Hard particles for incorporation into sintered alloys and wear-resistant iron-based sintered alloys and manufacturing processes therefor

Abstract

Hard particles for incorporation into a sintered alloy, consisting of: more than 25 to 60% Mo by mass; 3 to 15 mass% Mn; and the remainder consisting of Fe and unavoidable impurities.

Description

STATE OF THE ART 1. Field of the invention

[0001] The invention relates to hard particles suitable for incorporation into sintered alloys. The invention particularly relates to hard particles suitable for increasing the wear resistance of sintered alloys, to a wear-resistant iron-based sintered alloy containing the hard particles, and to a process for producing this sintered alloy. 2. Description of the state of the art

[0002] Sintered alloys with an iron-containing matrix are commonly used in valve seats, for example. Hard particles can be incorporated into sintered alloys to further increase the wear resistance of the sintered alloy. Generally, the hard particles are incorporated into sintered alloys as follows: A powder of the hard particles is mixed into a powder containing a low-alloy or stainless steel composition to create a mixed powder. This mixed powder is used to form a green compact. The green compact is then sintered to produce the sintered alloy.

[0003] JP 2001 - 181 807 A describes hard particles containing, expressed in mass %, Mo: 20 to 60%, C: 0.2 to 3%, Ni: 5 to 40%, Mn: 1 to 15%, and Cr: 0.1 to 10%, with the remainder being Fe and unavoidable impurities. It also states that Co, for example, can be added to these hard particles.

[0004] Using these hard particles, the adhesion between the hard particles and the ferrous matrix, which is the base material, can be improved when producing a sintered alloy with an iron-containing matrix. Adhesive wear can also be inhibited because the Mo forms a thin oxidation layer on the hard particles.

[0005] The amount of Mo in solid solution in the hard particles can be increased by adding Ni to the hard particles, as described in JP 2001-181807 A. This has the effect of improving the oxidation properties of the added Mo and can thus improve wear resistance. In addition, Co has a low stacking fault energy, and due to this, the addition of Co to the hard particles can increase the hardness of the hard particles and improve wear resistance. However, if the hardness of the hard particles has been increased by the addition of Co before densification, the formability into the green compact can be impaired. In addition, Ni and Co are more expensive than other elements, which leads to high raw material costs for hard particles to which Ni and / or Co have been added.

[0006] Taking these points into account, ferro-molybdenum (Fe-Mo-Si) hard particles, for example, are low-cost because they do not contain cobalt or nickel. In the case of ferro-molybdenum (Fe-Mo-Si) hard particles, the hard particles themselves have high hardness due to the incorporation of Si. However, when ferro-molybdenum is compacted and sintered with an iron-based powder as the matrix, a thin Si oxidation layer is formed. The formation of the thin Si oxidation layer can result in the inhibition of solid-liquid diffusion between the hard particles and the iron-containing matrix during sintering. The adhesion strength of the hard particles to the iron-containing matrix can then be lower, and the wear resistance of the sintered alloy can be reduced. Since the oxidation of Mo is inhibited by the oxidation of Si, the formation of a thin Mo oxidation layer on the hard particles is also hindered.Consequently, adhesive wear may ultimately be promoted due to cracking of the thin Si oxidation layer during sliding due to exposed iron.

[0007] DE 24 56 781 A1 discloses a ferroalloy containing mixed carbides, which contains up to 25 mass% each of chromium, molybdenum and manganese individually or together, as well as up to 10 mass% carbon, the remainder being iron. SUMMARY OF THE INVENTION

[0008] The invention provides hard particles for incorporation into sintered alloys that can cost-effectively increase the wear resistance of the sintered alloy, which is achieved by sintering the green compact, while increasing the formability of the green compact prior to sintering. The invention also provides a wear-resistant iron-based sintered alloy containing the hard particles and a process for producing this sintered alloy.

[0009] It is desirable to increase the amount of C in solid solution to increase the hardness of the hard particles incorporated into a sintered alloy without using Co. However, if the amount of C in solid solution is increased during the preparation of the hard particles, a carbide will form with Mo, and consequently, the generation of Mo oxide may be inhibited. Furthermore, if the hardness of the hard particles before compaction is too high, formability during compaction will be impaired, and consequently, the mechanical strength of the resulting sintered alloy may ultimately decrease.

