U.S. patent application number 16/587724 was filed with the patent office on 2020-01-23 for cemented carbide containing tungsten carbide and iron alloy binder.
The applicant listed for this patent is U.S. Army Research Laboratory ATTN: RDRL-LOC-I. Invention is credited to Kristopher A. Darling, Robert J. Dowding, Billy C. Hornbuckle, Steven M. Kilczewski, Heather A. Murdoch, John J. Pittari, III, Jeffrey J. Swab.
Application Number | 20200024702 16/587724 |
Document ID | / |
Family ID | 69161656 |
Filed Date | 2020-01-23 |
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United States Patent
Application |
20200024702 |
Kind Code |
A1 |
Pittari, III; John J. ; et
al. |
January 23, 2020 |
CEMENTED CARBIDE CONTAINING TUNGSTEN CARBIDE AND IRON ALLOY
BINDER
Abstract
A sintered cemented carbide body including tungsten carbide, and
a substantially cobalt-free binder including an iron-based alloy
sintered with the tungsten carbide. The iron-based alloy is
approximately 2-25% of the overall weight percentage of the
sintered tungsten carbide and iron-based alloy. The tungsten
carbide may be approximately 90 wt % and the iron-based alloy may
be approximately 10 wt % of the overall weight percentage of the
sintered tungsten carbide and iron-based alloy. The tungsten
carbide may comprise a substantially same size before and after
undergoing sintering. The iron-based alloy may be sintered with the
tungsten carbide using a uniaxial hot pressing process, a spark
plasma sintering process, or a pressureless sintering process. The
sintered tungsten carbide and iron-based alloy has a hardness value
of at least 15 GPa and a fracture toughness value of at least 11
MPa m.
Inventors: |
Pittari, III; John J.;
(Belcamp, MD) ; Kilczewski; Steven M.; (Belcamp,
MD) ; Swab; Jeffrey J.; (Fallston, MD) ;
Darling; Kristopher A.; (Havre De Grace, MD) ;
Hornbuckle; Billy C.; (Aberdeen, MD) ; Murdoch;
Heather A.; (Baltimore, MD) ; Dowding; Robert J.;
(Abingdon, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
U.S. Army Research Laboratory ATTN: RDRL-LOC-I |
Adelphi |
MD |
US |
|
|
Family ID: |
69161656 |
Appl. No.: |
16/587724 |
Filed: |
September 30, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15807604 |
Nov 9, 2017 |
|
|
|
16587724 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22F 3/10 20130101; B22F
2003/1051 20130101; B22F 3/16 20130101; C22C 29/08 20130101; C22C
29/067 20130101; C22C 1/051 20130101; B22F 3/14 20130101; B22F
2998/10 20130101; B22F 2998/10 20130101; C22C 33/0257 20130101;
B22F 2009/043 20130101; B22F 2999/00 20130101; B22F 2009/043
20130101; B22F 2202/03 20130101; B22F 2999/00 20130101; C22C
33/0257 20130101; B22F 1/0044 20130101; B22F 2998/10 20130101; C22C
1/05 20130101; B22F 2009/043 20130101; B22F 3/02 20130101; B22F
2003/1051 20130101; B22F 2998/10 20130101; C22C 1/05 20130101; B22F
2009/043 20130101; B22F 3/02 20130101; B22F 3/10 20130101; B22F
2998/10 20130101; C22C 1/05 20130101; B22F 2009/043 20130101; B22F
3/04 20130101; B22F 3/10 20130101 |
International
Class: |
C22C 29/08 20060101
C22C029/08; C22C 1/05 20060101 C22C001/05 |
Goverment Interests
GOVERNMENT INTEREST
[0002] The embodiments herein may be manufactured, used, and/or
licensed by or for the United States Government without the payment
of royalties thereon.
Claims
1. A sintered cemented carbide body comprising: tungsten carbide;
and a substantially cobalt-free binder comprising an iron-based
alloy sintered as a matrix with/around the tungsten carbide
particles throughout the body, wherein the sintered tungsten
carbide and iron-based alloy comprises a hardness value of at least
15 GPa.
2. The sintered cemented carbide body of claim 1, wherein the
iron-based alloy is between 0% and 49.999% of the overall weight
percentage of the sintered tungsten carbide and iron-based
alloy.
3. The sintered cemented carbide body of claim 1, wherein the
tungsten carbide comprises approximately 90 wt % and the iron-based
alloy comprises approximately 10 wt % of the overall weight
percentage of the sintered tungsten carbide and iron-based
alloy.
4. The sintered cemented carbide body of claim 1, wherein the
tungsten carbide comprises a substantially same size before and
after undergoing sintering.
5. The sintered cemented carbide body of claim 1, wherein the
sintered tungsten carbide and iron-based alloy comprises a fracture
toughness value of at least 11 MPa m.
6. The sintered cemented carbide body of claim 1, wherein the
iron-based alloy comprises a solid solution phase without a
graphite or M.sub.6C phase.
7. The sintered cemented carbide body of claim 1, wherein the
iron-based alloy binder comprises an element that forms a
grain-refining carbide (e.g., Zr leads to ZrC, V leads to VC).
8. The sintered cemented carbide body of claim 1, wherein the
tungsten carbide comprises a microparticle size of approximately
0.5-20 .mu.m.
9. The sintered cemented carbide body of claim 1, wherein the
substantially cobalt-free binder comprises no more than 0.2 mass %
of cobalt.
