Note: Descriptions are shown in the official language in which they were submitted.
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POWDER METAL COMPOSITIONS FOR WEAR AND TEMPERATURE RESISTANCE
APPLICATIONS AND METHOD OF PRODUCING SAME
100011
This application, which claims priority to U.S. Application Serial No.
13/837,549, filed March 15, 2013, the contents of which are incorporated
herein by reference
in their entirety.
TECHNICAL FIELD
[0002]
This invention relates generally to a powder metal composition, and methods
of producing the powder metal composition from an iron based alloy.
BACKGROUND OF THE INVENTION
[0003]
High hardness prealloyed iron based powder, such as tool steel grade of
powders, can either be used alone or admixed with other powder metal
compositions in the
powder-metallurgy production of various articles of manufacture. Tool steels
contain
elements such as chromium, vanadium, molybdenum and tungsten which combine
with
carbon to form various carbides such as M6C, MC, M3C, M7C3, M23C6. These
carbides are
very hard and contribute to the wear resistance of tool steels.
[0004] The
use of powder metal processing permits particles to be formed from fully
alloyed molten metal, such that each particle possesses the fully alloyed
chemical
composition of the molten batch of metal. The powder metal process also
permits rapid
solidification of the molten metal into the small particles which eliminates
macro segregation
normally associated with ingot casting. In the case of highly alloyed steels,
such as tool steel,
a uniform distribution of carbides can be developed within each particle,
making for a very
hard and wear resistant powder material.
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100051 It
is common to create the powder through atomization. In the case of tool
steels and other alloys containing high levels of chromium and/or vanadium
which are highly
prone to oxidation, gas atomization is often used, wherein a stream of the
molten alloy is
poured through a nozzle into a protective chamber and impacted by a flow of
high-pressure
inert gas such as nitrogen which disperses the molten metal stream into
droplets. The inert
gas protects the alloying elements from oxidizing during atomization and the
gas-atomized
powder has a characteristic smooth, rounded shape.
[0006]
Water atomization is also commonly used to produce powder metal. It is
similar to gas atomization, except that high-pressure water is used in place
of nitrogen gas as
the atomizing fluid. Water can be a more effective quenching medium, so that
the
solidification rates can be higher as compared to conventional gas
atomization. Water-
atomized particles typically have a more irregular shape which can be more
desirable during
subsequent compaction of the powder to achieve a greater green strength of
powder metal
compacts. However, in the case of tool steels and other steels containing high
levels of
chromium and/or vanadium, the use of water as the atomizing fluid would cause
the alloying
elements to oxidize during atomization and tie these alloying elements up
making them
unavailable for reaction with carbon to form carbides. Consequently, if water
atomization
were employed, it may need to be followed up by a separate oxide reduction
and/or annealing
cycle, where the powder is heated and held at an elevated temperature for a
lengthy period of
time (on the order of several hours or days) and in the presence of a reducing
agent such as
powdered graphite, or other source of carbon or other reducing agent or by
another reducing
process. The carbon of the graphite would combine with the oxygen to free up
the alloying
elements so that they would be available for carbide formation during the
subsequent
sintering and tempering stages following consolidation of the powder into
green compacts. It
will be appreciated that the requirement for the extra annealing/reducing step
and the addition
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of graphite powder adds cost and complexity to the formation of high alloy
powders via the
water atomization process.
SUMMARY OF THE INVENTION
[0007] One
aspect of the invention provides a method of producing a powder metal
composition, comprising the steps of: providing a melted iron based alloy
including 3.0 to 7.0
wt. % carbon, 10.0 to 25.0 wt. % chromium, 1.0 to 5.0 wt. % tungsten, 3.5 to
7.0 wt. %
vanadium, 1.0 to 5.0 wt. % molybdenum, not greater than 0.5 wt. % oxygen, and
at least 40.0
wt. % iron; and atomizing the melted iron based alloy to provide atomized
droplets of the
iron based alloy.
