METHOD FOR PRODUCING AN IMPROVED LAYER PARTICULAR FOR A DRILL BIT

METHODE DE DEPOSITION D'UNE COUCHE SUPERFICIELLE DURE SUR DES OUTILS DE FORATION

English Abstract

ABSTRACT
An improved method is disclosed of producing an object, such as a
drill bit, comprising a core member having a compacted and sintered metal
powder layer thereon and a wear-resistant layer isostatically hot compacted
over portions of the powder layer. The method enables improved bonding be-
tween the layers and between the powder layer and the core, giving rise to
very high resistance to wear and to chipping.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A drill bit with cutting teeth for drilling in rock comprising:
a core member having raceways for rolling elements for rotation
of the drill bit,
a supporting layer exteriorly shaped to define cutting teeth and
formed of highly compacted sintered powder material of substantially 90% density,
isostatically bonded to said core member,
a wear resistant layer applied by thermal spraying and covering only
the portions of the cutting teeth which upon drilling directly contact the
rock,
said wear resistant layer and said supporting layer being iso-
statically hot compacted which combined are of substantially 99% density,
said layers having a solid bond therebetween providing a drill bit
of superior mechanical properties including high resistance to wear and chipping.
2. A drill bit as claimed in claim 1 wherein the said supporting layer
consists of a nickel-containing alloy steel.
3. A drill bit as claimed in claim 2 wherein said nickel-containing
alloy steel contains about 3.5% nickel.
4. A drill bit as claimed in claim 1 wherein said core member is formed
by mechanical operations and heat treatment into a bearing for the bearing
system of the drill bit.
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Description

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The invention relates to a method for producing an object on which
an exterior layer is applied by thermal spraying, followed by a heat treatment,
and to the object, in particular a drill bit, obtained pursuant to this
method.
Such a method is disclosed in British Patent 1,367,762. In applica-
tion of the method described above to objects wherein it is required that the
exterior layer applied be capable, in operation, of withstanding great vari-
able forces, for example, that it must be resistant to wear, however, it
happens that this layer sometimes chips off, thus shortening the life of the
object obtained.
The invention accordingly provides a method for producing an object
on which an exterior layer is applied by thermal spraying, followed by a heat
treatment, which method comprises applying by cold isostatic compacting a
layer of a suitable powder material on a core member, sintering the resultant
structure, thereafter applying an exterior layer which is a wear-resistant
layer, and then isostat;cally hot compacting the obtained structure.
It has been found that a suitable powder material for this purpose
is a nickel-containing alloy steel powder preferably containing 3.5% nickel
therein.
The invention in addition provides a drill bit with cutting teeth
provided with a wear-resistant layer, for drilling in rock.
In one embodiment of the method pursuant to the invention a supply
of powder material is introduced into a rubber mold and distributed, after
which the core member, which is usually formed of a bearing steel, is placed
in the powder, following which the powder is pressed on. The core member may
alternatively be placed in the mold first, after which the powder material is
introduced and pressed on. The mold is closed and is then isostatically com-
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pacted cold until a coherent member having a density of approximately 90% is
obtained. The compact removed from the mold is then sintered in a furnace.
After cooling the sintered object is coated with a wear-resistant layer by
thermal spraying, for example plasma spraying, after which the structure thus
obtained is isostatically compacted hot. This hot isostatic compacting may be
done by inserting the entire object in a thin-walled deep-drawn vessel or con-
tainer of low-carbon steel having a wall thickness of approximately 0.5 mm,
filled with a ceramic powder. This vessel is then heated and subjected to
pressure on all sides. After hot isostatic compacting the object may be
readily separated from the surrounding ceramic mass and cleaned by sand blast-
ing, This method proves to provide components with accurately shaped dimen-
sions comparable to those of a forged product.
When a drill bit for rock is produced in this fashion, after sinter-
ing not the entire surface of the cutting teeth but only the parts or surfaces
thereof which come directly into contact with the rock are coated with the
wear-resistant layer by thermal spraying. Pollowing the selective application
of the wear-resistant layer the preformed drill bit is subjected in its en-
tirety to hot isostatic compacting, as described above.
The invention will now be explained in greater detail by means of the
accompanying drawing, in which:
Figure 1 is a cross section of a drill bit produced according to a
preferred embodiment of the invcntion.
Figure 2 is a perspective view of a portion of this drill bit.
The drill bit 1 shown in Figure 1 is composed of a core member 3,
made of a bearing material, in which are applied races 2 for rolling elements
~not shown). On this core member 3, solid at the beginning, is applied in a
rubber mold a layer 4 of powder, which combination is then isostatically com-
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pacted cold. This operation takes place preferably under a pressure ofapproximately 6000 atmospheres at room temperature. Then the preformed drill
bit, isostatically compacted cold, is removed from the mold and sintered in a
sintering furnace at a temperature of approx. 1200C at 1 atnnosphere under
reduction by hydrogen for approx. 1 hour, which operations lead to a density
of approx. 90% of the compacted material. Then, by means of plasma spraying
technique, the wear-resistan~ layer 5 is applied on the layer 4 and the object
obtained is then inserted into a vessel or container and isostatically com-
pacted hot under a pressure of for example approx. 1600 atmospheres and at a
temperature of approx. 1100C for at least 2 hours. This operation results in
a density of the layers 4 and 5 of 99% and a very solid bond between the
layers.
It will be found by the method pursuant to the invention that the
mechanical properties of the drill bit thus formed are greatly improved, like
the bond between the layers 4 and 5, on the one hand, and between the layer 4
and the core member 3, on the other. By this means the desired effect of very
high resistance to wear and resistance to chipping of the cutting teeth is ob-
tained, combined with a core member which functionally has other possible
applications, such as, for example, the function of a bearing.
It is noted that the original solid core member 3, after mechanical
operations and heat treatmènt, acquires the shape, as represented in Figure 1,
in which the races 2 of the rolling elements are supplied.
It may be seen further from Figure 2 that not the entire surface of
the cutting teeth of the drill bit is provided with the wear-resistant layer 5,
but that the wear-resistant layer is applied only on the places or surfaces
where the tooth comes directly into contact with rock during operation.
Thus there is provided by the invention a device, such as a drill
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bit, which in principle consists of three parts, namely a significantly im-
proved cutting part 5, a supporting part 4 and a core or bearing part 3, which
parts are combined in an economically and technically advantageous manner such
that the said drill bit satisfies the requirements set.