10. (canceled)
11. A method of forming a tungsten carbide cemented body, the
method comprising: providing tungsten carbide; and sintering a
substantially cobalt-free binder comprising an iron-based alloy
binder with the tungsten carbide to form the cemented tungsten
carbide body, wherein the sintered tungsten carbide and iron-based
alloy comprises a hardness value of at least 15 GPa and a fracture
toughness value of at least 11 MPa m.
12. The method of claim 11, wherein the sintering comprises a
uniaxial hot pressing process.
13. The method of claim 11, wherein the sintering comprises a field
assisted sintering technology process.
14. The method of claim 11, wherein the sintering comprises a
pressureless sintering process.
15. The method of claim 11, wherein the iron-based alloy is
approximately 2-25% of the overall weight percentage of the
sintered tungsten carbide and iron-based alloy.
16. The method of claim 11, wherein the tungsten carbide comprises
approximately 90 wt % and the iron-based alloy comprises
approximately 10 wt % of the overall weight percentage of the
sintered tungsten carbide and iron-based alloy.
17. The method of claim 11, wherein the tungsten carbide comprises
a substantially same size before and after undergoing
sintering.
18. The method of claim 11, wherein the tungsten carbide comprises
an average microparticle size of approximately 0.5-20 .mu.m.
19. (canceled)
20. The method of claim 11, wherein the iron-based alloy binder
comprises zirconium.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of U.S.
Non-Provisional patent application Ser. No. 15/807,604 filed on
Nov. 9, 2017 which claims the benefit of U.S. Provisional Patent
Application No. 62/420,332 filed on Nov. 10, 2016, the contents of
which, in their entireties, are herein incorporated by
reference.
BACKGROUND
Technical Field
[0003] The embodiments herein generally relate to compositions of
matter, and more particularly to compositions of cemented carbide
materials with iron alloys.
Description of the Related Art
[0004] Tungsten carbide (WC) materials have become critically
important in many military and commercial engineering applications,
due to their unique combination of high strength, hardness, and
fracture toughness. The most common cemented carbides in use today
achieve these exceptional properties through the combination of
hard tungsten carbide particles within a ductile cobalt (Co)
matrix. Cobalt is a costly strategic material and is also an
environmentally hazardous material that has been classified as a
possible human carcinogen and toxic to aquatic life.
SUMMARY
[0005] In view of the foregoing, an embodiment herein provides a
sintered cemented carbide body comprising tungsten carbide; and a
substantially cobalt-free binder comprising an iron-based alloy
sintered with, and uniformly distributed around, the tungsten
carbide, wherein the sintered tungsten carbide and iron-based alloy
comprises a hardness value of at least 15 GPa and a fracture
toughness value of at least 11 MPa m. In the context of the
embodiments herein, substantially cobalt-free refers to the carbide
body containing no more than 0.5 mass % Co more preferably no more
than 0.2, still more preferably no more than 0.1 mass % Co, and
still more preferably no more than 0.05 mass % Co. The iron-based
alloy may be 0% and 49.999% of the overall weight percentage of the
sintered tungsten carbide and iron-based alloy. The tungsten
carbide may comprise approximately 90 wt % and the iron-based alloy
may comprise approximately 10 wt % of the overall weight percentage
of the sintered tungsten carbide and iron-based alloy. The tungsten
carbide may comprise a substantially same size before and after
undergoing sintering. The iron-based alloy may be sintered with the
tungsten carbide using a uniaxial hot pressing (HP) process. The
iron-based alloy may be sintered with the tungsten carbide using a
field assisted sintering technology (FAST) process. The iron-based
alloy may be sintered with the tungsten carbide using a
pressureless sintering (PS) process. The tungsten carbide may
comprise a microparticle size of approximately 0.5-20 .mu.m. The
iron-based alloy may comprise a solid solution phase without a
graphite or M.sub.6C phase. The iron-based alloy binder may
comprise zirconium. The substantially cobalt-free binder may
comprise a particle diameter of less than 100 nm.
[0006] Another embodiment provides a method of forming a cemented
tungsten carbide body, the method comprising providing tungsten
carbide; and sintering a substantially cobalt-free binder
comprising an iron-based alloy binder with the tungsten carbide to
form the cemented tungsten carbide body, wherein the sintered
tungsten carbide and iron-based alloy comprises a hardness value of
at least 15 GPa and a fracture toughness of at least 11 MPa m. The
sintering may comprise a uniaxial hot pressing (HP) process. The
sintering may comprise a field assisted sintering technology (FAST)
process. The sintering may comprise a pressureless sintering (PS)
process. The iron-based alloy may be approximately 2-25% of the
overall weight percentage of the sintered tungsten carbide and
iron-based alloy. The tungsten carbide may comprise approximately
90 wt % and the iron-based alloy may comprise approximately 10 wt %
of the overall weight percentage of the sintered tungsten carbide
and iron-based alloy. The tungsten carbide may comprise a
substantially same size before and after undergoing sintering. The
tungsten carbide may comprise an average microparticle size of
approximately 0.5-20 m. The substantially cobalt-free binder may
comprise a particle diameter of less than 100 nm. The iron-based
alloy binder may comprise zirconium.