[0008]
Another aspect of the invention provides a method of producing a sintered
article, comprising the steps of: providing a melted iron based alloy
including 3.0 to 7.0 wt.
% carbon, 10.0 to 25.0 wt. % chromium, 1.0 to 5.0 wt. % tungsten, 3.5 to 7.0
wt. %
vanadium, 1.0 to 5.0 wt. % molybdenum, not greater than 0.5 wt. % oxygen, and
at least 40.0
wt. % iron; atomizing the melted iron based alloy to provide atomized droplets
of the iron
based alloy, referred to as a powder metal composition. The method next
includes mixing the
powder metal composition with another powder metal; compacting the mixture to
form a
preform; and sintering the preform.
[0009]
Another aspect of the invention provides a powder metal composition,
comprising: 3.0 to 7.0 wt. % carbon, 10.0 to 25.0 wt. % chromium, 1.0 to 5.0
wt. % tungsten,
3.5 to 7.0 wt. % vanadium, 1.0 to 5.0 wt. % molybdenum, not greater than 0.5
wt. % oxygen,
and at least 40.0 wt. % iron, based on the total weight of the powder metal
composition.
[00010]
Another aspect of the invention provides a sintered powder metal composition,
comprising: 3.0 to 7.0 wt. % carbon, 10.0 to 25.0 wt. % chromium, 1.0 to 5.0
wt. % tungsten,
3.5 to 7.0 wt. % vanadium, 1.0 to 5.0 wt. % molybdenum, not greater than 0.5
wt. A oxygen,
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and at least 40.0 wt. % iron, based on the total weight of the sintered powder
metal
composition.
BRIEF DESCRIPTION OF THE DRAWINGS
[00011] These and other features and advantages of the invention will
become more
apparent to those skilled in the art from the detailed description and
accompanying drawing
which schematically illustrates the process used to produce the powder.
1000121 Figure 1 is a schematic drawing of a process for producing a
powder metal
composition.
[00013] Figure 2 is a graph illustrating hardness v. carbon content.
DETAILED DESCRIPTION
1000141 A process for producing high carbon, iron based alloy powder, also
referred to
as a pre-sintered powder metal composition, is schematically illustrated in
Figure 1. The
powder metal composition is inexpensively produced and has an elevated
hardness that is
believed to be above that typically achieved by either gas or conventional
water atomization
processes with comparable alloy compositions having lower carbon levels.
[00015] The process first includes preparing a batch 10 of an iron based
alloy. The
iron based alloy is fully alloyed with carbide-forming elements, including
chromium (Cr),
molybdenum (Mo), tungsten (W), and vanadium (V). The iron based alloy is
melted and then
fed to an atomizer 12. In the embodiment of Figure 1, the atomizer is a water
atomizer 12,
but could alternatively be a gas atomizer. Some properties can be improved
using gas
atomization over water atomization, for example better flow, apparent density,
and lower
oxygen content. In addition, the gas atomization provides droplets having a
generally round
shape.
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1000161 In
water atomization step of Figure 1, a stream of the molten batch 10 is
impacted by a flow of high-pressure water which disperses and rapidly
solidifies the molten
stream into fully alloyed metal droplets or particles of irregular shape. The
outer surface of
the metal particles may become oxidized due to exposure to the water and
unprotected
atmosphere. The atomized particles are passed through a dryer 14 and then onto
a grinder 16
where the particles are mechanically ground or crushed. A ball mill or other
mechanical
reducing device may be employed. If an oxide skin is formed on the atomized
droplets, the
mechanical grinding of the particles fractures and separates the outer oxide
skin from the
particles, and the ground particles are then separated from the oxide to yield
an atomized
powder metal composition 18 and oxide particles 20. The power metal particles
and/or oxide
particles may also fracture and thus be reduced in size. The powder metal
composition 18
may be further sorted for size, shape and other characteristics normally
associated with
powder metal.
