METHOD OF POWDER METALLURGICAL MANUFACTURING OF A COMPOSITE MATERIAL

Abstract: In a method of powder metallurgical manufacturing of a composite material containing particles in a metal matrix, said composite material having a high wear resistance in combination with a high toughness, the powder particles (I) of a first powder of a first metal or alloy having a high content of hard particles (HT) dispersed in the matrix of said first powder particles, are dispersed in a second powder consisting of particles (II) of a second metal or alloy having a low content of hard particles dispersed in the matrix of said second powder particles, wherein a mutual contact between the hard particles and/or between the particles of said first powder is substantially avoided, and the mixture of said first and second powders is transformed to a solid body through hot compaction.

Description: METHOD OF POWDER METALLURGICAL MANUFACTURING OF A COMPOSITE MATERIAL TECHNICAL FIELD The present invention relates to a method of powder metallurgical manufacturing of a composite material containing particles in a metal matrix, said composite material having a high wear resistance in combination with a high toughness. BACKGROUND OF THE INVENTION Wear resistant metal material conventionally consist of a solidified metal matrix in which hard particles such as borides, carbides, nitrides or intermetallic phases appear as inclusions. The wear resistance and the fracture toughness in such materials are usually highest when the hard particles are evenly dispersed in the metal matrix and when a net- like distribution is avoided. At a given amount of evenly dispersed hard particle the fracture strength of the material is reduced as the size of the hard particles is raised, while the fracture toughness is increased. This can be explained in the following way with reference to the accompanying Fig. la and lb. When the material is subjected to a tension or bending load, F, cracks are initially formed in the brittle hard particles, Fig. 1 A. These cracks are the greater, the greater the hard particles are, and propagate already at a low tension to fracture; in other words the fracture strength decreases as the sizes of the hard particles are raised. At a given content of hard particles, however, the mean spacing between the hard particles increases with the sizes of the hard particles, Fig. lb. Therefore, a plastic zone can be established in the metal matrix in front of a crack, avoiding further cracks in the hard particles, wherein the fracture toughness will increase in relation to the spacing between the hard particles. At a given content of hard particles and consequently at a given wear resistance, an improved fracture toughness is accompanied by an impaired fracture strength. BRIEF DISCLOSURE OF THE INVENTION It is the purpose of the present invention to provide a composite material containing particles in a metal matrix, wherein the material will have a high wear resistance in combination with a high fracture strength and fracture toughness. This can be achieved by a method defined in the characterizing part of the accompanying claim 1. Further characteristic features of the invention are disclosed in the subsequent claims and in the following description, wherein reference will be made to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. la and lb schematically describe the relationship between the sizes of the hard particles and the mechanical properties fracture strength and fracture toughness for a dispersion structure at a given content of hard particles Fig. 2a and 2b schematically illustrate a one step and a two step dispersion structure, respectively, at equal volume contents of hard particles, Fig. 3 shows a two step dispersion structure made from a mixture of a first powder I and a second powder π, and Fig. 4 is a graph diagram of the ratio between the mean diameters of a first an a second powder versus the volume content of the first powder I. DETAILED DESCRIPTION OF THE INVENTION According to the invention, the well-known dispersion structure of Fig. 2a, which is obtained by a one step procedure, wherein the hard particles HT in a metal matrix MM replaced by the