Sintered Alloy Having Wear Resistance At High Temperature

Abstract

The present invention relates to sintered alloys having wear resistance at high temperature composed of a copper-base alloy containing copper as a principal constituent and 4 to 15 percent of tin, 4 to 25 percent of nickel, and 4 to 25 percent of chromium.

Description

BACKGROUND OF THE INVENTION

The present invention relates to copper-base sintered alloys having wear resistance and corrosion resistance at high temperature. These alloys are most suitable as construction materials for internal combustion engines, particularly the valving components of the engine.

Lead is commonly used as an additive for gasoline used in running internal combustion engines. As is well known, the lead additive prevents engine knock. The lead additive also has a lubricating effect which is produced by the adhesion of the lead onto the surface of the valves or valve seat. On the other hand, leadfree gasoline and liquid propane gas (LPG) have been used as fuel in response to corrective measures to the air-pollution problem. However, the use of such leadfree fuels results in a loss of the lubricating effect attributable to the lead and causes wear of conventional valving materials like cast iron or heat resistant steel. Eventually, the output of the engine is lowered.

The present invention eliminates the above-mentioned defects by providing sintered alloys having excellent wear resistance and corrosion resistance at high temperature.

SUMMARY OF THE INVENTION

The present invention relates to copper-base sintered alloys having excellent wear resistance and corrosion resistance at high temperature. These alloys are particularly adapted for fabricating valving equipment as used in internal combustion engines. The alloys are characterized by their composition range and contain copper as a principal constituent, 4 to 15 percent tin, 4 to 25 percent nickel, and 4 to 25 percent chromium.

DETAILED DESCRIPTION OF THE INVENTION

The sintered alloys according to the present invention are alloys containing chromium dispersed in copper-tin-nickel matrixes, namely copper-base sintered alloys composed of copper as a principal constituent, 4 to 15 percent tin, 4 to 25 percent nickel, and 4 to 25 percent chromium. The density is more than 88 percent of the theoretical one.

Since the sintered alloys of the present invention have remarkable wear resistance and corrosion resistance at high temperature, these alloys are most suitable for valve seats of internal combustion engines in which leadfree gasoline or LPG is used as fuel. The alloys are also applicable as bearing materials, especially for bearings subjected to high temperatures, such as, bearings for hot rollers.

The following description explains the function of each constituent element and the reason for restricting the composition range of the alloys. It is noted that percentages of the elements are all shown by weight.

Among the constituents of the sintered alloys of the present invention, copper excels in heat conductibility and corrosion resistance. It is remarkable that this constituent forms a copper oxide in conjunction with oxygen at high temperatures which imparts a lubricating effect thereby contributing to the wear resistance.

Tin, as in the case of copper, forms a film oxide at high temperature thereby contributing to the wear resistance. It also advances the mechanical strength. However, such effect is little observed at less than 4 percent, and at more than 15 percent the mechanical strength sharply drops as well as the ability of holding chromium. Therefore, the range is determined as 4 to 15 percent for tin.

Nickel possesses a heat resistance and a corrosion resistance. It permeates into copper in the form of a solid solution thereby strengthening the matrix. However, less than 4 percent nickel has little effect, and more than 25 percent does not cause a remarkable effect considering the quantity added. Hence, the nickel composition is preferably 4 to 25 percent.

Chromium does not reduce in hardness even at high temperature. Also it forms a chromium oxide together with oxygen at high temperature thereby increasing the wear resistance. However, such effect is slight at less than 4 percent. With up to 35 percent chromium the wear resistance advances in proportion to the quantity added, but the mechanical strength diminishes. Therefore, the preferred maximum is 25 percent.

Concerning the density of the alloy, when it is somewhat less than 88 percent of the theoretical one within the above-mentioned range of the constituents, the radiation at high temperature deteriorates and permanent strain of the engine structures results quite soon after engine start-up. This seriously affects the lifetime of the engine. Therefore, the density of the alloys should be more than 88 percent.