[00017] The
batch 10 of the iron based alloy provided for atomization has a high
carbon content. In one embodiment, the iron based alloy includes at least 3.0
wt.% carbon, or
3.0 to 7.0 wt. % carbon, or 3.5 to 4.0 wt. % carbon, and preferably about 3.8
wt. % carbon,
based on the total weight of the iron based alloy. The amount of carbon
present in the iron
based alloy depends on the amount and composition of the carbide-forming
elements.
However, the carbon is preferably present in an amount sufficient to form
metal carbides
during the atomization process in an amount greater than 15 vol. %, based on
the total
volume of the powder metal composition 18.
1000181
Another reason for adding the excess carbon to the iron based alloy is to
protect the iron based alloy from oxidizing during the melting and atomization
steps. The
increased amount of carbon decreases the solubility of oxygen in the melted
iron based alloy.
The amount of carbon also ensures that the matrix in which the carbides
precipitates reside is
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one of essentially austenite and/or martensite, particularly when the levels
of Cr and/or V are
high.
[00019] The
"low" oxygen content is an amount not greater than 0.5 wt. %, based on
the total weight of the iron based alloy. In one embodiment, the oxygen
content is not greater
than 0.3 wt. %, for example 0.2 wt. %. Depleting the oxygen level in the melt
has the benefit
of shielding the carbide-forming alloy elements, such as chromium (Cr),
molybdenum (Mo),
tungsten (W), and vanadium (V), from oxidizing during the melting or
atomization steps, and
thus being free to combine with the carbon to form carbides.
[00020] The
chromium (Cr), molybdenum (Mo), tungsten (W), and vanadium (V) of
the iron based alloy are also present in amounts sufficient to form the metal
carbides in an
amount of at least 15.0 vol. A), based on the total volume of the powder
metal composition
18. For cost reasons, there is also desire to increase the amount of some of
the carbide-
forming alloy elements over others. Thus, while Mo is an excellent choice for
forming very
hard carbides with a high carbide density, it is presently very costly as
compared, to say, Cr.
To develop a low cost tool grade quality of steel that is at least comparable
in performance to
a more costly and conventional M2 grade of tool steel, the iron based alloy
can include a
relatively high level of Cr, lower level of Mo, and increased amount of C. The
amount of W
and V can vary depending upon the desired amount of carbides to be formed.
Increasing the
amount of carbide forming alloying elements in the iron based alloy can also
increase the
amount of carbides formed in the matrix during the atomizing step. In
addition, the Cr, Mo,
W, and V are preferably present in amounts sufficient to provide exceptional
wear resistance
at a reduced cost, compared to other powder metal compositions.
[00021] In
one embodiment, the iron based alloy includes 10.0 to 25.0 wt. %
chromium, preferably 11.0 to 15.0 wt. % chromium, and most preferably 13.0 wt.
%
chromium; 1.0 to 5.0 wt. % tungsten, preferably 1.5 to 3.5 wt. % tungsten, and
most
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preferably 2.5 wt. % tungsten; 3.5 to 7.0 wt. % vanadium, preferably 4.0 to
6.5 wt. %
vanadium, and most preferably 6.0 wt. % vanadium; 1.0 to 5.0 wt. % molybdenum,
preferably 1.0 to 3.0 wt. % molybdenum, and most preferably 1.5 wt. %
molybdenum.
[00022] The
iron based alloy can optionally include other elements, which may
contribute to improved wear resistance or enhance another material
characteristic. For
example, the iron based alloy can include at least one of cobalt (Co), niobium
(Nb) titanium
(Ti), manganese (Mn), sulfur (S), silicon (Si), phosphorous (P), zirconium
(Zr), and tantalum
(Ta). In one embodiment, the iron based alloy includes at least one of the
following: 4.0 to
15.0 wt. % cobalt; up to T0. wt.% niobium; up to 7.0 wt. A) titanium; up to
2.0 wt. %
manganese; up to 1.15 wt. % sulfur; up to 2.0 wt. % silicon; up to 2.0 wt. %
phosphorous; up
to 2.0 wt. % zirconium; and up to 2.0 wt. % tantalum. In one embodiment, the
iron based
alloy contains pre alloyed sulfur to form sulfides or sulfur containing
compounds in the
powder. Sulfides, for example MnS and CrS, are known to improve machinability
and could
be beneficial to wear resistance.
