Note: Descriptions are shown in the official language in which they were submitted.
~ .
1~777~8
METHOD OF PRODUCING HIGH CARBON ~RD ALLOYS
The present invention relates to an improved method
for making high carbon hard alloys by the use of powder
metallurgy techniques and, in certain embodiments thereof
of forming a heat or quench hardenable steel. The pre-
sent invention also relates to an improved sintering
method for powder metallurgy techniq~les.
The ~rior art discloses the making of a heat harden-
able steel using powder techniques wherein an atomized
pre-alloyed powder is compressed and then sintered in
the presence of a carbonaceous reducing agent. The
sintered product is also mechanically worked so as to
effect a density substantially equivalent to the steel
in its cast and wrought state.
Alloys of the type to which the present invention
relates contain carbon ranging between about 0.6 to
5.0% by weight. In accordance with the teachi.ngs of the
prior art ? the metal powders employed are alloyed prior
to sintering. That is to say the base metal or metals
are melted with the alloying elements to form the
desired alloy and thereafter atomized.
The alloyed powders containing the requisite carbon
content to form the desired alloy are extremely hard
and abrasive. The prior art teaches that these hard
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powders are not easily compressed In fact, it is
believed that even after annealing, the powders retain
their abrasiveness and hardness and thereby limit cold
compactibility. It should be readily apparent that
this prior art method has the disadvantage of pro-
ducing an abrasive powder which required annealing
to render it more suitable for compaction.
By the present invention, it is proposed to provide
an improved method of forming a high carbon, hard alloy
using a powder metal technique which is simpler to per-
foL^m and which will yield uniform predictable results.
Another feature of the invention is directed to
a method of sinterin~ a Powder metal high carbon~ hard
alloy to substantially full density.
In one embodiment of the invention, the method is
directed to the formation of a high carbon, heat harden-
able steel.
This is accomplished by a method wherein a powdered
metal is formed by atomization of metal composition con-
taining the elements for forming the desired alloy, butlow in carbon content which may be maintained below 0.2%
by weight. The resulting powdered metal thus formed
with the carbon maintained at a minimum is readily com-
pactible without annealing. The carbon required to obtain
th~ dQsired analysis is provided by lampblack or graphite
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whlch is blended with the powder metal. The blended
powder metal and carbon ;s then compacted and sintered
so that the lampblack or graphite is diffused into
the metal powder. Additional carbon to that required
to achieve the desired alloy analysis may be provided
to compensate for oxygen reduction which occurs during
sintering as a result of the reaction of carbon with
oxide fo~ned during atomization of the powder metal.
The lampblack or graphite added may vary from about
0.6 to 5.0% by weight of the blend to be compacted:
of this amount of carbon from about 0.6 to 4.5% by
weight is added to achieve the desired product analysis
while 0% to about 0.5% carbon may be included to com-
pensate for the oxygen reduction~
The metal powders blended with the lampblack or
graphite form hard and abrasion resistant products,
and contain alloying quantities of one or more of ~he
elements chromium, vanadium, tungsten and molybdenum
so that a hard carbide is formed with such element or
elements. In preferred embodiments, the alloy contains
a quantity of iron so that complex carbides of iron
with one of the elements, chromium, vanadium, tungsten
or molybdenum may be formed.
In accordance with another aspect of the invention,
compact is sintered at a ~emperature just above the
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solidus temperature for the alloy. It has been found that
a "high density" alloy will result; that is, as herein used~
of such density that no further densification as by peening
or forging is required for use. Density in the range of
97V/o to 100% of theoretical density may be considered high
density. It has been found that at such sintering tempera-
ture, distor~io~ is minimal (i.e. the parts retain their ~;
shape) and dimensional shrinkage is predictable so ~hat
finished products can be produced and held within the
desired dimensional tolerances without further processing.
In particular, the present invention provides
the method of producing a high carbon, heat and abrasion
resistant alloy having a final composition including at
least 1% of at least one of the elements of the group con-
sisting of chromium~ vanadium, molybdenum and tungsten,
the elements of this group being characterized by a major
portion of the carbides thereof remaining undissolved at
elevated temperatures, said method comprising the steps of:
atomizing a melt having an initial composition
which includes at least 1% by weight of one of the elements
of the group consisting o chromium, vanadium, molybdenum
and tungsten and a carbon content of less than 0~2~/o by
weight thereby to limit the formation of the carbides of
said elements of said group to a level substantially below
that present in said final composition and thereby to form
a cold compactible powder,
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blending said cold compactible powder with carbon
particles of sufficient quantity to fo~n a blend which has
a final composition of at least about 0 6% by weight carbon,
without further treatment compressing said blend
into a green compact blank at a pressure in excess of 20 tsi;
and
heating said green compact blank at a temperature
and for a time sufficient to cause carbon diffusion and thus
to provide carbides of at least some of the elements of said
group of a quantity sufficient to impart hardness and abra-
sion resistance to said final composition.
