Hardfacing compositions and hardfacing coatings formed by pulsed plasma-transferred arc

A new hardfacing composition is disclosed. In addition, a pulsed plasma transferred arc method for depositing hardfacing material with a higher content of carbide and lower dilution by a substrate metal is disclosed. The disclosed method produces a hardfacing coating which has a strong metallurgical bond to the substrate metal. The method includesthe following steps: (1) establishing a transferred plasma arc between an electrode and an area of a work piece, (2) optionally preheating the work piece to at least 250.degree. F., (3) forming a plasma column of inert gas in the arc by passing an electrical current between the electrode and the work piece, (4) feeding a stream of hardfacing material in powder form into the plasma column, and (5) pulsing the current between a pre-selected high pulse current value and low pulse current value while feeding the powdered hardfacing material. In this method, the low pulse current is selected to be sufficiently high to melt at least one component of the hardfacing material. The pulse rate and the high pulse current are selected to minimize the formation of a weld pool on the metal substrate during the hardfacing process. This method has applications in hardfacing any metallic work piece which requires wear resistance or erosion resistance. This method is especially effective in hardfacing roller cone surfaces and the milled teeth of a rock bit for erosion and wear protection.

FIELD OF THE INVENTION 
This invention relates to hardfacing compositions and hardfacing coatings 
on a metallic work piece and more particularly to deposition of a 
hardfacing coating on roller cone surfaces of a rock bit. 
BACKGROUND OF THE INVENTION 
Earth boring or contacting devices, such as rock bits used in petroleum and 
rock drilling applications, include wear or erosion surfaces exposed to 
erosive wear due to contact with geological formations. Two types of rock 
bits are commonly used: tungsten carbide inserts (TCI) rock bits and 
milled tooth rock bits. 
A TCI rock bit is utilized to drill a hard formation because of the 
enhanced ability of tungsten carbide inserts to penetrate hard formations. 
However, the tungsten carbide inserts are mounted in a relatively soft 
metal, e.g., steel, that forms the body of the cutter cone. This 
relatively soft metal cutter body which holds the inserts in place may be 
abraded or eroded away when subjected to a high abrasive drilling 
environment. This abrasion or erosion occurs primarily due to the presence 
of cuttings from the formation, the direct blasting effect of the drilling 
fluid utilized in the drilling process, and the rolling and sliding 
contact of the cone body or cone shell with the formation. When the 
material supporting the inserts is eroded or abraded away to a substantial 
extent, the drilling forces being exerted on the inserts may either break 
the inserts or force them out of the cutter cone when they engage the 
formation. As a result, the bit may no longer be effective in cutting the 
formation. Moreover, the loose inserts that break off from the cutter cone 
may damage other inserts and the cutter cone, and eventually may lead to 
failure of the cutter cone. 
When drilling relatively soft but abrasive formations, individual cutting 
inserts may penetrate entirely into the abrasive formation, causing the 
formation to come into contact with the cutter cone or cone shell. When 
this contact occurs, the relatively soft cone shell material will erode 
away, namely at the edges of the surface lands, until the previously 
embedded portion of the insert becomes exposed and the retention ability 
in the cone shell is reduced, which may result in the loss of the insert 
and reduction of the life of the bit. To protect the cutter cone from 
erosion, hardfacing material, such as tungsten carbide, has been applied 
to the cone surfaces by a variety of methods. 
Milled tooth rock bits are another important type of rock bits used in 
petroleum and mining drilling applications. A milled tooth bit has a 
roller cone with teeth protruding from the surface of the cone for 
engaging the rock. The teeth are made of hardened steel and generally are 
triangular in a cross-section (taken in a plane perpendicular to the axis 
of the cone). The principal faces of such a milled tooth that engage the 
rock usually are dressed with a layer of hardfacing material to increase 
wear-resistance. 
With respect to hardfacing cone surfaces of rock bits for erosion 
protection, different approaches have been developed with varying degree 
of success. For example, small, flat-top compacts made of hard material 
may be placed in the vulnerable cutter shell areas by a silicate bonding 
agent to prevent erosion. Thermal spraying, plasma arc, and welding arc 
also may be used to coat the exposed surfaces, including the inserts, of a 
cutter cone with a wear resistant material. 
Although hardfacing coatings in accordance with existing methods protect 
cones from erosion to some extent, they are relatively unsatisfactory in 
their erosion protection performance. It is recognized that a good 
hardfacing coating preferably has a high carbide content and possesses 
strong bonding to the cone surface. 
In a high energy deposition process, such as thermal spraying, plasma arc, 
and welding arc, dissolution of the carbides in the hardfacing material 
may occur extensively due to the long duration of high temperatures the 
carbide is subjected to. In addition, the high temperature in a plasma arc 
or welding arc process may cause the substrate metal to melt and diffuse 
into the hardfacing material. On the other hand, in thermal spraying and a 
low energy process (e.g., use of silicate bonding agents), the bonding 
between the hardfacing and the cone surface may be relatively weak. 
Therefore, there exists a need for a method capable of depositing a 
hardfacing coating with a relatively higher carbide content and low 
substrate dilution while, at the same time, achieving strong metallurgical 
bonding to the substrate metal. 
SUMMARY OF THE INVENTION 
In one aspect, the invention relates to a method of depositing a hardfacing 
material on a metal substrate. The method includes: (1) forming a plasma 
column immediately adjacent to an area of a metal substrate; (2) feeding a 
hardfacing material into the plasma column; (3) pulsing an electrical 
current through the plasma at a pulse rate; and (4) controlling the 
pulsing of the electrical current to minimize the formation of a weld pool 
in the area so that a hardfacing coating is deposited in the area of the 
metal substrate. Optionally, the method may include pre-heating the metal 
substrate to a temperature of at least 250.degree. F. In some embodiments, 
the act of controlling includes controlling the pulse rate. In other 
embodiments, the electrical current may be pulsed between a pre-determined 
high pulse current and a pre-determined low pulse current. In some 
embodiments, the act of controlling includes controlling the 
pre-determined high pulse current. Furthermore, it may include controlling 
the pulse rate and the predetermined high pulse current. In some 
embodiments, the pre-determined low pulse current is selected to be high 
enough to melt at least one component of the hardfacing material. In other 
embodiments, the pre-determined low pulse current is in the range of about 
30-100 amps. In some embodiments, the high pulse current is in the range 
of about 80-250 amps. In some embodiments, the pulse rate is at least 20 
cycles per second. 