[0010] A first embodiment of the invention relates to hard particles for incorporation into sintered alloys. The hard particles, which are incorporated as a starting material in the sintered alloy, consist of more than 25 to 60 mass% Mo, 3 to 15 mass% Mn, and the remainder consisting of Fe and unavoidable impurities.

[0011] Since the hard particles do not contain C or Co, the hard particles according to the invention are softer than conventional hard particles for incorporation into sintered alloys. Consequently, the density of the molded part is increased during densification and the contact area with the iron-based powder, which is the matrix starting material, is increased. Consequently, the diffusion of iron from the iron-containing matrix into the hard particles is increased. The adhesion of the hard particles to the iron-containing matrix is ​​thus enhanced, and the mechanical strength of the sintered alloy can then be increased.

[0012] The Mo present in the hard particles forms Mo carbide, which increases the hardness of the hard particles and improves wear resistance. Since the Mo carbide and the Mo in solid solution in the hard particles form a thin Mo oxidation layer, the Mo is effective in increasing solid lubricity. If the amount of Mo is less than the above-specified lower limit, the solid lubricity is insufficient due to the thin Mo oxidation layer on the hard particles, and adhesive wear of the sintered alloy is promoted. If the above-specified upper limit is exceeded, the adhesion with the iron-containing matrix decreases during sintering. This leads to a decrease in the mechanical strength of the sintered alloy.

[0013] During sintering, the Mn present in the hard particles diffuses efficiently from the hard particles into the matrix of the sintered alloy and is therefore effective in improving the adhesion between the hard particles and the matrix. The Mn also brings about an increase in austenite in the matrix.

[0014] If the Mn content is much lower than the above-specified lower limit, little diffusion into the matrix occurs, and the adhesion between the hard particles and the matrix is ​​reduced. The density of the sintered alloy decreases if the Mn content is much higher than the above-specified upper limit.

[0015] A second embodiment of the invention relates to hard particles for incorporation into sintered alloys. The hard particles incorporated as starting material in the sintered alloy consist of more than 25 to 60 mass% Mo, 3 to 15 mass% Mn, more than 0.01 to 0.5 mass% C, and the remainder consisting of Fe and unavoidable impurities.

[0016] The formation of carbide by C and Mo in the hard particles is inhibited by limiting the amount of C addition to no more than 0.5 mass%. Consequently, the amount of Mo in solid solution in the hard particles can be increased even without the addition of Ni.

[0017] If the amount of C added exceeds 0.5 mass%, carbide formation easily occurs between Mo and C. As a result, the hard particles become harder, resulting in impaired compactability and reduced adhesion to the iron-containing matrix. Consequently, the mechanical strength of the sintered alloy may decrease.

[0018] A third embodiment relates to a wear-resistant iron-based sintered alloy obtained by mixing a powder composed of the above hard particles with an iron-based powder that serves as the matrix to disperse the hard particles, and then sintering. The sintered alloy contains 15 to 60 mass% of hard particles.

[0019] According to the third aspect, both the mechanical strength of the sintered alloy and its wear resistance can be improved because the sintered alloy contains 15 to 60 mass% of hard particles with respect to the sintered alloy.

[0020] If the hard particle content of the sintered alloy is less than 15 mass%, the wear-resistant effect of the hard particles may not be sufficiently manifested due to insufficient hard particle content. On the other hand, if the hard particle content of the sintered alloy exceeds 60 mass%, the proportion of the iron-containing matrix will be lower. This may make it impossible to retain the hard particles in the sintered alloy with sufficient bonding strength. Consequently, in a wear-generating environment, such as a contact / sliding environment, the hard particles may escape from the sintered alloy, and this may promote wear of the sintered alloy.

[0021] A fourth aspect relates to a method for producing a wear-resistant iron-based sintered alloy, which uses the above-described hard particles for incorporation into sintered alloys. In the method according to the fourth aspect for producing a sintered alloy, a mixed powder is prepared by mixing an iron-based powder, which will become the matrix, with 0.2 to 2 mass % of graphite powder and 15 to 60 mass % of a powder composed of the above-described hard particles. The mixed powder is compacted and then sintered while carbon (C) in the graphite powder diffuses into the hard particles.

