EP1534871B1 - Use of a hard metal alloy - Google Patents

Abstract

The invention relates to the use of a hard metal alloy for parts which are subject particularly to torsion in addition to wear. Said hard metal alloy substantially contains tungsten carbide having an average grain size of less than 1.2 µm and 13 to 23 percent by weight of binder metal which comprises one or several metals that is/are selected among the group consisting of cobalt, iron, and nickel.

Description

The invention relates to the use of a hard metal alloy for screwdriver bits.

Such screwdriver bits are often referred to as screwdriver bits are received in a holder or directly in the screwing tool and usually have a hexagonal-shaped receiving shaft and a tip, also referred to as the output, on. The shape or profile of the tip is matched to the head shape of the screw to be screwed. In particular, screws with slotted heads, with Phillips heads of different shapes, and with inner Torx heads are widely used.

The screwdriver bits are primarily made of steel, which is usually cured to a hardness between 54 and 62 HRC. At this hardness, such screwdriver bits made of steel usually have sufficient toughness to accommodate the stress occurring during the screwing on torsion without damage.

Despite the relatively high hardness, it often comes in response to the profile of the screwdriver bit to a rapid wear and damage the tread edges and thus premature wear and / or damage to the screw heads.

It has therefore been tried in the past, the wear resistance of the surface of the screwdriver bits at least in the profile area by hard coatings or brazed carbide reinforcements continue to improve.

For example, describes the DE 40 29 734 C2 the formation of an anti-slip coating on screwdriver bits, wherein Reibstoffteilchen be applied in the arc process of an electrode on the hardened work surface of the screwdriver bit.

However, all these measures are complicated and expensive and can damage the structure of the screwdriver bit in the case of coating due to the long temperature effects during the coating process. In the screwdriver bits reinforced by soldered carbide reinforcements, the shear strength of the solder joint is often too low.

The JP 02 204592 A , the DE 296 17 040 U as well as the US-A-4753678 describe fine-grained tungsten carbide hard metal alloys as the main constituent with cobalt binder or with a binder system of iron, cobalt and nickel.

As parts for which these hard metal alloys are used, drilling tools or cutting tools are described which operate at high speeds or high cutting speeds and which do not have to transmit high torques and are primarily claimed for mechanical wear and only to a small extent on Torosion ,

The FR-A-2 469 250 describes more generally a carbide material such as tungsten carbide without further details of the exact composition for the production of specially designed screwdriver bits.

Wholly made of tungsten carbide screwdriver bits are still not used in practice because of concerns about too low torsional strength in practice.

The object of the present invention is to provide for screwdriver bits, a material that exceeds the previously known materials for such applications in its properties.

According to the invention, this is achieved by using a hard metal alloy consisting essentially of tungsten carbide with an average particle size of less than 1.2 μm and from 13 to 23% by weight binder metal selected from one or more metals from the group of cobalt, iron and nickel.

The use of this special hard metal alloy brings significant improvements in wear resistance and at the same time excellent results in terms of torsional strength. This was surprising in particular because not particularly hard carbide alloys, as expected, have good torsional strength, but rather an alloy group which has only below-average values with regard to toughness. The torsional strength depending on the specific shape and size of the manufactured part is in the range of 1,800 to 2,400 N / mm ² .

The fact that the alloy consists essentially of tungsten carbide means that minor amounts of other hard materials, in particular other carbides, of the order of up to about 10% by weight may be present in the alloy without significantly changing the advantageous properties.

A cemented tungsten carbide hard metal alloy having a mean grain size in the range from 0.7 to 0.9 μm and from 13 to 17% by weight cobalt binder has proven particularly suitable for the use according to the invention.

Additionally improved properties of the hard metal alloy according to the invention are present when a certain amount of coarse grain is present in the hard metal alloy.

An advantageous order of magnitude has proven to be a proportion of up to 200 grains / mm ² with a mean particle size in the range of 6 to 15 μm.

The formation of the coarse grain fraction is effected by a slight sintering of the hard metal alloy during production. Without forming a coarse grain fraction sintering of the hard metal alloy according to the invention takes place at a temperature of about 1400 ° C, during a period of about 60 minutes. The sintering to form the coarse grain fraction is achieved by increasing the sintering temperature to about 1440 ° C and extending the sintering time to about 90 minutes.

The highest levels of transferable torques are achieved when the screwdriver bits are manufactured by metal powder injection molding.

For this purpose, granules are produced from the hard metal powder mixture and an organic binder, such as waxes or polymers, by mixing and heated in an injection molding machine to temperatures between about 100 and 200 ° C and injected through a press into correspondingly executed molds. After cooling the mixture, the blanks, which already have excellent strength, are ejected from the injection mold and then debinded in respective ovens and sintered to form a liquid phase of the binder metal portion. The resulting volumetric shrinkage is on the order of about 50%, the achieved sintering density at nearly 100% of the theoretical density.

Through the use of metal powder injection molding, targeted fillets can be incorporated in the injection mold on particularly vulnerable edges on the screwdriver bit, which allows high torques to be transmitted without breakage hazard due to the reduced notch effect on the screwdriver bit.

