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.