The invention relates to a method of producing a wire-drawing die, in which method a core mounted in an annulus is secured in a meta+, housing and the core is provided with a drawing passage. The core may consist of a material such as polycrystalline diamond, polycrystalline cubic boron nitride or a mixture thereof.

In this connection "polycrystalline diamond" must be understood to mean an aggregate of synthetic diamond. Polycrystalline diamond is commercially available under various designations ("Compax - General Electric Company USA, "Syndite" - De Beers Industrial Diamond Division). On delivery, the aggregate of synthetic diamond is usually attached to a cemented carbide substrate (for example WC + Co). The substrate may be flat or annular. In the latter case the aggregate of synthetic diamond fills the opening in the annular substrate. The last- mentioned embodiment is usually used for the production of wire-drawing dies. However, the heat conductivity of cemented carbides is relatively low, which may be a drawback for this use. It is further necessary to use special tools for the production of each individual size of the synthetic diamond-cemented carbide annular combination. Sometimes the cemented carbide annulus must be after-treated in order to render it possible to secure it in a metal housing, for example by means of shrinking or pressing.

Polycrystalline cubic boron nitride is also commercially available ("Amborite"- De Beers Industrial Diamond Division and "Barozon CBN " - General Electric Company USA).

Wire-drawing dies having an aggregate of synthetic diamond mounted in a cemented carbide annulus are commercially available. Usually the core mounted in a cemented carbide annulus is fitted in a metal housing by means of a shrinking or-pressing operation. For one type of wire-drawing die, the polycrystalline diamond core with cemented carbide annulus is provided with an envelope of brass (37.8% by weight of Zn, 3.4% by weight of Pb, remainder Cu) by means of an upsetting operation in such a way that the raised edges of the envelope are just clear from the drawing passage after the latter has been formed. The core mounted in the annulus is fitted by cold pressing in a metal housing consisting of austenitic chromium-nickel steel, a plug of austenitic chromium-nickel steel also being pressed into the housing.

It is an object of the invention to provide a method of producing a wire-drawing die, in which the starting point may be a core which is not mounted in a cemented carbide annulus, the core being subjected to a permanent compressive stress to reduce the sensitivity to tearing of the core at tensile stresses such as may occur during the drawing of metal wire. This object is accomplished by means of a method which is characterized in that the core is clamped in an annulus of a metal alloy which can be strengthened by means of a deformation and/or heat treatment, the annulus being strengthened during clamping and that the core-annulus combination is fitted in a metal housing. A cylindrical core is preferably used in this method because such a shape ensures the most uniform stress distribution possible in the core and a uniform heat dissipation. The metal housing may have the customary cylindrical shape. The core can be fixed in the metal housing in a conventional manner using for example a retaining plug.

Strengthening of the annulus has for its object to increase the elasticity limit τ_0.2, which results in the annulus exerting a permanent radial compressive stress on the core. This causes the tensile stress in the core to be reduced during drawing and, consequently, the sensitivity to tearing ofthe core material. Preferably, the material of the annulus consists of a metal alloy which, when heated to a temperature of some hundreds of degrees Celsius above the ambient temperature, such as may occur in certain circumstances during the use of the wire-drawing die, does not lose the strength it has obtained or whose strength increases still further, for example by means of a coherent or incoherent dispersion hardening.

The method according to the invention preferably uses alloys which have a good heat conductivity, so that the heat generated during drawing or supplied by the hot wire can be dissipated and the core is not heated to an impermissible high temperature and/or is loaded to an impermissible extent by temperature stresses.

In practice high temperatures, as high as, for example, 450⁰C with tungsten and 600⁰C with some kinds of steel, may be produced in the wire-drawing die during the drawing of wires.

The method according to the invention can be carried into effect as follows. A cylinder consisting of a metal alloy which can be strengthened,having an axial bore with a larger diameter than the diameter of the core is placed in the central opening of a metal housing of a suitable shape. A core is placed in the bore of the cylinder. The dimensions of the cylinder, the core and the metal housing can be chosen so that the cylinder can be deformed to a sufficient extent to clamp the core. This method can be carried into effect with either a preheated or an unheated cylinder. In case an unheated cylinder is used, the desired increase of the elasticity limit and the associated hardness is usually already obtained by means of a cold deformation. When a preheated cylinder is used, it must be strengthened to a sufficient degree by, for example, precipitation hardening.

Alloys which are suitable for use in the method according to the invention are, for example, brass (copper- zinc alloys); at elevated temperatures, these alloys lose, however, the strength obtained from cold deformation rather rapidly.

Other alloys which are suitable for use are, for example, hardenable aluminium alloys, such as an aluminium zinc alloy having the composition 5.5% by weight of Zn, 0.15% by weight of Mn, 2.5% by weight of Mg, 1.6% by weight of Cu, 0.25% by weight of Cr, remainder Al and an aluminium silicon alloy consisting of 1.0% by weight of Si, 0.7% by weight of Mn, 0.9% by weight of Mg, 0.15% by weight of Cr, remainder Al, hardenable iron alloys consisting, for example, of 2.0-3.25% by weight of Ni, 1.00-1.80% by weight of Cr, 0.15-0.35% by weight of Si, 0.40-0.10% by weight of Mn, 0.18% by weight of C, 0.60% by weight of Mo, remainder Fe, and, for example, 12.75% by weight of Cr, 8% by weight of Ni, 2.25% by weight of Mo, 1.15% by weight of Al, remainder Fe.