Brié Description of_the Drawin~
The drawing is a phase diagram in relation to the
carbon content of a typical high carbon hard alloy, and
specifically for a M2 tool steel having by weight 6%
tungsten, 5% molybdenum, 2% vanadium and 4% chromium.
Description of Specific Embodiments
Tool Steels
One suitable alloy class ormed comprises heat
hardenable alloy tool steels wherein the powder metals to
be blended with the lampblack or graphite have the follow-
ing analysis:
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~777~L8
Carbon About 0.2%
Silicon About 1%
Manganese About .25%
Sulfur About O04%
(0,05 to 0.5% for free
machining gr~des)
Phosphorus About 0.04% maximum
Chromium About 2 to 9%
Vanadium About 0.5 to 7%
Cobal~ Optional up to about 15%
Tungsten Optional up to about 24%
Molybdenum Optional up to about 12%
Iron Balance
Atomization of the composition is carried out in
the well-known manner in which a molten stream of the
composition is poured through an area wherein it is im-
pinged by a fluid such as liquid, as for example water;
or gas, as for example steam; nitrogen; compressed air
and the like. The impingement serves to disperse the
falling molten metal into find particles which drop into
a liquid medium such as water wherein the particles are
quenched. The size and contour of the particles may be
controlled by conventional and well-known means. The
composition of the metal powder thus formed in accordance
with the present invention has less than about 0. 2~/o
carbon content. In the absence of a substantial quantity
of carbon in the particles, the formation of any signi-
ficant amount of hard carbides in a ferrite matrix does
,.
not occur in the prior art.
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The required carbon, in the form of lampblack or
graphite to achieve the desired tool stelel c~mposition
is then blended with the metal powder. This blend of
powder metal and carbon contains at least sufficient
carbon to produce a compacted product having the desired
tool steel analysis. To this end, at least about .6%
by weight of lampblack or graphite is blended with the
powder metal. Generally the amount of lampb]ack or
graphite added will be in excess of that required to
achieve the desired analysis in the final composition.
The excess carbon is used during sintering to reduce the
oxides formed on the particles during atomization.
The blend of the metal powder and carbon is cold
compacted at compacting pressures of about 2810 to 8425
kg per sq. cm. in a die having a suitable lubricant on
the die wall. As an alternative, the powder may be
mixed with a lubricant, for example 3/4% by weight
Acrawax "C" made by Glyco Chemical Co., and no die wall
lubrication is necessary. The shape of the article to
be formed from the powder metal blend determines the
particular method of compaction or die shape to be used.
Conventionally, the compacted ~end would initially
preferably be sintered in a hydrogen or non-oxidizing
atmosphere or in a vacuum at a tempera~ure ranging
bfftween about 1093C and 1205C for sintering to occur.
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In accordance with the present invention it has been ound
that the graphite will diffuse into the powder metal par-
ticles. The sintered compacts may thereafter be peened ~o
densify the surface and thereby to minimize oxidation which
occurs during the preheating for forging as more fully to
be explained hereinafter. The compacts which are intended
for use as a tool, for as example as a gear hob 3 tool bit
and the like are further compr~ssed into greater density
and shaped into the desired configuration by forging. The
compacts are preheated in suitable atmosphere for forging
at a temperature o~ between about 1093C to 1177C (2000F
to 2150F) and thereafter forged. After forging, the
articles are heat treated at temperatures and periods to
achieve a desired range of hardness. The final hardness
and mechanical characteristics are achieved by well-known
quench hardening and tempering procedures.
In accordance with another feature of the present
invention, sintering can be carried out just above the
solidus temperature where there is a sufficient amount of
liquid phase present so that a high density sintered com-
pact will result. Thus it has been found that test sinter-
ing various heats of M2 tool steel between the solidus and
liquidus temperature will result in a high density alloy
as follows:
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~7~7~
% Theoretical
Heat Chemical Analysis of M2 Heat _Density
No. C Mn Si Cr V W Mo 1227C 1238C
5 hrs. 5 hrs.
1.04 .071.0~ 4.0 ~.2 6.2 4.7 97
2 1.15 .06.76 3.8 2.2 6.5 4.8 97
3 1.16 .051.22 3.8 2.3 ~.8 4.9 98 97
4 1.18 .031005 3.9 2.3 6.5 5.0 98 97
The following are specific examples of the method
10 of the present invention applied to tool steels:
EXAMPLE N0. 1
1. A heat of steel corresponding to an AISI M2 high
speed steel composition except for carbon content was
water atomized and screened into a -100 mesh powdered
metal having the following analysis:
C -0.023% Mo-4.60%
Mn-0.24% V -1.87%
Si-0.68% W -6.48%
P -0.009% 02-0.15%
2. The powdered metal was blended with 1.00% by
20 weight natural graphite to achieve the necessary carbon
content to form the desired tool steel composition. The
powdered metal was cold compacted in a closed die at
8425 kg per sq. cm. using a molybdenum disul~ide grease
as a die wall lubricant. The powder metal was compacted
into blanks of 8.9 cm in diameter by 3.2 cm high. The
density of the blanks was about 6.5 gm/cc or 80% of the
theoretical density.
