EP3822379B1

EP3822379B1 - Sintered alloy and method for producing same

Info

Publication number: EP3822379B1
Application number: EP19834991.2A
Authority: EP
Inventors: Daisuke Fukae; Hideaki Kawata
Original assignee: Showa Denko Materials Co Ltd
Current assignee: Resonac Corp
Priority date: 2018-07-11
Filing date: 2019-07-10
Publication date: 2022-07-06
Anticipated expiration: 2039-07-10
Also published as: EP3822379A1; CN112368409A; WO2020013227A1; JPWO2020013227A1; CN112368409B; EP3822379A4; JP7248027B2

Description

TECHNICAL FIELD

The present invention relates to a sintered alloy suitable such as for a turbocharger turbo component and a method for producing the same, and particularly relates to a sintered alloy suitable for a nozzle body or the like which is required to have a heat resistance, a corrosion resistance, and a wear resistance and a method for producing the same.

BACKGROUND ART

Generally, in a turbocharger attached to an internal-combustion engine, a turbine is rotatably supported by a turbine housing connected to an exhaust manifold of the internal-combustion engine and a plurality of nozzle vanes are rotatably supported so as to surround an outer periphery side of the turbine. An exhaust gas flowing into the turbine housing flows into the turbine from the outer periphery side thereof, and is then discharged in an axial direction, and at that time, rotates the turbine. The rotation of a compressor provided to the same shaft on a side opposite to the turbine compresses air supplied to the internal-combustion engine.
The nozzle vane is rotatably supported by a ring-shaped component called a nozzle body or a mounting nozzle. The shaft of the nozzle vane passes through the nozzle body and is connected to a link mechanism. Then, driving of the link mechanism rotates the nozzle vane and the opening degree of a flow path through which the exhaust gas flows into the turbine is adjusted.
Turbo components provided in the turbine housing such as the above described turbo component for a turbocharger, that is, a nozzle body (a mounting nozzle) and a plate nozzle attached thereto contact with the exhaust gas that is a high temperature corrosive gas. Accordingly, these components are required to have the heat resistance and the corrosion resistance, and are also required to have the wear resistance to cope with a sliding contact with the nozzle vane. For this reason, conventionally, as a material for forming the turbo components, for example, high-chrome cast steel, heat-resistant steel defined as type SCH22 in JIS standards, a wear resistant material obtained by applying a chromium surface treatment to a heat-resistant alloy to improve the corrosion resistance or the like is used.
On the other hand, in a powder metallurgy method also, a sintered alloy for the application to various types of machinery components has been developed, and a heat-and-wear-resistant sintered alloy for the application to the above described turbo components has been proposed (see Patent Document 1). In the powder metallurgy method, a sintered alloy having a special metallic structure that is not formed from molten steel obtained by casting or the like can be obtained.
Patent Document 2 relates to a sintered alloy including, in percentage by mass, Cr: 10.37 to 39.73, Ni: 5.10 to 24.89, Si: 0.14 to 2.52, Cu: 1.0 to 10.0, P: 0.1 to 1.5, C: 0.18 to 3.20 and the balance of Fe plus unavoidable impurities; a phase A containing precipitated metallic carbide with an average particle diameter of 10 to 50 µm; and a phase B containing precipitated metallic carbide with an average particle diameter of 10 µm or less, wherein the phase A is randomly dispersed in the phase B and the average particle diameter DA of the precipitated metallic carbide in the phase A is larger than the average particle diameter DB of the precipitated metallic carbide of the phase B.
Patent Document 3 relates to a component for turbocharger applications, in particular in diesel engines, which consists of an iron-based alloy having an austenitic base structure which comprises a carbide structure.

PRIOR ART DOCUMENTS

PATENT DOCUMENT

Patent Document 1: Japanese Patent Application Publication No. 2013-199695
Patent Document 2: EP 2 772 558 A2
Patent Document 3: WO 2013/059104 A1

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

However, the further improvement in the wear resistance of the turbocharger component that slidingly contacts with another member such as a nozzle body that slidingly contacts with the nozzle vane is desired to prevent the adhesive wear with another member. In addition, oxidation easily proceeds by steam contained in a high-temperature exhaust gas, and therefore, the further improvement in the corrosion resistance is also desired. Further, to meet the demand for complicated shapes, the improvement in a machining property (the machinability) is desired.
As described above, an object of the present invention is to provide a sintered alloy that is suitable for the application to a turbocharger component and has excellent wear resistance, corrosion resistance and machinability and a method for producing the same by the further improvement in an iron-based sintered alloy.

MEANS TO SOLVE THE PROBLEMS

To solve the above described problem, one aspect of the present invention provides a sintered alloy as defined in claim 1.
Further, one aspect of the present invention provides a method for producing a sintered alloy as defined in claim 4.
Advantageous embodiments are described in the dependent claims.

EFFECTS OF THE INVENTION

It is possible to suppress affection to other material properties while improving the machinability of a sintered alloy based on a composition design of an iron-based alloy, and this enables the provision of a sintered alloy that has excellent wear resistance, corrosion resistance and machinability and is suitable for the application to a turbocharger component and a method for producing the same.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Carbide particles dispersed in a metallic structure of a sintered alloy are likely to be obstacles for cutting during machining, and if the amount of carbide particles increases, machinability of the sintered alloy is lowered. Accordingly, to improve the machinability of the sintered alloy, the suppression in the generation of the carbide particles is considered to be effective. According to this objective, the sintered alloy having a composition with a low proportion of carbide was investigated, and it was revealed that while the reduction in the amount of carbide is effective for the improvement in the machinability, a pinning effect by carbide is lowered, a crystal grain of an iron alloy matrix becomes coarse, and accordingly a oxidation resistance is reduced.
For this reason, an approach to remedy the above described drawback was investigated, and it was found that in a case of the solid solution of copper to the iron alloy matrix, an oxidation resistance of the matrix in a steam environment is improved and the improvement in a corrosion resistance is possible. Further, the solid solution of the copper to the matrix hardens a soft austenite and suppresses the adhesion between the matrix and another member, and accordingly it was found that the improvement in the machinability is possible. Further, it becomes evident that the reduction in the sintering temperature suppresses the growth of the crystal grain and increases the oxidation resistance. In this regard, a liquid phase formation at a low temperature is possible with respect to a copper-phosphorus alloy powder, and thus, the use of the copper-phosphorus alloy powder as a raw material for introducing copper becomes effective means for setting a sintering temperature to be low.
By taking the above described factor into consideration, the present invention provides a sintered alloy with its oxidation resistance, corrosion resistance, and machinability being improved by arranging designs of an alloy composition so that functions of a component forming the sintered alloy are exerted at a favorable balance. Component compositions of the sintered alloy and the raw material powder according to the present invention are described below.

