Field of the Invention

This invention relates to a composite component comprising a body and a "hard metal" part, and more particularly to a component having a body cast-in-place about the hard metal part.

Background of the Invention

"Hard metals" are employed to provide wear resistance and/or abrasion resistance to tools. Typical hard metals are tungsten carbides sintered with cobalt and/or nickel but the term includes carbides of other metals such as titanium, titanium and chromium and also metallic nitrides and borides and the like. Cast alloys based on cobalt, chromium and tungsten are likewise included.

Conventional processes for the manufacture of components comprising a steel or alloy steel body and a hard metal" insert rely on the brazing or silver soldering of the shaped insert on to a prepared location on the body of the component. The most significant

disadvantage of these methods is the time-consuming and expensive brazing or soldering operation. An example is a rotary drill bit commonly used for the installation of roof bolts in underground mines which is usually manufactured by brazing a sintered tungsten carbide insert onto the cutting faces of the bit. The faces to which the insert is soldered must be carefully prepared (i.e. flat and free from asperities) and prior to the brazing operation, the tungsten carbide inserts must be accurately located on the faces.

A number of methods have been proposed which seek to avoid the disadvantages of brazing by casting in place the "hard metal" part in the body. However, methods proposed to date have either been unduly complex and costly to implement or have resulted in components having a limited ability to withstand thermal and mechanical stresses and in use have had an unacceptably short service life. As a result brazing, with its attendant disadvantages, has remained in use in commercial production and has been preferred over cast-in methods of forming tools with hard metal inserts.

An object of the present invention is to provide a method of manufacture of a component having a cast body and a hard metal insert which avoids or at least ameliorates the above mentioned disadvantages.

A further object is to provide machine tools and other components having cast-in carbide or "hard metal"

parts having improved resistance to mechanical or thermal stress.

The expression "cast-in-place" or "casting in place" is used to describe a process in which the body is formed by casting a body about the insert. The insert is thus retained in the cast body and is said to be "cast-in". The expression "cast-in-place" is also used to describe the metal body. It will be understood that these expressions do not imply that the "hard metal" part is formed by casting.

Summary of the Invention

According to a first aspect, the invention consists in a "hard metal" part adapted to be cast-in as an insert to a metal body, the part being provided with a metal coating which is substantially fugitive during casting of the body.

For preference the coating metal is copper, nickel or chromium or a combination thereof and is applied to a specially prepared "hard metal" part by electrodeposition. Desirably, the thickness of the coating is less than 10 microns.

For preference also the body component is a graphitic cast iron having a carbon equivalent of from 3 to 6%.

According to a second aspect the invention consists in a method of manufacture of a "hard metal" part adapted to be an insert in a cast-in-place metal body

component comprising the step of depositing a metal coating on the surface of the part whereby to enclose the part within said metal coating, the metal and the thickness of the coating being selected so that the coating is fugitive during casting-in-place of the body component.

Parts produced by the method may be embedded in a body by casting the body about the part and the invention extends to include the combination of the part with the body.

It is believed that use of the method achieves a metallurgical bond between the "hard metal" and the cast metal body, the coating metal being fugitive in the sense that it is consumed or alloyed during the casting step. The metal coating serves to protect the carbide from oxidation during casting.

Selection of appropriate casting metals may be used to prevent migration of carbon from the carbide into the casting metal.

Description of preferred embodiment

An embodiment of the invention will now be described by way of example only.

The example to be described is the manufacture of a rotary bit of the type commonly used in underground mines such as coal mines for the drilling of overhead holes into which are inserted bolts for retention of the mine roof. However, it will be understood that the

invention is not limited to use in tools of that kind.

Shaped sintered carbide inserts were first prepared by conventional techniques. However, the sintering step was conducted on a bed of alumina instead of the usual bed of graphite. It has been found that beds of graphite which are used in conventional sintering of carbides result in surface irregularities which prevent a sufficiently thin non-porous metal coating to be applied. By sintering on a bed of alumina, surface imperfections are substantially reduced.

After sintering, the inserts were then rumbled or otherwise treated to remove residual evidence of surface defects or asperities arising from the pressing and sintering process.

The carbide inserts were then flash copper coated by electroplating and then a coating of nickel was applied by electroplating using conventional nickel on copper electroplating techniques.

