GB2032457A - Hard Alloy Powder - Google Patents

Abstract

This invention provides a hard alloy powder having the composition: from 3 to 20% by weight of boron; not greater than 3% by weight of aluminium; not greater than 5% by weight of silicon; not greater than 2.5% by weight of oxygen; at least one metal selected from Cr, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co and Ni, and, when present, said metal being incorporated in the following amounts, from 5 to 35% by weight of Cr, from 1 to 35% by weight of Mo, from 0.5 to 30% by weight of W and less than 15% by weight of each of Ti, V, Nb, Ta, Hf, Zr, Co and Ni; and the balance, apart from incidental constituents and impurities, being at least 10% by weight of Fe. A process for the preparation of the hard alloy powder by melting the components and either water or gas atomizing is described and exemplified and the use of such a hard alloy powder in the preparation of a sintered hard alloy is also described and exemplified. Alternative methods of powder manufacture i.e. melting, solidifying and pulverizing, or by powder techniques are disclosed.

Description

SPECIFICATION Hard Alloy Powder This invention relates to a hard alloy powder comprising a boride or multiple boride of iron in which a part of the iron is substituted by at least one metal selected from Cr, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co and Ni, and to a method of making such a hard alloy powder.

Cemented carbides, especially materials comprising tungsten carbide as a hard phase, have hardness and strength, and have been widely used for metal cutting tools, metal moulds and the like.

However, tungsten, an important constituent of cemented carbides, is becoming increasingly scarce and therefore the cost of this raw material is becoming very high.

Furthermore, the specific gravity of such cemented carbides may be as high as 13 to 1 5.

Moreover, these cemented carbides have a very poor corrosion and oxidation resistance at high temperatures.

On the other hand, A Stellite (Registered Trade Mark) cobalt-based alloy comprising W and Cr, has a good corrosion resistance and a high oxidation resistance, but is poor in hardness and wear resistance. In order to improve the corrosion resistance, oxidation resistance at high temperature, high specific gravity, high cost of raw materials and so forth, we have proposed a sintered hard alloy, prepared from a hard alloy powder comprising iron boride or an iron multiple boride in which a part of the iron boride is substituted by a non-ferrous boride or multiple boride (See for example U.S. Patent Specification No. 3,999,952).

Improved corrosion resistance, oxidation resistance at high temperature, low specific gravity and low cost of raw materials are thereby attained but the sintered hard alloy described in this Specification does not have a comparable strength to that of the above cemented carbides.

This is mainly due to the nature of the hard alloy powder which is the main raw material of the sintered hard alloy.

We have now found that the inclusion of useful boride-forming elements such as Cr, Mo, W, V, Nb, Ta, Ti, Zr, Hf, Co and Ni, into this hard alloyed powder makes the sintered hard alloy stronger and harder; the strength of said sintered hard alloy can for example be improved to a level comparable with that of cemented carbides. In contrast, Al and C in small amounts in the hard alloy powder make the hard alloy powder brittle, with the result that the sintered alloy becomes poor in strength.

Thus, according to one aspect of the present invention, there is provided a hard alloy powder having the composition: from 3 to 20% by weight of boron; not greater than 3% by weight of aluminium; not greater than 5% by weight of silicon; not greater than 2.5% by weight of oxygen; at least one metal selected from Cr, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co and Ni, and, when present, said metal being incorporated in the following amounts, from 5 to 35% by weight of Cr, from 1 to 35% by weight of Mo, from 0.5 to 30% by weight of W and less than 15% by weight of each of Ti, V, Nb, Ta, Hf, Zr, Co and Ni; and the balance, apart from incidental constituents and impurities, being at least 10% by weight of Fe.

The present invention also provides a method of making such a hard alloy powder by atomization of the molten alloy with water or an inert gas.

Sintered hard alloys prepared from the hard alloy powders of the present invention essentially comprise a boride or multiple boride as a hard phase and a metal or alloy as a binder phase.

The hard phase is composed of an iron boride in which Fe is partially replaced by at least one metal selected from Cr, Mo, W, Ti, V, Nb, Hf, Ta, Zr, Co and Ni.