In the present invention, the matrix of the sintered alloy is the iron alloy matrix having a composition of austenite stainless steel, and has a metallic structure in which pores are dispersed in the matrix and a carbide particle is precipitated and dispersed. The austenite stainless steel is an iron alloy obtained by the solid solution of chromium and nickel to y iron, and is high in a corrosion resistance and a heat resistance, and has a coefficient of thermal expansion that is equivalent to a general austenitic heat-resistant material.
In the austenite stainless steel, the oxidation resistance in a steam environment is improved by suppressing the growth of crystal grains. In the present invention, the austenite stainless steel forming the matrix of the sintered alloy is composed of crystal grains each having an average crystal particle size of 10 to 50 µm. The suppression of the growth of the crystal grains to such particle size range is possible by sintering at a temperature lower than a conventional sintering temperature, and a sintering temperature is set in a range from 1050 to 1160 °C. To enable sintering at a low temperature, the use of a component that generates a liquid phase at a low temperature is useful. In this regard, the copper-phosphorus alloy powder enables sintering to proceed at a comparatively low temperature in a range from 1050 to 1130 °C. Accordingly, the use of the copper-phosphorus alloy powder effectively acts on the decrease in the sintering temperature and is useful in suppressing the growth of the crystal grains. By the proceeding of the preferable sintering by such temperature setting, a density of the sintered alloy becomes about 6.8 to 7.4 Mg/m³.
The austenite forming the matrix of the sintered alloy can be generated by using iron alloy powders obtained by the solid solution of chromium or nickel to iron as main raw materials. That is, an austenitizing element (chromium, nickel) is introduced into the raw material powder by alloying with iron to prepare the iron alloy powder. This causes the austenitizing elements to be uniformly distributed in the iron alloy matrix to exert the corrosion resistance and the heat resistance. Examples of other components alloyed with iron include silicon and it functions as an antioxidizing agent of chromium while preparation of iron-chromium alloy powders.
If the iron alloy matrix contains at least 12 % by mass of chromium (Cr), it exhibits a corrosion resistance against an oxidizing acid. When a factor that a part of chromium contained in the iron alloy powder used as the raw material is precipitated as carbide during sintering is taken into consideration, it is preferable if a chromium content of the iron alloy powder is set to be at least 15 % by mass so that sufficient chromium is remained in the iron alloy matrix. However, if the chromium content of the iron alloy powder exceeds 30% by mass, a brittle σ phase is formed, and a compressibility of the iron alloy powder may be significantly damaged. Therefore, according to the invention, the chromium content of the iron alloy powder used as the raw material is 15 to 30 % by mass. In the preferable sintered alloy that can be obtained by using such powders, a proportion of chromium in an overall composition is, according to the invention, 13.86 to 27.72 % by mass. More preferably, the proportion of chromium is at least 16.88 % by mass and not more than 23.10 % by mass, and most preferably, is at least 18.48% by mass and not more than 20.33% by mass.
The iron alloy matrix containing at least 3.5 % by mass of nickel (Ni) exhibits a corrosion resistance against non-oxidizing acids. Further, if the iron alloy powder having a nickel content of at least 7% by mass is used, such effects are preferably exerted and an oxidation resistance is imparted to the thus obtained sintered alloy. However, if the iron alloy powder having nickel content of more than 24 % by mass is used, effects of the corrosion resistance and the oxidation resistance do not change much. Nickel is also an expensive material, and thus, considering the production cost, the nickel content of the iron alloy powder used as the raw material is, according to the invention, 7 to 24 % by mass and the iron alloy powder containing preferably 10 to 22% by mass of nickel is used. The nickel content in an overall composition of the sintered alloy obtained by using such raw material is, according to the invention, 6.47 to 20.33% by mass. In a regard of the oxidation resistance, the nickel content is preferably at least 7.39 % by mass and in a regard of the economic efficiency, the nickel content is preferably not more than 18.48 % by mass.
Steel with an austenite structure has a crystallographically high atomic density, and thus, its corrosion resistance is superior to that of steel with a ferrite structure. For this reason, it is preferable to use an iron alloy powder with its chromium content and nickel content being appropriately adjusted as the raw material in order that the iron alloy matrix obtained after the sintering preferably has the austenite structure. Specifically, in an annealed structure diagram of an Fe-Cr-Ni based alloy in which an abscissa axis represents the amount of chromium (% by mass) and an ordinate axis represents the amount of nickel (% by mass), the austenite structure is formed in a region where the amount of nickel is larger than at a broken line that connects point A (Cr:15, Ni:7.5), point B (Cr:18, Ni:6.5), and point C (Cr:24, Ni:18). Accordingly, a composition of the iron alloy powder used as the raw material may be adjusted so that the amount of chromium and the amount of nickel become those in such region.
The iron alloy powder contains chromium that is easily oxidized, and thus, silicon is added as a deoxidizing agent to molten metal for preparing the iron alloy powder. For this reason, the iron alloy matrix contains silicon (Si). Further, the solid solution of silicon to the iron alloy matrix has effects of enhancing the oxidation resistance and the heat resistance of the matrix and the effects are remarkable when a silicon content is at least 0.46 % by mass. However, the iron alloy powder having the silicon content of more than 3.0 % by mass is hard, and thus, a compressibility is significantly impaired. An increase in oxides generated from silicon impairs the proceeding of sintering and reduces a strength of the sintered alloy. Accordingly, the silicon content of the iron alloy powder used as the raw material is, according to the invention, 0.5 to 3.0 % by mass. The silicon content in an overall composition of the sintered alloy produced by using such iron alloy powder is, according to the invention, 0.46 to 2.77 %.by mass. From the viewpoint of the oxidation resistance, the silicon content in an overall composition may be preferably at least 0.74 % by mass and not more than 1.85 % by mass.
Sintering of the iron alloy powder having the large chromium content is less likely to proceed. Accordingly, to promote the sintering, phosphorus (P) that generates a eutectic liquid phase of iron-phosphorus-carbon is used and it is blended in the form of a phosphorus alloy powder. In the present invention, as the phosphorus alloy powder, either one or both of the iron-phosphorus alloy powder and the copper-phosphorus alloy powder can be used. If the iron-phosphorus alloy powder with the phosphorus content of less than 10 % by mass is used, the amount of generated liquid phase is small and the sintering is less likely to proceed sufficiently. On the other hand, if the powder with the phosphorus content of more than 30% by mass is used, the iron-phosphorus alloy powder becomes hard, and thus, the compressibility of the raw material powder is significantly impaired. Accordingly, the phosphorus content of the used iron-phosphorus alloy powder is, according to the invention, 10 to 30 % by mass. If the copper-phosphorus alloy powder with the phosphorus content of less than 5 % by mass is used, the amount of generated liquid phase is small and the sintering is less likely to proceed sufficiently. On the other hand, if the copper-phosphorus alloy powder with the phosphorus content of more than 25 % by mass is used, the copper-phosphorus alloy powder becomes hard and the compressibility of the raw material powder is significantly impaired. Further, there is a case where the generated liquid phase easily flows out of a sintered body before it is sufficiently diffused. Accordingly, the phosphorus content of the used copper-phosphorus alloy powder is, according to the invention, 5 to 25 % by mass. Further, in an overall composition of the sintered alloy, if a proportion of phosphorus is less than 0.15 % by mass, the amount of generated liquid phase is insufficient and a sintering promoting effect becomes poor. On the other hand, if the proportion of phosphorus is more than 1.95 % by mass, the sintering proceeds excessively and the sintered alloy is densified, and if it exceeds the upper limit of a density as the sintered alloy, pores are reduced. This makes it difficult to suppress a plastic flow of the matrix and reduces a wear resistance. In addition, excess phosphorus alloy powders are likely to flow out to the outside as the liquid phase, and if the liquid phase flows out, portions where the phosphorus alloy powders were present become pores (so-called Kirkendall voids), and coarse pores are formed in the iron alloy matrix, and therefore, a corrosion resistance is lowered. Further, if the sintering proceeds excessively by an increase in the generated eutectic liquid phase, the growth of chromium carbide is promoted and the precipitated chromium carbide becomes coarse. Therefore, the phosphorus alloy powder is blended with the raw material powder so that the proportion of phosphorus in the overall composition of the sintered alloy becomes, according to the invention, 0.15 to 1.95 % by mass. More preferably, the phosphorus alloy powder may be blended such that the phosphorus content is at least 0.60 % by mass and not more than 1.50 % by mass and more appropriately, the phosphorus content is at least 0.60 % by mass and not more than 1.05 % by mass.
In the sintered alloy according to the present invention, the solid solution of copper (Cu) to the matrix improves the oxidation resistance and the corrosion resistance. At the same time, the machinability is improved because copper hardens soft austenite and suppresses the adhesion of the matrix. The copper can be blended with the raw material powder as the copper powder or the copper-phosphorus alloy powder. If the total amount of phosphorus for promoting the sintering is blended as the iron-phosphorus alloy powder, copper is blended in the form of the copper powder. If a part or all of phosphorus is not blended in the form of the iron-phosphorus alloy powder, the copper-phosphorus alloy powder is used so that the phosphorus content of the raw material powder has the above described appropriate compositional proportion of phosphorus. If the amount of copper introduced by the copper-phosphorus alloy powder is insufficient, or if the copper-phosphorus alloy powder is not used, the copper powder is used. According to the invention the copper content in the overall composition of the sintered alloy is 0.85 to 11.05 % by mass because the oxidation resistance and the corrosion resistance are favorable. More preferably, the copper content is at least 3.40 % by mass and not more than 8.50 % by mass, and the more appropriate copper content is at least 3.40 % by mass and not more than 5.95 % by mass.
Accordingly, one powder or a combination of two or more powders can be used as a blending material for blending phosphorus and copper as shown by the following five forms. In a case where the copper-phosphorus alloy powder (P: 5 to 25 % by mass) is used as the form of (5), if the copper-phosphorus alloy powder is blended at the proportion of 1.0 to 13 % by mass of the raw material powder, the amount of copper can be adjusted in the range from 0.75 to 12.35 % by mass and the amount of phosphorus can be adjusted in the range from 0.05 to 3.25 % by mass in the overall composition. The copper-phosphorus alloy powder causes a eutectic liquid phase to be generated at a lower temperature than the iron-phosphorus alloy powder, and thus, a decrease in a sintering temperature is possible and it is preferable in suppressing the growth of crystal grains. From the perspective of preferably generating the liquid phase, the used copper-phosphorus alloy powder according to the invention has a phosphorus content of at least 5 % by mass and not more than 25 % by mass and the used copper-phosphorus alloy powder preferably has a phosphorus content of at least 10 % by mass and not more than 20 % by mass. In addition, the machinability of the obtained sintered alloy tends to improve more when the copper-phosphorus alloy powder is used than when the iron-phosphorus alloy powder is used, and thus, the forms of (2) to (5) using the copper-phosphorus alloy powder are preferable. In addition, in the forms of (1) and (5), the used amount of blending material can be easily determined based on a compositional proportion of phosphorus and copper.

(1) The combination of the iron-phosphorus alloy powder and the copper powder.
(2) The combination of the iron-phosphorus alloy powder and the copper-phosphorus alloy powder
(3) The combination of the iron-phosphorus alloy powder, the copper-phosphorus alloy powder, and the copper powder.
(4) The combination of the copper-phosphorus alloy powder and the copper powder.
(5) The copper-phosphorus alloy powder.