[00023] The
remaining balance of the iron based alloy provided for atomization is iron.
In one embodiment, the iron based alloy includes at least 40.0 wt. % iron, or
50.0 to 81.5
wt% iron, and preferably 70.0 to 80.0 wt. % iron.
[00024] If
the atomization process is a water atomization process, a stream of the
melted iron based alloy is impacted by a flow of high-pressure water which
disperses and
rapidly solidifies the melted iron based alloy stream into fully alloyed metal
droplets of
irregular shape. Preferably, each atomized droplet possesses the full iron
based alloy
composition, including 3.0 to 7.0 wt. % carbon, 10.0 to 25.0 wt. % chromium,
1.0 to 5.0 wt.
% tungsten, 3.5 to 7.0 wt. % vanadium, 1.0 to 5.0 wt. % molybdenum, and at
least 40.0 wt. %
iron. The outside surface of the droplets may become oxidized due to exposure
to the water
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and unprotected atmosphere. However, the high carbon content and low oxygen
content
considerably limits the oxidization during the atomizing step.
[000251 In
the as-atomized state, the carbide-forming alloys may be present in a super
saturated state due to the rapid solidification that occurs during atomization
(ex. vanadium).
The unoxidized super saturated state of the alloying elements combined with
the high carbon
content allows carbides (ex. M8C7 V-rich carbides) to precipitate and fully
develop very
quickly (within minutes) during the subsequent sintering stage without the
need for an
extended prior annealing cycle (hours or days). The nanometric carbides
present in the as-
atomized powders grow to a micrometric size after sintering. However, the
powder metal
composition 18 can be annealed if desired, for example, from 1 to 48 hours at
temperatures of
about 900 ¨ 1100 C, or according to other annealing cycles if desired. The
annealing can be
carried out both before grinding and after grinding the powder metal
composition 18. It is
understood that annealing is not mandatory, but is optional.
100026] The
atomized droplets are then passed through a dryer and into a grinder
where they are mechanically ground or crushed to remove the oxide skin, and
then sieved.
Even if little or no oxide skin is present, the mechanical grinding step may
also be used to
fracture and reduce the size of the powder metal droplets. The hard and very
fine nano-
structure of the droplets improves the ease of grinding. A ball mill or other
mechanical size
reducing device may be employed. If an outer oxide skin is formed on the
atomized droplets
during the atomization step, which typically occurs during water atomization,
the mechanical
grinding fractures and separates the outer oxide skin from the bulk of the
droplets. The
ground droplets are separated from the oxide skin to yield the powder metal
composition 18
and oxide particles 20. However, the carbide-folining elements of the droplets
are protected
from oxidation by the high carbon content during the melting and atomizing
steps. The pre-
sintered powder metal composition 18 may be further sorted for size, shape and
other
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characteristics normally associated with powder metal. In certain cases, such
as when gas
atomization is used, the outer oxide skin is minimal and can be part of the
powder metal
composition and tolerated without removal, thus making grinding optional in
some cases for
at least the purpose of breaking the outer oxide layer. However, the grinding
can still be used
to reduce the size of the powder metal composition.
[00027] The
composition, in wt. %, of the pre-sintered powder metal composition 18 is
the same as the composition of the iron based alloy described above, prior to
melting and
atomization. The powder metal composition 18 typically includes 10.0 to 25.0
wt. %
chromium, preferably 11.0 to 15.0 wt. % chromium, and most preferably 13.0 wt.
