GB2184382A - Securing inserts - Google Patents

Abstract

A rock drill bit having hard metal or like inserts 3 is fabricated by positioning the inserts in body sockets 2 providing a non- interference fit, and thereafter compacting the body 1 to secure the inserts in the sockets by the application of isostatic pressure to the body. Securement may be enhanced by causing a diffusion bond to occur between an insert and its socket wall. The body may be given a coating layer 5 that is secured to and densified on the body by the compaction. Because the compaction of the body improves its properties it may be made of less costly material and/or cast instead of being forged, without sacrifice of performance. The inserts 3 may be tungsten carbide and the body 1 may be alloy steel. A nickel film 4 may be interposed between the inserts and the body. Isostatic pressure is applied through a duct 12 of a capsule 10. The fabrication method may also be used to secure valve seat inserts and to secure bearing races in support structures. The method may include heat treatment. <IMAGE>

Description

SPECIFICATION Securing inserts THIS INVENTION concerns the securing of inserts such as hard metal and carbide inserts in component bodies and whilst of general applicability is especially applicable to the securement of hard metal or carbide inserts in tool bodies, for example the tool bodies of rock drills and the like. For convenience the invention will be discussed and described in terms of this application.

Rock drills, especially rotary percussive rock drills, utilise tools or bits of a variety of forms but in general these comprise a wrought or forged steel body in which hard metal or carbide inserts are secured. In the case of the so-called "button bit" in which the inserts are of generally cylindrical form, these are traditionally secured by a mechanical/interference fit in accurately drilled holes formed in the wrought body.Reliable securing of the inserts thus depends upon precision drilling of the sockets to receive the inserts so that the overall manufacturing process involves a substantial amount of precision machining of a wrought and heat treated steel bar and even then the securement of the inserts depends critically upon such factors as tolerances and surface roughness of the sockets for the inserts, with the considerable risk that inadequate securement and support for an insert will occur and be undetected until a premature service failure occurs. Thus the traditional techniques for manufacture of such rock drill bits are costly and difficult to control to the extent necessary to assure reliable fixation of the inserts.

An objective of the present invention is therefore to provide a method for securing hard metal or like inserts in a tool or other component body that avoids these disadvantages of the traditional techniques and that can lead, therefore, to a less expensive product of enhanced service life and reliability. A further objective is to provide a method as aforesaid that enables high performance tools to be fabricated from lower grade and/or less costly materials than has hitherto been thought to be essential.

In accordance with the invention, a method of fabricating a component having hard metal or like inserts is characterised by providing a component body with at least one socket to receive an insert as a noninterference fit therein; positioning an insert in such socket; and thereafter compacting the body, to secure the insert in the socket, by the application of isostatic pressure to the body.

Because the compaction step in this method, conducted by the application of isostatic pressure to the body, will normally lead to densification and enhancement of the mechanical properties of the body material, the performance normally attainable only by use of wrought, e.g. forged, material for the body is obtainable by the use, instead, of lower grade, e.g. cast, material. Similarly, in many cases the properties of the insert will be enhanced by densification and/or closure of pores therein, by the compaction step.

Moreover, because it is possible to accomplish significant dimensional changes by the compaction step the socket may be formed to provide, initially, substantial clearance when the insert is placed therein, without detriment to secure fixing of the insert by the compaction step. Accordingly, the fit tolerances prior to compaction may be significantly greater than those required by the traditional techniques, so minimising the need for precision machining. Furthermore, inserts of other than of cylindrical configuration can be secured in complementarily shaped sockets by the method of the invention.

While the method of the invention may be utilised to produce only the equivalent of a mechanical connection between an insert and its accommodating socket, the invention has the advantageous capability to provide an enhanced fixing of the insert, by accomplishing a diffusion bond between the socket wall and the insert. Thus it is known that under appropriate conditions of temperature and pressure, dissimilar materials can be diffusion bonded together and, for instance, we have found that a tungsten carbide insert can be satisfactorily diffusion bonded into an alloy steel body socket by the method of the invention.

The conditions appropriate for diffusion bonding can lead to migration of alloying components to the bond region and the formation of undesirable phases in this region. However, in accordance with a further feature of the invention, these undesirable events may be avoided or at least mitigated by providing an intermediate layer between the insert and the socket wall, the intermediate layer being of an appropriate material to form a diffusion bond both with the insert material and with the body material. Thus in the case of securing a tungsten carbide insert in an alloy steel body socket, an intermediate layer of nickel is highly beneficial. This intermediate layer may be established in various ways, such as by application as a coating layer on the insert before its insertion into the socket.Alternatively the intermediate layer may be provided by nickel foil wrapped about the insert or inserted as a liner for the socket, prior to introduction of the ihsert into the socket.