[0022] The wear resistance and mechanical strength of the sintered alloy are enhanced by increasing the hard particle powder content to 15 to 60 mass% according to this manufacturing process. Furthermore, the hardness of the hard particles can be increased because the carbon in the graphite powder diffuses into the hard particles.

[0023] A valve seat can be formed using the sintered alloy constructed in this way. In a high-temperature environment, a mixed wear mode consisting of adhesive wear during contact, as described above, and abrasive wear during sliding can occur. Even in such a case, the hardness of the hard particles for the valve seat can be increased without compromising the existing solid lubrication properties of the hard particles. This can significantly improve the wear resistance of the valve seat compared to that previously available.

[0024] The invention can inexpensively increase the wear resistance of the sintered alloy, which is provided by sintering the green compact, while increasing the formability to form the green compact before sintering. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The features, advantages and technical and industrial significance of exemplary embodiments of the invention are described below with reference to the accompanying drawings, in which like numerals designate like elements, wherein: Fig. 1 is a table to describe the hard particles according to Examples 1 to 7 and Comparative Examples 1 to 6; Fig. 2 is a table to describe the sintered alloys according to Examples 8 to 25 and Comparative Examples 7 to 17; and Fig. 3 is a diagram to describe the wear test in the examples and comparative examples. DETAILED DESCRIPTION OF EMBODIMENTS

[0026] The hard particles of this embodiment are hard particles for incorporation into a sintered alloy, which are incorporated as a starting material in a sintered alloy. The hard particles of this embodiment have a higher hardness than the matrix of the sintered alloy. The hard particles are composed of more than 25 to 60 mass% Mo and 3 to 15 mass% Mn, with the remainder being Fe and unavoidable impurities.

[0027] These hard particles can be produced by an atomization process in which a melt with the above-mentioned composition is atomized. Another method for producing hard particles involves converting a solid, formed by solidifying the melt, into a powder by mechanical pulverization. A gas atomization process or a water atomization process can be selected for the atomization process. Producing hard particles by gas atomization ensures, among other things, excellent sintering behavior, as round hard particles are obtained, and the gas atomization process is therefore more preferred.

[0028] The lower and upper limits for the composition of the hard particles can be changed to appropriate values. These appropriate values ​​can be determined in accordance with the reasons for the compositional restrictions described below, as well as in accordance with the hardness, solid lubricity, adhesion, cost, and importance of the various properties of the target part or structure.

[0029] When Mo is first included in the composition of the hard particles, the hardness of the hard particles and the wear resistance increase due to the formation of Mo carbide. Furthermore, the Mo carbide and the solid-solution Mo in the hard particles form a thin Mo oxidation layer, and thus the Mo is effective in improving solid lubricity.

[0030] If the Mo content is less than 20 mass%, the hard particles have a high initial oxidation temperature, and the generation of Mo oxide is inhibited. Consequently, the wear resistance of the sintered metal ultimately decreases. On the other hand, if the Mo content exceeds 60 mass%, the adhesion between the hard particles and the iron-containing matrix is ​​reduced during sintering. According to the invention, the Mo content is more than 25 to 60 mass%. A more preferred Mo content for the hard particles is 25 to 55 mass%.

[0031] The Mn in the composition of the hard particles diffuses efficiently from the hard particles to the matrix of the sintered alloy during sintering and is therefore effective in improving the adhesion between the hard particles and the matrix. Mn also appears to be effective in increasing austenite in the matrix.

[0032] If the Mn content is less than 20 mass%, little Mn diffuses into the matrix, thus reducing the adhesion between the hard particles and the matrix. If the Mn content is much higher than the above-specified upper limit, the density of the sintered alloy decreases. A more preferred Mn content for the hard particles is 3 to 12 mass%.

[0033] The C in the composition of the hard particles combines with Mo to form Mo carbide, which is effective for increasing the hardness of the hard particles and wear resistance. However, due to limitations on the amount of C added, the hard particles in this embodiment are softer than conventional hard particles. As a result, the density of the molded part can be increased from compaction onward, and the contact area with the iron-based powder, which is the matrix raw material, increases. Consequently, the diffusion of iron from the iron-containing matrix into the hard particles increases. This has the effect of increasing the mechanical strength of the sintered alloy.