For a number of uses it is advisable to use hardenable copper alloys having good heat conductivity, such as copper-chromium alloys (0.3-1.2% by weight of Cr, 0-0,2% by weight of Zr, remainder Cu), copper-beryllium alloys (1,9% by weight of Be, 0-0.6% by weight of (Co + Ni) remainder Cu and 0.4-0.7% by weight of Be, 2-2.8% by weight of Co, 0-0.5% by weight of Ni, remainder Cu), copper-nickel silicon alloys (0.6-2.5% by weight of Ni, 0.5-0.8% by weight of Si, remainder Cu) and further copper-cadmium alloys (0-7-1.3% by weight of Cd, remainder Cu and 0.5-1.0% by weight of Cd, 0.2-0.6% by weight of Sn, remainder Cu). These copper-cadmium alloys can be strengthened by cold deformation, but when heated neither does their strength increase, nor do they lose strength obtained by deformation.

With an alloy consisting of 0.6% -1.0% by weight of Cr, 0.1% by weight of Zr, remainder Cu, which proved in practice to be very satisfactory in the method according to the invention, the limit of elasticity τ_0.2 increases from 27 kg/mm² to 40 kg/mm² for a deformation of 20%. After prolonged heating, for example for 20 hours at approximately 400°C, a τ_0.2 of 50 kg/mm² is obtained, which indicates a coherent dispersion hardening.

Another alloy which is strengthened to a high extent on deformation is brass consisting of 37% by weight of Zn, remainder Cu. On 20% deformation the τ was found to have increased from 15 kg/mm to 65 kg/mm². However, it appears that on prolonged heating at 400°C, the τ_0.2 decreases again to the initial value of ₁₅ kg/mm². Therefore this alloy is not so suitable for use in wire drawing dies according to the invention, intended for the drawing of those metals which release much heat during drawing and which have a poor heat conductivity, or which are drawn at elevated temperatures, such as tungsten, molybdenum and some steels.

In a further embodiment of a method according to the invention, the core is first pressed into a heated annulus and the annulus is heated until the desired strengthening has been obtained. Thereafter, the annulus containing the core is pressed into the metal housing using a cold deformation process and enclosed therein by means of one or more retaining plugs.

It is alternatively possible to place the core in the metal housing and to press a preheated cylinder with an axial bore at an elevated temperature into the housing.

The above-mentioned alloys, except the copper- zinc and the copper-cadmium alloys, can be used for this purpose.

Preferably, the metal housing consists of a rust-resistant, workable alloy such as a ferritic chromium steel, for example AISI 430 or an austenitic chromium-nickel steel, (for example AISI 302'or'304). The drawing passage can be formed in a manner which is customary in this technology, for example by means of laser drilling or spark erosion prior to or after the annulus holding the core has been secured in the metal housing.

The method according to the invention will now be further explained with reference to the accompanying drawing.

In the drawing:

• Fig. 1 is a cross-sectional view of a portion of a pressing device in which a metal housing with a core and a loose cylinder have been positioned.
• Fig. 2 is a cross-sectional view of a wire-drawing die obtained by means of the method described with reference to Fig. 1.
• Fig. 3 is a cross-sectional view of a portion of a pressing device for hot-pressing a core in an annulus.
• Fig. 4 is a cross-sectional view of a pressing die.
• Fig. 5 is a cross-sectional view of an annulus with a core prior to pressing.
• Fig. 6 is a cross-sectional view of an annulus with pressed-in core.
• Fig. 7 is a cross-sectional view of a finished wire-drawing die.
• Fig. 8 is a cross-sectional view of a metal housing, including a ring and a core, prior to pressing.

EMBODIMENT I:

Producing a wire-drawing die by means of a cold pressing operation (Figs. 1 and 2). A cylinder 4 having a 3.6 mm diameter axial bore is located around a polycrystalline diamond core 5 having-a diameter of 3.0 mm and is pressed into a cavity of a metal housing 6 consisting of ferritic chromium steel (AISI 430), by means of a simple hydraulic press, a portion of whose pressing blocks 1 and 2 are shown in Fig. 1, and a die 3. The dimensions of the cylinder 4, which consists of 0.6% by weight of Cr, 0.1% by weight Zr, remainder Cu, were chosen so that the cylinder 4 was deformed for 20% before it clamped the core 5. The total force applied was 2000 kgf. Thereafter, a retaining plug 7, also consisting of ferritic chromium steel (AISI 430) was pressed into the opening of the metal housing 6 and a draw passage 8 was made in the core 4 by laser drilling (Fig.2).