Because diffusion bonding of an insert into its socket gives a particularly secure fixing of the insert, the latter may have a length significantly less than that required to achieve satisfactory fixation by purely mechanical means as accomplished by traditional fabrication techniques.

Thus by the method of the invention signifi cant savings of costly insert material may be achieved without detriment to the service properties of the component fitted with the insert.

The conditions required for accomplishing compaction of the body to secure the insert in its socket, with or without diffusion bonding, by the use of isostatic pressure are also apposite to securement and densification of an coating material applied to the component body. Thus, for example, a wearresistant coating, for instance stellite material, may be applied to the component body as and where required thereon, prior to the compaction step which when subsequently performed will cause that coating material to become firmly adhered to and densified upon the body surface to enhance the service properties of the latter. If the compaction conditions are such as to achieve a diffusion bond between the insert and the body they will usually be appropriate also for accomplishing a diffusion bond between the coating material and the body.

Such coating of the component body is especially useful in the case of rock drill or like tools that are subject to particularly arduous and, especially, abrasive service conditions.

The coating material may for instance be applied to the body by plasma spraying techniques.

Many components, such as rock drill bits, to which the invention is applicable have a head portion and a shank portion the respective ideal properties of which are different.

The traditional fabrication techniques form the head and shank as a single component from a material whose properties offer the best compromise for the conflicting requirements of the head and shank.

By the method of the present invention the compacting step may be utilised also for accomplishing the connection of a head element to a shank element formed separately and, if required, of different materials selected to have the specific properties required of each element of the body.

Thus, for instance, a rock drill bit may have its head formed of cast alloy steel capable of developing the required properties, after compaction, for its service duty while the shank may be formed of cast or wrought alloy steel of different composition and capable of devel oping the (different) properties required of the shank in service.

The head may be provided with a suitable socket to receive an end of the shank, the compaction step serving to unite the head to the shank by, for instance, accomplishing a diffusion bond therebetween. Prior to the compaction step the shank may be attached to the head by welding to assure correct alignment of the components until fully se cured by the compaction step.

In preferred practice of the method of the invention, the insert(s) disposed in body socket(s), -and at least that part of the body containing the socket(s), are enclosed in a flexible capsule that is evacuated prior to the encapsulated body being subjected to isostatic pressure to accompl ish the compaction step. In this way the formation of gas entrapments and occlusions in the socket(s) is avoided, as is also contamination of the fluid, used to apply the isostatic pressure, by entrapped gas in the socket(s). Moreover, such encapsulation enables the applied isostatic pressure to act to close surface-connected pores in the body and/or insert materials, to enhance the surface properties thereof.

Preferably a ceramic cover is enclosed within the capsule to overlie the insert(s) and to provide a clear path for evacuation of gas from the socket(s) by evacuation of the capsule and, additionally, to prevent unwanted bonding of the capsule material to the insert(s) or to the body by the compaction step.

The compaction step will normally involve the application of elevated temperature as well as elevated isostatic pressure. In suitable cases, the temperature applied, constantly or in stages, and the heating and/or cooling rates may be chosen for accomplishing required heat treatments of the component material(s).

The accompanying drawing is a diagrammatic half-axial section of a rotary percussive rock drill bit in the course of fabrication by the method of the invention.

The drawing iilustrates a rock drill bit head 1 formed of EN40B alloy steel cast to shape with suitably oversized sockets 2 to receive relatively short carbide inserts 3, typically of tungsten carbide containing 6% cobalt carbide and formed by the conventional powder metallurgy techniques. In a typical application, the inserts 3 might be of circular section with a diameter of 12 mm, the sockets 2 in the as-cast head 1 having a diameter of about 13 mm, loosely to receive these inserts and a nickel film interlayer 4 having a thickness in the range 0.025 to 0.075 mm and a purity of 99.9%.

The end surface of the head 1 (and also its peripheral surface if required) is provided with a wear-resistant layer 5 of, for instance, a cobalt based alloy applied by, e.g., plasma spraying techniques.