Thus, according to a further aspect of the present invention there is provided a sintered hard alloy comprising a hard phase and a metal or alloy binder phase wherein said hard phase comprises at least one iron boride in which iron is partially replaced by at least one metal selected from Cr, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co and Ni, and said binder phase comprises at least one metal selected from Cr, Fe, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co, Ni, Cu and alloys thereof, or one or more alloys composed mainly of said metals.

The structure of the boride is generally of the MB or the M2B type (in which M indicates a metal element) and the structure of the multiple boride is generally of the MXNyB type (in which M and N indicate different metal elements).

Boron is a basic component of the hard alloy powder of this invention and forms the hard phase borides and multiple borides as described in the foregoing.

The boron content in the hard alloy powder of the invention is from 3 to 20% by weight, preferably from 5 to 16% by weight.

When the boron content is in the range from 3 to 12% by weight, the hard alloy powder mainly consists of an M2B type boride and/or multiple boride. When the boron content is from 12 to 20% by weight, the hard alloy powder mainly consists of an MB type boride and/or multiple boride.

The sintered hard alloy of the invention also contains a binder phase which combines with the hard phase to improve the strength of the sintered hard alloy.

The binder phase comprises at least one metal selected from Cr, Fe, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co, Ni, Cu, and alloys thereof, or alloys composed mainly of these metals.

If the boron content is lower than 3% by weight, the hardness of the hard alloy powder does not reach the desired high level. Accordingly, the lower limit of the boron content is 3% by weight, preferably 5% by weight.

In contrast, if the boron content is too high, the hard alloy powder becomes brittle, resulting in a reduction of the shear strength, and it is difficult to obtain sufficient strength. Accordingly, in this invention, the upper limit of boron content is 20% by weight, preferably 16% by weight.

The iron content of the hard alloy powder is at least 10% by weight. The Vickers hardness (Hv) of iron boride is about 1300 to 1700 for Fe2B, and about 1800 to 2000 for FeB. As iron is one of the lowest cost metals which forms a boride or multiple boride, the iron content is preferably as high as possible, taking into account the wear resistance and strength of the hard alloy powder.

Cr forms stable and hard borides, of which the Vickers hardness is about 1 300 for Cr2B and about 1200 to 2000 for CrB. Cr boride also improves the corrosion resistance and oxidation resistance so that they are comparable to those of stainless steels or heat resistant steels, and its hardness is further enhanced and high hardness and high toughness may be maintained, even at high temperatures. The preferred Cr content is from 5 to 35% by weight. When the Cr content is lower than 5% by weight, the foregoing effects are not so pronounced. In contrast, when the Cr content is higher than 35% by weight, the increased benefits due to Cr are small.

Mo and W form stable borides not only at room temperature but also at high temperatures.

Moreover, the hardness of their borides is very high, for example, the hardness of Mo28 is about Hv 1660, that of MoB is about Hv 1750 to Hv 2350, that of W2B is about Hv 2420 and the hardness of WB is about Hv 3750. When Mo and/or W are included in the hard alloy powder of the invention, the resulting sintered hard alloy has a good wear resistance.

Furthermore, when W is included in the hard alloy powder of the invention, the resulting sintered hard alloy is very hard, and comparable to that of cemented carbides. Such sintered hard alloys may be useful for the manufacture of cutting tools, e.g. for cutting JIS SNCM-2 type steels.

When Cr, Wand Mo are included in the hard alloy powder simultaneously, not only the machinability but also the high corrosion resistance, high heat resistance and high oxidation resistance are superior to those of cemented carbides.

The Mo content in the hard alloy powder of this invention is from 1 to 35% by weight, and preferably from 5 to 30% by weight.

When the Mo content is lower than 1% by weight, the foregoing effects of Mo are not especially remarkable. In contrast, if the Mo content is higher than 35% by weight, the heat resistance and oxidation resistance are reduced.

The W content is from 0.5 to by weight. If the W content is lower than 0.5% by weight, the foregoing effects of W are not especially remarkable. Since W is more expensive than Mo and the world W resources are now decreasing, increasing the W content in the hard alloy powder makes it more costly. Therefore the upper limit of W content is desirably 30% by weight, preferably 20% by weight in view of the improvement of, for example, wear resistance, toughness and high cost.

When Co is included in the hard alloy powder of the invention, it forms a stable boride and/or multiple boride, and therefore wear resistance may be improved. The Co content in the hard alloy powder is generally less than 15% by weight.