In another aspect, the invention relates to a method of manufacturing a 
hardfaced roller cone. The method includes (1) providing a roller cone; 
(2) forming a plasma column immediately adjacent to an area susceptible to 
erosion or wear on the roller cone; (3) feeding a hardfacing material into 
the plasma column; (4) pulsing an electrical current through the plasma at 
a pulse rate; and (5) controlling the pulsing of the electrical current to 
minimize the formation of a weld pool in the area so that a hardfacing 
coating is deposited in the desired area of the roller cone. 
In yet another aspect, the invention relates to a method of manufacturing a 
hardfaced rock bit. The method includes (1) providing a rock bit having a 
roller cone, the rock bit including an area susceptible to wear or 
erosion; (2) forming a plasma column immediately adjacent to the area; (3) 
feeding a hardfacing material into the plasma column; (4) pulsing the 
electrical current through the plasma at a pulse rate; and (5) controlling 
the pulsing of the electrical current to minimize the formation of a weld 
pool in the desired area and to deposit a hardfacing coating in the area 
of the rock bit. 
In yet another aspect, the invention relates to a hardfaced earth-boring 
device. The earth-boring device is manufactured by the following method: 
(1) providing an earth-boring device having a metal substrate; (2) forming 
a plasma column immediately adjacent to an area of the metal substrate; 
(3) feeding a hardfacing material into the plasma column; (4) pulsing the 
electrical current through the plasma at a pulse rate; and (5) controlling 
the pulsing of the electrical current to minimize the formation of a weld 
pool in the area so that a hardfacing coating is deposited in the desired 
area of the metal substrate. 
In yet another aspect, the invention relates to a hardfaced roller cone. 
The hardfaced roller cone is manufactured by the following method: (1) 
providing a roller cone; (2) forming a plasma column immediately adjacent 
to an area susceptible to erosion or wear on the roller cone; (3) feeding 
a hardfacing material into the plasma column; (4) pulsing the electrical 
current through the plasma at a pulse rate; and (5) controlling the 
pulsing of the electrical current to minimize the formation of a weld pool 
in the area so that a hardfacing coating is deposited in the desired area 
of the roller cone. 
In yet another aspect, the invention relates a hardfaced rock bit. The 
hardfaced rock bit is manufactured by the following method: (1) providing 
a rock bit having a roller cone where the rock bit includes an area 
susceptible to wear or erosion; (2) forming a plasma column immediately 
adjacent to the area; (3) feeding a hardfacing material into the plasma 
column; (4) pulsing the electrical current through the plasma at a pulse 
rate; and (5) controlling the pulsing of the electrical current to 
minimize the formation of a weld pool in the area where a hardfacing 
coating is being deposited on the rock bit. 
In yet another aspect, the invention relates to a hardfacing composition. 
The hardfacing composition includes (1) one or more carbides selected from 
the group consisting of single crystal WC, eutectic WC/W.sub.2 C, and 
sintered WC/Co; and (2) a cobalt and chromium alloy matrix including about 
65% by weight of cobalt, about 27% by weight of chromium, and about 6% by 
weight of molybdenum. Alternatively, the hardfacing composition may 
include (1) vanadium carbide; (2) one or more carbides selected from the 
group consisting of cast tungsten carbide and sintered tungsten carbide; 
and (3) a metallic matrix material component. 
In yet another aspect, the invention relates to a hardfacing coating formed 
by the following method. The method includes (1) providing a hardfacing 
composition; and (2) heating the composition to a temperature sufficient 
to melt at least one component of the composition so that a hardfacing 
coating is deposited on a metal substrate. The hardfacing composition 
includes one or more carbides selected from the group consisting of single 
crystal WC, eutectic WC/W.sub.2 C, and sintered WC/Co, and a cobalt and 
chromium alloy matrix including about 65% by weight of cobalt, about 27% 
by weight of chromium, and about 6% by weight of molybdenum. 
In yet another aspect, the invention relates to a hardfacing coating formed 
by the following method. The method includes (1) providing a hardfacing 
composition; and (2) heating the composition to a temperature sufficient 
to melt at least one component of the composition so that a hardfacing 
coating is deposited on a metal substrate. The hardfacing composition 
includes vanadium carbide, one or more carbides selected from the group 
consisting of cast tungsten carbide and sintered tungsten carbide, and a 
metallic matrix material component. 
In yet another aspect, the invention relates to a hardfacing coating over a 
substrate. The hardfacing coating includes (1) a carbide phase including a 
primary carbide and a secondary carbide; and (2) a binder matrix, the 
carbide phase being dispersed in the binder matrix. In some embodiments, 
the primary carbide of the carbide phase may include one of single crystal 
WC, eutectic WC/W.sub.2 C and sintered WC/Co, and the secondary carbide of 
the carbide phase may include one of VC, TiC, Cr.sub.3 C.sub.2, Cr.sub.7 
C.sub.3 and Cr.sub.23 C.sub.6. Alternatively, the primary carbide of the 
carbide phase includes one of VC, eutectic WC/W.sub.2 C, and sintered 
WC/Co. In other embodiments, the binder matrix includes a metallic matrix 
and a non-metallic composition. The metallic matrix may include one of 
cobalt, nickel, iron and alloys thereof. It may further include one or 
more of the following materials: silicon, aluminum, boron, tungsten, 
molybdenum, tantalum, and/or another transition metal. The non-metallic 
composition may include a secondary carbide and a boride. It may further 
include an Eta phase. The secondary carbide may include one of VC, TiC, 
Cr.sub.3 C.sub.2, Cr.sub.7 C.sub.2, and Cr.sub.23 C.sub.6. The boride may 
include one of CrB, TiB.sub.2, and ZrB.sub.2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the invention provide hardfacing compositions and hardfacing 
coatings with a relatively high carbide content, a low dilution by a 
substrate metal, and strong metallurgical bonding to the substrate metal 
by a pulsed plasma-transferred arc process. 