%
chromium; 1.0 to 5.0 wt. % tungsten, preferably 1.5 to 3.5 wt. % tungsten, and
most
preferably 2.5 wt. % tungsten; 3.5 to 7.0 wt. % vanadium, preferably 4.0 to
6.5 wt. %
vanadium, and most preferably 6.0 wt. % vanadium; 1.0 to 5.0 wt. % molybdenum,
preferably 1.0 to 3.0 wt. % molybdenum, and most preferably 1.5 wt. %
molybdenum.
[00028] The
powder metal composition 18 also includes at least 3.0 wt. % carbon, or
3.0 to 7.0 wt. % carbon, or 3.5 to 4.0 wt. % carbon, and preferably about 3.8
wt. % carbon.
The carbon is present in an amount sufficient to provide metal carbides in an
amount of at
least 15 vol. %, based on the total volume of the powder metal composition 18.
[00029] As
the amount of carbon in the powder metal composition 18 increases so
does the hardness of the powder metal composition 18. This is because greater
amounts of
carbon form greater amounts of carbides during the atomization step, which
increases the
hardness. The amount of carbon in the powder metal composition 18 is referred
to as carbon
total (Cm).
[00030] The
powder metal composition 18 also includes a stoichiometric amount of
carbon (Csmch), which represents the total carbon content that is tied up in
the alloyed
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carbides at equilibrium. The type and composition of the carbides vary as a
function of the
carbon content and of the alloying elements content.
[00031] The
Cstoich necessary to form the desired amount of metal carbides during
atomization depends on the amount of carbide-forming elements present in the
powder metal
composition 18. The Csioich for a particular composition is obtained by
multiplying the
amount of each carbide-forming element by a multiplying factor specific to
each element. For
a particular carbide-forming element, the multiplying factor is equal to the
amount of carbon
required to precipitate 1 wt. % of that particular carbide-forming element.
The multiplying
factors vary based on the type of precipitates formed, the amount of carbon,
and the amount
of each of the alloying elements. The multiplying factor for a specific
carbide will also vary
with the amount of carbon and the amount of the alloying elements.
[00032] For
example, to form precipitates of (Cr23.5Fe7.3V63 iM03.2W29)8C7, also
referred to as M8C7, in the powder metal composition 18, the multiplying
factors of the
carbide-forming elements are calculated as follows. First, the atomic ratio of
the M8C7
carbide is determined: 1.88 atoms of Cr, 0.58 atoms of Fe, 5.05 atoms of V,
0.26 atoms of
Mo, 0.23 atoms of W, and 7 atoms of C. Next, the mass of each element per one
mole of the
M8C7 carbide is determined: V = 257.15 grams, Cr = 97.76 grams, Fe = 32.62
gams, Mo =
24.56 grams, W = 42.65 gams, and C = 84.07 grams. The weight ratio of each
carbide-
forming element is then determined: V = 47.73 wt. %, Cr = 18.14 wt. %, Fe =
6.05 wt. %,
Mo = 4.56 wt. %, W = 7.92 wt. %, and C = 15.60 wt. %. The weight ratio
indicates 47.73
grams of V will react with 15.60 grams of C, which means 1 gram of V will
react with 0.33
grams of C. To precipitate 1.0 wt. % V in the M8C7 carbide you need 0.33 wt. %
carbon, and
therefore the multiplying factor for V is 0.33. The same calculation is done
to detei wine the
multiplying factor for Cr = 0.29, Mo = 0.06, and W = 0.03.
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[00033] The Cstoich in the powder metal composition 18 is next determined
by
multiplying the amount of each carbide-forming element by the associated
multiplying factor,
and then adding each of those values together. For example, if the powder
metal composition
18 includes 4.0 wt. % V, 13.0 wt. % Cr, 1.5 wt. % Mo, and 2.5 wt. % W, then
Csto,ch =
(4.0*0.33) + (13.0*0.29) + (1.5*0.06) + (2.5*0.03) = 5.26 wt. %.