dispersed structure achieved by a two step procedure, Fig. 2b. The two step dispersion structure of the invention, Fig. 2b, contains regions with a dense dispersion of fine, hard particles in a first metal matrix MM I, wherein these regions which are rich of fine, hard particles in their turn appear as a dispersion of inclusions in second metal matrix MM II, which is essentially lacking hard particles. The two step dispersion micro structure of the invention has a high fracture strength because of its small hard particle diameters in the first metal matrix MM I and also a high fracture toughness because of the large spacing between the hard particles in the second matrix MM Π. In the following, the advantages of the micro structure obtained by the two step dispersion in comparison with the one step dispersion micro structure will be explained with reference to an embodying example. At the manufacturing of the material accordin to the example, there was used as starting materials, gas atomised steel powders having alloy compositions shown in Table 1. Table 1: Chemica composition of used steel powders Metal Powder Content in weight-% C Cr Mo W Co V MP 1.28 4.2 5.0 6.4 8.5 3J MP I 2.3 4.2 7.0 6.5 10.5 6.5 MP Π 0.4 5.0 1.4 - - 1.0 The steel alloys also contained about 0.4 % Si, about 0.3 % Mn, and nitrogen and other impurities in amounts normal for high speed steels, balance iron. Test materials were made by hot isostatic pressing, and the materials were hardened and tempered to a hardness of about 900 HV30. The conventional one step dispersion structure was formed by metal powder MP and contained a fine dispersion of carbides having a mean diameter d of about 1 μm, representing a volume content of about 16%. The two step dispersion structure of the invention according to Fig. 3 was made from a mixture of metal powder MP I and MP II. In powder MP I there is formed a fine dispersion of carbides having a mean diameter di of about 1 μm, representing a volume content of about 30%. It is mixed with powder MP π, which is essentially lacking carbides, such that the carbide content in the test samples amounted to about 16 vol.-%. The structure regions formed of powder MP II contained about 2 vol.-% of fine carbides, and can be referred to as almost void of carbides, while the regions formed from powder MP I contained about 30 vol.-% of carbides, in other words they were rich of carbides. In order to achieve a dispersion of MP I particles in the bulk of MP D particles, the mean powder particle diameters Di and Dπ of the powders MP I and MP II, respectively, shall be selected such that the ratio Di/Dπ is increased with increasing volume content of powder MP I and such that it will lie above the border curve in Fig. 4, and preferably in the shadowed (obliquely lined) area A above the curve C in Fig. 4. In the example embodying the invention, indicated by E in Fig. 4,there was chosen a ratio D,/Dπ = 5. The test material having a dispersed structure made conventionally in one step and the dispersion structure made according to the invention in two steps had, when subjected to static bending, a fracture strength of about 3000-3200 MPa. In wear experiments, wherein the materials were subjected to wear against bound flint grains of mesh size 80 under a load of 1.31 N/mm 2 , the wear resistance of both the materials was measured to between 7.5 x 10 4 and 8 x 10 4 . Both the test materials in other words exhibited at an average about equal fracture strengths and wear resistances. The fracture toughness of the test material made in two steps according to the invention, however, was measured to 15 MPa/m which is more than 40% over the value for the conventional material made in one step, which was measured to only 10.5 MPa/m. Two die inserts were made of the test material of the invention, made in two steps, and the die inserts were shrunk into a cold forging tool for forming screws from a steel wire. In comparison to the conventional high speed steel S 6-5-2, which is being used according to prior art, the quantity of screws which was manufactured in the tool was increased with a factor 8 when working an annealed wire and with a factor 6.5 when working a cold drawn wire.

