CH617371A5 - Mixture of metal powders - Google Patents

Abstract

This mixture, which can be used for manufacturing particles made from an iron alloy by conventional powder metallurgy techniques, has the following composition: Nickel 0.5 to 4 % by weight Manganese 0.5 to 6 % by weight Carbon (graphite) 0.05 to 1 % by weight Iron and usual complement to 100 % by impurities weight. The nickel and the magnanese are in the form of a powdered binary alloy having a nickel/manganese ratio between 15/85 and 65/35 by weight. The mixture may furthermore contain copper (up to 5 % by weight) and boron (up to 0.4 % by weight).

Description

The present invention relates to improvements in the composition of mixtures of metal powders, with which ferrous alloy articles can be produced by implementing the techniques of powder metallurgy. In particular, the invention relates to a metallic powder of this category, making it possible to manufacture articles having a significantly improved wear resistance characteristic.

English Patent No. 975322 describes and claims a mixture of metallic powders, from which articles of ferrous alloy can be obtained by the usual methods of powder metallurgy, this mixture being composed in the following manner:

Components

Nickel

Copper

Manganese

Boron

Carbon (graphite)

Common iron and impurities

Weight% 0.5 to 6 0.5 to 5 0.5 to 4 0.01 to 0.4 0.05 to 1.5

100% supplement

The addition of manganese as an elementary powder in said mixture has the disadvantage that manganese oxidizes easily, thereby giving a layer which is very difficult to reduce. The boron content of this mixture is required in order to reduce this oxide, since the presence of any trace of oxide prevents alloying and diffusion and, moreover, adversely affects the properties, particularly the wear resistance of the objects or articles made with said mixture of powders. However, the presence of boron fails to give a satisfactory solution to the problem posed by oxidation, when objects having a high resistance to wear are requested.

It has now been discovered that this problem, associated with the presence of manganese in the above-mentioned mixture, can be solved and that articles with improved wear resistance can be obtained by adding the content of nickel and manganese. of the mixture, only in the form of a binary alloy having a nickel / manganese weight ratio which can vary between 15/85 and 65/35. The use of this binary alloy makes it possible to vary the composition of the powder mixture, a variation which can go as far as outright exclusion of boron.

According to the present invention, a mixture of metallic powders is provided, from which ferrous alloy articles can be obtained by implementing the well-known method of powder metallurgy, the mixture being composed as follows:

Components% by weight

20 Nickel 0.5 to 4

Manganese 0.5 to 6

Carbon (graphite) 0.05 to 1.5

Copper (if applicable) up to 5

Boron (if applicable) up to 0.4

25 Common iron and impurities 100% supplement

mixture in which nickel and manganese are added in the form of a powder constituted by a binary alloy, having a nickel / manganese weight ratio of between 15/85 and 65/35. It is advantageous that the contents by weight of nickel, manganese and carbon in the mixture according to the invention are respectively the following:

Nickel 1.4 to 2.8%

Manganese 2.1 to 3.9%

35 Carbon 0.5 to 1.25%

However, it is preferable that, in order to obtain a product offering maximum resistance to wear, the percentages by weight of these constituents are:

40 Nickel 1.9 to 2.8

Manganese 3.0 to 3.9

Carbon 0.5 to 1.25

(especially 0.65 to 1.0)

An advantage of the use of a binary nickel / man-45 ganèse alloy, such as that which was mentioned above, results from the fact that this alloy, taken in the specified range, is present in liquid phase at a temperature close to 1150 ° C which is,

as is known, a commonly used sintering temperature; this is why its diffusion can easily develop in so the mass of the sintered composition. It is preferable to choose a nickel / manganese alloy with a nickel / manganese ratio between 20/80 and 55/45, each of these alloys being in the liquid phase at the temperature of 1100 ° C or, even better, to take a alloy with continuous solubility with a nickel / 55 manganese ratio of approximately 40/60, alloy which has a melting point close to 1025 ° C. However, it is understood that higher sintering temperatures reaching up to 1350 ° C can be used to obtain higher diffusion rates.

Preferably, the particle size of the binary alloy 60 is such that all of the particles of the alloy must pass through a BSS (British Sieve Series) mesh of 200 mesh (76 µm). But it is specially indicated that all the constituents of the mixture, except the iron powder, pass entirely through the BSS sieve of 300 mesh (53 (j.).