[0007] These and other aspects of the embodiments herein will be
better appreciated and understood when considered in conjunction
with the following description and the accompanying drawings. It
should be understood, however, that the following descriptions,
while indicating preferred embodiments and numerous specific
details thereof, are given by way of illustration and not of
limitation. Many changes and modifications may be made within the
scope of the embodiments herein without departing from the spirit
thereof, and the embodiments herein include all such
modifications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The embodiments herein will be better understood from the
following detailed description with reference to the drawings, in
which:
[0009] FIG. 1A is a ternary phase diagram for a Fe--Ni--Zr system
used as a binder material for cemented WC;
[0010] FIG. 1B is a pseudo-binary phase diagram of the
W--C--Fe--Ni--Zr system showing the desired carbon range for
processing wherein deleterious phases (M.sub.6C, graphite) are
avoided;
[0011] FIG. 2A is a scanning electron microscope (SEM) image of the
cross-section of sample HP-12 displaying the graded mesostructure
observed in all HP specimens;
[0012] FIG. 2B is a negative of the SEM image of FIG. 2A;
[0013] FIG. 3 is a SEM image of the cross-section of specimen HP-17
showing the large binder pools that tended to form in the core
region of the produced puck;
[0014] FIG. 4A is a SEM image of the cross-section of sample SPS-2
revealing the presence of a graded mesostructure due to
processing;
[0015] FIG. 4B is a negative of the SEM image of FIG. 4A;
[0016] FIG. 5A is a transmission electron microscope (TEM) bright
field image of WC with the iron-based binder showing the binder
pool between WC grains;
[0017] FIG. 5B is a scanning transmission electron microscope
(STEM) bright field image highlighting the zirconium-based carbide
particles present within a binder pool between WC grains;
[0018] FIG. 6 is a flow diagram illustrating a method according to
an embodiment herein; and
[0019] FIG. 7 is a graph illustrating example binder melting
temperatures as a function of the percentage of weight of the
overall carbide and iron-based alloy.
DETAILED DESCRIPTION
[0020] The embodiments herein and the various features and
advantageous details thereof are explained more fully with
reference to the non-limiting embodiments that are illustrated in
the accompanying drawings and detailed in the following
description. Descriptions of well-known components and processing
techniques are omitted so as to not unnecessarily obscure the
embodiments herein. The examples used herein are intended merely to
facilitate an understanding of ways in which the embodiments herein
may be practiced and to further enable those of skill in the art to
practice the embodiments herein. Accordingly, the examples should
not be construed as limiting the scope of the embodiments
herein.
[0021] The term "approximately" as used herein means within plus or
minus five percent, more preferably within plus or minus two
percent, and still more preferably within plus or minus one
percent.
[0022] The term "substantially" as used herein means for the most
part or to a significant degree or amount and that a dimension,
time duration, shape, or other adjective may vary slightly from
what is described due to physical imperfections, power
interruptions, variations in machining or other manufacturing, and
the like.
[0023] The term "uniformly" as used herein means that the
dispersion of first phase is sufficiently uniform such that the
mechanical properties of the bulk material would not vary
significantly from those of a material having a completely uniform
dispersion.
[0024] The embodiments herein provide a new cemented carbide
material containing tungsten carbide particles with a iron-based
alloy as the binder. Referring now to the drawings, and more
particularly to FIGS. 1A through 7, there are shown exemplary
embodiments.
[0025] The embodiments herein replace the strategic, yet hazardous
and costly material, cobalt, which is commonly added to tungsten
carbide to form a cemented carbide material with a iron-based alloy
material, and without degradation in the material properties or
performance of the material, for example, in military applications
such as with armor-piercing penetrators. The fabrication techniques
provided by the embodiments herein allow one to achieve a cemented
carbide containing tungsten carbide particles with an iron alloy
binder matrix between the tungsten carbide particles that has a
fine-grained crystalline microstructure. The refined microstructure
of the iron alloy imparts a higher degree of toughness to the
cemented carbide by promoting more uniform deformation, compared
with a cobalt binder phase conventionally used in cemented carbide
core material.
[0026] The composition of matter of the iron alloy binder may
contain any number of transition metal elements in any proportion,
including, but not limited to, nickel, zirconium, molybdenum,
tantalum, titanium, and in a variety of combinations depending on
the properties and performance required of the densified cemented
carbide. The amount of iron alloy binder in the cemented carbide
can range from 0% and 49.999% by weight depending on the properties
and performance desired. Additionally, the cemented carbide may
contain other additives to promote densification or control grain
growth.
[0027] The embodiments herein provide a technique to produce
non-hazardous tungsten carbide bodies by supplanting the cobalt
binder phase with an iron-based alternative. The binder alloy may
be tuned for a fundamentally new approach, wherein carbides are
introduced through precipitation from the liquid binder alloy
interacting with the carbon in the tungsten carbide-binder system.
The alloying elements chosen for the iron-based binder are expected
to form a specific carbide phase, as a thermodynamically stable
state, which is a known WC grain refiner. This ability to
incorporate carbides through phase formation, rather than as a
separate additive, provides a unique method for carbide
distribution that takes advantage of more economically feasible and
traditional ball-milling processes.
[0028] Due to the extremely high melting point of tungsten carbide
(.apprxeq.2785.degree. C.), fully dense tungsten carbide bodies are
extremely difficult to produce without the inclusion of an additive
that will melt at far lower temperatures in order to cement the WC
particles together to form a dense body. This process is known as
sintering or, more specifically, liquid phase sintering (LPS).