If the Co content is too high, primary crystals may grow very quickly during liquid sintering, and thus the strength of this sintered hard alloy is reduced. The influence of Ni is similar to that of Co, and the Ni content is generally less than 15% by weight.

Ti, V, Nb, Ta, Hf and Zr are metals of Group IV-a or V-a of the Periodic-Table according to Mendeleev, and all form stable borides and/or multiple borides when included in the hard alloy powder according to the invention.

For example, the hardness of TiB is about Hv 2700 to Hv 2800, and that of VB2 is about Hv 2080 to Hv 2800. If suitable amounts of Ti, V, Nb, Ta, Hf and Zr are present in the hard alloy powder, each preferably being employed in an amount of less than about 15% by weight, the wear resistance and strength of the sintered hard alioy is improved, not only at room temperature but also at high temperatures.

In the sintered hard alloy according to the -invention, a binder phase is present in addition to a hard phase. The hard alloy powder of the invention forms the raw material of the hard phase; in contrast the binder phase comprises at least one metal selected from Cu, Ni, Co, Fe, Cr, Mo, W, Ti, Zr, V, Nb, Ta and Hf, and/or alloys of these metals, andlor alloys composed mainly of these metals.

Cu and Cu alloys generally have a relatively low melting point and Cu does not readily form a boride. It is considered that the Cu or Cu alloy is molten at the sintering temperature to form a liquid phase which is effective for increasing the density of the resulting sintered hard alloy.

Binder phase constituent elements other than Cu, Co, Fe and Ni generally have melting points higher than that of iron boride.

However, it is believed that as the sintering temperature is raised, the elements form a eutectic liquid phase with iron boride and hence liquid sintering is made possible. Consequently, the resulting sintered alloy is generally free of pores and it attains a substantially full density of 100% and becomes sufficiently dense and compact.

Although dimensional shrinkage on liquid sintering may be as high as 10 to 20%, uniform shrinkage may be accomplished without collapse of its shape by controlling the sintering temperature and the metal content. The sintered hard alloy consists of both the hard phase and the binder phase.

We have found that the control of the amounts of Al, Si, 0 and C in the hard alloy powder of the invention is very important to give superior strength to the sintered hard alloy produced therefrom.

Any Al which is present in the hard alloy powder appears to combine with B and oxygen during liquid sintering, which leads to a reduction in the rate of liquid sintering.

Accordingly, a uniform shrinkage or a full density of 100% cannot be obtained, and the strength of the sintered hard alloy deteriorates unless the Al content is limited. Thus, the Al content in the hard alloy powder should be not greater than 3% by weight, and preferably less than 1% by weight.

When C is present in the hard alloy powder it combines with oxygen during the liquid sintering to form CO or CO2 gas, which gas forms micropores in the sintered hard alloy The C content in the hard alloy powder should advantageously therefore be kept at not greater than 2% by weight, and preferably less than 1% by weight.

Oxygen combines with Al, C and other metals which are present in the hard alloy powder of the invention such as Cr, Ti, V, Nb and so forth, to form oxides. These oxides make the sintered hard alloy brittle. Accordingly, the oxygen content in the hard alloyed powder should not be greater than 2.5% by weight.

When Si is present in the hard alloy powder it accelerates the rate of liquid sintering while Al slows down the rate of liquid sintering. It is believed that the wettability and fluidity of the sintered hard alloy during liquid sintering is improved when Si is present in the hard alloy powder. When the Si content is less than 0.3% by weight, the effect of Si is little. In contrast, if the Si content is more than 5% by weight, the sintered hard alloy becomes brittle. Accordingly, the hard alloy powder of the invention should have an Si content of not greater than 5% by weight, and preferably from 0.3 to 5% by weight.

From an industrial viewpoint, the hard alloy powder may be most advantageously produced by the so-called water atomizing or gas atomizing method, which method comprises forming a molten alloy comprising Fe, ferro-boron and the desired additive element metals, and then ietting the molten alloy fall from small holes while atomizing the fine streams of molten alloy by high pressure water jets or argon or nitrogen gas jets projected from suitable nozzles.