[0034] Furthermore, limiting the addition of C to the hard particles makes it possible to inhibit the formation of Mo carbide while increasing the amount of Mo in solid solution without, for example, incorporating Ni. Consequently, the formation of a thin Mo oxidation layer is facilitated. The wear resistance of the resulting sintered alloy can therefore be improved.

[0035] When incorporating C into the hard particles, it is preferable to incorporate no more than 0.5 mass% of C. By adding C to the hard particles, the hardness of the hard particles can be increased. By limiting the C added to the hard particles to no more than 0.5 mass%, the formation of a carbide from Mo and C is suppressed. The amount of Mo in solid solution in the hard particles can also be increased without the addition of Ni.

[0036] The average particle diameter of the hard particles can be selected according to, among other things, the type and application of the iron-based sintered alloy. For example, the average particle diameter of the hard particles can be 20 to 250 µm.

[0037] The hard particles are mixed with the iron-based powder to disperse the powder composed of hard particles for incorporation into a sintered alloy into the iron-based powder forming the matrix. The hard particle content at this time is preferably 10 to 60 mass% relative to the mixed powder as a whole.

[0038] Due to their distribution in the matrix of the sintered alloy, the hard particles form a hard phase that increases the wear resistance of the sintered alloy. The wear resistance of the sintered alloy will be unsatisfactory if the proportion of hard particles relative to the sintered alloy is less than 10% by mass. If the proportion of hard particles relative to the sintered alloy exceeds 60% by mass, the sintered alloy exhibits increased aggressiveness toward a counterpart or structure, and the retention of the hard particles in the sintered alloy is also impaired.

[0039] The mixed powder contains 15 to 60 mass% of the powder composed of hard particles and 0.2 to 2 mass% of graphite powder. The remainder of the powder is an iron-based powder (for example, pure iron powder or low-alloy steel powder), which becomes the matrix of the wear-resistant iron-based sintered alloy. The low-alloy steel powder, for example, is an Fe-C-based powder. This low-alloy steel powder is composed of 0.2 to 5 mass% of C, the remainder being Fe and unavoidable impurities, and 100 mass% of the low-alloy steel powder.

[0040] The mixed powder is molded into a green compact. As mentioned above, the hard particles of this embodiment are softer than conventional hard particles. Consequently, the hard particles of this embodiment provide increased density for the compacted molded part and can increase the contact area with the iron-based powder, which is the matrix starting material.

[0041] This green compact is sintered. At this point, the diffusion of iron from the iron-containing matrix into the hard particles increases. Since the addition of carbon to the hard particles is more limited than with conventional hard particles, the carbon in the graphite powder also diffuses into the hard particles, and the hard particles then experience an increase in hardness.

[0042] A sintering temperature of approximately 1050 to 1250°C, and in particular approximately 1100 to 1150°C, can be used. The sintering time at this sintering temperature can be 30 to 120 minutes, and even better, 45 to 90 minutes. A non-oxidizing atmosphere, such as an inert gas atmosphere, can be used as the sintering atmosphere. A nitrogen atmosphere, an argon gas atmosphere, or a vacuum atmosphere can be used for the non-oxidizing atmosphere.

[0043] Furthermore, the matrix of the iron-based sintered alloy resulting from sintering preferably contains a pearlite structure to ensure its hardness. The pearlite structure can be a pearlite structure, a pearlite-austenite mixed structure, a pearlite-ferrite mixed structure, or a pearlite-cementite mixed structure. The content of low-hardness ferrite is preferably low to ensure wear resistance. The hardness of the matrix can be approximately HV 120 to 300. The hardness of the matrix can be adjusted by, for example, the composition of the matrix, the heat treatment conditions, and the addition amount of carbon powder. This composition and hardness are not limited to the numerical value ranges specified above, as long as the adhesion between the hard particles and the matrix is ​​not reduced and as long as the wear resistance is not reduced.The process described above can provide a sintered alloy consisting of about 6 to 25 mass% Mo, about 1 to 5 mass% Mn and not more than 2 mass% C, the balance being iron and unavoidable impurities.