FIG. 1 illustrates one embodiment of the pulsed plasma-transferred arc 
process. The apparatus generally includes a torch 20, and non-consumable 
electrode 28, a pilot arc welding power supply 16, a main arc power supply 
15 and a pulse controller 17. The torch 20 generally is of conventional 
construction and includes a first passage 21 for receiving plasma-forming 
gas 12, a second passage 13 for receiving a flow of powdered hardfacing 
material 11 and a third passage 22 concentric to the second passage for 
receiving a flow of shielding gas 30. Additionally, the torch has a 
restricting orifice 26 at the end of the passage 21 through which a flow 
of inert gas is directed toward a work piece or substrate 25. Optionally, 
the torch may include a collimating orifice 27 for controlling the shape 
of plasma column 23. 
As illustrated in FIG. 1, electrode 28 and restricting orifice 26 are 
connected to the pilot arc power supply 16. A primary arc is generated 
between electrode 28 and restricting orifice 26 when an arc current is 
supplied by the pilot arc power supply 16. Generally, the electrode 28 is 
connected to the negative terminal of the power source 16 while the 
restricting orifice 26 of the torch is connected to the positive terminal 
of the power supply 16. 
The main arc power supply 15 is connected to and between the electrode 28 
and the substrate 25 in order to provide the arc current to establish a 
plasma column between the electrode 28 and the substrate 25. The electrode 
28 is connected to the negative terminal of the power supply 15 while the 
substrate 25 is connected to the positive terminal of the power supply 15. 
After a primary arc of inert gas 12 is established between electrode 28 
and substrate 25, a current is supplied by the main arc power supply 
between electrode 28 and substrate 25 to create a plasma column 23. A 
powder of hardfacing material 11 is introduced into the second passage 13 
by a connecting tube 14. This usually is achieved by use of an inert gas 
as a carrier for the hardfacing powder 11. The hardfacing powder typically 
includes one or more metallic components and one or more non-metallic 
components. Once inside the second passage, the hardfacing powder is 
carried into the plasma column by the inert gas inside the torch. While in 
the plasma column 23, at least one component of the hardfacing material is 
melted by the plasma column and a layer 24 of the hardfacing material 11 
is subsequently deposited on the substrate. To protect the hardfacing 
material and substrate from oxidation and contamination, a shielding gas 
30 is introduced into the third passage 22 by a connecting tube 29. During 
the deposition process, the current supplied by the main arc power supply 
15 to electrode 28 and substrate 25 is pulsed by a pulse controller 17 
between a high pulse current and a low pulse current. 
In some embodiments, the following steps are used to deposit a layer of 
hardfacing material on a metallic work piece: (1) establishing a 
transferred plasma arc between an electrode and an area of the work piece, 
(2) forming a plasma column of inert gas in the arc by passing an 
electrical current between the electrode and the work piece, (3) feeding a 
stream of the hardfacing material in powder form into the plasma column, 
and (4) pulsing the current at a selected pulse rate between a 
pre-selected high pulse current value and low pulse current value while 
the powdered hardfacing material is fed into the plasma column. 
Optionally, the work piece may be preheated to a temperature of at least 
250.degree. F. 
The low pulse current generally is selected to be high enough to melt at 
least one component of the hardfacing material, whereas the pulse rate and 
the high pulse current are selected so that formation of a weld pool of 
the substrate metal is minimized during the deposition process. 
This differs from existing pulsed plasma-transferred arc processes in which 
a sizable weld pool is formed on the substrate. The formation of a weld 
pool is undesirable because the carbides in the hardfacing material tend 
to dissolve in the weld pool, resulting in changes to the chemical 
composition and morphology of the deposited hardfacing material. Such 
changes may alter or degrade the mechanical properties of the final work 
piece. Furthermore, the formation of a weld pool may cause the molten 
metal to diffuse into the hardfacing coating, resulting in substrate 
dilution of the coating. 
Generally speaking, the higher the carbide content in a hardfacing coating, 
the better its wear resistance and erosion resistance. By melting at least 
one component of the hardfacing material and minimizing the formation of a 
weld pool on the surface of the substrate, embodiments of the invention 
advantageously preserve the carbide phase in the hardfacing material and 
reduce substrate dilution, while allowing the hardfacing material to be 
strongly bonded to the substrate. 
This metallurgical bonding is further strengthened by including an optional 
step of preheating the work piece to a temperature of at least 250.degree. 
F. before the hardfacing material is deposited. The preheating may be 
achieved by any method, for example, by a torch or an oven. This step 
reduces the otherwise large difference between the ambient temperature of 
the work piece and the temperature of the hardfacing material in the 
plasma column; this large temperature difference may induce cracking of 
the hardfacing coating. It is observed that the preheating step has 
reduced the cracking of the hardfacing coating after solidification and 
subsequent cooling to ambient temperature. This has led to increased 
mechanical strength, including enhanced wear and erosion resistance, of 
the coating. 
FIG. 2 illustrates a graph of the current versus time used in one 
embodiment. The current supplied to the electrode 28 and the substrate 25 
is pulsed between a low pulse current level and a high pulse current 
level. Preferably, the current is pulsed at a rate of at least 20 cycles 
per second. The low pulse current is in the range of about 30 amps to 
about 100 amps, and the high pulse current is in the range of about 80 
amps to about 250 amps. 
In contrast, a conventional prior art plasma-transferred arc process 
utilizes a constant current between the electrode and the work piece, as 
illustrated in FIG. 3. In the prior art plasma-transferred arc process, a 
constant current level was selected to create a plasma plume between the 
electrode and the work piece. This current level usually is high enough to 
dissolve a substantial portion of the carbide in the hardfacing material 
and to cause the metal substrate to melt and diffuse into the hardfacing 
material. The dissolution of carbide and dilution by the substrate metal 
may adversely affect the wear resistance and erosion resistance of the 
hardfacing coating. 
Dilution of a hardfacing material by a substrate metal may be indicated by 
"dilution rate", which is defined as the weight percentage of substrate 
metal which has diffused into the hardfacing material. Because the 
formation of a weld pool is reduced or minimized in embodiments of the 
invention, the dilution rate of the hardfacing coating by substrate metal 
typically is in the range of about 1-10%. 
In the embodiments of the invention, the energy input, typically in the 
range of 1-6 kW, is lower than a conventional plasma-transferred arc 
process because the current is pulsed. This relatively low energy input 
results in lower temperatures in the plasma column so that the carbides in 
the hardfacing material are not melted or dissolved to a significant 
extent. Furthermore, the finer carbide particles, which may be beneficial 
to erosion protection, remain in the hardfacing coating. 
In some preferred embodiments, a plasma column is established between the 
electrode and an area with a width greater than 1/8 inch of the work 
piece; thus, a large area of the work piece may be coated by each pass. 