Water atomization or gas atomization processes also result in the alloying elements, such as B, Cr, Mo, W, Ti and the like, being uniformly distributed. At the same time, the microstructure of a hard alloy powder so produced exhibits a very fine grain size.

It is also possible to utilise a method of manufacture in which the alloy is melted, solidified to form a boride alloyed ingot and pulverizing it mechanically, or alternatively a method comprising mixing ferro-boron powder with boride powders of other elements. We have found however that a sintered hard alloy formed from a hard alloy powder produced by water or gas atomization, as the raw material, has the best strength and hardness.

According to a further aspect of the invention there is provided a process for the manufacture of a hard alloy powder which comprises the steps of: a. preparing a mixture of materials comprising from 3 to 20% by weight of boron; not more than 5% by weight of silicon; at least one metal selected from Cr, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co and Ni in the following amounts, 5 to 35% by weight Cr, 1 to 35% by weight, Mo, 0.5 to 30% by weight W, and up to 1 5% by weight of each of Ti, V, Nb, Ta, Hf, Zr, Co and Ni;; and the balance being at least 10% by weight of Fe, b. melting said mixture to form a molten alloy in an atmosphere containing less than 30% by volume of oxygen, c. discharging a stream of said molten alloy in an atmosphere of an inert gas, and d. impinging water or an inert gas under - pressure against said molten alloy stream to atomize the stream of said molten alloy.

In the process of the invention, the starting materials are melted in an atmosphere comprising O2 in an amount less than 30% by volume, and preferably from 5 to 30% by volume. It is believed that any Al present in the starting materials, is preferentially oxidized and rises to the surface of the molten alloy as slag. The molten alloy is then allowed to fall from small holes in an inert gas atmosphere such as for example nitrogen, argon and like, so as to prevent oxidation of B and the desired additive elements during water or gas atomization.

The molten alloy is formed into particles by sheets or curtains of water or inert gas. The water or the inert gas is advantageously under a high pressure and impinges against the stream of the molten alloy to form particles thereof or to atomize the stream. The water or the gas is preferably directed against the stream of molten alloy at an angle of 10 to 200 from the vertical.

The water which is used for the atomization is conveniently at a pressure above 40Kg/cm2.

There is no maximum pressure limit for the water and normally, the maximum pressure is based on practical consideration of the pumping equipment used.

According to a still further aspect of the present invention there is provided a process for the preparation of a sintered hard alloy according to the invention which comprises mixing a metal or alloy powder with a hard alloy powder according to the invention, forming the mixture into a green compact, and sintering the green compact to thereby partially form a local liquid phase in the compact, said metal or alloy being selected from at least one of Cr, Fe, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co, Ni, Cu and alloys thereof, or one or more alloys composed mainly of these metals.

For the preparation of sintered hard alloys according to the invention, the desired metal or alloy powder forming the binder phase is mixed with the so prepared hard alloy powder, and the mixture is conveniently ground or pulverized to fine particles using a ball-mill or a vibration ballmill. The obtained mixture is conveniently formed into a green compact having a desired shape using a press or cold isostatic press. The green compact is sintered in a vacuum or in an atmosphere of hydrogen, argon or nitrogen gas, to thereby partially form a local liquid phase in the compact, whereby the density of the sintered body may be increased substantially to the maximum density of 100%.

In addition, it is possible to obtain a sintered body of high density using a hot isostatic press process or a hot press, either alone or in combination with a separate liquid sintering process.

The sintered hard alloy incorporating a hard alloy powder according to the invention generally has a Rockwell A scale hardness of 80 to 94 and a shear strength of 50 to 280 Kg/mm2, as measured according to the test method of JIS H5501 on tips of cemented carbide alloy.

The sintered hard alloy incorporating a hard alloy powder according to the invention may be used in those applications where high speed steels and cemented carbide alloys have heretofore been employed. More specifically, the sintered hard alloy of the invention may be used, for example, in the production of tools, dies or punches for drawing, ironing or swaging metals which are to be used at room temperature and at elevated temperatures; metal moulds for cold or hot working; cutting tools and heat resistant alloy articles to be used at high temperatures. In addition, it may be used when a high rust resistance, a high oxidation resistance, a high hardness and a high wear resistance may be required.