Carbon is blended with the raw material powder as a graphite powder, and during the heating of carbon, generates a eutectic liquid phase of iron-phosphorus-carbon to promote the sintering. If carbon diffused from the eutectic liquid phase of iron-phosphorus-carbon to the iron alloy matrix is combined with chromium, the carbon is dispersed and precipitated in the matrix as chromium carbide. If the amount of precipitated carbide is large, the machinability is lowered, and thus, in the present invention, the precipitation of carbide is controlled by appropriately adjusting a blending amount of graphite powders. According to the invention, the graphite powders are blended such that a proportion of carbon of the overall composition becomes 0.20 to 1.00 % by mass. If the proportion of carbon exceeds 1.00 % by mass of the overall composition, the machinability is lowered because even if the sintering proceeds by the generation of an iron-phosphorus-carbon eutectic liquid phase, a large amount of carbide is formed. In addition, the reduction in the amount of chromium solid solution in the matrix lowers the heat resistance and the corrosion resistance. If the proportion of carbon is less than 0.20 % by mass of the overall composition, a promotion effect of the sintering and the wear resistance by carbide may not be obtained. A carbon content according to the invention is at least 0.20 % by mass and not more than 1.00 % by mass of the overall composition and more preferably the carbon content is at least 0.4 % by mass and not more than 0.80 % by mass of the overall composition.
Further, the precipitation of chromium carbide can be suppressed by blending an element (hereinafter referred to as a carbide-forming element) having a higher ability to generate carbide than chromium with the raw material powder if necessary. The carbide-forming element preferentially reacts with graphite than chromium during sintering to generate carbide and at least a part of the carbide-forming element exists as carbide in the sintered alloy. This suppresses a reduction in a chromium concentration in the iron alloy matrix, and thus, there are effects of improving the heat resistance and the corrosion resistance of the matrix. In addition, the generation of alloy carbide by the reaction of the carbide-forming element with carbon contributes to the improvement in the wear resistance. The carbide-forming element is selected from the group consisting of molybdenum, vanadium, tungsten, niobium, and titanium to be used and can be used alone or in a combination of two or more. However, if more than 3.23 % by mass of the carbide-forming element of the overall composition is blended, the compressibility of the raw material powder is lowered, and thus, the carbide-forming element may be arbitrarily blended in a range of not more than 3.23 % by mass of the overall composition of the sintered alloy. Preferably, the content of the carbide-forming element may be at least 0.46 % by mass and not more than 2.77 % by mass of to the overall composition. When two or more elements are used in combination, it is sufficient if the total amount thereof may be the above described compositional proportion.
According to the invention an overall composition of the sintered alloy has 13.86 to 27.72 % by mass of Cr, 6.47 to 20.33 % by mass of Ni, 0.85 to 11.05 % by mass Cu, 0.46 to 2.77 % by mass of Si, 0.15 to 1.95 % by mass of P, and 0.20 to 1.00 % by mass of C, with the remainder composed of Fe and unavoidable elements. According to an embodiment an overall composition of the sintered alloy has 13.86 to 27.72 % by mass of Cr, 6.47 to 20.33 % by mass of Ni, 0.85 to 11.05 % by mass of Cu, 0.46 to 2.77 % by mass of Si, 0.15 to 1.95 % by mass of P, 0.20 to 1.00 % by mass of C, and not more than 3.23 % by mass of a carbide-forming element, with the remainder composed of Fe and unavoidable elements. The carbide-forming element is at least one selected from the group consisting of molybdenum, vanadium, tungsten, niobium, and titanium, and may be used alone, or in a combination of two or more of these.
In the sintered alloy prepared based on the above described compositional proportion, a carbide is precipitated and dispersed in the iron alloy matrix in which pores are dispersed and the carbide may be generated from iron, chromium, and the above described carbide-forming element. The sintered alloy, according to the invention, has a density οf 6.8 to 7.4Mg/m³. When the sintered alloy slides over a mated member, carbide reduces the contact between the iron alloy matrix and the mated member and suppresses a plastic flow of the matrix, and thus, it contributes to the wear resistance. In the present invention, to improve the machinability, an alloy composition is designed so that the proportion of carbide becomes a predetermined amount or less. For this reason, it is preferable that carbides are dispersed and precipitated as a large amount of grains without coarsening so that functions of carbide are efficiently exerted. Specifically, if carbide becomes coarse with its size exceeding 10 µm, by ubiquitous carbide, a region where carbide does not exist, that is a region where the plastic flow is not suppressed increases, and the wear resistance is significantly reduced. In addition, it has an adverse effect on the machinability of the sintered alloy either. However, if a dispersed carbide has a size of less than 1 µm, it does not substantially have a function of suppressing a flow of the matrix. For this reason, carbide particles each having a maximum diameter (maximal value of particle size) in a range from 1 to 10 µm are preferable. In a structure cross section of the sintered alloy, it is preferable if a proportion (an area proportion) of an area of the carbide particles each having a maximum diameter in a range from 1 to 10 µm occupied in an area of all of the carbide particles is at least 90 %. In the present invention, the promotion of the sintering of a phosphorus alloy (particularly, a copper-phosphorus alloy) can set a heating temperature during sintering to be low and the coarsening of the carbide particles is suppressed.
To a maximal value of a particle size of each carbide particle, a long length of a particle portion that is determined to be maximum in an image by image analysis software (WinROOF manufactured by MITANI CORPORATION) when measuring the particle size from the image of a particle cross section by an image analysis of a metallic structure cross section by using the image analysis software is applied. In addition, to an average crystal particle size of crystal grains of the iron alloy matrix, a value obtained by measuring an area of the austenite matrix and the number of crystal grains in the cross section through the image analysis of the metallic structure cross section, by calculating an average area (number average) of the crystal grains from these values and by converting them as an area circle equivalent diameter by an approximate calculation is applied.

The mixed powder is prepared by blending the raw material so that a proportion of each component becomes the above described compositional proportion of the sintered alloy and this mixed powder is used as a raw material powder for molding. By compression molding the mixed powders, a green compact can be obtained and a sintered body obtained by heating the green compact at a sintering temperature is the above described sintered alloy.
According to the invention, the blending material (a second raw material) for blending phosphorus and copper in any one form of the above described forms of (1) to (5) and the graphite powder was uniformly mixed to the above described iron alloy powder (a first raw material) containing nickel, chromium, and silicon to prepare the mixed powder. At this time, the mixing proportion, according to the invention, are adjusted so that the mixed powders contains 0.15 to 1.95 % by mass of phosphorus, 0.85 to 11.05 % by mass of copper, and 0.20 to 1.00 % by mass of carbon. The obtained mixed powders is used as the raw material powders for molding. If a combination of two or more powders (the forms of (1) to (4)) is used as the second raw material, the two or more powders may be individually added to the preparation of the mixed powders or may be added to it after uniformly mixing the two or more powders.
In an embodiment, the raw material powder in which the carbide-forming element is blended so that its proportion becomes not more than 3.23 % by mass of the overall composition is used. The carbide-forming element is selected from molybdenum, vanadium, tungsten, niobium, and titanium and may be used alone or in a combination of two or more of these. The carbide-forming element can be used in a state of being alloyed with an iron alloy that is the first raw material. That is, in the second embodiment, the iron alloy powder containing nickel, chromium, silicon and the carbide-forming element may be used as the first raw material. If the proportion of the carbide-forming element is more than 3.23 % by mass of the overall composition, the compressibility of the iron alloy powder is reduced and it becomes difficult to mold the raw material powder to have a desired green density. Accordingly, the proportion of the carbide-forming element may be not more than 3.23 % by mass of the overall composition. Note that, an effect by the addition of the carbide-forming element becomes apparent at a stage where the proportion of the carbide-forming element is about 0.92 % by mass, and thus, the use of the carbide-forming element in the range from 0.92 to 3.23 % by mass is preferable. However, this is an arbitrarily component, and thus, the carbide-forming element may be used in a range from 0 to 3.23 % by mass in accordance with a design of the alloy composition and particularly, in accordance with a compositional proportion of chromium and carbon.

Pores among powder particles in the green compact obtained by the compression molding remain in the sintered alloy after sintering also, and if the amount of pores is large, the strength and the wear resistance are reduced. However, in the sintered alloy used as a turbocharger component, a passivation film of chromium is formed on a surface of the sintered alloy and an inner surface of each pore by oxygen contained in a high-temperature exhaust gas, and this improves the wear resistance and the corrosion resistance. The passivation film of chromium is hard and is firmly fixed to the surface of the sintered alloy, and it has an effect of preventing the iron alloy matrix from being adhered to a mated member. Accordingly, if an appropriate amount of pores are dispersed in the sintered alloy, each pore with its inner surface being covered by the passivation film of chromium has an effect of preventing a plastic flow of the iron alloy matrix and a wear resistance of the sintered alloy is improved. In consideration of this point, a density of the sintered alloy is, according to the invention, 6.80 to 7.40 Mg/m³ and if the density exceeds 7.40Mg/m³, in association with a decrease in the amount of pores, an effect of the passivation film may not be obtained and the wear resistance is lowered. If the density is less than 6.80Mg/m³, a strength of the sintered alloy is lowered and the wear resistance is reduced. It is preferable if the density is at least 7.00Mg/m³ and not more than 7.40Mg/m³, and it is more appropriate if a sintering density is at least 7.20Mg/m³ and not more than 7.40Mg/m3.