Claims: CLAIMS 1. Method of powder metallurgical manufacturing of a composite material containing particles in a metal matrix, said com osite material having a high wear resistance in combination with a high toughness, characterized in that the powder particles (I) of a first powder of a first metal or alloy having a high content of hard particles (HT) dispersed in the matrix of said first powder particles, are dispersed in a second powder consisting of particles (II) of a second metal or alloy having a low content of hard particles dispersed in the matrix of said second powder particles, that a mutual contact between the hard particles and/or between the particles of said first powder is substantially avoided, and that the mixture of said first and second powders is transformed to a solid body through hot compaction. 2. Method according to claim 1, characterized in that the mean diameter of the hard particles is less than a fourth of the mean diameter of the particles of said first powder. 3. Method according to claim 1 or 2, characterized in that the powder particles of the first powder contains more than 10 vol.-% of hard particles, and that the powder particles of the second powder contains less than 10 vol.-% of hard particles. 4. Method according to claim 3, characterized in that the powder particles of the first powder contains 10-20 voL-% of hard particles, and that the powder particles of the second powder contains less than 5 vol.-% of hard particles. 5. Method according to claim 1 or 2, characterized in that the powder particles of the first powder contains more than 20 vol.-% of hard particles, and that the powder particles of the second powder contains less than 10 voL-% of hard particles. 6. Method according to claim 5, characterized in that the powder particles of the second powder contains less than 8 vol.-% of hard particles. 7. Method according to any of claims 1-6, characterized in that the ratio (DiDπ) between the mean diameter (Di) the powder particles of the first powder and the mean diameter (Dπ) of the powder particles of the second powder is selected in dependency of the proportion of said first powder in a mixture of said first and second powders and is caused to lie in the shadowed (obliquely lined) area in the graph diagram in the accompanying Fig.4. 8. Method according to any of claims 1-7, characterized in that the hard particles consist of the type of compounds, phases or elements which belong to the group consisting of carbides, nitrides, borides, oxides, intermetallic phases and silicon. 9. Method according to claim 8, characterized in that the carbides, nitrides and/or borides essentially occur as compounds of carbon, nitrogen and/or boron on on hand, and one or more of the elements belonging to the group consisting of Fe, Ni, Cr, Mo, W, V, Nb, Ti, Ta, B, Si on the other hand. 10. Method according to claim 8, characterized in that the oxides essentially occur as compounds of oxygen and one or more of the elements belonging to the grou consisting of Ca, Mg, Al, Si, Cr, Ti, Zr, Y, Ce and La. 11. Method according to any of claims 1-10, characterized in that the first and second metals or alloys are aliminium alloys and that the hard particles to at least a significant degree are formed as primary or eutectic precipitation of silicon, Si. 12. Method according to any of claims 1-11, characterized in that the hard particles in the powder particles are established at the solidification of droplets of said first and second metals or alloys to form powder particles or at a heat treatment subsequent to said solidification. 13. Method according to claim 12, characterized in that at least the first powd is prepared by a process including gas atomization of the molten first metal or alloy to form particles having substantially spherical shape, that the powder particles of said firs and second powders, prior to mixing them with each other, are caused to have different particle size distributions and that the mean diameter (DJ of said first powder is caused to be larger than the mean diameter (Dπ) of said second powder. 14. Method according to claim 13, characterized in that also the second powd is prepeared by a process including gas atomization of the molten second metal or alloy to form particles having substantially spherical shape. 15. Method according to any of claims 1-10, characterized in that the powder particles are shaped by agglomeration of finer powder particles to adopt the approxima shapes of compact spheres. 16. Method according to any of claims 1-10, characterized in that the powder particles are shaped by agglomeration of finer powder particles to adopt compact polyedric shapes. 17. Method according to any of claims 12-16, characterized in that at least one of said first and second powders is prepared by a process including sieving of a bulk of powder to provide a powder having selected sizes. 