Some applications require hardfacing coatings in a relatively large area 
of a metallic substrate. It should, however, be understood that it also is 
possible to narrow the plasma column to hardface an area with a width less 
than 1/8 inch. The thickness of the hardfacing coatings generally are at 
least 0.020 inch, with a preferred range of about 0.030 to about 0.300 
inch. It should, however, be understood that almost any thickness may be 
obtained by the embodiments of the invention. 
The hardfacing material used in embodiments of the invention generally 
includes a metallic component and a non-metallic component. The metallic 
component can be any suitable metal or combination of an alloy of metal, 
such as iron, steel, nickel-based alloys, and the like. In addition, the 
metallic component also may include silicon, aluminum, boron, and a small 
amount of refractory metals (such as tungsten, molybdenum, tantalum, or 
other transition metals). The non-metallic component generally includes a 
hard material, such as a carbide, boride, or nitride. 
In some embodiments, the hardfacing material composition includes carbide 
as the non-metallic component and an alloy based on cobalt, iron, and/or 
nickel as the metallic component. The carbide may include a primary 
carbide composition and, optionally, a secondary carbide composition. The 
primary carbide composition includes one or more of sintered tungsten 
carbide (e.g., WC/Co), cast tungsten carbide (i.e., eutectic WC/W.sub.2 
C), and macro-crystalline or single crystal tungsten carbide (i.e., WC). 
The secondary carbide composition may include one or more of vanadium 
carbide (VC), chromium carbide (i.e., Cr.sub.3 C.sub.2, Cr.sub.7 C.sub.3, 
or Cr.sub.23 C.sub.6), and titanium carbide (i.e., TiC). In addition, 
borides, such as CrB, TiB.sub.2 and ZrB.sub.2, may be included if desired. 
In other embodiments, macro-crystalline tungsten carbide (WC) may be 
replaced by vanadium carbide (VC). 
Generally, the carbide (including both the primary carbide and the optional 
secondary carbide) present in a hardfacing material composition is in the 
range of about 30-60% by weight, and the metallic component present is in 
the range of about 40-70% by weight (i.e., the balance of the 
composition). The average carbide particle size may range from about 15 
.mu.m to about 600 .mu.m, with a preferred range of about 30-200 .mu.m. 
The primary carbide composition is in the range of about 70-100% by weight 
of the carbide, and the optional carbide composition is in the range of 
about 0-30% by weight of the carbide. It should be noted that a hardfacing 
composition according to embodiments of the invention is not limited to 
these preferred numerical ranges; other ranges also may be used. 
The hardfacing composition may be in the form of powder, i.e., carbide 
particles mixed with a metallic powder. Alternatively, the hardfacing 
composition may be in the form of tube rod (i.e., carbide particles are 
placed in a metallic tube). Other geometries, such as a wire, are also 
possible. It is to be noted that the hardfacing material may be used to 
hardface any metallic substrate that requires wear resistance, erosion 
resistance, or both. 
During the hardfacing process, a hardfacing material is transformed to a 
hardfacing layer or coating. The term "hardfacing material" is used herein 
to refer to a hardfacing composition before it is applied in the 
hardfacing process, whereas the term "hardfacing layer" or "hardfacing 
coating" is used to refer to the coating deposited by the hardfacing 
process. Because a hardfacing coating is formed from a hardfacing 
material, the composition of the hardfacing coating may not be 
dramatically different from that of the hardfacing material. During the 
hardfacing process, a small percentage of carbide may dissolve in the 
metallic component, and substrate material may diffuse into the hardfacing 
coating. Some chemical reactions or alloying also might occur. Therefore, 
in general, the carbide content in a hardfacing coating may be lower than 
the corresponding hardfacing material. Moreover, the carbide particle size 
in the hardfacing coating may be smaller than that of the hardfacing 
material. 
The hardfacing coating deposited by the embodiments of the invention has 
the following specific properties. The hardfacing coating includes two 
phases: a carbide phase and a binder matrix. Preferably, the carbide phase 
is uniformly dispersed in the binder matrix, which is continuous. FIGS. 4A 
and 4B illustrate the microstructure of a hardfacing coating obtained in 
one embodiment. The photomicrographs indicate a particulate phase 
uniformly dispersed throughout a continuous matrix. Analysis indicates 
that the particles are the carbide phase and the continuous matrix is the 
binder matrix. 
The volume occupied by the carbide phase generally is in the range of about 
30-65%, with a preferred range of about 30-60%. The carbide phase includes 
a primary carbide and, optionally, a secondary carbide. The primary 
carbide content falls within the range of about 70-100% by volume of the 
carbide phase. The primary carbide includes one or more of single crystal 
WC, eutectic WC/W.sub.2 C, and sintered WC/Co. The secondary carbide is 
the balance of the carbide phase; it is generally in the range of about 
0-15% by volume of the carbide phase. The secondary carbide phase includes 
one or more of the following materials: VC, TiC, Cr.sub.3 C.sub.2, 
Cr.sub.7 C.sub.3, and Cr.sub.23 C.sub.6. As indicated in FIGS. 4A and 4B, 
the shape of the carbide phase may be angular, irregular, rounded, or 
spherical. The size of the carbide phase generally is within the range of 
about 15-600 .mu.m with a preferred range of about 30-200 .mu.m. 
The volume of the binder matrix, being the balance of the hardfacing 
coating, is generally in the range of about 35-70%. The binder matrix 
includes a metallic component and non-metallic component. The metallic 
component may contain cobalt, nickel, iron, or mixtures or alloys thereof. 
It may further include silicon, aluminum, boron, and/or a small amount of 
refractory metals (such as tungsten, molybdenum, tantalum, or other 
transition metals). The non-metallic component includes a secondary 
carbide and a boride. The total volume content of non-metallic component 
in the binder matrix is between about 7-42% with a preferred range of 
about 8-30%. The secondary carbides may include one or more of VC, TiC, 
Cr.sub.3 C.sub.2, Cr.sub.7 C.sub.3, and Cr.sub.23 C.sub.6. The borides may 
include one or more of CrB, TiB.sub.2, and ZrB.sub.2. The particle size of 
the secondary carbides and the borides is between about 10-50 .mu.m. The 
shape of the particles may be angular, irregular, rounded, or spherical. 