To obtain the sintered alloy having the above described density, a compression molding of raw material powders is conducted such that a density of a green compact becomes, according to the invention, 6.00 to 6.80 Mg/m³. The use of the phosphorus alloy powders proceeds the sintering by the formation of a liquid phase at a low temperature, and thus, by heating a green compact having such density to 1050 to 1160 °C, the sintering proceeds, and accordingly, the sintered alloy having a density of about 6.80 to 7.40Mg/m³ can be obtained. Setting a heating temperature during sintering to be within the above described temperature range suppresses the growth of crystal grains in the iron alloy matrix and causes an average crystal particle size in the iron alloy matrix to be 10 to 50 µm. If the heating temperature is less than 1050 °C, the sintering hardly proceeds, and if the heating temperature exceeds 1160 °C, a coarse crystal grain having a particle size of more than 50 µm is likely to grow in the iron alloy matrix after the sintering. More preferably, the sintering temperature may be at least 1100 °C and not more than 1140 °C.
Generally, in a production of the sintered alloy having a high chromium content, a chromium-containing alloy powder from which a passivation film of a surface is removed is used as a raw material so that the sintering proceeds actively and the sintering is conducted in a vacuum atmosphere or a reduced pressure atmosphere. In this regard, in the present invention, the sintering proceeds well at a relatively low temperature by the generation of liquid phases by the phosphorus alloy powders, and thus, activity during the sintering can be maintained if it is conducted in a non-oxidizing atmosphere and the sintering is possible in a normal pressure environment. Accordingly, it is not necessary to adjust a pressure environment to a vacuum environment or a reduced-pressure environment and turbocharger components can be produced at a low cost in a non-oxidizing environment as the same as during the production of general sintered components.
Note that, as a sintering atmosphere, if a gas containing about 10% by volume or more of nitrogen is used, hard nitride (mainly chromium nitride) is formed on a surface of the sintered alloy and an inner surface of the pores, and this is preferable because wear resistance of the sintered alloy can be improved. Examples of an atmosphere gas containing nitrogen include a nitrogen gas, a mixed gas of nitrogen and hydrogen, an ammonia decomposition gas, a mixed gas obtained by mixing nitrogen with the ammonia decomposition gas, and a mixed gas obtained by mixing hydrogen with the ammonia decomposition. In this case, the amount of nitrogen introduced from the atmosphere to the sintered alloy is extremely small and is substantially the same as the amount of unavoidable impurities contained in the sintered alloy.
In accordance with the above, the sintered alloy having a density of 6.8 to 7.4Mg/m3 is obtained and the obtained sintered alloy has a compositional structure in which pores and precipitation particles of carbide are dispersed in the iron alloy matrix having a composition of austenite stainless steel. The solid solution of copper hardens the austenite structure and the wear resistance and the corrosion resistance of the iron alloy matrix are improved. The structure of the iron alloy matrix becomes a fine crystal grain having an average crystal particle size of about 10 to 50 µm by a low sintering temperature and the corrosion resistance and the oxidation resistance of the matrix are improved. In addition, each carbide particle has a particle size of about 1 to 10 µm and a proportion of an area of carbide having a particle size of more than 10 µm is less than 10% of an area occupied by carbide in a structure cross section. The sintered alloy is produced based on a compositional proportion determined so as to suppress the amount of precipitated carbide, and therefore, a proportion of carbide particles occupying the metallic structure cross section is not more than 10 % by area and the machinability of the sintered alloy is improved. Moreover, even if the amount of precipitated carbides itself is decreased, the adhesive wear of the matrix can be prevented by the carbide particles being finely dispersed, and the suppression in the growth of crystal grains in the matrix by a pinning effect of carbide is also effective.

[Example 1]

(Sample numbers 1 to 39)

As iron alloy powders, alloy powders (an average particle size: 70 µm) containing chromium, nickel, and silicon at a compositional proportion shown in Table 1 were prepared. In addition, as copper-phosphorus alloy powders, the copper-phosphorus alloy powders (an average particle size: 40 µm, as a remainder, containing copper and unavoidable impurities) having a phosphorus content as shown in Table 1 were prepared. A median size based on a particle size distribution measurement was applied to an average particle size of the powders. These alloy powders and graphite powders (an average particle size: 10 µm) were uniformly mixed at a blend proportion as shown in Table 1 to obtain mixed powders having a overall composition as shown in Table 2. The mixed powders were used in the following operation as raw material powders for molding.
The raw material powders were filled in a mold hole and the raw material powders were compression molded at a pressure of 600 MPa by using a punch, and accordingly, two types of green compacts that are a columnar green compact and a disk-shaped green compact were formed. A dimension of the columnar green compact has an outer diameter of 10 mm, and a height of 10 mm, and a dimension of the disk-shaped green compact has an outer diameter of 24 mm and a height of 8 mm. In sample numbers 4, and 9 to 39, the amount of raw material powders at which a density of a green compact becomes 6.4 Mg/m³ was calculated in advance, in sample numbers 1 to 3 and 5 to 8, the amount of raw material powders at which a density of a green compact takes values shown in Table 2 was calculated, and the amount of raw material powders to be filled in the mold hole was adjusted by a weighing capacity. In addition, a density of the obtained green compact was confirmed based on an Archimedes method.
The obtained two types of green compacts were heated at 1130 °C in a mixed gas atmosphere of hydrogen and nitrogen and sintered for 60 minutes at the same maintained temperature, and thereafter, the green compacts were cooled to a room temperature. At this time, an average cooling rate from the sintering temperature to 300 °C was 12°C/minute. In this manner, sintered bodies of sample numbers 1 to 39 were produced.
By using the obtained sintered body as the sintered alloy sample, a density, the amount of wear, a crystal grain size of the iron alloy matrix, and the thickness of an oxide film were measured by the following operations. Measurement results are shown in Table 3.

A density of the sintered alloy was measured in accordance with a sintered density test method for a metal sintered material as defined in Japanese Industrial Standard (JIS) Z2505 by using the columnar sintered alloy sample. The amount of wear of the sintered alloy was measured as the amount of wear in a roll-on-disk frictional and wear test by using the disk-shaped sintered alloy sample. In the roll-on-disk frictional and wear test, the sintered alloy sample was used as a disk material, the reciprocal sliding relative to a mated member was conducted for 15 minutes at 700 °C, and the amount of wear of the disk material was measured. As the mated member, a roll (an outer diameter: 15 mm and a length: 22 mm) obtained by applying a chromizing treatment to SUS316L equivalent materials of the JIS standard was used.

Further, the columnar sintered alloy sample was cut, a cross section of the sample was mirror-polished, and the cross section was corroded with aqua regia (nitric acid: hydrochloric acid = 1: 3), and thereafter, a metallic structure of the cross section was inspected under a microscope at a magnification of 200 times to inspect a structure of the matrix. At this time, an image analysis of the structure cross section was conducted by using WinROOF manufactured by MITANI CORPORATION as image analysis software, the image was binarized, an area of a matrix of austenite was measured, and the number of crystal grains in the matrix was measured, and accordingly, an average area of the crystal grains was calculated. An average crystal particle size of the crystal grains was determined by converting the value into an area circle equivalent diameter.

In addition, a strip-shaped sintered alloy sample having a major axis of 20 mm, a minor axis of 10 mm, and a height of 3 mm was cut out from the columnar sintered body having an outer diameter of 24 mm and a length of 8 mm by the machining. This sintered alloy sample was left in an atmosphere containing steam (temperature: 860 °C, test atmosphere: 8%, steam/air) for 100 hours, and then, is collected and cut, and as similar to the measurement of the above described crystal grains, a cross section of the sample was treated and a metallic structure of the cross section was inspected under a microscope. In the cross sectional inspection, the thickness of an oxide film was measured. In this measurement, three portions of an oxide film portion of a cross section image were arbitrarily selected to measure the thicknesses of those, and an average value of the measured values is displayed.