18. Method according to any of claims 1-17, characterized in that the ratio between the mean diameters of the particles of the first and second powders satisfy the expression 1.6 < < 30, where ~ DII ~ Dj is the mean diameter of the particles of the first powder, and Dπ is the mean diameter of the particles of the second powder. 19. Method according to claim 18, satisfying the expression 2 <^ <10. ~ DII ~ 20. Method according to claim 19, satisfying the expression 3 < — < 8. ~ DII ~ 21. Method according to any of claims 1-20, characterized in that said first and second metals or alloys consist mainly of any of the elements belonging to the group consisting of Fe, Ni, Co, Cu and Al and that at least said first alloy is alloyed to provide harder particles and desired features. 22. Method according to any of claims 1-21, characterized in that the hot compaction is carried out through any of the following techniques: vacuum sintering, pressure sintering or hot isostatic pressing. 23. Method according to any of claims 1-22, characterized in that the first metal or alloy is an alloy which contains, expressed in weight-%, more than totally 1% of C, N, B, and O; 0-2 Mn, 0-3 Si, and more than totally 15% of metals having a high afiBnrty to C, N, B, and O to form carbides, nitrides, borides, and/or oxides, said metals including Cr, Mo, W, V, Nb, Ta, Zr, Ti, and Al, and that the second metal or alloy contains less than totally 1% of C, N, B, and O, 0-2 Mn, 0-3 Si, and less than totally 15% of said metals having a high affinity to C, N, B, and O, balance in both said first and said second alloy iron, cobalt and nickel and incidental impurities and accessory elements in normal amounts. 24. Method according to claim 23. characterized in that said first alloy contains more than totally 1.5 % of C, N, B, and O, and totally more than 18% of said metals having a high affinity to C, N. B, and O. 25. Method according to claim 24, characterized in that said first alloy contains more than totally 2.0% of C, N, B, and O, and totally more than 22% of said metals having a high affinity to C, N, B, and O. 26. Method according to claim 23, wherein the second alloy contains less than totally 0.9% of C, N, B, and O, and less than totally 14% of said metals having a high affinity t C, N, B, and O. 27. Method according to claim 26, wherein the second alloy contains less than totally 0.6% of C, N, B, and O, and less than totally 10% of said metals having a high affinity to C, N, B, and O. AMENDED CLAIMS [received by the International Bureau on 15 July 1996 (15.07.96); original claims 1-27 replaced by amended claims 1-26 (4 pages)] 1. Method of powder metallurgical manufacturing of a composite material containing particles in a metal matrix, said composite material having a high wear resistance in combination with a high toughness, characterized in that the powder particles (I) of a first powder of a first metal or alloy having a high content of hard particles (HT) dispersed in the matrix of said first powder particles, are dispersed in a second powder consisting of particles (II) of a second metal or alloy having a low content of hard particles dispersed in the matrix of said second powder particles, the ratio (D-/Dιi) between the mean diameter (Di) the powder particles of the first powder and the mean diameter (Dπ) of the powder particles of the second powder being selected in dependency of the proportion of said first powder in a mixture of said first and second powders and caused to lie in the shadowed (obliquely lined) area in the graph diagram in the accompanying Fig.4, that a mutual contact between the hard particles and/or between the particles of said first powder is substantially avoided, and that the mixture of said first and second powders is transformed to a solid body through hot compaction. 2. Method according to claim 1, characterized in that the mean diameter of the hard particles is less than a fourth of the mean diameter of the particles of said first powder. 3. Method according to claim 1 or 2, characterized in that the powder particles of the first powder contains more than 10 vol.-% of hard particles, and that the powder particles of the second powder contains less than 10 vol.-% of hard particles. 4. Method according to claim 3, characterized in that the powder particles of the first powder contains 10-20 vol.-% of hard particles, and that the powder particles of the second powder contains less than 5 vol.-% of hard particles. 5. Method according to claim 1 or 2, characterized in that the powder particles of the first powder contains more than 20 vol.-% of hard particles, and that the powder particles of the second powder contains less than 10 vol.-% of hard particles. 6. Method according to claim 5, characterized in that the powder particles of the second powder contains less than 8 vol.-% of hard particles. 