65 With regard to iron powder, it is preferable that all the particles pass through the BSS 100 mesh screen (152 (i), with 75% of these particles passing through the BSS 200 mesh screen (76 n) and 50% in the 300 mesh BSS sieve (53 n).

3

617,371

As described and claimed in pending British patent application No. 46670/75 in the name of the owner,

it is particularly recommended that at least 80% of the particles of binary alloy can pass through the BSS sieve of mesh 325 (about 44 µi). As explained in this application, this selection of the particle size ensures that the sintered product retains a high austenite content. It has been shown that the wear resistance of articles produced from metallic powders is directly proportional to the austenite content maintained, and this can be explained by the fact that the austenite breaks during the application of energy to form martensite and therefore produce an increase in hardness.

The carbon is preferably added in the form of a very fine graphite powder (micronized graphite) and in a percentage by weight of between 0.45 and 1.5.

Iron is preferably added in the form of a soft iron powder. A small fraction of the iron content can be replaced by the same weight of one or more other components, as long as these do not adversely affect the tensile strength and ductility of articles produced with these powder mixtures . The quantity of iron thus replaced must not, however, exceed 5% by weight of the total weight of the mixture. Below is a list of the elements which can be added in this way, the figures in brackets indicating the average content limit:

Al (1%), B (0.3%), Cr (5%), Mg (1%), Nb and / or Ta (4%),

P (0.3%), Si (1%), Ti (1%), W (4%), V (0.3%), Zr (0.6%),

Se (0.6%) and Pb (0.5%).

Copper can also be added to the composition and, when this is the case, its content should not exceed 5% by weight. The addition of copper has a favorable effect on the tensile strength of the mixture of sintered metal powders, but little or no effect on its resistance to wear which is an important characteristic of the composition of metal powders according to the invention. The copper is preferably added in the form of an elementary powder with a particle size such that all of the powder is preferably passed through a 100 mesh BSS sieve (152 | x).

It is an advantage for the composition of metallic powders according to the invention that it is not essential to add boron, which acts as a flux, although this constituent can be added if desired. When boron is added, its content can reach 0.4% by weight of the total weight of the composition. It can be introduced in the amorphous state or in the form of one or more key alloys (for example ferrobore) or alternatively in the form of one or more chemical compounds of boron, such as metallic borates (for example copper borate).

The powder mixture according to the present invention can be used to manufacture ferrous alloy articles by the traditional techniques of powder metallurgy. Thus, after weighing the various ingredients used in the composition of the mixture of metal powders, these ingredients are carefully and completely mixed in order to obtain a perfectly homogeneous mixture, to which lubricants such as paraffin wax, stearates, etc., which are well known. those skilled in the art can be incorporated in suitable proportions. The resulting mixture is agglomerated in a mold under a pressure of at least 2300 bars, then the agglomerate thus formed is ejected from the mold and sintered in a protective atmosphere (preferably ammonia and propane) at a temperature included between 1100 and 1350 ° C for at least 5 min.

The following examples are given by way of illustration of the invention, but it is understood that they do not impose any restriction in its field of application. All the percentages given have been calculated by weight and the temperatures are expressed in degrees centigrade.

Example 1:

A composition of metallic powders was obtained by completely mixing 4.0% of a binary nickel / manganese alloy (40/60 ratio), 1.0% of elementary copper powder, 0.8% of micronized graphite, the complement 100% being made with soft iron. The particle size of the soft iron powder was such that all passed through the 100 mesh BSS sieve (152 n), 75% passed through the 200 mesh BSS sieve (76 | i) and 50% passed through the BSS sieve 300 mesh (53 | i). All the other constituents were of particle size such that all of these constituents passed through a 300 mesh BSS sieve (53 n). A percentage of zinc stearate equal to 0.7% was added to the mixture as a lubricant.

Test samples in the form of test pieces with a diameter of 31 mm were produced from the above powder mixture, first by agglomeration to a gross specific gravity of 6.8 g / cm3, then by sintering at 1140 ° C for 30 min in a neutral atmosphere of ammonia and propane. The resulting sintered test pieces had an average surface hardness of the order of 210 (Vickers HV hardness 5 kg) and an average austenite content of approximately 9 with a standard deviation of the content approximately equal to 2.75. This result was therefore clearly better than that which one would have expected with a test tube obtained by the traditional technique, given, moreover, that the mixture did not contain boron as flux.

Example 2:

Four compositions of metallic powders were prepared, each having the same elementary composition, namely, 1.6% of nickel, 2.4% of manganese, 1.25% of carbon, 1.0% of copper, the complement to 100% having been made up of iron.