[0044] In this embodiment, the valve seat of the exhaust valve of an internal combustion engine is formed by the above-mentioned sintered alloy. The valve seat of the exhaust valve of an internal combustion engine is used in a high-temperature atmosphere. Consequently, the wear occurring on the valve seat is a combination of adhesive wear when the valve and the valve seat are in contact and abrasive wear due to sliding between the valve seat and the valve. Even in such a case, the hardness of the hard particles can be increased without impairing the solid lubrication ability of the hard particles. The wear resistance of the valve seat can therefore be further improved over that conventionally available.

[0045] In the following, together with Comparative Examples 1 to 17 and Examples 1 and 20 not belonging to the invention, Examples 2 to 19 and 21 to 25 which embody the invention in detail are described. - Examples 1 to 7 -

[0046] Powders composed of hard particles were prepared using the process described below. The hard particles consisted of 20 to 60 mass% Mo, 3 to 15 mass% Mn, and 0 to 0.5 mass% C, with the remainder being Fe and unavoidable impurities. An alloy powder was produced by mixing a melt containing the Fig. 1, a gas atomization process was carried out using an inert gas (nitrogen gas). Classification of the alloy powder into the 45 µm to 180 µm range then produced a powder of the hard particles. - Comparison example 1 -

[0047] A powder composed of hard particles was prepared as in Examples 1 to 7. The difference between Comparative Example 1 and Examples 1 to 7 was that in the former case, 1.5 mass% C was added to be outside the range of 0.0 to 0.5 mass% C. - Comparison examples 2 and 3 -

[0048] As in Examples 1 to 7, powders composed of hard particles were prepared. The difference between Comparative Examples 2 and 3 and Examples 1 to 7 was that 15 mass% Mo was added in Comparative Example 2 and 70 mass% Mo in Comparative Example 3, in each case outside the range of 20 to 60 mass% Mo. - Comparison example 4 -

[0049] As in Examples 1 to 7, a powder composed of hard particles was prepared. The difference between Comparative Example 4 and Examples 1 to 7 was that in the former case, 1.5 mass% C was added to be outside the range of 0 to 0.5 mass% C, and 12 mass% Ni was also added. - Comparison Example 5 -

[0050] As in Examples 1 to 7, a powder composed of hard particles was prepared. The difference between Comparative Example 5 and Examples 1 to 7 was as follows: alloy lumps containing 63 mass% Mo were prepared to be outside the range of 20 to 60 mass% Mo, and additionally containing 1.1 mass% Si. Furthermore, the preparation was carried out by pulverizing the alloy lumps. Conventional ferro-molybdenum hard particles were produced by the preparation method of Comparative Example 5. - Comparison Example 6 -

[0051] As in Examples 1 to 7, a powder composed of hard particles was prepared. The difference between Comparative Example 6 and Examples 1 to 7 was that the hard particles were prepared based on the preparation conditions used for Comparative Example 6 in Fig. 1 are specified. -- Measurement of the initial oxidation temperature --

[0052] The hard particle powders according to Examples 1 to 7 and Comparative Examples 1 to 6 were heated in the atmosphere to induce oxidation, and the temperature at which the weight gain accompanying the oxidation showed a sharp burst was measured. This temperature at which the weight gain showed a sharp burst was taken as the temperature at which oxidation began. These results are shown in Fig. 1 is specified. -- Hardness test --

[0053] The hardness of the hard particles according to Examples 1 to 7 and Comparative Examples 1 to 6 was measured using a micro Vickers hardness tester and a measuring load of 0.98 N (0.1 kgf). These results are shown in Fig. 1 is specified. --- Result 1 ---

[0054] As in Fig. As shown in Figure 1, a thin oxidation layer was formed more easily by Mo in the hard particles according to Examples 2 to 7 than in the hard particles according to Comparative Example 1. The reason for this seems to be the absence of C addition or the addition of only a small amount of C.

[0055] In addition, the hard particles according to Examples 1 to 7 have a lower hardness than the hard particles of Comparative Examples 1 and 4. The reason for this seems to be that the formation of Mo carbide in the hard particles is hindered due to the absence of C addition or the addition of only a small amount of C.

[0056] Silicon is added to the hard particles according to Comparative Example 5, and Co is added to the hard particles according to Comparative Example 6. This appears to have imparted higher hardness to the hard particles according to Comparative Examples 5 and 6 than to the hard particles according to Examples 1 to 7. Based on this, it is assumed that the hard particles according to Examples 1 to 7 have higher formability during densification than the hard particles according to Comparative Examples 1 and 3 to 6.