7. Method according to any of claims 1-6, characterized in that the hard particles consist of the type of compounds, phases or elements which belong to the group consisting of carbides, nitrides, borides, oxides, intermetallic phases and silicon 8. Method according to claim 7, characterized in that the carbides, nitrides and/or borides essentially occur as compounds of carbon, nitrogen and/or boron on o hand, and one or more of the elements belonging to the group consisting of Fe, Ni, C Mo, W, V, Nb, Ti, Ta, B, Si on the other hand. 9. Method according to claim 7, characterized in that the oxides essentially occur as compounds of oxygen and one or more of the elements belonging to the gro consisting of Ca, Mg, Al, Si, Cr, Ti, Zr, Y, Ce and La. 10. Method according to any of claims 1-9, characterized in that the first and second metals or alloys are aliminium alloys and that the hard particles to at least a significant degree are formed as primary or eutectic precipitation of silicon, Si. 11. Method according to any of claims 1-10, characterized in that the hard particles in the powder particles are established at the solidification of droplets of said first and second metals or alloys to form powder particles or at a heat treatment subsequent to said solidification. 12. Method according to claim 11, characterized in that at least the first pow is prepared by a process including gas atomization of the molten first metal or alloy to form particles having substantially spherical shape, that the powder particles of said fi and second powders, prior to mixing them with each other, are caused to have differe particle size distributions and that the mean diameter (Di) of said first powder is cause to be larger than the mean diameter (Dπ) of said second powder. 13. Method according to claim 12, characterized in that also the second pow is prepeared by a process including gas atomization of the molten second metal or all to form particles having substantially spherical shape. 14. Method according to any of claims 1-9, characterized in that the powder particles are shaped by agglomeration of finer powder particles to adopt the approxim shapes of compact spheres. 15. Method according to any of claims 1-9, characterized in that the powder particles are shaped by agglomeration of finer powder particles to adopt compact polyedric shapes. 16. Method according to any of claims 11-15, characterized in that at least one of said first and second powders is prepared by a process including sieving of a bulk of powder to provide a powder having selected sizes. 17. Method according to any of claims 1-16, characterized in that the ratio between the mean diameters of the particles of the first and second powders satisfy the expression 1.6 < < 30, where ~ DII ~ D] is the mean diameter of the particles of the first powder, and Dπ is the mean diameter of the particles of the second powder. 18. Method according to claim 17, satisfying the expression 2 <^<10. ~ DII ~ 19. Method according to claim 18, satisfying the expression 3 < — < 8. _ DII ~ 20. Method according to any of claims 1-19, characterized in that said first and second metals or alloys consist mainly of any of the elements belonging to the group consisting of Fe, Ni, Co, Cu and Al and that at least said first alloy is alloyed to provide harder particles and desired features. 21. Method according to any of claims 1-20, characterized in that the hot compaction is carried out through any of the following techniques: vacuum sintering, pressure sintering or hot isostatic pressing. 22. Method according to any of claims 1-21, characterized in that the first metal or alloy is an alloy which contains, expressed in weight-%, more than totally 1% of C, N, B, and O; 0-2 Mn, 0-3 Si, and more than totally 15% of metals having a high affinity to C, N, B, and O to form carbides, nitrides, borides, and/or oxides, said metals including Cr, Mo, W, V, Nb, Ta, Zr, Ti, and Al, and that the second metal or alloy contains less than totally 1% of C, N, B, and O, 0-2 Mn, 0-3 Si, and less than totally 15% of said metals having a high affinity to C, N, B, and O, balance in both said first and said sec alloy iron, cobalt and nickel and incidental impurities and accessory elements in norm amounts. 23. Method according to claim 22. characterized in that said first alloy contai more than totally 1.5 % of C, N, B, and O, and totally more than 18% of said metals having a high affinity to C, N. B, and O. 24. Method according to claim 23, characterized in that said first alloy contai more than totally 2.0% of C, N, B, and O, and totally more than 22% of said metals having a high affinity to C, N, B, and O. 25. Method according to claim 22, wherein the second alloy contains less than totally 0.9% of C, N, B, and O, and less than totally 14% of said metals having a high affinit C, N, B, and O. 26. Method according to claim 25, wherein the second alloy contains less than totally 0.6% of C, N, B, and O, and less than totally 10% of said metals having a high affinit C, N, B, and O.

Inventors: BERNS HANS, BERNS, HANS
Applicants: BERNS HANS, ERASTEEL KLOSTER AB, KOEPPERN GMBH & CO KG, BERNS, HANS, ERASTEEL KLOSTER AKTIEBOLAG, KOEPPERN GMBH & CO. KG
Application year: 1996
Main classification: C22C1
IPC codes: C22C1, C22C29, C22C33