[00034] In addition, the powder metal composition 18 includes a Ctot /
Cstotch amount
less than 1.1. Therefore, when the powder metal composition 18 includes carbon
at the upper
limit of 7.0 wt. %, the Cstoich will be equal to or less than 6.36 wt. %
carbon. The Ctot / Cgoich
ratio will vary depending on the amount of alloying elements for a fixed
carbon content, but
the Ctot / Cstoich ratio will remain less than 1.1.
[00035] Table 1 below provides examples of other carbide types that can be
found in
the powder metal composition 18, and multiplying factors for Cr, V, Mo, and W
for generic
carbide stoichiometry. However, the metal atoms in each of the carbides listed
in the table
could be partly replaced by other atoms, which would affect the multiplying
factors.
[00036] Table 1
Example of Multiplying factor
Element Carbide type
stoichiometry fm (v0/0/w%)
Cr35Fe35C3 0.20
Cr M7C3 Cr4Fe3C3 0.17
(Cr34Fe66)7C3 0.29
V M8C7 (V63Fe37)8C7 0.33
Mo3Fe3C 0.04
Mo M6C
Mo2Fe4C 0.06
W3Fe3C 0.02
M6C
W2Fe4C 0.03
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1000371 The
metal carbides are formed during the atomization process and are present
in an amount of at least 15.0 vol. %, but preferably in an amount of 40.0 to
60.0 vol. %, or
47.0 to 52.0 vol. %, and typically about 50.0 vol. %. In one embodiment, the
powder metal
composition 18 includes chromium-rich carbides, molybdenum-rich carbides,
tungsten-rich
carbides and vanadium-rich carbides in a total amount of about 50.0 vol. %.
[00038] The
metal carbides have a nanoscale microstructure. In one embodiment, the
metal carbides have a diameter between 1 and 400 nanometers. As alluded to
above, the
carbides can be of various types, including M8C7, M7C3, MC, M6C, M23C6, and
M3C, wherein
M is at least one metal atom, such as Fe, Cr, V. Mo, and/or W, and C is
carbon. In one
embodiment, the metal carbides are selected from the group consisting of:
M8C7, M7C3, M6C;
wherein M8C7 is (V63Fe37)8C7; M7C3 is selected from the group consisting of:
(Cr34Fe66)7C3;
Cr3.5Fe35C3, and Cr4Fe3C3; and M6C is selected from the group consisting of:
Mo3Fe3C,
Mo2Fe4C, W3Fe3C, and W2Fe4C. The microstructure of the powder metal
composition 18
also includes nanoscale austenite, and may include nanoscale martensite, along
with the
nanoscale carbides.
[00039] In
one embodiment, the powder metal composition 18 consists essentially of
3.0 to 7.0 wt. % carbon; 10.0 to 25.0 wt. % chromium; 1.0 to 5.0 wt. %
tungsten; 3.5 to 7.0
wt. % vanadium; 1.0 to 5.0 wt. % molybdenum; not greater than 0.5 wt. %
oxygen; a balance
of iron, and incidental impurities in an amount not greater than 5.0 wt. %,
preferably not
greater than 2.0 wt. %. However, the powder metal composition 18 can
optionally include
other elements, which may enhance material characteristics. In one embodiment,
the powder
metal composition includes at least one of cobalt, niobium, titanium,
manganese, sulfur,
silicon, phosphorous, zirconium, and tantalum. For
example, the iron based alloy can
include at least one of 4.0 to 15.0 wt. ,4) cobalt; up to 7.0 wt.% niobium;
up to 7.0 wt. %
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titanium; up to 2.0 wt. % manganese; up to 1.15 wt. % sulfur; up to 2.0 wt. %
silicon; up to
2.0 wt. % phosphorous; up to 2.0 wt. % zirconium; and up to 2.0 wt. %
tantalum. In one
embodiment, the powder metal composition 18 contains pre alloyed sulfur to
form sulfides or
sulfur containing compounds in the powder. Sulfides, for example MnS and CrS,
are known
to improve machinability and could be beneficial to wear resistance.