The non-metallic component may include an Eta phase and a trace amount of 
oxides, which are a by-product of the welding process. Eta phase is a 
carbide phase of the formula W.sub.3 M.sub.3 C or W.sub.6 M.sub.6 C, where 
M is Fe, Co, or Ni. The particle size of the Eta phase generally is less 
than 20 .mu.m, and the particle shape can be crystal-like, irregular, or 
dendritic. The Eta phase generally is formed during the hardfacing 
process. 
Another embodiment of the invention relates to automating the placement of 
hardfacing material onto the cone surfaces. This is particularly important 
when hardfacing material is applied to the cone surface in intricate 
patterns between the inserts. In preferred embodiments, insert holes are 
drilled after the cones have been hardfaced. Therefore, it is important 
that deposition of the hardfacing material not interfere with the 
subsequent insert hole-drilling operation. One method of automation is to 
use numerically controlled ("NC") or computer numerically controlled 
("CNC") machines to move the plasma torch to place the hardfacing material 
directly onto predetermined areas of the cone cutter which are susceptible 
to erosion. The machines can be programmed using any conventional 
computer-aided manufacturing techniques to place the hardfacing material 
sufficiently away from where the insert holes will be drilled. 
Alternatively, NC or CNC machines may be used to move the cone relative to 
a plasma torch. 
When a hardfacing material is placed on the cone surface between areas 
which will become insert holes, a "start" mark on the cone may be 
necessary to ensure proper setup for the hole-drilling process to be 
synchronized with the hardfacing process. Other suitable methods to ensure 
a proper zero or circumferential starting location also may be used. A 
small start hole in the cone which interfaces with a tooling fixture zero 
point is one possible method. Another acceptable method is to use a 
machine with index plates which are timed to be in phase with the 
subsequent hole-drilling operation. The machine then may be set up to 
place hardfacing material onto the cone surface and automatically index to 
the next circumferential location. This allows insert holes to be drilled 
in the intended areas. A "start" mark may be required for proper setup of 
the subsequent insert hole-drilling operation. 
In some embodiments, only circumferential bands of hardfacing material in 
the cone grooves adjacent to the insert lands are deposited. It is 
entirely possible to do this by a robot. Manufacturing parameters such as 
speed and feed rate may be optimized to achieve the desired hardfacing 
thickness and consistency. 
The following examples illustrate embodiments of the invention and are not 
restrictive of the invention as otherwise described herein. 
EXAMPLE 1 
This example demonstrates that embodiments of the invention are capable of 
producing hardfacing coatings with satisfactory wear resistance. In this 
example, all hardfacing compositions were fed in the form of powder 
through a pulsed-PTA torch into a plasma column. All hardfacing powders 
included a carbide in the form of particles mixed with a powdered metal 
matrix material (i.e., the metallic component). The metal matrix material 
included alloys of iron, nickel, or cobalt designated by AISI 316, AISI 
304, AISI 309, AISI 410, 17-4 PH, Stellite 21, Ultimet, and BNi-2. The 
composition of these metal matrix materials in terms of weight percentage 
are listed in Table 1, and their powders were produced by an argon 
atomizing process. 
TABLE 1 
______________________________________ 
Composition of Metal Matrix Material (wt. %) 
Compo- 
sition C B Si Cr Cu Mo W Ni Co Fe 
______________________________________ 
AISI 0.02 0 0 17 0 2.5 0 11 0 69.48 
316 
AISI 0.02 0 0 19 0 0 0 10 0 70.98 
304 
AISI 0.04 0 0 23 0 0 0 13 0 63.96 
309 
AISI 0.10 0 0 12.5 0 0 0 0 0 87.4 
410 
17-4 PH 0.07 0 0 15.4 4.4 0 0 4.5 0 75.63 
Stellite 0.25 0 0 27 0 6 0 1.5 65.25 0 
21 
Ultimet 0.06 0 0 26 0 5 2 9 54.94 3 
BNi-2 0.06 3.12 4.5 7 0 0 0 84.82 0 0.5 
______________________________________ 
In addition, vanadium steel also was used as a metal matrix material. It 
typically includes about 74% iron, 3.4% carbon, 0.5% manganese, 0.9% 
silicon, 5.2% chromium, 14.5% vanadium, 1.3% molybdenum, and 0.1% sulfur. 
Fourteen hardfacing powder samples were made according to Table 2. 
TABLE 2 
______________________________________ 
Composition of Hardfacing Powder Samples (wt. %) 
Matrix 
Sample Wt. 
No. WC/Co WC/W.sub.2 C 
VC Si/Mn % Matrix Material 
______________________________________ 
1 26 30 4 3 37 AISI 316 
2 26 30 4 3 37 AISI 304 
3 26 30 4 3 37 AISI 309 
4 26 30 4 3 37 AISI 17-4-PH 
5 40 13 4 3 40 AISI 17-4-PH 
6 26 30 4 3 37 AISI 410 
7 40 13 4 3 40 AISI 410 
8 40 13 6 3 38 AIS1 410 
9 41 16 4 0 38 BNi-2 
10 40 15.8 3.7 3 37.5 BNi-2 
11 40 16 4 0 40 Stellite 21 
12 35 18 4 3 40 Stellite 2l 
13 40 13 4 3 40 Ultimet 
14 40 13 4 3 40 Vanadium 
Steel 
______________________________________ 
In addition, a fifteenth sample was made for use in a manual oxyacetylene 
hardfacing process. This sample contained about 42% WC/Co particles in the 
size range of from about 420 to 590 .mu.m, 12% WC/W.sub.2 C, 6% WC, and an 
AISI 1008 steel tube rod as the balance. The sample was made for use as a 
baseline for comparison. 
Fourteen hardfacing coatings were obtained by the pulsed-PTA method from 
Samples No. 1-14. The fifteenth hardfacing coating was made by a manual 
oxyacetylene welding process from Sample No. 15. All the hardfacing 
coatings subsequently were subjected to a low stress abrasion test and a 
high stress abrasion test. The low stress abrasion test was conducted in 
accordance to ASTM G65, and the high stress abrasion test was conducted in 
accordance to ASTM B611. 