[Table 1]

Sample number	Iron alloy powder					Copper-phosphorus alloy powder			Graphite powder
	Blend proportion	Composition % by mass				Blend proportion % by mass	Composition % by mass		Blend proportion % by mass
	Blend proportion	Fe	Cr	Ni	Si	Blend proportion % by mass	Cu	P	Blend proportion % by mass
1	Remainder	Remainder	12.00	8.00	0.80	7.00	Remainder	15.00	0.60
2	Remainder	Remainder	15.00	8.00	0.80	7.00	Remainder	15.00	0.60
3	Remainder	Remainder	18.00	8.00	0.80	7.00	Remainder	15.00	0.60
4	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	0.60
5	Remainder	Remainder	22.00	8.00	0.80	7.00	Remainder	15.00	0.60
6	Remainder	Remainder	25.00	8.00	0.80	7.00	Remainder	15.00	0.60
7	Remainder	Remainder	30.00	8.00	0.80	7.00	Remainder	15.00	0.60
8	Remainder	Remainder	35.00	8.00	0.80	7.00	Remainder	15.00	0.60
9	Remainder	Remainder	20.00	0.00	0.80	7.00	Remainder	15.00	0.60
10	Remainder	Remainder	20.00	7.00	0.80	7.00	Remainder	15.00	0.60
4	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	0.60
11	Remainder	Remainder	20.00	12.00	0.80	7.00	Remainder	15.00	0.60
12	Remainder	Remainder	20.00	16.00	0.80	7.00	Remainder	15.00	0.60
13	Remainder	Remainder	20.00	20.00	0.80	7.00	Remainder	15.00	0.60
14	Remainder	Remainder	20.00	22.00	0.80	7.00	Remainder	15.00	0.60
15	Remainder	Remainder	20.00	24.00	0.80	7.00	Remainder	15.00	0.60
16	Remainder	Remainder	20.00	8.00	0.20	7.00	Remainder	15.00	0.60
17	Remainder	Remainder	20.00	8.00	0.50	7.00	Remainder	15.00	0.60
4	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	0.60
18	Remainder	Remainder	20.00	8.00	1.50	7.00	Remainder	15.00	0.60
19	Remainder	Remainder	20.00	8.00	2.00	7.00	Remainder	15.00	0.60
20	Remainder	Remainder	20.00	8.00	3.00	7.00	Remainder	15.00	0.60
21	Remainder	Remainder	20.00	8.00	3.50	7.00	Remainder	15.00	0.60
22	Remainder	Remainder	20.00	8.00	0.80	0.00	Remainder	15.00	0.60
23	Remainder	Remainder	20.00	8.00	0.80	1.00	Remainder	15.00	0.60
24	Remainder	Remainder	20.00	8.00	0.80	4.00	Remainder	15.00	0.60
4	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	0.60
25	Remainder	Remainder	20.00	8.00	0.80	10.00	Remainder	15.00	0.60
26	Remainder	Remainder	20.00	8.00	0.80	13.00	Remainder	15.00	0.60
27	Remainder	Remainder	20.00	8.00	0.80	15.00	Remainder	15.00	0.60
28	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	0.30	0.60
29	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	5.00	0.60
30	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	10.00	0.60
4	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	0.60
31	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	20.00	0.60
32	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	25.00	0.60
33	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	30.00	0.60
34	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	0.10
35	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	0.20
36	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	0.40
4	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	0.60
37	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	0.80
38	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	1.00
39	Remainder	Remainder	20.00	8.00	0.80	7.00	Remainder	15.00	1.50

[Table 2]

Sample number	Overall composition % by mass							Green compact Density Mg/m³	Sintering temperature °C
Sample number	Fe	Cr	Ni	Cu	Si	P	C	Green compact Density Mg/m³	Sintering temperature °C
1	Remainder	11.09	7.39	5.95	0.74	1.05	0.60	6.5	1130
2	Remainder	13.86	7.39	5.95	0.74	1.05	0.60	6.6	1130
3	Remainder	16.63	7.39	5.95	0.74	1.05	0.60	6.5	1130
4	Remainder	18.48	7.39	5.95	0.74	1.05	0.60	6.4	1130
5	Remainder	20.33	7.39	5.95	0.74	1.05	0.60	6.2	1130
6	Remainder	23.10	7.39	5.95	0.74	1.05	0.60	6.0	1130
7	Remainder	27.72	7.39	5.95	0.74	1.05	0.60	5.8	1130
8	Remainder	32.34	7.39	5.95	0.74	1.05	0.60	5.6	1130
9	Remainder	18.48	0.00	5.95	0.74	1.05	0.60	6.4	1130
10	Remainder	18.48	6.47	5.95	0.74	1.05	0.60	6.4	1130
4	Remainder	18.48	7.39	5.95	0.74	1.05	0.60	6.4	1130
11	Remainder	18.48	11.09	5.95	0.74	1.05	0.60	6.4	1130
12	Remainder	18.48	14.78	5.95	0.74	1.05	0.60	6.4	1130
13	Remainder	18.48	18.48	5.95	0.74	1.05	0.60	6.4	1130
14	Remainder	18.48	20.33	5.95	0.74	1.05	0.60	6.4	1130
15	Remainder	18.48	22.18	5.95	0.74	1.05	0.60	6.4	1130
16	Remainder	18.48	7.39	5.95	0.18	1.05	0.60	6.4	1130
17	Remainder	18.48	7.39	5.95	0.46	1.05	0.60	6.4	1130
4	Remainder	18.48	7.39	5.95	0.74	1.05	0.60	6.4	1130
18	Remainder	18.48	7.39	5.95	1.39	1.05	0.60	6.2	1130
19	Remainder	18.48	7.39	5.95	1.85	1.05	0.60	6.0	1130
20	Remainder	18.48	7.39	5.95	2.77	1.05	0.60	5.8	1130
21	Remainder	18.48	7.39	5.95	3.23	1.05	0.60	5.6	1130
22	Remainder	19.88	7.95	0.00	0.80	0.00	0.60	6.4	1130
23	Remainder	19.68	7.87	0.85	0.79	0.15	0.60	6.4	1130
24	Remainder	19.08	7.63	3.40	0.76	0.60	0.60	6.4	1130
4	Remainder	18.48	7.39	5.95	0.74	1.05	0.60	6.4	1130
25	Remainder	17.88	7.15	8.50	0.72	1.50	0.60	6.4	1130
26	Remainder	17.28	6.91	11.05	0.69	1.95	0.60	6.4	1130
27	Remainder	16.88	6.75	12.75	0.68	2.25	0.60	6.4	1130
28	Remainder	18.48	7.39	6.98	0.74	0.02	0.60	6.4	1130
29	Remainder	18.48	7.39	6.65	0.74	0.35	0.60	6.4	1130
30	Remainder	18.48	7.39	6.30	0.74	0.70	0.60	6.4	1130
4	Remainder	18.48	7.39	5.95	0.74	1.05	0.60	6.4	1130
31	Remainder	18.48	7.39	5.60	0.74	1.40	0.60	6.4	1130
32	Remainder	18.48	7.39	5.25	0.74	1.75	0.60	6.4	1130
33	Remainder	18.48	7.39	4.90	0.74	2.10	0.60	6.4	1130
34	Remainder	18.58	7.43	5.95	0.74	1.05	0.10	6.4	1130
35	Remainder	18.56	7.42	5.95	0.74	1.05	0.20	6.4	1130
36	Remainder	16.52	7.41	5.95	0.74	1.05	0.40	6.4	1130
4	Remainder	18.48	7.39	5.95	0.74	1.05	0.60	6.4	1130
37	Remainder	18.44	7.38	5.95	0.74	1.05	0.80	6.4	1130
38	Remainder	18.40	7.36	5.95	0.74	1.05	1.00	6.4	1130
39	Remainder	18.30	7.32	5.95	0.73	1.05	1.50	6.4	1130

[Table 3]

Sample number	Sintered body Density Mg/cm³	Iron alloy matrix Average crystal particle size µm	Amount of wear µm	Thickness of oxide film µm	Remark
1	7.2	30	27	30	Cr amount: Outside lower limit
2	7.2	25	25	10
3	7.2	23	23	8
4	7.2	21	22	5
5	7.1	21	23	3
6	7.0	21	22	5
7	6.9	20	22	10
8	6.8	20	22	30	Cr amount: Outside upper limit
9	6.8	20	22	45	Ni amount: Outside lower limit
10	7.0	20	22	10
4	7.2	21	22	5
11	7.2	21	22	5
12	7.2	21	22	5
13	7.3	22	22	5
14	7.3	22	22	5
15	7.4	22	22	5
16	7.3	18	22	32	Si amount: Outside lower limit
17	7.2	20	22	10
4	7.2	21	22	5
18	7.1	23	22	5
19	7.0	26	22	10
20	6.9	30	22	20
21	6.7	35	22	50	Si amount: Outside upper limit
22	6.6	28	20	100	Cu-P amount: Outside lower limit
23	6.9	26	21	15
24	7.1	23	22	7
4	7.2	21	22	5
25	7.3	27	18	12
26	7.4	32	15	20
27	7.0	-	-	-	Excessive liquid phases Cu-P amount: Outside upper limit
28	6.6	18	24	86	P amount: Outside lower limit
29	6.9	19	23	13
30	7.1	20	22	8
4	7.2	21	22	5
31	7.3	28	20	10
32	7.4	33	20	18
33	7.0	-	-	-	Excessive liquid phases P amount: Outside upper limit
34	6.8	35	50	75	C amount: Outside lower limit
35	7.0	29	35	15
36	7.1	25	28	7
4	7.2	21	22	5
37	7.2	20	18	10
38	7.3	18	10	20
39	7.4	18	5	60	C amount: Outside upper limit