[0029] To demonstrate the feasibility of producing dense, cemented
tungsten carbide bodies utilizing the chosen iron-based binder,
three different manufacturing processes are described below:
uniaxial hot pressing (HP), field assisted sintering technology
(FAST), and pressureless sintering (PS). Each technique provides
features to facilitate densification of the powder compacts. Other
manufacturing processes such as hot isostatic pressing (HIP) and
flash sintering, for example, may be utilized in accordance with
the embodiments herein. However, for the purposes of describing
experimental procedures, the HP, FAST, and PS processes are
described below. Furthermore, specific equipment and test
parameters are described below for the purposes of describing the
experimental procedures that were used. However, the embodiments
herein are not limited to these specific equipment and parameters
and other comparable equipment and different test parameters may be
utilized in accordance with the embodiment herein.
[0030] In a hot pressing process, the sintering may be induced by
three parameters: pressure, time, and temperature. Adding pressure
and temperature to the powder system reduces the sintering time and
temperatures required to produce sufficiently densified bodies.
Field assisted sintering technology allows for the application of
pulsed DC current (in addition to pressure, temperature, and time)
directly through a graphite die and, in the case of conductive
samples, the powder compact. This current produces highly localized
internal heat (Joule heating), in contrast to the external nature
of heat generation by heating elements in hot pressing, which
facilitates the densification of the powder compact. In variant of
FAST processing, external heating elements are used to assist in
eliminating thermal gradients. This is often called a "hybrid
system". Pressureless sintering does not require a graphite punch
and die like HP and SPS do; rather, the sample is formed into a
green compact and then placed into a furnace on a near-frictionless
(or low friction) bed (to allow for shrinkage). Since there is no
external pressure, as in the case of HP and SPS, higher processing
temperatures may be selected to facilitate and enhance
densification.
[0031] Sintering processes are conducted across different
compositions, hold times, applied pressures and temperature ranges
to demonstrate the efficacy of the embodiments herein. Based upon
preliminary experimentation, these four parameters are identified
to have the greatest influence on the microstructure and properties
of the final body. Individual summaries of the results from each
manufacturing technique are provided below.
[0032] The tungsten carbide powders used were obtained from Global
Tungsten & Powders Corp. (GTP) located in Towanda, Pa. Two
different GTP tungsten carbide powders used for the experiment
were: SC40S and SC04U. Other tungsten carbide powders may also be
used in accordance with the embodiments herein. These materials
possess particle sizes differing by a single order of magnitude and
are considered, for these purposes to be "coarse" (mean particle
size of approximately 4 .mu.m) and "fine" (mean particle size of
approximately 0.6 .mu.m), respectively.
[0033] The binder alloy utilized was a nanostructured, iron-based
alloy developed and processed in-house through mechanical alloying.
The iron-nickel-zirconium (Fe--Ni--Zr) alloy was formulated to be
an oxide-dispersed strengthened (ODS) steel alloy, where each
constituent played a vital role in obtaining the final product. In
the W--C--Fe--Ni--Zr system, zirconium's affinity for carbon is
exploited to form zirconium carbide (ZrC), which was predicted by
thermodynamic modeling calculations. The iron-zirconium carbide
interface exhibits very low lattice mismatch, and increases the
critical stress required for crack propagation. Additionally,
Fe--ZrC composites have promising abrasive wear resistance.
Furthermore, zirconium substantially lowers the melting point of
the binder system, improving liquid phase formation during
sintering. Binder powders from two different milling techniques
were produced; one from room temperature milling and another from a
cryomilling technique. Cryogenic milling helps the particles
maintain their small grain structure by diminishing the effects of
recovery and recrystallization. It also reduces agglomerate
formation typically seen in room temperature milling of soft
materials.
[0034] The Fe-8Ni-4Zr and Fe-8Ni-lZr (at. %) alloys were
synthesized by high energy mechanical alloying in a SPEX 8000D
shaker mill. The appropriate amounts of Fe, Ni, and Zr starting
powders (available from Alfa Aesar, Ward Hill, Mass.), which were
-325 mesh and 99.9%, 99.8%, and 99.5% pure, respectively, with a
total weight of 10 g were loaded into hardened steel vials (SPEX
model 8007) along with milling media (440.degree. C. stainless
steel balls) at a ball-to-powder ratio of 10-to-1 by weight, and
then sealed inside a glove box containing argon atmosphere (oxygen
and moisture are less than 1 ppm). Room temperature ball milling
was carried out at 950 revolutions per minute for a total of 20
hours. No processing agents such as sodium chloride, stearic acid,
or other organics were utilized. After milling, the vials were
again placed inside the glove box and the powders removed and
stored until enough consecutive milling runs were completed,
thereby generating enough powder (approximately 250 g) for the
consolidation experiments. In general, the powders were
deagglomerated having individual particle sizes between 10 .mu.m
and 100 .mu.m. X-ray diffraction studies revealed the as-milled
powder to have a microstructure consisting of a Fe--Ni--Zr
supersaturated body centered cubic (BCC) solid solution having an
average matrix grain size of .about.10 nm. Table 1 shows the bulk
hardness of the binder powders before combination and processing
with WC.
TABLE-US-00001 TABLE 1 Density and hardness comparison of the
nano-iron-based alloy to cobalt Binder Density (g/cc) Hardness
(GPa) Nano-iron-alloy 7.9 .apprxeq.10 Cobalt 8.9 .apprxeq.1
[0035] In general, the powders were mixed as a 90 wt % tungsten
carbide to 10 wt % iron-alloy composition, unless otherwise noted.
This ratio was selected as the baseline because it directly
compares to the composition of tungsten carbide-cobalt materials
used in armor-piercing cores, which are used for property
comparisons.