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3. The blanks were heated in a hydrogen àtmosphere
to 982C, held for one-hal hour to equalize temperature,
and sintered at 1149C for one hour at temperature.
4. The sintered blanks were shot peened for 10
minutes to densify the surface and minimize internal
oxidation during preheating of the blanks for forging.
S. The blanks were preheated in air for forging in
the temperature range of 1093C to 1149C.
6. They were forged on a high energy rate forging
press to a final density of 8.09 gm/cc (99.3% theoretical).
7. Thereafter the forged blanks were annealed at
871C - 900C for four (4) hours and allowed to slow
cool. Annealed hardnesses ranged from Rockwell C 15-25.
8. The blanks were preheated for hardening at
816C for 30 minutes, austenitized at 1232C for 4
minutes, and oil quenched.
9. The blanks were double tempered at 552C for
two ~2) hours.
Hardened properties included-
Hardness - Rc 65
Intercept grain size - 25
The resulting tool steel is capable of being used
as gear hobs, cutters, mills and the like.
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EXAMPLE N0. 2
1. A heat of steel corresponding to an AISI M2 high
speed (represenked by Heat No. 3 in the preceding table)
except for carbon content was water atomized and screened
into a -100 mesh powder having the following analysis:
C -0.03% Mo-4.~/O
Mn-0.07% V -2.2%
Si-1.04% W -6.2%
Cr-4.0% 02-.20%
2, The powder was blended with 1.15 percent by
weight natural graphite (1.0% to meet analysis specifi-
cation and .15% to compensate for oxygen reduction) O.l~h
molybdenum disulfide lubricant was also added to the
blend with 1% Acrawax C.
~ . The powder was cold compacted at /U2~ kg per
sqO cm. into blanks 2.54 cm in diameter by 2.54 cm thick.
The density of the blanks was about 6.3 gm/cc or 77% of
theoretical density.
4. The blanks were burned off at 482C for 60
minutes under an atmosphere of nitrogen ~U gr per sq. cm.
gage pressure).
5. The compact was then sintered at 1238C for 5
hours in a vacuum. The sintering temperature selected
was just above the solidus line into the liquid ~
austenite ~ carbide re~ion of the phase diagram. As
appears from the drawing, the solidus point for a
similar steel at the final carbon content of 1~04~/o is
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~L~777~8
approximately 1227C; the liquidus point is approxi-
mately 1427C.
6 Cooling from the sintering temperature was
carried on by g~s fan cooling with low dew point ni-
trogen.
A heat density tool steel product resulted with
minimum distortion of shape such that a fillished pro-
duct is produced within usable tolerances without
further processing of the sintered compact. Holding
at an intermediate temperature to equalize the tem-
perature throughout the load during sintering, as in
Example No. 1, did not appear necessary. The density
was 7.9 gm/cc or about 97% of theoretical.
EXAMPLE N0. 3
1. A heat of steel corresponding to an AISI M2
high speed tool steel composition exc~pt for carbon
content was water atomized and screened into a -40
mesh powder having the following analysis:
C -0.052% Mo-4. 91~/o
Mn-0.33% V -1.93%
Si-0.92% W -6.48%
Cr-4.46% 0~-0.20%
2 The -40 mesh powder was bIënded with 1.10 per-
cent, by weight of natural graphite, ~.95% to meet
analysis specifieation and .15% to compensate for oxygen
redu~ion). The powder blend was processed as ~ollow~:
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3. The powder was cold compacted at 7022 kg per
sq. cm. using a molybdenum sulfide grease as a die wall
lu~ricant into blanks of the following dimensions:
Dia Height Weight Density
7.62 cm 12.7 cm 3.62 kg 6.3 gm/cc
(7~/O of theoretical)
4. The blanks were sintered in vacuum as described
below:
a) 982C for one (1) hour to equalize the
ternperature throughout the load.
b) 1149C for one and one-half hour at tem-
perature.
c) Cooled to 871C and held for one hour.
d) Rapid cool to room temperature using ni-
trogen backfill system.
e) Vacuum level maintained at 100 microns or
less.
5. Blanks were shot peened for 15 minutes to densify
the surface.
6. Preheated for forging in air at 816C-871C
and then at 1093C-1149C for 10 minutes maximum time.