Briefly, in the ASTM G65 test, abrasive particles, e.g., semi-rounded 50/70 
mesh (210/300 .mu.m) silica sand, were fed between a test material (such 
as the hardfacing coating) and a rotating chlorobutyl rubber wheel. The 
test material was pressed against the rotating wheel at a specific force 
of 130 N (30 pounds). The rotating speed of the wheel was about 200.+-.10 
rpm, and the sand flow rate was about 380 to 430 g/min. The weight loss of 
the test material was measured by weighing each sample before and after a 
6000 revolution test and then converted to volume loss (in cubic 
millimeters per 1000 revolutions). The smaller the wear number is, the 
better wear resistance the material has. The stress exerted on the 
abrasive particles was low enough to not crush the abrasive particles in 
this test. 
In the ASTM B611, the abrasive particles used in the test were 30 mesh (590 
.mu.m) angular aluminum-titanium oxide. They were fed and crushed between 
an annealed AISI 1020 steel wheel and the test material. Some degree of 
impact on the test material was encountered. The abrasive particles in the 
slurry were fed between the test material and the wheel by the rotating 
wheel at 100.+-.10 rpm. The test material was pressed against the rotating 
wheel with a 20 kg force for 1000 revolutions. A wear number was obtained 
and expressed as the reciprocal of the volume loss (in cubic millimeter) 
of the test material per revolution. In this test, the larger the number 
is, the better the wear resistance is. 
The testing results for the fifteen hardfacing coatings are summarized in 
the following Table 3. 
TABLE 3 
______________________________________ 
Wear Resistance of Hardfacing Coatings 
Hardfacing Low Stress Abrasion 
High Stress Abrasion 
Coating Number Wear Number Wear Number 
______________________________________ 
1 1.7-1.9 2.2 
2 1.3-1.7 1.5-2.2 
3 1.8 2.3 
4 2.1-2.3 1.9-2.0 
5 2.1 2.5 
6 2.0 2.7 
7 1.8 3.0 
8 2.0 2.4 
9 2.3-2.9 2.0-2.3 
10 2.2 2.0-3.1 
11 1.4 3.3 
12 1.5 2.8 
13 2.0 2.4 
14 1.6 2.5 
15 1.8-2.0 2.6-3.0 
______________________________________ 
It is to be noted that Hardfacing Coating No. 15, as produced by a manual 
oxyacetylene welding process, has been used in commercial products and 
serves as a baseline for comparison. The above table indicates that 
Hardfacing Coatings No. 7 and No. 14 have low-stress wear numbers and 
high-stress wear numbers similar to Hardfacing Coating No. 15. On the 
other hand, Hardfacing Coatings No. 11 and No. 12 exceeded the performance 
of Hardfacing Coating No. 15. 
EXAMPLE 2 
This example shows that hardfacing coatings deposited by embodiments of the 
invention have higher carbide contents, more uniform distribution of 
carbide particles, and lower substrate dilution than those by a non-pulsed 
plasma-transferred arc process. 
Two hardfacing samples were prepared on a AISI 4815 steel substrate: one by 
a pulsed plasma-transferred arc method in accordance to an embodiment of 
the invention ("pulsed-PTA hardfacing") and another by a 
plasma-transferred arc method of prior art ("PTA hardfacing"). Both 
hardfacings were deposited in the following manner. 
A powder mixture designated as "HM 22" was used as the hardfacing 
composition. The powder mixture included about 22.3% by weight of sintered 
WC/Co pellets, 28.7% by weight of spherical cast WC-W.sub.2 C, 12.8% by 
weight of macro-crystalline WC, and the balance 410 stainless steel. The 
particle size of the sintered WC/Co pellets was in the range of about 74 
to 297 .mu.m, the spherical cast WC/W.sub.2 C was about 74 to 149 .mu.m, 
and the macro-crystalline WC was about 44 to 74 .mu.m. 
For the PTA hardfacing, the powder mixture was fed into a plasma column at 
a rate of about 40 g/min. The voltage and current used were about 26 volts 
and 115 amps, respectively. 
For the pulsed-PTA hardfacing, the powder mixture also was fed into a 
plasma column at a rate of about 40 g/min. The voltage was about 26 volts. 
The high pulse current was about 115 amps and the low pulse current was 
about 80 amps. The current was pulsed at a pulse rate of about 100 cycles 
per second with about 65% duration time at the high pulse current. 
After both samples were obtained, photomicrographs were taken at a 
cross-section of the hardfacing and steel substrate. FIG. 5 is a 
cross-sectional view of the pulsed-PTA hardfacing on a steel substrate. 
FIG. 6 is a cross-sectional view of the PTA hardfacing on a steel 
substrate. In both figures, the steel substrate is labeled as "S," the 
sintered WC-Co pellets are labeled as "A," and the spherical cast 
WC-W.sub.2 C pellets are labeled as "B." 
Perusal of the figures reveals several microstructural differences between 
the pulsed-PTA hardfacing and the PTA hardfacing. First, there are more 
sintered WC-Co and WC-W.sub.2 C pellets in the pulsed-PTA hardfacing than 
the PTA hardfacing. Secondly, the carbide particles are larger and more 
uniformly distributed in the pulsed-PTA hardfacing than the PTA 
hardfacing. Moreover, the interface between the substrate and the 
hardfacing coating for the pulsed-PTA hardfacing is less corrugated or 
wavy than for the PTA hardfacing. The corrugation is believed to be 
associated with melting of the substrate metal. Upon melting, portions of 
the substrate metal may diffuse into the hardfacing coating during the 
deposition process, resulting in substrate dilution. Less corrugation 
generally indicates less substrate melting and lower substrate dilution of 
the hardfacing coating. 
It should be noted that dilution of the substrate metal usually occurs in 
hardfacing and welding as a result of the high energy input of a plasma 
process. Due to the high energy, an area of the substrate melts. The 
molten metal in this area may alloy with the hardfacing material and then 
re-solidify. This area is called "diluted area." As mentioned above, a 
dilution rate is defined as the weight percentage of substrate metal which 
has difffised into the hardfacing coating. Alternatively, it may be 
defined as the percentage of the diluted area over the total area exposed 
to a hardfacing process. It may be measured by examining a cross-section 
of the hardfacing coating and the substrate in an optical microscope. In 
this example, the dilution rate as measured by optical microscopy for the 
PTA process was about 21.4%, whereas the dilution rate for the pulsed-PTA 
was in the range of about 1.7-5.8%. 
