Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
The present invention relates to the field of wear resistant
castings and their manufacture. More specifically, the present
invention relates to the field of wear resistant earthworking
castings and penetration resistant security devices.
In the field of earthworking equipment, the useful lifetime
of the teeth contacting the formation being worked is important
to the economic success of the work being performed.
The lifetime of these teeth are affected by the environment
in which they operate. Typically, the environments encountered
may produce conditions of abrasive wear, impact loading,
temperature variation, vibration and corrosion at the teeth
surface, all factors which tend to reduce the life~ime of the
tooth or tool. The high cost in terms of downtime and tool cost
for the replacing of worn out and broken tools has led to the
development of a wide variety of tools designed to provide
improvements in their in-service lifetimes.
In some cases, these improved tool designs have -încluded
the embedding of carbide into the tool working surface through
casting processes (see~ for example, United States Patent Nos.
4,024,902 and 4,140,170).
These casting techniques present problems when it is desired
to produce castings having relatively thin cross sections or
when it is desired to place carbide particles on the surface of
a vertically extending appendage, as well as a horizontal
portion, of a casting.
In order to minimize dissolution of the carbide particles
during casting, and the resulting brittle eta phase (M6C or M12C
carbide containing tungsten and iron) produced at the carbide-
steel interfaces, the cemented carbide particles utilized
typically should have a size oE at least 1/8 inch. Increasing
the si2e of the particles reduces -the carbide-steel interface
area However, in thin sections of a casting having a thickness
only slightly larger than the carbide size, the carbides can act
in conjunction with -the mold to rapidly and excessi~ely chill
the molten metal flowing between the carbides and thereby cause
incomplete filling in these thin sections.
It is also i~lpractical to hold large cemented carbide
particles uniformly dispersed along a vertical section of a
casting without filling that section with carbide from the
bottom up so as to hold the carbides in position during casting.
This can lead to the aforementioned voids and/or incomplete
filling due to excessive chilling of the melt.
Australian Patent No. AU-Bl-31362/77 attempts to avoid the
aforementioned casting problems by milling a heat treatable low
alloy steel powder together with a tungsten carbide powder or
tungsten molybdenum solid solution carbide powder, and then
pressing and sintering to subs-tantially full density a compact
of the resulting mixture. Low alloy steel is then cast around
the sintered steel-carbide compact to form a finished component.
This Australian patent, however, limits the steel powders used
to low chromium content steel.
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BRIEF SUI~MARY OF THE INVENTION
According to the present invention, a tough wear resistant
body having carbide particles with a size greater than 400 mesh
embedded substantially within a first metallic matrix are described.
The above composite of carbide particles and first metallic
matrix is bonded to a second metallic matrix, Preferably, the
carbide particles are cemented carbide particles, most
preferably containing tungsten carbide. Preferably3 the carbide
particles comprise 30 to 80 w/o of the composite and have a size
greater than 4~ mesh.
Preferably, the second metallic matrix substantially
surrounds the composite of carbide particles and first metallic
matrix.
Preferablyi the first metallic matrix is composed of steel,
preferably~ stainless steel, and most preferably an austenitic
stainless steel.
Preferably, the second metallic matrix is composed of
steel, preferably, a low alloy steel or austenitic steel, and
most preferably an austenitic stainless steel.
It is also preferred that the cemented carbide particles
utilized contain principally tungsten carbide and a binder
selected from cobalt, nickel, their alloys with each other, or
their alloys with other metals.
It has also been found that where the first metallic matrix
is austenitic stainless steel~ the first matrix may be less than
90 percent dense or as low as 75 to 85 percent dense,
_~_
Also provided7 according to the present invention, is a
process in which the carbide particles are blended with the Eirst
metallic matrix powders, and the blend is then isostatically
compacted and sintered. A second metallic matrix or molten metal
is then bonded to said compact. The molten metal may be cast
substantially around the compact or, depending on the application,
such as in providing a wear surface, the molten metal may not
completely incorporate the composite.
It is, therefore, an object of the present invention to
minimize the brittle phases produced when casting molten metal
around carbides~
It is, therefore, also an object of the present invention
to provide a product having excellent wear, corrosion and drill
resistant properties as well as good toughness.