In particular, the hard alloy powder of this invention may be used for production of composite metal materials by laminating it to other metal substrates or spray coating it to metal substrates. Moreover, the hard alloy powder of this invention may be used for the production of composite metal materials, which comprises the hard alloy powder as a dispersed hard particle phase in a matrix phase composed of a metal or self-fluxing alloy. For example, a process for the production of such a dispersed composite material is by a known powder metallurgy technique of which the hard alloy powder and matrixing metal or alloy powder are mixed, then filled into a mould, and then heated until the matrix powder is fused. In the process of heating, the matrix powder fuses and surrounds the hard alloy powder.

In order to segregate the hard alloy powder in such a dispersed composite material, the mould may, if desired, be rotated at a high speed during the heating process, thereby making use of the difference in specific gravity between the hard alloy powder and the matrixing powder. The dispersed composite material according to the invention may be used where a high resistance and a high hardness are required. When the hard alloy powder is used for such dispersed composite materials or in spray coating, it is important to note that the content of Al, O, C and Si, included in the hard alloy powder, has a great influence on its properties such as toughness and hardness.

The invention will now be illustrated in the following Examples, which Examples are not intended to limit the scope of the invention.

Example 1 The following raw materials (having the weight percent compositions shown) for the preparation of a hard alloy powder were charged to a high frequency induction furnace:- Ferro-boron (B 20.0%, Al 1.57.3%, Si 0.9 1.4%, Fe balance); Electrolytic chromium metal (Cr 99.8%, Al 0.004%, Si 0.003%); Tungsten metal (W 99.84%, C 0.01%, Si 0.003%); Molybdenum metal (Mo 99.93%, C 0.01%, Al 0.004%); Ferro-vanadium (V 83.53%, C 0.12%, Si 1.15%, Al 1.5%, Fe balance); Electrolytic Iron (C 0.001%, Si 0.002%, Fe balance); and Silicon metal (Si 98.49%, Al 0.26%, C 0.03%).

The materials are melted to form a molten alloy, and the atmosphere in the fumace is argon gas mixed 20% by volume of 02. The molten alloy is transferred to a tundish, and molten alloy allowed to flow downward by gravity through an outlet nozzle having an internal diameter of 12 mm. Two oppositely directed streams of water, positioned at a downward angle of 15C relative to the axis of the molten alloy stream, are impinged against the alloy to atomize the molten alloy. The water is under a pressure of 70 Kg/cm2, and the atmosphere below the outlet nozzle is nitrogen gas.

The resulting atomized hard alloy powder has the following chemical analysis in weight percent.

per cent Boron 8.4 Chromium 8.8 Molybdenum 5.0 Tungsten 14.8 Vanadium 1.5 Aluminium 0.00 Silicon 0.77 Oxygen 0.28 Carbon 0.08 Manganese 0.10 Iron balance This is an M2B type hard alloyed powder.

Another hard alloy powder of MB type is prepared and atomized by the foregoing method, and has the following chemical analysis in weight percent: per cent Boron 15.0 Chromium 5.6 Tungsten 14.0 Vanadium 1.52 Aluminium 0.27 Silicon 1.87 Oxygen ' 0.33 Carbon 0.09 Manganese 0.11 Iron balance The thus obtained powders were mixed with Mo powder, Ni powder and an alloy powder (60% Cr, 20% V, balance Fe). The mixing ratio is as follows in weight percent: percent MB type hard alloy powder 40 M2B type hard alloy powder 10 Mo powder 44 Ni powder 1 60% Cr, 20% V, balance Fe alloy powder 5 The mixture is wet-milled for 1 68 hours in a ball-mill in the presence of ethyl alcohol solution and is then dried in a nitrogen gas atmosphere.

The dried particulate mixture is compactformed into a metal mould of a size of 5.2 mmxl0.4mmx32 mm under a moulding pressure of 1.5 ton/cm2. (In the subsequent Examples, green compacts had the same size as mentioned above, unless otherwise indicated).

The green compact was liquid sintered at 1 2000C in a vacuum of 10-3 mmHg for 30 minutes.

A compact sintered hard alloy having a shear strength of 1 90 Kg/mm2, a Rockwell A scale hardness (HRA) of 90.2 and a specific gravity of 7.96 g/cm3 is obtained.