In one example, the nickel was deposited under the following conditions:

Nickel Sulphate (NiSo4.7H20) - 30-40 oz/gallon Nickel Chloride (NiC12.6H20) - 6 oz/gallon Boric Acid (H3B03) - 4 oz/gallon Current density 25-50 amperes/sq.ft. Temperature 80-90°F.

The copper coating had a thickness of from 0.1 to 1.5 micron and preferably about 1 micron or less, and

the nickel coating a thickness of 1 - 9 microns and preferably about 5 microns or less. The coating was adherent, non-porous and encapsulated the carbide insert.

The metal coated carbide inserts were then embedded in a drill body which was cast about the carbide inserts using conventional investment casting techniques as follows. The carbide parts were first glued into position on wax patterns and the patterns "treed-up" in the normal manner in preparation for investment casting.

The "trees" were then invested with refractory in the usual manner. After melting out the wax, the refractory shells were fired at a temperature of 700°C for 1 1/2 hours and then metal was cast into the shells immediately.

The casting metal used was a magnesium inoculated spheroidal graphite cast iron having a nominal composition of C 3.5%; Si 1.5%; P 0.015%; S 0.010%; Ni 5%, by weight. The casting temperature was 1600°C.

Examination of the resultant wing bits showed that a satisfactory metallurgical bond had been achieved and that the strength of the bond was comparable with that obtained by conventional brazing techniques. The cutting edges of the carbide insert were satisfactorily retained.

As previously stated it is believed that the metal coating on the carbide part serves to protect the part during the casting step but is fugitive during casting

being lost by oxidation or alloying with the casting metal.

Suitable coating metals are copper nickel and chromium. However, other metals could be employed. Thin coatings of the kind required could be deposited by vacuum deposition or other known techniques.

The casting metal should be a graphitic cast iron adjusted so that the carbon equivalent (C.E.) as defined by the formula:

C.E. = % C + 0.3 (% Si + %P) is in the range of 2.5% to 6.0%.

It has been found that selection of a C.E. in that range inhibits migration of carbon from the carbide to the steel.

Instead of normal grey iron, inoculated or spheroidal graphite (S.G. or ductile iron) may be used depending on the mechanical properties required for the particular component. The irons may also contain various alloying elements, particularly nickel, chromium, molybdenum, tungsten and vanadium either singly or as multiple alloying elements.

Strict control over the temperature of the mould at the firing stage, and also during pre-heating prior to casting, is highly desirable and the preferred temperature (range) in both stages is 650 - 750°C. The casting temperature of the molten iron is similarly an important factor in the attainment of a satisfactory

metallurgical bond at the hard metal/metal interface and it is the combination of the carefully controlled mould pre-heat and casting temperatures which ensures the success of the bond. The preferred range for casting is 1500 - 1650°C.

It will be understood that a usual temperature and time for the firing of investment casting moulds (or shells) is 1000°C for 1 1/2 hours. This temperature and time is recommended so that the refractory mould attains a maximum strength to resist breakage during handling and casting.

However, in the present process, the metal coated tungsten carbide inserts are inserted in the shell during the preparation of the shell, i.e., attached to the wax pattern and thence coated with refractory during the dip-coat procedure involved in the manufacture of investment casting shells. Tungsten carbide oxidises very rapidly at temperatures above about 450°C.

It has been established during the development of the present process that the copper-nickel coating can give oxidation protection to the tungsten carbide for the following times and temperatures:

650°C - 1 1/2 hours

700°C - 1 hour

750°C - 1/2 hour Thus, to attain sufficient strength in the shell for the process and to ensure that no oxidation of the

tungsten carbide occurs, careful control over the conditions for firing and pre-heating is important. It has been found that even very superficial oxidation prevents the formation of a metallurgical bond between the carbide and the metal. Furthermore, it is important to operate at the highest feasible temperature for preheating, also to ensure a satisfactory bond.

The use of as high a casting temperature as possible is also necessary to obtain a satisfactory metallurgical bond. The minimum at which a bond was obtained during development was 1500°C, metal temperature, but only a minority of the inserts were bonded. At 1650°C, approximately, a 90% success rate was obtained. Below 1500°C, no bonding occurred.

Although the invention has been herein described with reference to electrodeposition of a metal coating on the "hard metal" part, the metal coating can be applied by vapourphase deposition, chemical deposition, or other methods.

As will be apparent to those skilled in the art from the teaching hereof, the invention may be embodied in other forms and all such are deemed to fall within the scope hereof.