EXAMPLE 3 
To measure the erosion resistance of hardfacing coatings deposited in 
accordance with the above embodiments, a slurry jet erosion test was 
conducted on three samples. Samples No. 1-4 were made in accordance with 
the above embodiments, whereas Sample No. 5 was prepared by a super-D gun 
process as described in U.S. Pat. No. 5,535,838. 
Sample No. 1 included a coating deposited using tube rod designated as "70 
M" by a pulsed plasma-transferred arc process in accordance with 
embodiments of the invention. The 70 M tube rod contained about 65% by 
weight of macro-crystalline WC with particle size in the range of about 75 
to 177 .mu.m and 35% by weight of steel of the AISI 1008 type. The 
macro-crystalline WC particles were primarily triangular pellets. 
Sample No. 2 included a coating deposited using tube rod designated as "HM 
16" by a pulsed plasma-transferred arc process in accordance with the 
above embodiments. The HM 16 tube rod contained 44.7% by weight of 
spherical sintered WC-Co pellets with particle size of about 420 to 600 
.mu.m, 12.8% by weight of spherical cast WC/W.sub.2 C with particle size 
of about 74 to 149 .mu.m, 6.4% by weight of single crystal WC with 
particle size of about 30 .mu.m, and AISI 1008 steel as the balance. 
Sample No. 3 included a coating deposited using tube rod designated as 
"HM-18VM" by a pulsed plasma-transferred arc process in accordance with 
the above embodiments. The HM-18VM tube rod contained about 23.3% by 
weight of sintered WC-Co pellets with particle size of about 74 to 297 
.mu.m, about 30% by weight of cast spherical WC/W.sub.2 C with particle 
size of about 74 to 149 .mu.m, 6.7% by weight of VC with particle size of 
about 20 .mu.m, and AISI 1008 steel as the balance. 
Sample No. 4 included a coating deposited using tube rod designated as 
"ST-70S" by a pulsed plasma-transferred arc process in accordance with the 
above embodiments. The ST-70S tube rod contained about 68.4% by weight of 
cast spherical WC/W.sub.2 C with particle size of about 74 to 177 .mu.m, 
and AISI 1008 steel as the balance. 
The resulting hardfacing coating deposited from the HM 16 tube rod 
contained about 32% by volume sintered WC/Co, 9% by volume of spherical 
cast WC/W.sub.2 C and 5% by volume single-crystal WC, and a steel matrix 
as the balance. The particle size of the sintered WC/Co was in the range 
of about 420 to 600 .mu.m. The spherical cast WC/W.sub.2 C had a particle 
size of about 75 to 149 .mu.m. Both had a spherical shape. Furthermore, 
the particle size of the single-crystal WC was in the range of about 10 to 
30 .mu.m. The deposition conditions for Sample No. 1 and Sample No. 2 are 
summarized in Table 4. 
TABLE 4 
______________________________________ 
Deposition Conditions for Samples No. 1 and No. 2 
Sample No. 1 Sample No. 2 
______________________________________ 
Hardfacing Material 
70 M HM 16 
Substrate Geometry Disk Disk 
4" diameter 4" diameter 
21/2" thickness 21/2" thickness 
Substrate Alloy AISI 4815 AISI 4815 
Voltage (Volt) 28 27.5 
Material Feed Rate (g/min) 10 10 
Shield Gas Flow Rate (cfh) 20 20 
Current Gas Flow Rate (cfh) 4.5 5 
Pulse Rate (cps) 100 100 
High Pulse Current (A) 85 80 
Low Pulse Current (A) 47 40 
Thickness (inch) 0.060 0.060 
Carbide Content (volume %) 50 46 
______________________________________ 
Samples No. 3 and No. 4 were deposited under similar conditions. All of the 
above samples were tested in a slurry jet erosion test, along with a 
sample of carburized AISI 4815 steel. 
The slurry jet erosion test is a fluid erosion test in which a hardfacing 
coating is subjected to impingement of a stream of abrasive grit. The 
amount of material removed during the test, which is indicative of erosion 
rate, is calculated in terms of inches per hour. The erosion rate 
generally correlates with erosion performance of the hardfacing in 
drilling applications. 
In this example, the test apparatus included a mud pump, a 121/4" bit body 
with a hardfacing sample to be tested, and two 16/32" 97 series nozzles. 
The abrasive grit was drilling mud with 0.5% US silica F110 sand with a 
particle size of about 70-110 .mu.m. The nozzle stand off distance was 
approximately 1" and the angle of impingement was about 11 degrees. The 
flow rate of the drilling mud was about 350 feet per second. The test 
duration varied from 15 to 30 minutes, depending on the material tested. 
The erosion rate was normalized against a standard tungsten carbide coupon 
designated as "Matrix P-90". A representative composition for Matrix P-90 
includes about 64% macro-crystalline WC, about 33% cast tungsten carbide, 
about 1% nickel, and about 2% iron. Table 2 shows different erosion rates 
for the four samples. 
TABLE 5 
______________________________________ 
Erosion Rate of Hardfacing Samples 
Carbide Content 
Erosion Rate 
Coating (wt. %) (in/hr) 
______________________________________ 
Sample No. 1 50 0.0003 
(70 M) 
Sample No. 2 46 0.0002 
(HM 16) 
Sample No. 3 49 0.0003 
(HM-18VM) 
Sample No. 4 52 0.0001 
(ST-70S) 
Sample No. 5 80 0.0004 
(Super D-Gun) 
Carburized AISI 4815 Steel 0 0.0115 
______________________________________ 
The data show that the hardfacing coatings deposited by the pulsed 
plasma-transferred arc process according to the above embodiments have 
better erosion resistance than the one made by a super D-gun process, 
although the super D-gun process produces a good, wear-resistant 
hardfacing. It is surprising that Sample No. 1, which includes only 50% 
carbide, is more erosion resistant than Sample No. 5 (made by a super 
D-gun process, which contains 80% carbide). It is further noted that 
Sample No. 4 is more erosion resistant than Sample No. 1, although they 
include a similar amount of carbide. This indicates that the type and 
shape of tungsten carbide in a hardfacing coating may have a significant 
influence on its erosion properties. 
EXAMPLE 4 
This example shows how a rock bit with roller cones could be hardfaced in 
accordance with the present embodiments. FIG. 7 illustrates three roller 
cones of a rock bit overlaid with hardfacing material according to one of 
the above embodiments. The three roller cones are labeled as 76A, 76B and 
76C. Although the insert configuration on one cone is different from that 
of another in FIG. 7, it is entirely acceptable to manufacture a rock bit 
with three identical cones. There are gage row inserts 75 and inner row 
inserts 78. These inserts are circumferentially spaced around the surface 
of the cone. Generally, there is more than one row of inner row inserts. 