[0057] In addition, the hard particles according to Examples 1 to 7 have a lower initial oxidation temperature than the hard particles of Comparative Example 6, thus exhibiting increased oxidizability. This is due to the higher amount of Mo, which has a low initial oxidation temperature (approximately 340°C for a particle size of 80 to 200 mesh), and the lower amount of Cr, which has a high initial oxidation temperature (approximately 500°C for a particle size of 145 mesh).

[0058] The hard particles according to Comparative Example 2 have a lower Mo content than those in Examples 1 to 7, and consequently, the formation of a thin Mo oxidation layer is hindered. Consequently, the wear resistance of the sintered alloy ends up being lower (see Comparative Example 9 below). - Examples 8 to 19 -

[0059] A mixed powder was prepared by mixing the following: 15 to 60 mass% of a powder composed of the above-described hard particles according to Example 2 and 0.2 to 2 mass% of graphite powder, with the remainder being pure iron powder to form the matrix. Specifically, the powder consisting of the hard particles, the graphite powder, and the pure iron powder was mixed into the Fig. 2 specified proportions using a mixer to prepare a mixed powder which served as a mixing raw material.

[0060] The mixed powder blended as described above was placed in a mold and compacted with a compaction force of 78.4 × 10 7 Pa (8 tonf / cm 2) to form a ring-shaped green compact (test specimen). The green compact was sintered at 1120°C for 60 minutes in an inert atmosphere (nitrogen gas atmosphere) to form a sintered alloy (valve seat) corresponding to the test specimen. - Examples 20 to 25 -

[0061] Sintered alloys (valve seats) were prepared by following the procedure described in Examples 8 to 19. Examples 20 to 25 differed from Examples 8 to 19 mainly in two respects. Examples 20 to 25 used the hard particles according to Examples 1 and 3 to 7. In Examples 20 to 25, the sintered alloys were prepared by mixing the powder composed of the hard particles, the graphite powder, and the pure iron powder into the Fig. 2 specified proportions, followed by sintering. - Comparison Example 7 -

[0062] A sintered alloy (valve seat) was prepared by following the same procedures as in Examples 8 to 19. Comparative Example 7 differed from Examples 8 to 19 in that it used, as the hard particles, a powder composed of the hard particles of Comparative Example 1 (hard particles to which 1.5 mass% C was added so as to be outside the range of 0 to 0.5 mass% C). - Comparison Example 8 -

[0063] A sintered alloy (valve seat) was prepared by following the procedures in Examples 8 to 19. Comparative Example 8 differed from Examples 8 to 19 in that it used, as the hard particles, a powder composed of the hard particles of Comparative Example 3 (hard particles to which 70 mass% Mo was added so as to be outside the range of 20 to 60 mass% Mo). - Comparison Example 9 -

[0064] A sintered alloy (valve seat) was prepared by following the same procedures as in Examples 8 to 19. Comparative Example 9 differed from Examples 8 to 19 in that it used, as the hard particles, a powder composed of the hard particles of Comparative Example 2 (hard particles to which 15 mass% Mo was added so as to be outside the range of 20 to 60 mass% Mo). - Comparison Example 10 -

[0065] A sintered alloy (valve seat) was prepared using the same procedure as in Examples 8 to 19. The difference from Examples 8 to 19 was the use of a powder composed of hard particles containing 40 mass% Mo, 0 mass% Mn, and 1.5 mass% C (hard particles prepared to be outside the range of 3 to 15 mass% Mn) as the hard particles. Comparative Example 10 corresponds to the hard particles specified in the previously described JP 2001-181807 A. - Comparison examples 11 and 12 -

[0066] Sintered alloys (valve seats) were produced by following the procedure described in Examples 8 to 19. The difference to Examples 8 to 19 is that the proportion of hard particle powder, as described in Fig. 2, based on the mixed powder, the proportion of hard particle powder was adjusted to a range of 15 to 60 mass%. The proportion of hard particle powder in Comparative Example 11 was 65 mass%, while the proportion of hard particle powder in Comparative Example 12 was 10 mass%. - Comparison examples 13 and 14 -