From wire drawing experiments performed with wire-drawing dies thus obtained, it was found that when tungsten wire (for example, starting diameter of the wire 650_/um, diameter of the hole of the drawing die 490_/um), as well as copper wire (for example starting diameter of the wire 1000 µm hole diameter 900 µm and wire diameter 1100 µm, hole diameter 1000/um) were drawn, service lives were obtained which were at least equal, but were in most cases considerably longer than for synthetic diamonds fitted in cemented carbide rings.

EMBODIMENT II:

In a manner similar to that described in Embodiment I, a wire drawing die was produced from the same materials. However, the cylinder 4 was preheated to a temperature of 625°C. The cylinder 4 was not strengthened by cold deformation, but was directly strengthened by means of a coherent precipitation hardening operation, for which the cylinder with core was heated, after deformation, for a further 5 minutes at 6₂₅⁰C. The properties of the wire drawing dies obtained in this manner do not materially differ from those of the dies described in Embodiment I.

EMBODIMENT III:

By means of a device whose component parts which are important for the description of this embodiment are shown in outline in Fig. 3(partly cross-sectional) and Fig. 4 (cross-sectional), a wire-drawing die was produced by pressing a synthetic diamond core into the opening of a heated annulus. The annulus is not materially deformed, as is the case in Embodiments I and II.

The device comprises a hydraulic press, the drawing showing a portion of the pressing block 30, provided with a fixed upper die 31 and a movable lower die 32 and a tube oven 33. Furthermore, Fig. 3 shows a divided die 34/35 having a movable moulding die 36. The die 34/35 is positioned on a dish 37, which is supported by the rod 38 and connected thereby to the movable lower die 32. This construction was opted for to reduce the heat dissipation from the die 34/35 to the lower die 32.

Fig. 4 shows the die 34/35 in cross-section. The lower die 34 comprises a central opening 39, one end of which is of such a shape that it forms a support 40 for an annulus 43. (Fig.5). The upper die 35 has a central opening 41 in which a moulding die 36 can be moved up and down.

A core made of synthetic diamond is fitted in an annulus in the following manner. The lower die 32 is outside the oven 33 during mounting. The lower die 34 is placed on the dish 37. Thereafter an annulus 43 consisting of, for example, 0.6% by weight of Cr, 0.1% by weight Zr, remainder Cu (Fig. 5) is positioned on the surface 40 in the lower die 34. A core 42 made of synthetic diamond is placed in the annulus 43, one end of the opening 44 having been widened somewhat for this purpose (the diameter of the synthetic diamond 42 is 3.00 mm, the diameter of the opening 44: 2,65 mm, the diameter of the widened portion 3.03 mm). The upper die 35 is now placed on the lower die 34 and the moulding die 36 is introduced into the opening 41. The lower die 32 is moved up so far that the moulding die 36 contacts the upper die 31. The mould is heated by eans of the oven 33 to a temperature of 625⁰C (the temperature of the mould 34/35 is measured by means of a thermo-couple, not shown). Thereafter, the lower die 32 is raised still further until the synthetic diamond 42 has been pressed into the annulus-43; this is effected substantially pressure-free at the above-mentioned temperature. During heating and pressing of the synthetic diamond 42 into the annulus 43, the atmosphere in the volume enclosed by the oven 33 was weakly reducing, for which purpose a mixture of nitrogen and hydrogen (21%) was passed into this volume. After pressing, the annulus 43 with the core 42 was cooled to ambient temperature in the same atmosphere. Fig. 6 shows the annulus 43 with the pressed-in core 42. The combination thus obtained was then after-treated so that the axis of the assembly coincides as closely as possible with the axis of the core 42. Thereafter, the combination 42/43 was cold-pressed into the opening of a metal housing 45 (Fig. 7) consisting of ferritic chromium steel (AISI 430). Thereafter, the retaining plug 46, consisting of ferrite chromium steel (AISI 430) was applied by pressing and the core 42 was provided with a drawing passage by laser drilling.

EMBODIMENT IV:

A further embodiment of the method according to the invention is shown schematically in Fig. 8. A metal housing 80 was placed in a press. The metal housing 80 held a core 81 of, for example, polycrystalline diamond and a hardenable metal annulus 82, placed on top of the core.

The diameter of the aperture in the annulus was less than the diameter of the core 81. The annulus was pressed, while being deformed, around the core 82 in the metal housing by means of a press (not shown) which had a cylindrical die. The combination of the metal housing, core and annulus was preferably at a temperature between 400 and 700 ^o C, for example 550°C.

In this embodiment of the method, the annulus 82 may have been provided with two ring-shaped edges at the side facing the core, edge 84 having for its function to centre the core when the annulus 82 was brought into position, edge 85 being pressed during the pressing operation into a recess 86 in the metal housing, which ensured a secure mechanical connection of the annulus 82 in the metal housing 80. The materials mentioned in the preceding embodiments may be used in this embodiment.

It is, of course, possible to place the annulus first in the metal housing and to press a core in the annulus thereafter.

It appeared that, in practice, the dies obtained by means of the method according to the invention are suitable for drawing tungsten and molybdenum wire, copper wire, stainless steel wire and so-called tyre cord (steel wire coated with a brass layer).