[00040] The remaining balance of the powder metal composition 18 is iron.
In one
embodiment, the powder metal composition includes at least 40.0 wt. % iron, or
50.0 to 81.5
wt. % iron, and preferably 70.0 to 80.0 wt. % iron. The powder metal
composition has a
melting point of about 1,235 C (2,255 F). It will be completely melted at
about 1,235 C
(2,255 F), but may include a small fraction of a liquid phase at a
temperature as low as
1,1500 C. The melting point of the powder metal composition 18 will vary as a
function of
the carbon content and alloying element content.
[00041] The powder metal composition 18 typically has a microhardness of
800 to
1,500 Hy50. Figure 2 illustrates the hardness of the powder metal composition
without
annealing compared to the carbon content, and indicates the hardness increases
with
increasing amounts of carbon. Table 2 below also provides the hardness values
for varying
amounts of carbon, both before and after annealing, when the powder metal
composition
includes 13.0 wt. Ã1/0 chromium, 2.5 wt. % tungsten, 6.0 wt. % vanadium, 1.5
wt. %
molybdenum, 0.2 wt. A) oxygen, 70.0 to 80.0 wt. % iron, and impurities in an
amount not
greater than 2.0 wt. %. The data shows that the hardness of the powder metal
composition
increases with increasing amounts of carbon, both without annealing and after
annealing. It
should be noted that the amount of carbon content is the amount before
annealing. The
carbon content may decrease slightly during annealing, for example it may
decrease up to
0.15 wt. %. However, the hardness values still increase with increasing
amounts of carbon.
[00042] Table 2
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Carbon Content Hardness Before Annealing Hardness After Annealing
(wt %)
3.66% 975 HVO.025 450 HVO.025
3.03% 900 HVO.025 407 HVO.025
2.70% 810 HVO.025 382 HVO.025
1000431 The
hardness can be essentially maintained through sintering and tempering,
although some of the excess carbon contained in the powder metal composition
above that
needed to develop the carbides may diffuse out of the hard powder metal
composition if
admixed with another ferrous powder composition having a lower carbon content.
This
excess carbon diffusion has the added benefit of eliminating or at least
decreasing the need
for additions of carbon-rich powders (e.g., powdered graphite) that is
sometimes added
during compaction and sintering for control of microstructure and property
enhancement. In
addition, prealloyed carbon will reduce the tendency for graphite segregation
which can
occur with separate graphite additions.
1000441 The
powder metal composition 18 is typically compacted and sintered to form
an article that can be used in various applications, particularly automotive
components. Prior
to sintering, the powder metal composition 18 is preferably admixed with
another powder
metal or a mixture of other powder metals. The other powder metals can include
unalloyed,
low alloyed, or alloyed steel powder, as well as non-ferrous powder. In
addition, small
amounts of other metals or components could be present in the mixture.
1000451 In
one embodiment, the mixture includes 10.0 to 40.0 wt. % of the powder
metal composition 18, and preferably at least 20.0 wt. % of the powder metal
composition 18.
The mixture also includes 30.0 to 90.0 wt. %, of the other powder metal, but
typically
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includes about 60.0 to 80.0 wt. % of the other powder metal. Next, the mixture
is compacted
and then sintered.
[00046] The
high carbon powder metal composition can be annealed prior to sintering.
Annealing increases the compressibility of the powder metal composition 18
thereby
allowing more of the powder metal composition 18 to be used in the mixture, or
to press to
higher green density. The amount of powder metal composition 18 in the mixture
can be
increased to amounts greater than 40.0 wt. %, for example up to 60.0 wt. %,
when the powder
metal composition 18 is annealed. However, thermal processing, such as
extended annealing
or oxide reduction, of the powder metal material is not required prior to
sintering, as is
necessary with other powder metal compositions with low carbon levels to
reduce oxygen
and produce the appropriate microstructure.