Another object of the present invention is to provide a
process by which an earthworking tool or penetration resistant
security device can be fabricated.
BRIEF DESCRIPTION OF THE DRAWINGS
The exact nature of the present invention will become more
~O clearly apparent upon reference to the following detailed speci-
fication, reviewed in conjunction with the following drawings:
Figure 1 shows an isometric view of a cast lock box
according to the present invention.
Figure 2 shows a cross section of the embodiment shown in
Figure 1 viewed along arrows II II.
Figure 3 shows a cross section through a mold cavity used
to produce the Figure 1 embodiment of the present invention
2()~
Figure 4 shows a cross section through an embodiment of a
digger tooth according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention, 30 to 80 weight
percent of carbide particles are blended with 70 to 20 weight
percent of steel powder to produce a substantially uni~orm
mixture o carbide and steel. The carbide particles used are
preferably cemented tungsten carbides having a size of 400 mesh
or, more preferably, greater than 40 mesh. Most preferably,
these cemented carbide particles should have a siæe of -6+12
mesh (U. ~. Sieve Series), or 0.13 between ,066 inches~ respec-
tively.
It has been found that sintered composites containing
cemented carbide particles within this most preferred size range
are resistant to penetration by drilling.
Further improvements in wear resistance and drill pene~ra-
tion resistance may be obtained by utilizing carbide particles
having a bimodal size distribution. In this embodiment oE the
invention~ the size of the smaller carbide particles is selected
so as to allow them to fit into the interstices formed between
the larger carbide particles, thereby further increasing wear
resistance.
The cemented carbide may have a metallic binder selected
from cobalt, nickel, or cobalt-nickel alloys. In addition to the
tungsten carbide, the cemented carbide may contain lesser amounts
of other carbides, such as tantalum carbide, niobium carbide,
hafnium carbide, zirconium carbide and vanadium carbide.
~ ~ ~ 2 ~ ~ ~
Crushed and screened scrap cemented carbide has been found to
be suitable for use in this process.
While tungsten carbide particles of greater than -400 mesh
may be substituted for all or part of the cemented carbide
particles in the composite, tungsten carbide powder is not
preferred since it bonds less readily to the steel, tends to
fracture easily and generally provides ~ess wear and impact
resistance than cemented tungsten carbides of the same
particle size.
The steel powder utilized in this invention may be an alloy
steel~ but is preferably a stainless steel because of their
greater resistance to corrosion. However, most preferred of
the stainless steels are the austenitic stainless steels because
of their high wear and impact resistance from room temperature
down to cryogenic temperatures. Of the austenitic stainless
steels, AISI types 301, 302, 30~ and 304L grades are preferred
because of their high work hardening rates.
In addition to the carbide and steel powders in the charge,
organic binders are also added to prevent segregation and
produce uniform distribution of the carbides during blending
and retention of the uniform mixture after blending.
After blending, the mixture of powders is compacted by
uniaxially pressing in a die or isostatic pressing in a preform
mold, preferably at approximately 35,000 psi, but not less than
10,000 psi.
After compaction, the compact is then sintered at a
temperature preferably below the melting point of the steel and,
~z~
most preferably, in the range of 1900 degrees Fahrenheit to
2250 degrees Fahrenheit ~or 20 to 90 minutes, thereby avoiding
the formation of eta phases at the cemented carbide-steel
interface, and still providing a strong metallurgical bond
between the cemented carbide and the steel.
In most cases, the bond between the steel and cemented
carbide takes the form of an alloy layer at the cemented
carbide-steel interface. This layer is principally comprised
of cobalt and iron and is typically less than 40 microns thick.
This bond is important to the secure retention of the coarse
cemented carbide particles within the steel matrix.
It has been found that the as sintered compacts utilizing
austenitic stainless steel powder generally exhibit inter-
connected microporosity and have a steel binder density of less
than 90 percent of theoretical and, more typically, 75 percent
to 85 percent of theoretical. To increase the density of the
compacts' hot isostatic pressing, infiltration or increased
compacting pressures may be employed. These processes will also
result in improved carbide retention in the composite. The
infiltrant used may be selected from any of the copper base or
silver base brazing materials that wet both stainless steel
and carbide.
