

1 Gear 1a Tooth surface (Load load surface)
1b Inner peripheral surface P Rough air hole

Claims (5)

A sintered machine part having a load surface to which a load is applied,
Raw material powder comprising an iron-based coarse powder having an average particle size of 60 μm or more and an iron-based fine powder having a particle size less than the square root √area _max of the estimated maximum pore envelope area of a sintered sample formed of the coarse powder Is formed by sintering and sintering an iron-based sintered body, the amount of fine powder in the raw material powder is 5 to 20 wt%, and the sintered density of the iron-based sintered body is 7.6 g / cm ³ or more. And
The √area _max was determined as a predicted volume from the portion corresponding to the load-loading surface of the sintered sample to a depth of 30% when the depth at which the stress due to the load reaches 100%. Sintered mechanical parts characterized by   The sintered machine part according to claim 1, wherein partially diffused alloy steel powder is used as the coarse powder.   The sintered machine part according to claim 2, wherein an Fe-Ni-Mo system is used as the partial diffusion alloy steel powder.   The sintered machine part according to any one of claims 1 to 3, wherein the fine powder is the same iron-based powder as the coarse powder.   The sintered machine part according to any one of claims 1 to 3, wherein an iron-based powder different from the coarse powder is used as the fine powder.