In each case, nickel and manganese were added in the form of a binary alloy having a nickel / manganese ratio equal to 40/60. The particle size of the binary alloy differed in each powder, as will be explained below. In each case, carbon was added in the form of micronized graphite, copper as elemental copper and iron as soft iron. The particle sizes of graphite, copper and iron were identical to those of Example 1.

The binary alloy in powder 1 was an atomized nickel / manganese powder which did not pass through the BSS 100 mesh screen (152 n). In powder 2, the binary alloy was a ground powder with a particle size distribution such that 0.2% of the powder did not pass through the 140 mesh BSS sieve (105 | X approximately), 5.2% did not did not pass through the 200 mesh BSS sieve, 38.0% did not pass through the 325 mesh BSS sieve (approximately 44 | i), 31.0% did not pass through the 400 mesh BSS sieve (approximately 37 x) and 25.6% passed through the 400 mesh BSS sieve (about 37%). In the case of powder 3, the binary alloy was an atomized powder with a distribution of particle dimensions such that 0.1% did not pass through the BSS sieve of 140 mesh, 0.2% did not pass through the BSS sieve of mesh 200, 10.3% did not pass through the BSS mesh of mesh 325, 26.0% did not pass through the BSS mesh of mesh 400 and 63.4% passed through the BSS mesh of mesh 400. Finally, in in the case of powder 4, the binary alloy was an atomized powder which passed entirely through the BSS sieve of mesh 400, this powder having been obtained by sieving the powder 3.

Each of these powders was thoroughly mixed and 0.7% zinc stearate was added to the mixture as a lubricant. Test pieces with a diameter of 31 mm were made from each of the powders, by agglomeration to a gross specific gravity of 6.8 g / cm 3 then by sintering at 1140 ° C for 30 min in a neutral ammonia / propane atmosphere. The test pieces were then subjected to hardness tests and an austenite content assay. The results of these tests were given in Table 1:

5

10

15

20

25

30

35

40

45

50

55

60

65

617,371

Table 1

Powder No.

1

2

3

4

Medium surface hardness

191.9

213.3

229.3

236.6

(HV 5 kg)

Average austenite content

6.57

8.96

15.5

17.15

Standard deviation of austenite content

2.63

2.71

2.73

1.68

The results appearing in Table 1 show that all the powders had better properties than those of the powders obtained by the traditional methods, taking especially into account the omission of boron in the mixtures according to the invention. However, it appears that the particle size of the powder constituted by the binary alloy has a very clear effect on the hardness and on the residual austenite content (and, consequently, on the wear resistance). Powder 3, in which 89.4% of the binary alloy is passed through the 325 mesh BSS sieve, is clearly superior to powders 1 and 2, but less than powder 4, which contains a binary alloy powder of which all (100%) is passed through the 400 mesh BSS sieve. Not only is the average austenite content of powder 4 higher than that of powder 3, but also the standard deviation is lower, that is, there are fewer variations in the value of the austenite content.

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Claims (8)

617,371 1. Mixture of metallic powders, usable for the manufacture of ferrous alloy articles having the following composition: Nickel 0.5 to 4% by weight Manganese 0.5 to 6% by weight Carbon (graphite) 0.05 to 1.5% by weight Iron and common impurities Complement to 100% by weight characterized by the fact that nickel and manganese are in the form of a powdery binary alloy having a nickel / manganese weight ratio of between 15/85 and 65/35. 2. Mixture according to claim 1, characterized in that it contains copper in an amount not exceeding 5% by weight. 2 3. Mixture according to claim 1, characterized in that it contains boron in an amount not exceeding 0.4% by weight. 4. Mixture according to claim 1, characterized in that the nickel, manganese and carbon are present according to the following weight percentages: Nickel 1.4 to 2.8 Manganese 2.1 to 3.9 Carbon 0.5 to 1.25 5. Mixture according to claim 4, characterized in that the said constituents are present according to the following weight percentages: Nickel 1.9 to 2.8 Manganese 3.0 to 3.9 Carbon 0.5 to 1.25 6. Mixture according to claim 5, characterized in that the carbon content is between 0.65 and 1.0% by weight. 7. Mixture according to one of claims 1 to 6, characterized in that the weight ratio nickel / manganese in the binary alloy is between 20/80 and 55/45. 8. A mixture according to claim 7, characterized in that said alloy is an alloy with continuous solubility having a nickel / manganese weight ratio equal to 40/60.