Additionally, there are heel row inserts (not shown) located on the heel 
surface (not shown) of the cone. Much of the erosion of cones typically 
occurs between the gage row inserts 75 and the heel row inserts (not 
shown). Furthermore, erosion may also occur at the lands between the gage 
row inserts and the lands between inner row inserts 78. It is also 
possible that erosion may occur in the grooves between successive inner 
row inserts 78. These areas on a cutter cone surface are collectively 
referred to as "area susceptible to erosion or wear". It should be 
understood that the term "area susceptible to erosion or wear" means any 
area on a roller cone that experiences significant erosion or wear when in 
use. It may also mean an area of a rock bit that experiences significant 
erosion or wear when in use. Such areas include, for example, the 
shirttail and the journal of a rock bit and the piston of a percussion 
bit. 
As illustrated by FIG. 7, a layer of hardfacing material deposited on 
different areas of the cone is represented by 70a, 70b, 70c, and 70d. 
Layers 70a, 70b, 70c, and 70d may be of the same or different hardfacing 
materials, depending upon the design of the rock bit. These layers are 
deposited on selected lands and grooves of the cutter cone surfaces. It 
should be understood that, in some applications, it is sufficient to coat 
only the lands or the grooves of a cutter cone surface. Preferably, the 
boundaries of a hardfacing coating is at least 1/16 inch away from the 
insert holes so that the hardfacing coating does not affect the subsequent 
placement of inserts in the holes by interference fit. 
Moreover, a hardfacing coating may take various shapes. For example, they 
include, but are not limited to, round, circular, elliptical, square, 
rectangular, trapezoidal, oblong, arched, triangular, annular, and any 
irregular shape. In the case where hardfacing is desired only in the 
grooves of a cutter cone surface, it is advantageous to deposit a 
continuous circumferential ring in the grooves. 
In addition to roller cones surfaces, the shirttails, the journals, and the 
pistons of a rock bit may be hardfaced by the pulsed plasma transferred 
arc process. In fact, any wear component of a rock bit may be hardfaced in 
this manner. 
EXAMPLE 5 
This example illustrates that the pulsed plasma-transferred arc process 
according to the above embodiments may also be applied to hardface milled 
tooth rock bits. 
FIG. 8 illustrates a typical milled tooth rock bit 80. It includes a steel 
body 85 having a threaded pin 81 at one end for connection to a 
conventional drill string. At the opposite side of the body, there are 
three roller cones 82. Each roller cone is rotatably mounted on a pin or 
journal (hidden) extending diagonally inwardly from one of the three legs 
83 which extend downwardly from the body of the rock bit. Typical teeth 84 
on such a cone are generally triangular in a cross-section taken in a 
plane perpendicular to the axis of rotation of the cone. 
FIG. 9 is a cross-sectional view of a tooth coated with a hardfacing 
material according to the above embodiments. Such a tooth has a leading 
flank 86 and a trailing flank 87 meeting in an elongated crest 88. The 
flanks of the teeth are covered with a hardfacing coating 89. Sometimes 
only the leading face of each tooth is covered with a hardfacing coating 
to create a self-sharpening effect. This effect is due to the differential 
erosion between the more wear resistant hardfacing on the leading face and 
the less wear resistant steel on the trailing face, and such differential 
erosion tends to keep the crest of the tooth relatively sharp. There also 
are times when the ends (i.e., the faces between the leading flank face 
and the trailing flank face) of a tooth are coated with a layer of 
hardfacing. 
Hardfacing is especially desirable in areas which are referred to as the 
gage surface. The gage surface generally is the flat surface at the heel 
of the cone which engages the sidewall of the wellbore as the bit is used. 
The gage surface includes the outer end of teeth 84 in the gage row of the 
teeth nearest the heel of the cone and may include additional areas nearer 
the axis of the cone than the root between the teeth. These areas are 
collectively referred to as "area susceptible to erosion or wear" for 
milled tooth rock bits. Again, the term "area susceptible to erosion or 
wear" means any area on a milled-tooth cone that experiences significant 
erosion or wear when in use. 
In addition to milled-tooth cones, the piston and the journal of a 
milled-tooth cone rock bit also may be hardfaced. These components are 
exposed to frictional contact wear and abrasion wear in use. 
As demonstrated above, the above embodiments are capable of producing 
erosion-resistant and wear-resistant hardfacing coatings on a metallic 
work piece. They are especially effective in producing hardfacing coatings 
on milled tooth rock bits and tungsten carbide insert rock bits. The 
hardfacing coatings may have higher carbide contents and lower substrate 
dilution. Furthermore, there may be stronger metallurgical bonding between 
the hardfacing coatings and the metallic substrate. Such coatings 
exhibited good wear and erosion resistance. 
While the invention has been disclosed with respect to a limited number of 
embodiments, numerous modifications and variations therefrom exist. For 
example, the hardfacing material may include any suitable materials for 
wear resistance and erosion resistance applications. Such materials may 
include hard metals, metal alloys, carbides, borides, nitrides, diamond 
grit, and diamond-like carbon. Although the hardfacing material was 
introduced into the plasma column in the form of powder, other forms, such 
as wire and tube rod, also are feasible. It should be understood that the 
invention is equally applicable to any other arc processes that can be 
pulsed, including, for example, the metal inert gas welding process. 
Furthermore, the invention also can be used to apply a hardfacing coating 
on any wear component of an earth-boring device, such as the surface of 
the steel body of a polycrystalline diamond compact insert-type drill bit 
and drill hammer head pistons in percussion bits. While single-crystal WC, 
eutectic WC/W.sub.2 C, and sintered WC/Co are preferred as the primary 
carbide, it should be understood that any hard carbide may be used in 
place of single-crystal WC, eutectic WC/W.sub.2 C or sintered WC/Co. Such 
carbides may include, for example, titanium carbide and chromium carbide. 
Furthermore, it is also conceivable that a third carbide phase may further 
enhance erosion and wear protection. Such a tertiary carbide may include 
any hard carbide material. Finally, the invention can be practiced in an 
order different from that described above. It is intended that the 
appended claims cover all such modifications and variations as fall within 
the true spirit and scope of the invention.