The sintered alloys of sample numbers 1 to 8 have different chromium contents. In any of the samples, a crystal grain in the iron alloy matrix is small and the growth of the crystal grain in the matrix is preferably suppressed. As the chromium content increases, tendencies of decrease in a crystal grain size, the amount of wear, and the thickness of the oxide film are observed, and in a case where the chromium content is at least 18.48 % by mass of an overall composition, sizes of the crystal grains become substantially constant and the amount of wear becomes substantially constant also. Even if the chromium content exceeds 30 % by mass, the amount of wear does not increase. This is considered to be caused by the suppression in the generation and the grain growth of chromium carbide due to the small amount of added graphite, and wear seems to be suppressed by maintaining the amount of chromium in the matrix and preventing a decrease in a strength of the matrix. However, in a range where the chromium content exceeds 20% by mass, the thickness of the oxide film increases. This is considered to be caused by a fact that a reduction in the compressibility of each iron alloy powder having an excessively high chromium content decreases densities of a green compact and a sintered body, and this allows easy proceeding of oxidation from a surface. From these results, a preferable chromium content for the sintered alloy so as to have both a wear resistance and an oxidation resistance can be considered to be in a range from at least 13.86 % by mass and not more than 27.72 % by mass, more preferably can be considered to be in a range from at least 16.88 % by mass and not more than 23.10 % by mass, and most preferably, can be considered to be in a rage from at least 18.48 % by mass and not more than 20.33 % by mass.
The sintered alloys of sample numbers 4, and 9 to 15 have different nickel contents, and in any of the samples, a crystal grain of the iron alloy matrix is small and the growth of crystal grains in the matrix is preferably suppressed. A fact that the thickness of an oxide film is rapidly decreased by the addition of nickel indicates that an oxidation resistance of the sintered alloy is improved by nickel. It is considered that an increase in a density of a sintered body is caused by a large specific gravity of nickel. From results in Table 3, it can be expected that there is no inconvenience in material properties even if nickel content is at least 22.18 % by mass (sample number 15), and if a nickel content is at least 6.47 % by mass, the sintered alloy having a wear resistance and an oxidation resistance can be obtained. In a regard to oxidation resistance, it can be said that it is more preferable if nickel content is at least 7.39 % by mass.
The sintered alloys of sample numbers 4, and 16 to 21 have different silicon contents. As a silicon content increases, the thickness of an oxide film rapidly decreases further, and from this fact, it is understood that silicon is effective for improving oxidation resistance. However, if silicon content exceeds 2.77 % by mass, the thickness of an oxide film rapidly increases. This is considered to be caused by a reduction in a density of the sintered alloy in association with a reduction in the compressibility of the raw material powders and the reduction in an oxidation resistance by coarsening of crystal grains, and this can be understood from tendencies found in densities of a green compact and a sintered body and a crystal grain size of the iron alloy matrix. The densities of the green compact and the sintered body decrease as the silicon content further increases, and this is considered to be caused by a reduction in a compressibility of the iron alloy powders. In addition, a crystal grain size of the iron alloy matrix increases as a silicon content further increases. From these results, it is considered that an oxidation resistance is lowered by an insufficient density of a sintered body and coarsening of crystal grains. Accordingly, it is favorable if a silicon content is at least 0.46 % by mass and not more than 2.77 % by mass, and the silicon content may be preferably set to be at least 0.74% by mass and not more than 1.85% by mass.
The sintered alloys of sample numbers 4, and 22 to 27 have different blend proportions of copper-phosphorus alloy powders, and therefore, contents of copper and phosphorus in an alloy composition change depending on a blend proportion. It is understood that a density of the obtained sintered alloy increases by the addition of the copper-phosphorus alloy powders and an increase in the proportion thereof and the sintering of the matrix is promoted by the copper-phosphorus alloy powders. In addition, from a significant decrease in the thickness of an oxide film, it is understood that an oxidation resistance is improved. Further, if a blend proportion of the copper-phosphorus alloy powders exceeds 4 % by mass, the amount of wear decreases and the wear resistance is improved. However, if a blend proportion of the copper-phosphorus alloy powders exceeds 13 % by mass, a sintering density decreases, and this is caused by the excessive generation of liquid phases during sintering. For this reason, in sample number 27, measurements of the crystal grain size and the material property were omitted. In results shown in Table 3, sintered alloys of sample numbers 4, and 23 to 26 in which a blend proportion of the copper-phosphorus alloy powders is 1.00 to 13.00 % by mass has favorable wear resistance and oxidation resistance. A copper content in these sintered alloys is 0.85 to 11.05 % by mass and the phosphorus content in these sintered alloys comes to be 0.15 to 1.95 % by mass, and thus, these ranges can be considered as preferable contents of copper and phosphorus. More preferably, the copper content is at least 3.40 % by mass and not more than 8.50 % by mass and the phosphorus content is at least 0.60 % by mass and not more than 1.50 % by mass and the more appropriate copper content is at least 3.40 % by mass and not more than 5.95 % by mass and the more appropriate phosphorus content is at least 0.60 % by mass and not more than 1.05 % by mass.
The sintered alloys of sample numbers 4, and 28 to 33 have different alloy compositions in the used copper-phosphorus alloy powders. In these samples, as a phosphorus content proportion increases, a copper content proportion further decreases, but in any of the samples, a copper content of the obtained sintered alloy is in the above described preferred range. A density of a sintered body in sample number 28 is relatively low and an oxide film is thick, and from these, it is considered that this is caused by an insufficient phosphorus content and a low sintering promoting effect. In addition, a decrease in a sintering density in sample number 33 is caused by the excessive generation of liquid phases during sintering. For this reason, in sample number 33, measurements of the crystal grain size and the material property were omitted. From these results, in the used copper-phosphorus alloy powders, it is preferable if a phosphorus content is at least 5 % by mass and not more than 25% by mass, and it is more preferable if the phosphorus content is at least 10 % by mass and not more than 20 % by mass.
The sintered alloys of sample numbers 4, and 34 to 39 have different carbon contents, and a carbon content is designed to be in a low range from 0.10 to 1.50 % by mass to enhance the machinability of the sintered alloy. If the carbon content decreases in this range, tendencies of a decrease in a density of the sintered body and a increase in a crystal grain size of the iron alloy matrix are observed, but, even if the carbon content is lowered down to 0.20 % by mass, the amount of wear and the thickness of an oxide film is kept to be low. That is, it is understood that a wear resistance and an oxidation resistance are maintained. In sample number 39, during the generation of iron-phosphorus-carbon eutectic liquid phases, the amount of chromium solid solution in the matrix was reduced, and thus, it is considered that an oxidation resistance of the matrix is reduced also. Therefore, a preferable carbon content is at least 0.20 % by mass and not more than 1.00 % by mass, and it is more preferable if the carbon content is at least 0.4 % by mass and not more than 0.80 % by mass.

[Example2]

(Sample numbers 40 to 46)

In sample number 4 of Example 1, mixed powders as shown in Table 4 were prepared similarly except that 3.00 % by mass of the iron-phosphorus alloy powders (phosphorus content: 35.00 % by mass, average particle size: 40 µm) and 0 to 13.00 % by mass of the copper powders (average particle size: 30 µm) were blended instead of blending the copper-phosphorus alloy powders. By using the mixed powders as the raw material powders for molding, the mixed powders were molded to the disk-shaped green compact and the columnar green compact as shown in Table 5 (a density of a green compact: 6.4 Mg/m³) by the same operation as in Example 1. These were sintered under the same condition as in Example 1 to produce a sintered alloy sample and a density, the amount of wear, a crystal grain size of the iron alloy matrix, and the thickness of an oxide film were measured. The results are shown in Table 6.