[0036] To guide and optimize the processing and composition of the
binder material provided by the embodiments herein, thermodynamic
modeling studies were conducted using Thermo-Calc 4.0 software and
a custom combination of the TCFE7, TCAL3, and TCMG3 databases to
investigate the expected phase compositions of the binder and the
anticipated binder and tungsten carbide phase interactions during
sintering. FIG. 1A illustrates the ternary phase diagram for the
Fe--Ni--Zr binder system at several possible processing
temperatures. The desired FCC and liquid phase regions are shaded
as indicated. As the processing temperature increases, the region
of single phase liquid (or complete melting) both increases in
compositional space and encompasses lower alloying (Ni, Zr)
additions. Processing temperatures and binder alloy composition
were chosen in order to enhance the liquid phase sintering effect
and avoid unwanted intermetallic phases.
[0037] The modeling studies also indicated the acceptable carbon
ranges necessary to avoid the formation of graphite and other
deleterious carbide phases (e.g. W3Fe3C, referred to as M.sub.6C),
as shown in FIG. 1B. This system has a slightly narrower acceptable
carbon range than the traditional WC--Co system; however, with the
modeling results the desired phase region can be correctly
targeted. The carbide, ZrC, is also expected to be present in the
complete system--this carbide is used as a grain refiner and
provides second phase strengthening of the binder phase, and does
not need to be avoided.
[0038] Differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA) was utilized in tandem to
investigate the phase transitions within the powder mixtures. These
analysis techniques help provide a fuller understanding of the
phase transformations and interactions of the tungsten carbide
powder and iron-alloy binder throughout the temperature range
required for the sintering procedures. Information acquired from
these techniques was used in conjunction with developed ternary
phase diagrams to guide the sintering processes.
[0039] Uniaxial Hot-Pressing (HP)
[0040] The raw tungsten carbide and iron-alloy powders are mixed in
glass jars for 5 to 10 minutes at approximately 50-60 G's using a
Resodyn LabRA.TM. ResonantAcoustic.RTM. mixer to homogenize the
powder mixture. Experimentally, thirty grams of the mixture is then
removed and placed in a graphite die with one-inch diameter
graphite punches. Two sheets of GraFoil.RTM. material (each sheet
is 0.5 mm in thickness) are placed above and below the powder
mixture (i.e., between the powder and punches) to aid in the
release of the part after hot pressing. A force of approximately
200 lbf was placed on the punches using a hydraulic Carver.RTM.
press. The punch and die set is then placed in an OXY-GON.RTM.
Bench Top Hot Press Furnace. Hot pressing studies were conducted
between 1000.degree. C. and 1150.degree. C. hold temperatures and
hold times ranging from 30 minutes to 3 hours. The load on the
punches is maintained at approximately 2000 lbf gauge throughout
the duration of the run. All runs are performed in a vacuum
environment.
[0041] In total, 18 samples were produced using the HP method. The
first eight runs explored the hold temperature- and time-space and
its resulting effect on the microstructure. The next ten runs
explored the effect of composition and WC grain size on
microstructure morphology. Coarse (4 .mu.m) and fine (0.6 .mu.m)
tungsten carbide powders are mixed with iron-alloy binder powder to
produce respective 5 wt % and 15 wt % Fe-binder mixtures (in
addition to the standard 10 wt % mixture) using the same procedure
as before to determine the effect of composition on the
densification, microstructure, and properties of the produced
bodies. Two additional runs were made at the 10 wt % iron-binder
composition for both coarse and fine WC powder using a cryo-milled
version of the iron-based binder to describe the effect on
microstructural morphology. The example parameters utilizing the HP
process include a 90:10 ratio of the WC:Fe alloy (wt %), between
approximately 1000-2000 lbf for the load on die, between
approximately 850-1150.degree. C. as the first hold temperature,
between approximately 10-180 minutes for the first hold time,
approximately 1115.degree. C. as the second hold temperature, and
between approximately 60-90 minutes for the second hold time.
[0042] Field Assisted Sintering Technology (FAST)
[0043] Field assisted sintering technology (FAST) may be turned to
as an alternative method to produce dense tungsten carbide bodies
with the selected iron-based alloy. All specimens for FAST were
produced using the 10 wt % binder composition.
[0044] Raw powders are consolidated following the same procedure
used during hot pressing. Samples are produced using a graphite
punch and die set, just as in the HP procedure. Three separate FAST
runs with different ramp stages and maximum amperages are made. The
parameters for each are provided in Table 2. Generally, FAST
furnaces are current-controlled rather than temperature-controlled
(as in HP), thus the maximum temperature reached during each run is
also presented. This temperature is measured using a pyrometer
focused on the outer surface of the die.
TABLE-US-00002 TABLE 2 Field assisted sintering technology
parameters for each sample Max. Temp. Sample Stage 1 Stage 2 Stage
3 (.degree. C.) FAST-1 200 A/min to 2000 A N/A -200 A/min to 1275 0
A FAST-2 200 A/min to 1475 A 150 A/min -200 A/min to 1280 to 1850 A
0 A FAST-3 100 A/min to 1475 A 150 A/min -200 A/min to 1247 to 1800
A 0 A Note: The pressure on the punches for all SPS tests is 100
lbf
[0045] Pressureless Sintering (PS)
[0046] Powder mixtures were produced using the same procedure as
for the HP and FAST methods with the exception of the substitution
of a ball milling procedure for the acoustic mixing. A ball milling
procedure is executed on a mixture of 90 wt % WC and 10 wt %
Fe-binder. This process "smears" the softer binder grains onto the
harder WC grains, producing a "coating" of binder on each WC
particle that creates a better dispersion of the binder amongst the
powder compact.