As described above, the sintered alloys of the present invention contain chromium which is hard and has excellent wear resistance at high temperature. The chromium is dispersed in massive form into copper-tin-nickel matrixes which are comparatively mild and have a good heat conductibility, wear resistance and corrosion resistance as well as a suitable material strength. Thus, these alloys consist of both the phase of hardness and that of mildness and have an excellent wear resistance particularly at high temperature. Furthermore, by providing the alloys with increased density, the heat conductibility is increased at high temperature and under heavy load thereby contributing to prevention of permanent strain.

It is further noted that the sintered alloys according to the present invention have the advantage of being simplified in their constituent materials and production method thereby maintaining easily a stable quality in mass production. The present invention is described as follows.

EXAMPLE 1

Electrolytic copper powder of under 100 mesh, atomized tin powder of under 100 mesh, electrolytic chromium powder of under 200 mesh and carbonyl nickel powder having an average particle size of 4.mu. are arranged and mixed so as to give copper 56 percent, nickel 20 percent, tin 4 percent and chromium 20 percent, each by weight percentage. Thereafter, the resulting mixture is formed under a forming pressure of 6 t/cm.sup.2 into a formed mass having a density of 7.4 g/cm.sup.3. Then, this formed mass is sintered at 900.degree.C. for 60 minutes in a neutral gas atmosphere, and the resultant sintered mass is again pressed under a pressure of 7 t/cm.sup.2 thereby increasing the density up to 7.6 g/cm.sup.3. A sintered alloy according to the present invention is obtained.

EXAMPLE 2

After the same powders as in Example 1 are arranged and mixed so as to give copper 74.8 percent, nickel 8.8 percent, tin 4.4 percent and chromium 12 percent, the mixture is formed under a forming pressure of 5 t/cm.sup.2 into a formed mass having a density of 7.5 g/cm.sup.3. Then,,the formed mass is sintered at 860.degree.C. for 60 minutes in a neutral gas atmosphere. The resulting sintered mass is again pressed to increase the density up to 8.1 g/cm.sup.3. A sintered alloy of the present invention is obtained.

The following table shows, along with those of the conventional materials, the results of the tests made relating to the characteristics and the quantities of wears at high temperature of the alloys of the present invention as obtained in Examples 1 and 2.

TABLE ______________________________________ Type of alloy Composition of Constituents (% by weight) Stensile Strength (kg/cm.sup.2) Hardness Hv(0.2) Quantities of wear ______________________________________ (mm) Alloys of the Present Invention - Example 1 56Cu-20Ni-4Sn-20Cr 24 400-450(Cr in the massive form) 120-140 (matrix) 0.37 Example 2 74.8Cu-8.8Ni-4.4Sn-12Cr 28 400-450(Cr in the massive form) 140-160 (matrix) 0.42 Comparison of Examples -Special Cast Iron Fe-3.5C-25Si-1Mn-0.5P-0.5Cr-0.5Mo-0.1V 40 250-300 7.42 Heat Resistant Steel Fe-0.4C-2Si-15Cr-15Ni-2W-0.6Mn 90 290-310 6.88 ______________________________________

In the above table, the quantities of wear are indicated by the worn away quantities in millimeters in the direction of height of the specimens measured after the testing continued for a fixed duration (100 hours) by the so-called "sliding high-cycle impact tester." This tester has a mechanism wherein 2,500 shocks a minute are given under a surface pressure of 30 kg/cm.sup.2 by means of a jig made of heat resistant steel while the angular specimens fixed to cast iron by special means are rotated 10 times a minute at an elevated temperature of 500.degree. to 550.degree.C.

Claims

1. A sintered copper based alloy compact having wear resistance at high temperature consisting essentially of 4 to 15 percent tin, 4 to 25 percent nickel, 4 to 25 percent chromium, the balance being copper, the alloy

2. A valve seat for an internal combustion engine comprising a sintered alloy as in claim 1.