[0067] Sintered alloys (valve seats) were produced by following the procedure described in Examples 8 to 19. The difference to Examples 8 to 19 is that the proportion of graphite powder was Fig. 2, based on the mixed powder, the proportion of graphite powder was set to within 0.2 to 2 mass%. The proportion of graphite powder in Comparative Example 13 was 0 mass%, while the proportion of graphite powder in Comparative Example 14 was 3 mass%. - Comparative examples 15 to 17 -

[0068] Sintered alloys (valve seats) were prepared using the same procedure as in Examples 8 to 19. Comparative Examples 15 to 17 differed from Examples 8 to 19 by using the hard particles according to Comparative Examples 4 to 6. -- Tensile test --

[0069] Test pieces of the sintered alloys according to Examples 8 to 25 and Comparative Examples 7 to 17 were prepared. The tensile strength was measured by conducting a tensile test (20°C condition) on the test pieces in accordance with JIS Z 2241. These results are shown in Fig. 2 is specified. -- Wear test --

[0070] The wear resistance was evaluated by testing the sintered alloys according to Examples 8, 14, 15 and 20 and Comparative Examples 7, 9 and 12 to 17 using the test apparatus in Fig. 3 a wear test was carried out. In this wear test, as in Fig. As shown in Figure 3, a propane gas burner 10 was used as a heat source, and a propane gas combustion atmosphere was created for a sliding area between an annular valve seat 12 made of the sintered alloy prepared as described above and a valve surface 14 of a valve 13. The valve surface 14 had been subjected to a Tenifer treatment on SUH 11. The temperature of the valve seat 12 was controlled at 250°C, and a load of 176 N (18 kgf) was applied by a spring 16 when the valve seat 12 was in contact with the valve surface 14. The valve seat 12 and the valve surface 14 were brought into contact at a rate of 2000 times / minute, and the wear test was run for 8 hours. The results are shown in Fig. 2 is specified. -- Hardness test --

[0071] The hardness of the hard particles for the sintered alloys according to Examples 15 to 17 and 25 and Comparative Examples 7, 13, 16, and 17 was measured using a micro Vickers hardness tester at a measuring load of 0.98 N (0.1 kgf). These results are shown in Fig. 2 is specified. --- Result 2: Addition amount for each element ---

[0072] The sintered alloys according to Examples 8 to 25 exhibited higher tensile strengths than the sintered alloys according to Comparative Examples 7 and 8, which used hard particles to which large amounts of Mo or C were added. This is believed to be because the hard particles (hard particles according to Examples 1 to 7) used in the sintered alloys according to Examples 8 to 25 were softer than the hard particles (hard particles according to Comparative Examples 1 and 3) used in the sintered alloys according to Comparative Examples 7 and 8, improved formability of the green compact.

[0073] The sintered alloy according to Comparative Example 15, which used the hard particles according to Comparative Example 4, had a higher tensile strength than that of Comparative Example 7, which lacked Ni, provided by the diffusion of Ni present in the hard particles into the matrix. The sintered alloy according to Example 16 had almost the same tensile strength as that of Comparative Example 15, despite the absence of Ni.

[0074] In the sintered alloys according to Examples 15 to 17, the hard particles according to Example 2 were used. The hardness of the hard particles according to Example 2 is higher after sintering. This appears to be because, due to the limitation of the C content for the hard particles according to Example 2, the carbon from the graphite powder is more easily subject to solid solution diffusion into the hard particles during sintering. On the other hand, in the case of Comparative Example 7, which used the hard particles of Comparative Example 1, the hardness of the hard particles after sintering is lower. This appears to be because, due to the higher C content in the hard particles according to Comparative Example 1 than in the hard particles according to Examples 1 to 7, the above-mentioned phenomenon is almost completely absent.

[0075] In the sintered alloy according to Comparative Example 9, the hard particles according to Comparative Example 2 were used. The hard particles in Comparative Example 9 had a lower Mo content than the hard particles of Examples 1 to 7. This appears to have led to higher abrasive wear in the sintered alloy according to Comparative Example 9 compared to the sintered alloys according to Examples 1 to 7.