Example 2 Two kinds of MB type hard alloy powder, incorporating ferro-boron, electrolytic chromium metal, tungsten metal and so forth as raw materials, are atomized by the same method described in Example 1. The resulting atomized MB type hard alloy powder has the following chemical analysis in weight percent: per cent Boron 14.3 Chromium 5.6 Tungsten 14.0 Aluminium 0.25 Silicon 1.71 Oxygen 0.30 Manganese 0.12 Carbon 0.08 Iron balance Another atomized MB type hard alloy powder prepared similarly had the following analysis in weight percent.

per cent Boron 14.0 Chromium 7.8 Vanadium 3.8 Aluminium 0.22 Silicon 1.30 Oxygen 0.32 Carbon 0.10 Manganese 0.09 Iron balance The two thus obtained powders are mixed with Mo powder, Ni powder and the M2B type hard alloy powder of Example 1.

The mixing ratio is shown as follows in weight percent: 15% MB type powder (composition 14.3% B; 5.6% Cr; 14.0% W alloy, etc.) 10% MB type powder (composition 14.0% B; 7.8% Cr; 3.8% V alloy, etc.) 30% M2B type powder (composition 8.4% B; 8.8% Cr; 5.0% Mo; 14.8% W; 1.5% V alloy, etc.) 44% Mo powder 1% Ni powder The mixture is wet milled for 1 68 hours in a ball-mill, dried in nitrogen gas and the compact formed under a moulding pressure of 1.5 ton/cm2.

The resulting green compact is sintered in a vacuum at 1 2250C for 30 minutes to obtain a sintered hard alloy having a shear strength of 1 98 Kg/mm2, a Rockwell A scale hardness (HRA) of 89.5 and a specific gravity of 8.10 g/cm3.

Example 3 MB type and M2B type hard alloy powders are atomized by the same method described in Example 1. The resulting atomized MB type hard alloyed powder has the following chemical analysis in weight percent per cent Boron 16.4 Chromium 11.0 Aluminium 0.30 Silicon 1.36 Oxygen 0.45 Carbon 0.07 Manganese 0.10 Iron balance The M2B type hard alloy powder has the following chemical analysis in weight percent: per cent Boron 9.0 Chromium 12.5 Aluminium 0.27 Silicon 0.95 Oxygen 0.31 Carbon 0.11 Manganese 0.09 Iron balance The powders obtained were mixed with Cr powder, Mo powder and Ni powder.The mixing ratio is shown as follows in weight percent: per cent MB type hard alloy powder 43 M2B type hard alloy powder 1 6 Cr powder 1 5 Mo powder 25 Ni powder 1 The mixture is wet milled for 168 hours in a ball-mill, dried in nitrogen gas and the compact formed under a moulding pressure of 1.5 ton/cm2.

The compact is sintered in a vacuum at 1200 C for 30 minutes to obtain a sintered hard alloy having a shear strength of 1 26 Kg/mm2 and a Rockwell A scale hardness (HRA) of 91.1.

This sintered hard alloy is measured for hardness at various temperatures, as shown in the accompanying Figure 1.

Figure 1 shows the Vickers hardness (100 g loaded), measured in a vacuum, of the product of this Example (No. 3) and also that of the cemented carbide D2 (WC7% CO) and the cemented carbide P10 (65% WC9% Co28% TiC+TaC).

Figure 1 shows that the product of this Example retains the highest hardness of the 3 products tested at elevated temperatures.

Example 4 An MB type hard alloyed powder was prepared by the same method described in Example 1. The resulting atomized MB type hard alloy powder has the following chemical analysis in weight percent: per cent Boron 14.0 Chromium 10.0 Tungsten 6.0 Aluminium 0.35 Silicon 1.72 Oxygen 0.31 Carbon 0.10 Manganese 0.08 Iron balance This hard alloy powder is mixed with Cr powder and Mo powder. The mixing ratio is shown as follows in weight percent: per cent MB type hard alloy powder 55 Cr powder 22.5 Mo powder 22.5 The mixture is wet milled for 1 68 hours in a ballmill, dried in nitrogen gas and a compact formed under a moulding pressure of 1.5 ton/cm2.

The compact was sintered in a vacuum at 1 2250C for 30 minutes to obtain a sintered hard alloy having a shear strength of 122 Kg/mm2 and a Rockwell A scale hardness (HRA) of 91.0.