The sintered compact is then positioned within a mold and
molten metal is poured around it to produce a casting. The
casting procedure used may be any of those well known to those
s~illed in the art. However, i~ is preferred that the casting
procedure described in United States Patent No. 4,024,90~ be
used. Preheating of the compact may be utilized prior to
pouring of the molten metal into the mold.
The molten metal may be a ferrous or nonferrous alloy and
is, preferably, steel. The type of steel utilizing need not
be identical to that contained in the compact. Where impact,
strength and corrosion properties are important~ the cast steel
is preferably an austenitic stainless steel. Low alloy and
austenitic manganese steels may also be utilized.
The cast steel forms a metallurgical bond with the steel
binder in the compact with a minimal amount of reaction with the
cemented carbides. The formation of eta phase is thereby
minimized since the surface area of the carbides coming into
contact with the molten steel has been minimized.
The use of the cemented carbide-steel compac~s also allows
the carbides to be bonded in a variety of concentrations,
positions and orientations both on the surface and beneath the
surface of castings.
The process and products according to the present invention
will become more apparent upon reviewing the following detailed
examples.
EXAMPLE N0. 1
A number of wear and impact resistant digger teeth 1 (see
Figure 4) having compacts 3 were fabricated. A uniformly blended
mixture composed of 60 w/o 1/~ inch to 3/16 inch cobalt
cemented tungsten carbide granules and 40 w.p minus 100 mesh
atomized 304L austenitic stainless steel powder (manufactured
by Hoeganaes Corporation o~ New Jersey) was prepared by dry
mixing with 1.25 w/o paraffin and w/o 0.75 ethyl cellulose.
The mixture was manually compacted into an elastomeric
polyurethane mold cavity of the desired compact shape (2 inches
long x 3/4 inch wide x 1/4 inch thick), dimensioned to allow
for cold isostatic powder compaction plus one percent sintering
shrinkage. Following cold isostatic ompaction at 35,000 psi,
the compacted preform was removed from the mold and ~acuum
sintered at 2100 degrees Fahrenheit for 60 minutes. The sintered
hodies were then placed in a sand mold that had eight recesses
formed to the required digger tooth shape. The ingredients to
produce an AISI 4340 low alloy steel were melted in an induc~ion
furnace, the compacts were preheated, and the steel cast into
the mold at 3050 degrees Fahrenheit to 3150 degrees Fahrenheit
to form the digger tooth shown in Figure 4 in which the 4340
steel 5 is bonded to two angularly related faces of the compact 3.
A metallographic examination disclosed that the stainless
steel matrix containing an austenitic structure with some
in~ergranular chromium carbides referred to as sensitiæakion,
which is typical of slow cooled austenitic stainless steels
after sintering. Sensitization can be eliminated by a subsequent
solution heat treatment. The cemented carbide-s-tainless steel
matri~ interfaces contained a continuous bond zone approximately
15 microns thick of an alloy principally composed of iron and
cobalt. The cemented carbide dispersed particles appeared Eree
of thermal cracking with a minimu~l amount of dissolution,
melting or degradation of the dispersed carbide phase at or
near the interfacial bouIldaries. There was some melting or
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blendlng of the stainless steel and some degradation of carbides
where the molten metal made contact with the carbides at the
surface of the compact. However, below the compact surface, the
interfacial carbide boundaries were generally sharp except for
the aforementioned iron-cobalt alloy diffusion zone. No
potentially harmful concentrations of eta phases were observed.
Test samples were repeatedly (five and six times) strick
wuth a ball peen hammer at room and at liquid nitrogen (-320
degrees Fahrenheit3 temperatures and found to have good impact
resistance with little evidence of brittle type fractures. It
should be noted, however, that with a higher weight percent of
cemented carbides in the co~posite, the impact resistance might
be reduced slightly, but its resistance to wear and drill
penetration would increase.
Micro hardness measurements of a section of the as cast
digger tooth showed average hardnesses (indentations) of about
75 R"C", 29 R"C" and 38 R"C" within a traverse of the cemented
carbide, 30~L stainless steel and 4340 steel (0.125 inch from
the stainless steel interfaces) respectively.