[0047] No external pressure is applied during sintering for this
technique, the green body density is increased to aid the
densification of the processed specimens. In order to achieve this,
the powder compact is first pressed in a hydraulic press, at a much
higher force than used for the HP or FAST samples. In one version
of the process some parts are removed from the die and placed in a
vacuum-sealed package to undergo a 60 ksi cold isostatic pressing
(CIP) procedure. The compacted green bodies are placed in an
alumina crucible on smooth alumina balls, which may be used to
create a near-frictionless surface to accommodate shrinkage during
densification. An alumina lid is placed on a crucible, and the
whole assembly is placed on a porous alumina setter inside of an
alumina tube contained within the furnace. The ends of the alumina
tube are capped off with gas inlet and outlet ports. Argon gas (of
initial 99.999% purity, referred to as ultra-high purity) flows
through a gettering furnace (increasing the purity to <1 ppm
O.sub.2) and then into the process tube. These conditions are
maintained throughout the duration of the sintering procedure.
[0048] The sintering temperatures for PS range from 1175.degree. C.
to 1475.degree. C. This higher temperature range is due to the fact
that there is no applied pressure to the part and additional
thermal input is needed to bring the sintering process to full
densification. This requires processing in a different region of
process space with lower pressure. Hold times range between two and
20 hours to control the microstructural development.
[0049] Characterization Techniques
[0050] The quality of all specimens was initially judged based on
their density in relation to the theoretical density (TD) for that
composition. Further, three benchmark property requirements were
selected for replacing WC--Co cermets: 2 kg Knoop hardness of 15
GPa as measured following the procedure outlined in ASTM C1326-13,
fracture toughness of 11 MPa(m.sup.1/2) as measured following the
procedure outlined in ASTM C1421-18, and flexure strength of 3 GPa
as measured following the procedure ASTM 1684-08. These baseline
values are selected to be equal to, or exceeding, current
properties as observed in WC-10% Co AP cores. Fabricated specimens
had dimensions sufficient for Knoop hardness and Palmqvist fracture
toughness measurements via indentation methods, while flexure
strength could not be obtained from these specimens.
[0051] Density Measurements
[0052] The density of the fabricated parts was measured by two
different procedures for comparison accuracy. An initial
measurement was made using Archimedes' method. The liquid used for
sample immersion was deionized water. Measurements made using the
Archimedes' method were verified using helium pycnometry.
[0053] Uniaxial Hot-Pressing
[0054] Hot pressing is demonstrated to produce sufficiently dense,
cemented tungsten carbide bodies using the iron-based binder alloy.
The first eight runs (HP-1 through HP-8) revealed that density
increased with hold temperature and pressure, and porosity
decreased. However, a substantial change in density is not be found
above a 1115.degree. C. hold temperature, hence this value was
selected as the optimal processing temperature for the two-stage
consolidation. It was further found that above 1115.degree. C.
there is "squeeze-out" of liquid material along the die-punch wall
that is a result of binder flow. The squeeze-out is further
evidence that conditions suitable for complete melting of the
binder have been reached.
[0055] The percent of the theoretical density for the various
compositions range from 88.0% to 97.8%, and generally increase with
hold temperature and die load, as noted previously. While the
bodies produced from optimal hot pressing parameters have high
percentages of theoretical densities, they all generally possess
one common artifact--a graded mesostructure. An example of this
feature is presented in FIGS. 2A and 2B.
[0056] The relative thickness and amount of each band varies in
each sample; and it is a persistent artifact present in all HP
samples. Further experiments where the graphite die and punches
were coated with boron nitride spray as an insulating barrier
against this reaction did not eradicate the result. This graded
mesostructure leads to variable properties across the regions
within the body. In general, there are two identifiable "layers"
within the mesostructure, however, sometimes more inter-layers are
apparent that exhibit mixed properties of the rim and core areas.
The rim regions are extremely porous. Core areas typically have
large pools of binder, sometimes up to the millimeter scale,
similar to those shown in FIG. 3.
[0057] The percent of the theoretical density observed tends to
increase with decreasing binder phase volume. It was observed that
samples produced using cryomilled binder also have significantly
lower percentages of theoretical density than their corresponding
samples made from room temperature milled binder powder. Binder
pooling also tends to increase in the samples containing cryomilled
binder.
[0058] The graded mesostructure leads to gradients in hardness
values based upon the location of the indent. Hardness values at
the rim region are approximately 16 GPa, while the core regions are
softer, exhibiting a hardness of only 13 GPa. The core is softer
due to the binder pooling issue. The binder phase is much softer
than the tungsten carbide phase seen in the majority of the rim
region. These results are still better than the conventional WC--Co
material (12.9 GPa), indicating that the Fe-alloy binder provided
by the embodiments herein are an improvement over the traditional
cobalt binder.
[0059] Field Assisted Sintering Technology
[0060] The percent of theoretical densities range between 90.8% and
94.9% for specimens produced using field assisted sintering
technology. The specimens exhibit similar mesostructures to the
samples made by the HP process; the same graded mesostructure is
present, as shown in FIGS. 4A and 4B. While the number of
gradations and their respective thicknesses may vary it is observed
that there are graded mesostructures present in all FAST
samples.