Particularly advantageous in the metal powder injection molding of screwdriver bits is when in the injection mold directly below the screwdriver tip several parallel, about 45 ° to the longitudinal axis extending, web-shaped elevations are incorporated. These bumps cause the material injected into the mold to be forced into a directional flow to the tip of the driver bit, resulting in a particularly good filling and uniform density in this area, which is subjected to the greatest stress in the application.

Furthermore, the metal powder injection molding alone by a specific surface finish of the injection mold allows a particularly structured design of the profile surface of the screwdriver bit, through which even with a lower contact force of the screwdriver bit slipping it can be largely prevented from the screw head, whereby the service life of the screwdriver bit is extended. An additional enhanced effect in this direction can be achieved through the Incorporation of ultrahard particles, eg diamond grains, in the course of the injection molding process in the injection mold reach.

The invention will be described in more detail below with reference to a production example.

Preparation example

To test the torsional strength, rod-shaped samples of a hard metal alloy according to the invention, referred to as samples 1 in Table 1, were prepared from 85% by weight tungsten carbide with a mean grain size of 0.7 μm, balance cobalt. The samples had a hexagonal profile with a cylindrical central part. The total length of the samples was 38 mm with a key width of the hexagonal profile of 5 mm. The cylindrical center portion of the samples had a length of 16 mm and a diameter of 3.8 mm. The resulting cross section of the central part corresponds approximately to the shear cross section of the screwdriver bits most frequently used in practice.

On a die press, a block with the dimensions 70 mm x 46 mm x 25 mm was prepared by pressing the powder mixture of the hard metal alloy according to the invention with a pressure of 220 MPa.

From the pressed block, the samples were cut with their hexagon basic shape from the middle part by means of diamond discs. Then the samples were sintered at 1420 ° C for 60 minutes.

After sintering, the 38 mm diameter cylindrical center section with a tolerance of ± 5 μm was ground into the hexagonal profile of the samples. The samples prepared in this way were tested for bending strength and torsional strength on appropriate test equipment. The values of the bending strength and torsional strength of the tested samples are shown in Table 1.

For comparison, similar samples of a hard metal alloy, consisting of 75 wt.% Tungsten carbide with a mean grain size of 3 microns, balance cobalt, referred to in Table 1 as samples 2, further from a Tungsten carbide, consisting of 85 wt.% Tungsten carbide with a mean grain size of 1.5 microns, remainder cobalt, referred to in Table 1 as samples 3, and finally from a hard metal alloy consisting of 80 wt.% Tungsten carbide with a mean grain size of 1, 5 microns, balance cobalt, referred to in Table 1 as samples 4, prepared and tested as the samples of the hard metal alloy according to the invention on bending strength and torsional strength. The mean values of the strengths of these samples are also listed in Table 1.

As a further comparison, similar samples of hardened steel, referred to as Sample 5 in Table 1, were prepared. The steel had a composition customary for the production of screwdriver bits. These samples were also tested for flexural strength and torsional strength and their averages are listed in Table 1. <u> Table 1 </ u> rehearse composition Flexural strength torsional strength [N / mm ² ] [N / mm ² ] 1 85% by weight of WC, 0.7 μm 3300 2300 (Invention) Rest Co 2 75% by weight WC, 3 μm 2500 1200 Rest Co 3 85% by weight WC, 1.5 μm 3400 1900 Rest Co 4 80% by weight WC, 1.5 μm 3100 1350 Rest Co 5 stole 4400 2300

It can be seen from the table that the hard metal alloy according to the invention has the best values in terms of torsional strength compared with the other hard metal alloys. Your values in the Torsional strength is comparable to the values of the steel alloy. Since carbide also has significantly better wear resistance properties compared to steel, there is a serious advantage of the hard metal alloy. It is particularly surprising that hard metal alloys with high toughness and good or even higher bending strength have worse, sometimes even significantly worse values in the torsional strength.

Claims (7)

1. Use of a hard metal alloy substantially comprising tungsten carbide with an average particle size of less than 1.2 µm, and 13 to 23 wt.% of binder metal, selected from one or more metals from the group of cobalt, iron and nickel, for screwdriver inserts.
2. Use of a hard metal alloy according to claim 1, characterised in that the hard metal alloy consists of tungsten carbide with an average particle size in the range of 0.7 to 0.9 µm, and 13 to 17 wt.% of cobalt.
3. Use of a hard metal alloy according to claim 1 or 2, characterised in that it has a coarse-grain fraction of up to 200 particles/mm² with an average particle size in the range of 6 - 15 µm.
4. A screwdriver insert made from an alloy according to one of claims 1 to 3.
5. A process for the production of a screwdriver insert according to claim 4, characterised in that the production takes place by metal powder injection moulding.
6. The process for the production of a screwdriver insert according to claim 5, characterised in that several parallel fin-shaped protrusions running at approximately 45° to the longitudinal axis of the screwdriver insert are machined into the injection mould directly beneath the screwdriver tip.
7. A screwdriver insert produced by a process according to claim 6, characterised in that it has several parallel grooves running at approximately 45° to the longitudinal axis of the screwdriver insert directly beneath the screwdriver tip.