EXAMæLE N0. 2
A drill resistant lock box 10 shown in Figure 1 was produced
by casting molten 4340 grade low alloy steel around ~intered
304L stainless steel-carbide plates (4 inches long x 2 1/2 inches
wide x 1/8 inch to 3/16 inch thick) and plates (3 1/4 inches
long x 2 1/2inches wide x 1/8 inch to 3/16 inch ~hick). The
position of one of the sintered plates 12 is shown by the
dashed lines. The plates were made by uniformly blending a
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mixture of 50.0 w/o -8+12 mesh cobal~ cemented tungsten carbide
chips, 50.0 w/o -100 mesh ~ISI 304L stainless s-teel powder, and
10.0 w/o of binders (Chloruthene Nu and 0.75 Ethyl Cellulose).
The matrix stainless s-teel powder containing the dispersed
hard carbide phase was packed in a polyurethane mold shaped to
the plate dimensions. The mold was then sealed, placed in a
rubber bag which was evacuated and sealed and then isostatically
pressed at 35,000 psi. The compacted plate, after being remo~ed
from the rubber bag and mold, was sintered in a vacuum furnace
at 2100 degrees Fahrenheit for 60 minutes.
The drill resistant plates were then positioned in the
front, back and sides of the lock box cavity in a mold~
Figure 3 shows a section through a sand mold 30 having a cavity
formed between a cope section 32 and a drag section 34
Sintered plates 12 are shown held in position in the side wall
cavities by nails 36 and 40 which are embedded in the drag
portion 34 of the mold 30. Cemented carbide particles 42 have
been laid on the bottom surface of the cavity. Prior to placing
the cope 32 on to the drag 34, the cemented carbide particles
42 and plates 12 were preheated, The cope 32 was then placed
into the drag 34 and molten 4340 low alloy steel was poured into
the mold cavity
The objective of the present invention in this security
application is to provide the lock box with 1/8 inch thick
sintered stainless steel-cemented carbide plates enveloped with
steel ~or pro~ection against drill penetration.
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It is a Eurther objective and novel feature of this
invention -that when making the loGk box that the plate or plates
will retain their shape and the carbide particles remain
uniformly dispersed in the plates when molten steel is cast
around -them filling the remaining lock box wall cavity After
the destruction of two masonary 1/8 diameter drill bits, the
front section 14 of the lock box 10 shown in Figure 1 was not
penetrated.
A section cut through the lock box containing the carbide-
stainless s~eel plate is shown in Figure 2. There was a ]ittlemelting of the stainless steel when the molten alloy steel was
cast around the sintered stainless steel carbide plate and the
carbides remained uniformly dispersed in the plate 12. There
was very little carbide degradation and a minimum of brittle
phases at the carbide-4340 steel interfaces. A metallurgical
bond was produced between the austenitic structure of the
stainless steel and the 43~0 cast steel structure. The carbide
particles 42 in the bottom wall 20 of the box may be replaced
by plates identical or similar to those shown in the side walls 22.
EXAMPLE N0. 3
Drill and impact resistant, 5/32 inch ~hick plates were
fabricated. Fifteen plates consisted of a uniformly blended
mixture of 60 w/o 3/32 inch to 1/8 inch (-8~12 mesh) cobalt
cemented tungsten carbide chips, 40 w/o minus 100 mesh 304L
stainless steel powder, 2 w/o of chlorothene Nu, 1 w/o ethyl
cellulose and 1/4 w/o armido wax. A second group of 15 plates
were made with 70 w/o ~-~+12 mesh) cemented carbide chips and
30 w/o (-100 mesh) 304L stainless steel powder similarly
blended. The armido wax and ethyl cellulose were added to the
powder blend during mixing as a pressing lubricant to prevent
segregation of the carbide particles during mixing and mold
filling. Next, the matrix powder containing the dispersed hard
carbide phase was packed in a preform mold made of polyurethane~
The packed mold with a suitable fitted cover was then sealed
and placed in a rubber bag or balloon which was evacuated~
sealed and isostatically pressed at about 35,000 psi. The plates
were then sintered in a vacuum furnace at 2100 degrees
Fahrenheit for 60 minutes.
These plates may now be incorporated into a casting using
the casting techniques previously described or any of the other
casting methods known in the art.
Modifications may be made within the scope of the appended
claims.
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