[Table 4]

Sample number	Iron alloy powder					Iron-phosphorus alloy powder			Copper powder	Graphite powder
	Blend proport ion	Composition % by mass				Blend proportion % by mass	Composition % by mass		Blend proportion % by mass	Blend proportion % by mass
	Blend proport ion	Fe	Cr	Ni	Si	Blend proportion % by mass	Fe	P	Blend proportion % by mass	Blend proportion % by mass
40	Remai nder	Remai nder	20.00	8.00	0.80	3.00	Remai nder	35.00	0.00	0.60
41	Remai nder	Remai nder	20.00	8.00	0.80	3.00	Remai nder	35.00	0.85	0.60
42	Remai nder	Remai nder	20.00	8.00	0.80	3.00	Remai nder	35.00	3.00	0.60
43	Remai nder	Remai nder	20.00	8.00	0.80	3.00	Remai nder	35.00	6.00	0.60
44	Remai nder	Remai nder	20.00	8.00	0.80	3.00	Remai nder	35.00	8.50	0.60
45	Remai nder	Remai nder	20.00	8.00	0.80	3.00	Remai nder	35.00	11.05	0.60
46	Remai nder	Remai nder	20.00	8.00	0.80	3.00	Remai nder	35.00	13.00	0.60

[Table 5]

Sample number	Overall composition % by mass							Green compact Density Mg/m³	Sintering temperature °C
Sample number	Fe	Cr	Ni	Cu	Si	P	C	Green compact Density Mg/m³	Sintering temperature °C
40	Remainder	19.28	7.71	0.00	0.77	1.05	0.60	6.4	1130
41	Remainder	19.11	7.64	0.85	0.76	1.05	0.60	6.4	1130
42	Remainder	18.68	7.47	3.00	0.75	1.05	0.60	6.4	1130
43	Remainder	18.08	7.23	6.00	0.72	1.05	0.60	6.4	1130
44	Remainder	17.58	7.03	8.50	0.70	1.05	0.60	6.4	1130
45	Remainder	17.07	6.83	11.05	0.68	1.05	0.60	6.4	1130
46	Remainder	16.68	6.67	13.00	0.67	1.05	0.60	6.4	1130

[Table 6]

Sample number	Sintered body Density Mg/cm³	Iron alloy matrix Average crystal particle size µm	Amount of wear µm	Thickness of oxide film µm	Remark
40	7.2	25	25	30	Cu amount: Outside lower limit
41	7.1	24	25	20
42	7.1	23	24	11
43	7.1	22	23	6
44	7.0	22	23	6
45	6.9	26	24	12
46	6.7	35	25	64	Cu amount: Outside upper limit

In sample number 43 of Table 6, the density of the sintered alloy was 7.1 Mg/m³, an average crystal particle size of the iron alloy matrix was 22 µm, the amount of wear was 23 µm, and the thickness of the oxide film was 6 µm. An overall composition of sample number 43 was substantially the same as that of sample number 4, and when these samples are compared, it is understood that they are also the same in properties of the sintered alloy. Accordingly, it is understood that the sintered alloy having a wear resistance and an oxidation resistance can be obtained similarly even if the iron-phosphorus alloy powders and the copper powders were used in combination instead of using the copper-phosphorus alloy powders.
In Table 6, tendencies of further decreases in the crystal grain size of the alloy matrix and the amount of wear and the thickness of the oxide film as the compositional proportion of copper increases are observed. This is considered to be due to the protection of the passivation film of a surface layer and the improvement in the oxidation resistance by the solid solution of copper to the matrix. However, if the proportion of copper further increases, all of the crystal grain size of the matrix and the amount of wear and the thickness of the oxide film start to increase, and thus, it is preferable if the compositional proportion of copper is set to be in a range from 0.85 to 11.05% by mass.
In sample numbers 40 to 46, as the compositional proportion of copper increases, the density of the sintered alloy decreases further. Such tendency is not observed in sample numbers 22 to 25 using the copper-phosphorus alloy powders, and it is considered that such tendency is related to a balance between the formation of liquid phases and proceeding of sintering. In this regard, it is considered that the use of the copper-phosphorus alloy powders is more preferable than the combination use of the iron-phosphorus alloy powders and the copper powders.

[Example3]

(Sample numbers 47 to 52)

The mixed powders that are the same as those in sample number 4 of Example 1 were prepared. By using the mixed powders as the raw material powders for molding, the raw material powders were molded to the disk-shaped green compact and the columnar green compact by repeating the same operations as in Example 1 except that the amount of the raw material powders to be filled in a mold hole is changed so that a molding density of the green compact takes values as shown in Table 7. These are sintered under the same condition to produce a sintered alloy sample. A density of the sample, the amount of wear, a crystal grain size of the iron alloy matrix, and the thickness of an oxide film were measured. The results are shown in Table 7.

(Sample numbers 53 to 58)

By repeating the same operation as in sample number 4 of Example 1, mixed powders were prepared. By using the mixed powders as raw material powders for molding, the raw material powders were molded to the disk-shaped green compact and the columnar green compact as the same as in Example 1. By using theses, a sintered alloy sample was produced under the same condition as in Example 1 except that a sintering temperature was changed to a temperature shown in Table 7. A density of the sample, the amount of wear, a crystal grain size of the iron alloy matrix, and the thickness of an oxide film were measured. The results are shown in Table 7.

[Table 7]

Sample number	Green compact Density Mg/cm³	Sintering temperature °C	Sintered body Density Mg/cm³	Iron alloy matrix Average crystal particle size µm	Amount of wear µm	Thickness of oxide film µm	Remark
47	5.8	1130	6.6	22	23	84	Density: Outside lower limit
48	6.0	1130	6.9	21	22	16
49	6.2	1130	7.0	21	22	10
4	6.4	1130	7.2	21	22	5
50	6.6	1130	7.3	21	22	4
51	6.8	1130	7.4	20	22	3
52	7.0	1130	-	-	-	-	Molding: Not Possible
53	6.4	1030	6.7	10	35	64	Sintering temperature: Outside lower limit
54	6.4	1050	6.9	18	25	12
55	6.4	1100	7.0	20	23	3
4	6.4	1130	7.2	21	22	5
56	6.4	1140	7.2	28	24	7
57	6.4	1160	7.2	34	28	10
58	6.4	1180	7.0	-	-	-	Excessive liquid phases Sintering temperature: Outside upper limit

From the results of sample numbers 4, and 47 to 52, it is understood that the sintered alloy obtained by sintering a green compact having a density in a range from 6.00 to 6.80 Mg/m³ has good wear resistance and oxidation resistance. If the density of the green compact is small, the oxidation resistance is reduced by the insufficient density of the sintered alloy. In sample number 52, molding of a green compact was difficult, and a green compact having a density of more than 6.80 Mg/m³ was not able to be obtained. From the results shown in Table 7, the density of the sintered body is preferably at least 6.90 Mg/m³ and not more than 7.40 Mg/m³, and an adjustment may be made such that the density becomes preferably at least 7.00 Mg/m³ and not more than 7.40 Mg/m³. It is more appropriate if the sintering density is at least 7.20 Mg/m³ and not more than 7.40 Mg/m³.
From the results of sample numbers 4, and 53 to 58, it is understood that as a sintering temperature becomes high, an average crystal particle size of the iron alloy matrix becomes larger and the proceeding of sintering is promoted by a raise in the sintering temperature. The sintered alloy obtained by sintering at a temperature in a range from at least 1050 °C and not more than 1160 °C has good wear resistance and oxidation resistance. In sample number 53 having the sintering temperature of less than 1050 °C, the wear resistance and the oxidation resistance are low. This is considered to be caused by facts that eutectic liquid phases are not sufficiently generated and the strength of the iron alloy matrix may not be obtained. In sample number 58 having the sintering temperature of more than 1160 °C, a sintering density is reduced due to the excessive generation of liquid phases during sintering, and therefore, the measurements of the crystal grain size and the material properties were omitted. It is considered that a more preferable sintering temperature is in a range from at least 1100 °C and not more than 1140 °C.

[Example4]

(Sample numbers 59 to 65)

Mixed powders were prepared in the same manner as sample number 4 of Example 1 except that the iron alloy powders were changed to iron alloy powders (average particle size: 70 µm) obtained by alloying molybdenum so that each iron alloy powder in sample number 4 has an overall composition as shown in Table 8. By using the mixed powders as the raw material powders for molding and by repeating the same operations as in Example 1, the raw material powders were molded to the disk-shaped green compact and the columnar green compact (molding pressure: 600 MPa). These were sintered under the same condition as in sample number 4 to produce a sintered alloy sample and a density, the amount of wear, a crystal grain size of the iron alloy matrix, and the thickness of an oxide film were measured. The results were shown in Table 8.
According to the results of sample numbers 4, and 59 to 65, in any of the samples, the sintered alloy has a stable wear resistance and an excellent oxidation resistance. However, it is observed that as a molybdenum content increases, densities of the green compact and the sintered body tend to decrease further. This is considered to be caused by a slight decrease in the compressibility of the iron alloy powders by the alloying of molybdenum, and therefore, it is considered that molybdenum that is a carbide-forming element is preferably blended in a range of not more than 3.23% by mass. More preferably, the molybdenum content may be in a range from at least 0.46% by mass and not more than 2.77% by mass.

[Example 5]

In order to examine the machinability of the sintered alloys of Sample Nos. 4, 22 to 27, and 34 to 46, a turning tool made of cemented carbide was prepared, and a cylindrical sintered alloy sample was used as follows turning. That is, lathe processing (cutting speed: 50 m / min, cutting depth: 0.2 mm, feed speed: 0.05 mm / rotation) is performed on the end face of the sample from the outer peripheral side to the inner peripheral side with a cutting tool, and the total cutting distance is reduced. At the stage when the height reached 1000 m, the wear amount of the flank of the cutting tool (tool wear amount) was measured. The measured values are shown in Table 9 as a guide for evaluating the machinability.