[0076] Based on these results, when C is added to the hard particles, its content is preferably no more than 0.5 mass%, and more preferably no more than 0.4 mass%. Furthermore, the Mo content in the hard particles is preferably 20 to 60 mass%, and more preferably 22 to 55 mass%.

[0077] In the sintered alloy according to Comparative Example 10, hard particles containing no Mn were used. Elemental analysis was performed on the sintered alloys according to Example 15 and Comparative Example 10. Diffusion of Mn into the iron-containing matrix of the sintered alloy of Example 15 was detected, while no diffusion of Mn into the iron-containing matrix of the sintered alloy of Comparative Example 10 was observed. Based on these results, it appears that the tensile strength of the sintered alloy is increased because, during sintering, the bond strength of the hard particles to the iron-containing matrix was increased by the diffusion of the Mn present in the hard particles into the iron-containing matrix. --- Result 3: Proportion of hard particle powder ---

[0078] The proportion of hard particles in the sintered alloy of Comparative Example 11 is larger than in Examples 8 to 25. Consequently, the contact between the hard particles increases during densification, and the sinterability between the hard particles and the iron particles forming the matrix is ​​reduced. This appears to lead to a reduction in the tensile strength of the sintered alloy of Comparative Example 11. On the other hand, the proportion of hard particles for the sintered alloy of Comparative Example 12 is smaller than in Examples 8 to 25. It can be presumed that this resulted in insufficient development of the wear-resistance effect of the hard particles. In view of this, the proportion of the hard particle powder relative to the mixed powder preferably ranges from 15 to 60 mass%, and more preferably from 20 to 55 mass%. --- Result 4: Proportion of graphite powder ---

[0079] The sintered alloy according to Comparative Example 13 has a higher amount of ferrite in the iron-containing matrix because the proportion of graphite powder in this case is smaller than in Examples 8 to 25. In the case of the sintered alloy of Comparative Example 14, the proportion of graphite powder is larger than in Examples 8 to 25, and the C in the hard particles undergoes an increase and some melting occurs. In both cases, this appears to have led to the decrease in the tensile strength of the sintered alloy in Comparative Examples 13 and 14. In view of this, it is considered that the proportion of graphite powder preferably ranges from 0.2 to 2 mass%, and more preferably from 0.5 to 2 mass%. --- Result 5 ---

[0080] The sintered alloys according to Comparative Examples 16 and 17 contain Si and consequently have lower tensile strength than the sintered alloys of Examples 8 to 25. The adhesion of the hard particles appears to be lower for the sintered alloys of Comparative Examples 16 and 17 because their hard particles are harder than for the sintered alloys according to Examples 8 to 25. This appears to have led to a greater degree of wear for the sintered alloys according to Comparative Examples 16 and 17 than for the sintered alloys of Examples 8, 14, 15, and 20.

[0081] The sintered alloy of the embodiments is well suited for use as a wastegate valve of a turbocharger or in the valve train (for example, the valve seat or valve guide) of a compressed natural gas or LPG engine used in high-temperature environments.

Claims

[1] Hard particles for incorporation into a sintered alloy, consisting of: more than 25 to 60 mass% Mo; 3 to 15 mass% Mn; and the remainder consisting of Fe and unavoidable impurities. [2] Hard particles for incorporation into a sintered alloy, consisting of: more than 25 to 60 mass% Mo; 3 to 15 mass% Mn; more than 0.01 to 0.5 mass% C; and the remainder consisting of Fe and unavoidable impurities. [3] Wear-resistant iron-based sintered alloy obtained by: Obtaining a mixed powder by mixing a powder composed of the hard particles according to claim 1 or 2 into an iron-based powder which becomes a matrix so that the hard particles are dispersed; and Sintering of the mixed powder, wherein the wear-resistant iron-based sintered alloy comprises 15 to 60 mass% of the hard particles with respect to the wear-resistant iron-based sintered alloy. [4] Process for producing a wear-resistant iron-based sintered alloy, comprising Obtaining a mixed powder in which an iron-based powder which becomes a matrix is mixed with 0.2 to 2 mass% of graphite powder and 15 to 60 mass% of powder composed of the hard particles according to claim 1 or 2; Compacting the mixed powder; and Sintering of the compacted mixed powder while carbon from the graphite powder diffuses into the hard particles.

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