The rock drill bit also comprises a shank 6 that may be formed of a different material from the head 1 or from the same EN40B alloy steel as the head. The head 1 is formed with a rear socket 7 to receive the shank 6 as a relatively loose fit (the clearance is exag gerated in the drawing) and while the head 1 and shank 6 are heid in their correct relative positions as by suitable jigging, the head is attached to the shank by a circumferential weld such as indicated at 8. The head has an axial vent 9 communicating with the socket 7 to allow of evacuation of the latter prior to the compaction step. The vent 9 may be constituted by a fluid flow passage in the head that communicates with a corresponding passage in the shank 6, in which case the rear end of the passage in the shank would be plugged to allow of its evacuation prior to the compaction step.

With the elements of the bit assembled as illustrated in the drawing, the front end of the head is enclosed within a flexible capsule 1 0 of, for instance, stainless steel and whose thickness has been exaggerated in the drawing. The capsule 10 is sealed to the head 1 as by brazing or welding at 11 and is fitted with a duct 12 by means of which the interior of the capsule may be evacuated.

To prevent the capsule 10 from bonding to the end surface of the head and to the inserts 3, a ceramic cover 13 is fitted over the front end of the head 1 prior to enclosure within the capsule 10. This cover 13 may be formed of alumina silicate fibre or be an aluminabased castable ceramic cast in place over the end of the head 1. Clearances between the cover 11 and the head and insert surfaces are exaggerated in the drawing.

Prior to compaction, the capsule is evacuated to withdraw as far as possible gas from the volume enclosed by the capsule, including the vent 9 and socket 7, whereafter the duct 12 is sealed. Thereafter compaction of the complete component is accomplished by the application of isostatic pressure thereto, this pressure being transmitted through the capsule to the enclosed end of the head 1 to accomplish compaction of the head portion within the capsule to secure the inserts 3 in their sockets 2.

Preferably the compaction is performed at elevated temperature, for instance at a temperature in the range 850 to 1 1750C dependent upon the precise materials of the various elements of rock drill bit. The applied pressure will typically lie within the range 20 to 140 MPa, being for instance about 105 MPa.

The isostatic pressure will be maintained for a suitable period, typically in the range 120 to 240 minutes, depending upon the section size of the component.

The temperature used in the compaction step may be chosen to accomplish a heat treatment of the component undergoing compaction and the rate of cooling of the component may likewise be chosen having regard to the properties to be developed in the component by heat treatment. Thus, for instance, cooling rates in the range 8"C per minute to 60"C per minute may be applied and varied from time to time during cooling.

Conveniently the isostatic pressure is applied by Argon gas.

While the invention has been discussed primarily in relation to its application to the fabrication of tools such as rock drill bits it is, as has been noted, of general applicability. Other appl ications are, for instance, the securing of valve seat inserts and the securing of bearing races in support structures.

Claims (16)

1. A method of fabricating a component having hard metal or like inserts, characterised by providing a component body with at least one socket to receive an insert as a noninterference fit therein; positioning an insert in such socket; and thereafter compacting the body, to secure the insert in the socket, by the application of isostatic pressure to the body.

2. A method according to claim 1, further characterised by accomplishing a diffusion bond between the socket wall and the insert.

3. A method according to claim 2, further characterised by providing an intermediate layer between the insert and the socket wall, the intermediate layer being of a material to form a diffusion bond both with the insert material and with the body material.

4. A method according to claim 3, further characterised in that the insert comprises tungsten carbide, the body comprises alloy steel and said intermediate layer is of nickel.

5. A method according to any preceding claim, further characterised by the step of providing a coating layer on the body surface and securing and densifying said layer by said compaction step.

6. A method according to claim 5, wherein said coating layer is formed of a wear-resistant coating material.

7. A method according to claim 6, further characterised by effecting a diffusion bond between the coating layer and said body.

8. A method according to any one of claims 5, 6 and 7, further characterised in that said coating layer is applied to the body by plasma spraying techniques.

9. A method according to any preceding claim, further characterised in that said compacting step is effective to unite two or more elements of said component.

10. A method according to claim 9, further characterised in that said compacting step serves to connect a head element to a shank element of said component.

11. A method according to any preceding claim, further characterised by enclosing in a flexible capsule at least that part of the body containing said socket; evacuating said capsule; and thereafter subjecting the encapsulated body to isostatic pressure to accomplish the said compacting.

12. A method according to claim 1 1, further characterised by positioning a ceramic cover within said capsule to overlie said insert.

13. A method according to any preceding claim, further characterised by the application of elevated temperature in said compacting step.

14. A method according to claim 1 3, further characterised in that said elevated temperature is applied to accomplish a heat treatment of the component material.

15. A method of fabricating a rock drill bit, substantially as herein described with reference to the accompanying drawing.

16. Every novel feature and every novel combination of features disclosed herein.