This sintered hard alloy was measured for rust resistance at a high temperature in an air atmosphere.

The cemented carbide D-2, the heat-resisting steel SUH-3 (C 0.4%, Cr 11%, Mo 0.1%, Si 2.2%, Fe balance) and Stellite No. 1 (C 2%, Cr 30%, W 1 2%, Co balance) were measured as comparative examples.

In the testing-method, the weight increase by oxidation is observed when these samples are heated at 1 0000C for various periods in air.

Figure 2 of the accompanying drawings shows the results of these tests.

In Figure 2, "A" is the product of this Example, "B" is Stellite No. 1, "C" is the heat-resisting steel of the SUH-3 type, and "D" is the cemented carbide of D-2 type.

As is shown in Figure 2, the sintered hard alloy of this Example shows only a small weight increase by oxidation.

Claims (14)

Claims

1. A hard alloy powder having the composition: from 3 to 20% by weight of boron; not greater than 3% by weight of aluminium; not greater than 5% by weight of silicon; not greater than 2.5% by weight of oxygen; at least one metal selected from Cr, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co and Ni, and, when present, said metal being incorporated in the following amounts, from 5 to 35% by weight of Cr, from 1 to 35% by weight of Mo, from 0.5 to 30% by weight of W and less than 15% by weight of each of Ti, V, Nb, Ta, Hf, Zr, Co and Ni; and the balance, apart from incidental constituents and impurities,being at least 10% by weight of Fe.

2. A hard alloy powder as claimed in claim 1 wherein the Si content is from 0.3 to 5% by weight.

3. A hard alloy powder as claimed in either of claims 1 and 2 containing from 5 to 35% by weight of Cr.

4. A hard alloy powder as claimed in claim 1 substantially as herein described in any of the Examples.

5. A process for the manufacture of a hard alloy powder according to claim 1 which comprises the steps of: a. preparing a mixture of materials comprising from 3 to 20% by weight of boron; not more than 5% by weight of silicon; at least one metal selected from Cr, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co and Ni in the following amounts, 5 to 35% by weight Cr, 1 to 35% by weight Mo, 0.5 to 30% by weight W, and up to 15% by weight of each of Ti, V, Nb, Ta, Hf, Zr, Co and Ni; and the balance being at least 10% by weight of Fe, b. melting said mixture to form a molten alloy in an atmosphere containing less than 30% by volume of oxygen, c. discharging a stream of said molten alloy in an atmosphere of an inert gas, and d. impinging water or an inert gas under pressure against said molten alloy stream to atomize the stream of said molten alloy

6. A process as claimed in claim 5 wherein the water or inert gas is impinged against the molten alloy stream at an angle of from 10 to 20 with respect to the longitudinal axis of said stream.

7. A process as claimed in either of claims 5 and 6, wherein the pressure of the water jet is above 40 Kg/cm2.

8. A process as claimed in claim 5 substantially as herein described in any of the Examples.

9. A sintered hard alloy comprising a hard phase and a metal or alloy binder phase wherein said hard phase comprises at least one iron boride in which iron is partially replaced by at least one metal selected from Cr, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co and Ni, and said binder phase comprises at least one metal selected from Cr,F, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co, Ni, Cu, and alloys thereof, or one or more alloys composed mainly of said metals.

10. A sintered hard alloy as claimed in claim 9 substantially as herein described in any of the Examples.

11. A process for the preparation of a sintered hard alloy as claimed in claim 9 which comprises mixing a metal or alloy powder with a hard alloy powder as claimed in claim 1, forming the mixture into a green compact, and sintering the green compact to thereby partially form a local liquid phase in the compact, said metal or alloy being selected from at least one of Cr, Fe, Mo, W, Ti, V, Nb, Ta, Hf, Zr, Co, Ni, Cu and alloys thereof, or one or more alloys composed mainly of these metals.

12. A process as claimed in claim 11 wherein the green compact is sintered in a vacuum or in an atmosphere of hydrogen, argon or nitrogen gas.

13. A process as claimed in claim 11 substantially as herein described in any of the Examples.

14. An article formed of or containing a sintered powder comprising a hard alloy powder as claimed in claim 1.