[0061] The FAST samples possess similar hardness profiles and
characteristics when compared to the HP samples. The core region is
softer, with hardness values around 13.5 GPa, and the rim region is
harder, around 16 GPa. Mean hardness values are presented in Table
3, along with the percentage of theoretical density measurements
and the maximum temperature reached during the FAST process.
TABLE-US-00003 TABLE 3 Density and hardness results for each of the
FAST samples Sample % TD Hardness, HK.sub.2 (GPa) Maximum
Temperature (.degree. C.) FAST-1 91 16 1275 FAST-2 95 15 1280
FAST-3 92 15 1247
[0062] Slightly higher temperatures are reached during FAST than
observed from HP. However, this does not result in any improvements
in the microstructure or density of the formed parts. Only the
coarse tungsten carbide powder with a 10 wt % iron-binder additive
was used to isolate the effects of FAST on the microstructural
morphology and to facilitate comparison of the results to those
from the HP process. The hardness shown in Table 3 is the overall
average hardness of the material with measurements made in both the
core and rim of the material.
[0063] Pressureless Sintering
[0064] Based on the findings of the HP and FAST studies, an example
composition may be the 2-25 wt % iron-alloy binder with the WC
having an approximate size ranging from between 0.5-20 microns. In
order to understand the effect of the ball mill step, samples of
raw powders (i.e. before milling) and milled powders are observed
under scanning electron microscopy (SEM). The milling has an
effect, as the hard WC particles are "coated" with the softer
binder alloy. As a secondary effect, the milling procedure also
reduces some of the harsh angular features of the WC particles,
which helps the packing efficiency and consolidation of the system
during sintering. The ball milling technique is postulated to
improve the dispersion of the binder with WC grains prior to and
during sintering. The HP technique was investigated using the
milled powder, however the reaction with the graphite die/punch
surfaces still occur, resulting in the same graded mesostructured
and no improvement in the percentage of the theoretical
density.
[0065] After the CIP process was complete, green densities were
measured based on sample geometry and mass. Generally, green body
densities for the samples were approximately 60% of the theoretical
density. Reaching green density values of this magnitude are
essential for producing a dense sample with an extremely high
percentage of the theoretical density.
[0066] On the whole, percentage of theoretical densities ranged
between 55.4% and 96.5% for all PS experiments. A sharp increase in
density was noted when increasing the green press load and moving
to the secondary CIP procedure providing greatly increased green
densities. Density values increased with longer hold times and
increased temperatures up to 1450.degree. C. and 10 hours (PS-14:
96.1% TD). Above this level no appreciable increase in density was
noted for either higher temperatures or longer hold times. These
observations are supported by thermodynamic calculations that
indicated that the processing temperature would need to be above
.apprxeq.1435.degree. C. for complete melting of the binder phase
in the case where there was no applied pressure. Therefore, a hold
temperature of 1450.degree. C. and hold time of 10 hours were
selected as the optimal processing conditions. The cryomilled
binder used in PS-10, without a ball milling step, resulted in a
density of 89.9% of TD, demonstrating that the cryomilled binder
does not produce a substantial improvement in the density under
identical sintering conditions. Sintering under vacuum did not
produce any improvement in properties.
[0067] In contrast to HP and FAST processed material, mechanical
properties are uniform across the cross-section, with the observed
hardness being above the baseline of the WC--Co materials.
Indentation toughness values increase with increasing percent of
theoretical density, further indicating that the WC--Fe-alloy
material is capable of exhibiting improved properties and
performance compared to traditional WC--Co materials at a similar
composition. Table 4 provides a comparison of the properties for
WC--Co and the optimal WC--Fe-alloy.
TABLE-US-00004 TABLE 4 Comparison of the properties for WC--Co and
the optimal WC--Fe- alloy Hardness Palmqvist Binder Amount HK2
Toughness, W.sub.K Density Material (wt %) (GPa) (MPa(m.sup.1/2))
(g/cm.sup.3 C--Co 11 13 12 14.4 WC--Fe-alloy 10 16 11 14.1
[0068] FIG. 5A is a TEM bright-field image showing an iron alloy
based binder pool between multiple WC grains taken from a PS
sample. The binder pool shown is one of the larger ones observed,
with dimensions of approximately 200 nm by 200 nm. The pool
contains fine particles within it. In order to help resolve the
particles, a scanning transmission electron microscopy (STEM) image
is shown in FIG. 5B on the same specimen, though from a different
binder pool. This enhanced imaging technique allows for the
particulates to appear as darker specks within the surrounding
binder pool of lighter contrast. This binder pool is slightly
larger than the one observed in the original TEM bright field image
of FIG. 5A. Nevertheless, within this binder pool, numerous
particles with diameters less than 100 nm are present. The size and
spacing of such particles provides solid evidence of their ability
to pin dislocations, which increases the strength and toughness of
the overall material in comparison to a binder pool without such
particulates. The fine particles are a zirconium-based carbide, due
to zirconium having a stronger affinity to react with carbon than
either iron or nickel.
[0069] It was demonstrated that sufficiently densified tungsten
carbide materials may be produced using the iron-based binder. The
2-25 wt % iron-binder composition was selected to be the
composition going forward.
[0070] The results of pressureless sintering are deemed to be
promising for iron-alloy based binders. Pressureless sintered
materials were observed to have a homogeneous microstructure and
properties, in contrast to the heterogeneous mesostructures
produced via the HP and FAST techniques. Hardness and indentation
toughness values are above the baseline values of comparable WC--Co
materials.