[00047] The
sintered powder metal composition preferably includes the metal carbides
finely and uniformly distributed throughout the powder metal composition. If
100% of the
sintered composition is formed with the powder metal composition 18, then the
metal
carbides are present in the sintered powder metal composition in an amount of
at least 15 vol.
%, and preferably 40.0 to 60.0 vol. %, or 47.0 to 52.0 vol. %, and typically
about 50.0 vol. %.
In one embodiment, the sintered powder metal composition includes chromium-
rich carbides,
molybdenum-rich carbides, tungsten-rich carbides, and vanadium-rich carbides
in a total
amount of about 50.0 vol. %. In another embodiment, the sintered powder metal
composition
includes the vanadium-rich carbides in an amount of about 5.0 to 10.0 vol. %
and chromium-
rich carbides in an amount of about 40.0 to 45.0 vol. %, based on the total
volume of the
sintered powder metal composition.
[00048] The
metal carbides of the sintered powder metal composition have a
microscale microstructure. In one embodiment, the vanadium-rich MC carbides
have a
diameter of about lf.tm, and the chromium-rich M7C3 carbides have a diameter
of about 1 to 2
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lam. The fine carbide structure may also provide a more homogeneous
microstructure. The
carbides can be of various types, including M7C3, M8C7, MC, M4C3, M6C, M23C6,
M6C5, and
M3C, wherein M is a metal atom and C is carbon. For example, the carbides can
include V-
rich carbides, such as M8C7, M4C3, M6C5, Nb-rich carbides, such as MCx, where
x varies
from 0.75 to 0.97; or Ti and Ta-rich carbides, such as MC. The microstructure
of the sintered
powder metal composition also includes microscale austenite, and may include
microscale
martensite, along with the microscale carbides.
[00049] Table 3 includes an example of the powder metal composition
prepared
according to the method of the present invention, and a commercial grade of M2
tool steel for
comparison.
[00050] Table 3. Compositions (in wt. %)
Cr V Mo W C Fe
Inventive 13 6 1.5 2.5 3.8 bal.
example
M2 4 2 5 6 0.85 bal.
[00051] The powder metal composition 18 was admixed with another powder
metal
and sintered. The powder metal composition was present in an amount of 20.0
wt. % and the
other powder metal was present in an amount of 80.0 wt. %, based on the total
weight of the
admixture. The powder metal composition 18 of the sintered admixture included
chromium-
rich carbides in an amount of about 40-45 vol. %, and vanadium-rich carbides
in an amount
of about 7 vol. (Yo, based on the total volume of the powder metal composition
18. The
chromium-rich carbides had a size of about 1-2 Inn and the V-rich carbides had
a size of
about I JIM. The surrounding matrix of the particles in which the carbides
were precipitated
was essentially austenitic with some areas of martensite and ferrite.
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[00052] The
microhardness of the admixture after sintering was in the range of about
800 to 1,500 Hv50. The inventive powder metal composition was admixed at 15
and 30 vol.
% with a primary low carbon, low alloy powder composition. The hardness of the
high
carbon particles stayed above 1000 Hv50 after compacting, sintering and
tempering. Some of
the carbon from the inventive composition diffused into the neighboring lower
carbon content
primary powder matrix material of the admix.
1000531
Controlling the sintering and tempering cycles allows one to control the
properties of the primary matrix, including varying amounts of ferrite,
perlite, bainite and/or
martensite. Additions, such as MnS and/or other compounds may be added to the
admix to
alter the properties of the admix, for example to improve machinability. The
inventive
powder metal composition remained essentially stable and the properties
essentially
uninhibited by subsequent heat treatments employed to develop the properties
of the primary
matrix material.
1000541 The
invention has been described in connection with presently preferred
embodiments, and thus the description is exemplary rather than limiting in
nature. Variations
and modifications to the disclosed embodiment may become apparent to those
skilled in the
art and do come within the scope of the invention. Accordingly, the scope of
invention is not
to be limited to these specific embodiments.
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