[Table 9]

Sample number	Overall 1 composition % by mass							Amount of tool wear µm	Remark
Sample number	Fe	Cr	Ni	Cu	Si	P	C	Amount of tool wear µm	Remark
22	Remain der	19.88	7.95	0.00	0.80	0.00	0.60	80	Cu amount, P amount: Outside lower limit
23	Remain der	19.68	7.87	0.85	0.79	0.15	0.60	75
24	Remain der	19.08	7.63	3.40	0.76	0.60	0.60	69
4	Remain der	18.48	7.39	5.95	0.74	1.05	0.60	56
25	Remain der	17.88	7.15	8.50	0.72	1.50	0.60	58
26	Remain der	17.28	6.91	11.05	0.69	1.95	0.60	80
27	Remain der	16.88	6.75	12.75	0.68	2.25	0.60	88	Cu amount, P amount: Outside upper limit
34	Remain der	18.58	7.43	5.95	0.74	1.05	0.10	40	C amount: Outside lower limit
35	Remain der	18.56	7.42	5.95	0.74	1.05	0.20	44
36	Remain der	18.52	7.41	5.95	0.74	1.05	0.40	50
4	Remain der	18.48	7.39	5.95	0.74	1.05	0.60	56
37	Remain der	18.44	7.38	5.95	0.74	1.05	0.80	65
38	Remain der	18.40	7.36	5.95	0.74	1.05	1.00	75
39	Remain der	18.30	7.32	5.95	0.73	1.05	1.50	190	C amount: Outside upper limit
40	Remain der	19.28	7.71	0.00	0.77	1.05	0.60	85	Fe -P + Cu
41	Remain der	19.11	7.64	0.85	0.76	1.05	0.60	75	Fe -P + Cu
42	Remain der	18.68	7.47	3.00	0.75	1.05	0.60	69	Fe -P + Cu
43	Remain der	18.08	7.23	6.00	0.72	1.05	0.60	56	Fe -P + Cu
44	Remain der	17.58	7.03	8.50	0.70	1.05	0.60	60	Fe -P + Cu
45	Remain der	17.07	6.83	11.05	0.68	1.05	0.60	72	Fe -P + Cu
46	Remain der	16.68	6.67	13.00	0.67	1.05	0.60	80	Fe -P + Cu

According to the results of sample numbers 4, 22 to 27, and 40 to 46, it is understood that, by the addition of copper, the amount of tool wear decreases and the machinability is improved. The machinability is good when copper content is at least 0.85 % by mass. However, if the addition amount of copper further increases, the amount of tool wear starts to increase, and this is considered to be caused by the excessive proceeding of the hardening of the matrix by the solid solution of copper. In the results of Table 9, it can be considered that the machinability of the sintered alloy is improved when the copper content is in a range from 0.85 to 11.05 % by mass.
In addition, according to the results of sample numbers 4, and 34 to 39, a tendency of an increase in the amount of tool wear as the carbon content increases is observed. That is, it is evident that setting of a blending amount of graphite to be lower than a conventional amount is effective for improving the machinability, and it is understood that the machinability can be improved and tool wear can be suppressed by setting the carbon content of the sintered alloy to be not more than 1.00 % by mass.

INDUSTRIAL APPLICABILITY

A sintered alloy having excellent oxidation resistance, heat resistance and wear resistance, and an improved machinability can be provided, and thus, such sintered alloy can be applied to a turbo component for a turbocharger and can be advantageously applied to a component such as a nozzle body which is required to have a durability against a high temperature corrosive gas.

Claims

A sintered alloy, comprising:
by mass, 13.86 to 27.72 % of Cr, 6.47 to 20.33 % of Ni, 0.85 to 11.05 % of Cu, 0.46 to 2.77 % of Si, 0.15 to 1.95 % of P, 0.20 to 1.00 % of C, and a remainder of Fe and unavoidable elements as an overall composition, preferably further comprising 3.23 % or less of a carbide-forming element, the carbide-forming element being at least one element selected from the group consisting of Mo, V, W, Nb, and Ti;

having a density of 6.8 to 7.4 Mg/m³; and

having a metallic structure containing an iron alloy matrix with pores dispersed within the iron alloy matrix and a carbide dispersed in the iron alloy matrix, the iron alloy matrix comprising crystal grains with an average crystal particle size of 10 to 50 µm, wherein the average crystal particle size is determined as specified in the description.
The sintered alloy according to claim 1, comprising:
by mass, 13.86 to 27.72 % of Cr, 6.47 to 20.33 % of Ni, 0.85 to 11.05 % of Cu, 0.46 to 2.77 % of Si, 0.15 to 1.95% of P, 0.20 to 1.00 % of C, 3.23 % or less of a carbide-forming element, and a remainder of Fe and unavoidable elements as an overall composition;

having a density of 6.8 to 7.4 Mg/m³;

the carbide-forming element being at least one element selected from the group consisting of Mo, V, W, Nb and Ti; and

having a metallic structure containing an iron alloy matrix with pores dispersed within the iron alloy matrix and a carbide dispersed in the iron alloy matrix, the iron alloy matrix comprising crystal grains with an average crystal particle size of 10 to 50 µm, wherein the average crystal particle size is determined as specified in the description.
The sintered alloy according to claim 1 or 2, wherein a nitride is formed on a surface of the sintered alloy and an inner surface of the pores.
A method for producing a sintered alloy as defined in claim 1, the method comprising:
preparing an iron alloy powder comprising, by mass, 15 to 30 % of Cr, 7 to 24 % of Ni, 0.5 to 3.0 % of Si, and a remainder of Fe and unavoidable impurities, preferably further comprising 3% or less of a carbide-forming element, the carbide-forming element being at least one element selected from the group consisting of Mo, V, W, Nb, and Ti;

preparing a blending material for blending phosphorus and copper, the blending material containing one selected from an iron-phosphorus alloy powder having a phosphorus content of 10 to 30 % by mass, a copper-phosphorus alloy powder having a phosphorus content of 5 to 25 % by mass, and a copper powder, or a combination thereof;

preparing a raw material by mixing the iron alloy powder, the blending material, and a graphite powder such that the raw material powder contains 0.15 to 1.95 % by mass of phosphorus, 0.85 to 11.05 % by mass of copper, and 0.20 to 1.00 % by mass of carbon;

compressing the raw material powder into a green compact with a density of 6.0 to 6.8 Mg/m³; and

heating and sintering the green compact at a temperature in a range from 1050 to 1160 °C in a non-oxidizing atmosphere,

wherein the sintered alloy obtained has a metallic structure containing an iron alloy matrix with pores dispersed within the iron alloy matrix and a carbide dispersed in the iron alloy matrix, the iron alloy matrix comprising crystal grains with an average crystal particle size of 10 to 50 µm, wherein the average crystal particle size is determined as specified in the description.
The method for producing a sintered alloy as defined in claim 2 according to claim 4, the method comprising:
preparing an iron alloy powder comprising, by mass, 15 to 30 % of Cr, 7 to 24 % of Ni, 0.5 to 3.0 % of Si, 3% or less of a carbide-forming element, and a remainder of Fe and unavoidable impurities, the carbide-forming element being at least one element selected from the group consisting of Mo, V, W, Nb, and Ti;

preparing a blending material for blending phosphorus and copper, the blending material containing one selected from an iron-phosphorus alloy powder having a phosphorus content of 10 to 30 % by mass, a copper-phosphorus alloy powder having a phosphorus content of 5 to 25 % by mass, and a copper powder, or a combination thereof;

preparing a raw material powder by mixing the iron alloy powder, the blending material, and a graphite powder such that the raw material powder contains 0.15 to 1.95 % by mass of phosphorus, 0.85 to 11.05 % by mass of copper, and 0.20 to 1.00 % by mass of carbon;

compressing the raw material powder into a green compact with a density of 6.0 to 6.8 Mg/m³; and

heating and sintering the green compact at a temperature in a range from 1050 to 1160 °C in a non-oxidizing atmosphere.
The method for producing the sintered alloy according to claim 4 or 5, wherein the blending material is a powder that contains phosphorus in a form of either one or both of the iron-phosphorus alloy powder and the copper-phosphorus alloy powder, and that contains copper in a form of either one or both of the copper powder and the copper-phosphorus alloy powder.
The method for producing the sintered alloy according to any one of claims 4 to 6, wherein the blending material is any one of (1) to (5) below:
(1) a combination of the iron-phosphorus alloy powder having the phosphorus content of 10 to 30 % by mass and the copper powder;

(2) a combination of the iron-phosphorus alloy powder having the phosphorus content of 10 to 30 % by mass and the copper-phosphorus alloy powder having the phosphorus content of 5 to 25 % by mass;

(3) a combination of the iron-phosphorus alloy powder having the phosphorus content of 10 to 30 % by mass, the copper-phosphorus alloy powder having the phosphorus content of 5 to 25 % by mass, and the copper powder;

(4) a combination of the copper-phosphorus alloy powder having the phosphorus content of 5 to 25 % by mass and the copper powder; and

(5) the copper-phosphorus alloy powder having the phosphorus content of 5 to 25 % by mass.
The method for producing the sintered alloy according to any one of claims 4 to 7, wherein the non-oxidizing atmosphere is a normal pressure atmosphere consisting of a mixed gas of nitrogen and hydrogen containing at least 10% by mass of the nitrogen or a nitrogen gas.