[0071] TEM investigations into the binder regions of the
liquid-phase sintered cermets revealed fine carbide particulates
contained within the binder pools. These particulates provide a
two-fold effect: (1) pinning the tungsten carbide grains from
growing, and (2) pinning the dislocation motion within the binder
phase. The overall results of the experiment are promising towards
the solution of achieving a dense cemented tungsten carbide
material with an iron-alloy binder material.
[0072] The carbides are introduced by precipitation from the liquid
binder chemically reacting with the C in the WC/binder system. The
alloying element(s) selected for the Fe-based binder are expected
to form a carbide as a thermodynamically stable phase;
incorporation of carbide through phase formation rather than as a
separate addition may represent a more controllable method for
carbide distribution that takes advantage of the economically
feasible traditional ball-milling processes.
[0073] FIG. 6 is a flow diagram illustrating a method 100 of
forming a tungsten carbide cemented body, the method 100 comprising
providing (101) tungsten carbide, and sintering (103) a
substantially cobalt-free binder comprising an iron-based alloy
binder with the tungsten carbide to form the tungsten carbide
cemented body, wherein the sintered tungsten carbide and iron-based
alloy comprises a hardness value of at least 15 GPa and a fracture
toughness of at least 11 MPa m. In examples, the sintering (103)
may comprise a uniaxial hot pressing process, a field assisted
sintering technology process, or a pressureless sintering process.
Some example iron-based alloy binders, which may be used in
accordance with the embodiments herein include: Fe--Ni, Fe--Zr,
Fe--Ni--Zr, Fe--V, Fe--Cr, Fe--Ta, Fe--Ti, Fe--Cu, Fe--Mn, Fe--Al,
Fe--Nb, Fe--Mn--Zr, Fe--Mn--Ta, Fe--Mn--Ti, Fe--Mn--Al, Fe--Mn--Cr,
Fe--Mn--V, Fe--Ni--Ta, Fe--Ni--Ti, Fe--Ni--Mn, Fe--Nb--Cr,
Fe--Al--Cr, Fe--Ni--Cr, Fe--V--Cr, Fe--V--Ta, Fe--V--Ti, Fe--V--Al,
Fe--V--Ni. FIG. 7 illustrates the example binder melting
temperatures as a function of the weight percentage of the overall
combined carbide and iron-based alloy. As an example, in the case
of a Fe--Ni--Zr binder, increasing the Zr content significantly
lowers the melting temperature, enabling less expensive liquid
phase sintering processes.
[0074] The iron-based alloy may be between 0% and 49.999% of the
overall weight percentage of the cemented tungsten carbide. The
cemented tungsten carbide may comprise approximately 90 wt % and
the iron-based alloy may comprise approximately 10 wt % of the
overall weight percentage of the cemented tungsten carbide. The
cemented tungsten carbide phase may have a grain size substantially
the same as the original microparticle size before undergoing
sintering (103). The tungsten carbide may comprise a substantially
same size before and after undergoing sintering. The tungsten
carbide may comprise an average microparticle size of approximately
0.5-20 .mu.m. The substantially cobalt-free binder may comprise no
more than 0.2 mass % of cobalt. The iron-based alloy binder may
comprise zirconium.
[0075] The embodiments herein may be utilized in many military and
commercial applications, including, but not limited to use as the
core material in armor-piercing projectiles used in numerous
military weapon systems, cutting tools for the cutting and/or
machining of steels, hard metals, metal alloys, and abrasion
resistant materials, knives and hammers, road scarfing inserts used
for the patching and replacement of asphalt and concrete roadways,
bearing and sealing applications, and inserts used in the mining
and drilling of rock and earthen material in the coal, oil, and gas
industries.
[0076] The microstructure of the tungsten carbide with the iron
alloy confirms the uniform distribution of the iron alloy around
the tungsten carbide particles thereby providing a reduced
contiguity of the tungsten carbide grains. Furthermore, mechanical
properties measured on the tungsten carbide-iron alloy material
provided by the embodiments herein meet or exceed those of the
conventional cemented tungsten carbide-cobalt material including
relative material density, and hardness. Sintering studies
conducted through pressureless sintering techniques demonstrated
that the processing technique provided by the embodiments herein is
capable of producing a homogeneous microstructure with a high
percentage of theoretical density along with improved hardness and
indentation toughness values.
[0077] The embodiments herein eliminate the need of using cobalt in
cemented carbide materials, which eliminates a potentially harmful
material particularly to human and aquatic life. Indeed, the
elimination of cobalt from the tungsten carbide-based cemented
carbide material will also significantly reduce overall processing
costs since the components of the iron-based alloy system are
relatively easier and less expensive to manufacture compared with
cobalt-based materials. This also eliminates some safety concerns
associated with the fabrication of cemented tungsten carbide
components containing cobalt.
[0078] The foregoing description of the specific embodiments will
so fully reveal the general nature of the embodiments herein that
others may, by applying current knowledge, readily modify and/or
adapt for various applications such specific embodiments without
departing from the generic concept, and, therefore, such
adaptations and modifications should and are intended to be
comprehended within the meaning and range of equivalents of the
disclosed embodiments. It is to be understood that the phraseology
or terminology employed herein is for the purpose of description
and not of limitation. Therefore, while the embodiments herein have
been described in terms of preferred embodiments, those skilled in
the art will recognize that the embodiments herein may be practiced
with modification within the spirit and scope of the appended
claims.
* * * * *