70 High energy rate forged to 8.05 gm/cc (99/O
theoretical~.
. .
EXAMPLE N0. 4
A lot of water atomized powder having the sarne
flnalysls as described in Example No. 3 was processed
as follows:
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~177'79~3
1. Blended with 1.10 pereent by weight natural
graphite (.95% to meet chemistry specifieation and .15%
to aeeount for oxygen reduetion), plus 1 percent by
weight "Aerawax C" as a lubrieant,
2. The powder blend was eold compacted at 7022
kg per sq. em. into a blank 7.62 cm. long by 1.27 em.
wlde by 2,29 em. thick bar weighing about 140 grams.
The green density was 6.5 gm/cc (80% theoretical density).
3. The blanks were burned off at 538C for one
(1) hour in nitrogen.
4. The blanks were sintered in vacuum as described:
a) Heat to 982C hold for thirty (30) minutes.
b) 1149C - sixty ~60) minutes
e) 760C - 2 hours
d) Baekfill with nitrogen
5. Shot peened for six ~) minutes.
6, Preheated in 1232C furnace for 5 minutes in
argon to heat blanks to 1149C - 1177C.
7. Forged in a closed die under 3 conditions, no
lateral flow, 10% lateral flow, and 20% lateral metal
flow. Final densities were:
. . .
Condition Density
No flow 7.96 gm/ee (97.3% theoretical)
lOV/o lateral flow 8.00 gm/cc (98.1% theoretical)
20% lateral flow 8.05 gm/cc (99% theoretical)
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1~777~13
8. Annealed at 900 C for four (4) hours to a
hardness of Rockwell C20.
9. Preheated for hardening at 816 C - 871 C for
thirty (30) minutes, then austenitized at 1218 C for
4 minutes and oil quenched.
10. Tempered 552 C for two hours.
11. Cooled in liquid~nitrogen for 30-60 minutes.
12. ~ouble tempered 552 C for two (2) hours.
EXAMPLE N0. 5
A heat of steel corresponding to an AISI T15 high
speed tool steel composition except for carbon content
was water at~mized and screened into a -100 mesh powder
having the following analysis:
C -0.042% V -4.25%
Mn-0.26% Co-4.71%
Si-Q.76% W -12.71%
Cr-4.05% 02-0.45%
The -100 mesh powder was blended with 1.90 percent
by weight natural graphite (1.5% to meet chemistry spec-
ification o~ tool steel desired and .33% to compensate
for oxygen reduction)~ The blended powder was processed
as follows:
1. The blend mixed with 1 percent by weight
"Acrawax C" to act as a lubricant.
2. The powder was cold compacted at 7022 kg per
~MI~:sa 3-4-7G
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~777~8
sq. cm. into 1.27 cm. x 7.62 cm. x 2.29 cm. bars weighing
140 grams. The blank density was 6.4 gm/cc (77% theo-
retical density).
3. The lubricant was burned off at 538C for one
~1) hour in nitrogen.
4, The blanks were sintered in vacuum maintained
at 100 microns or below as described:
a) Heated to 982C for thirty (30) minutes.
b) 1149C - sixty (60) minutes.
c) 760 C - ~wo (2) hours.
d) Backfill with nitrogen.
5. Shot peened for six (6) minutes to densify to
surface to minimize internal oxidation during preheating
for forging.
6. The shot peened b]ank was preheated between
about 1149C - 1177C in an inert atmosphere.
7, Forged
8. Annealed
9. Hardened and tempered
EXAMPLE N0. 6
A blend of powder having the chemical analysis as
Example No. 3 was processed as follows:
1. Blended with 1.3 percent Cby weight~ natural
graphite and 1 percent (by weight) "Acrawax C" for
lubrication.
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~17779L~
2. Cold compacted at 7022 kg per sq. cm. into
2.54 cm. diameter by 2.03 cm. thick slugs, weighing 45
grams to pro~ide a density of 6.3 gm/cc or about 77~/O
of theoretical density.
3. Burned off 538C for one hour in nitrogen.
4. Sintered as follows:
Slugs were inserted into a hydrogen at~
mosphere furnace at 1238C, held one hour,
and rapid cooled. The result was a high
density, finished product with usable ~o-
lerancesg having a density of 7.9 gm/cc or
about 97% of theoretical density.
High Carbon Stainless Steels
Another suitable steel alloy formed is a heat har-
denable, high carbon stainless steel such as that used
for cutlery corresponding to an AISI 440C steel. The
steel is characterized by a high carbon content in the
range o about .6 to 1.25% by weight and a high chromium
content in the range of about 12 to 27% by weight. The
steel is processed in similar manner as the tool steel
described above.
A heat hardenable high carbon stainless steel may
have the following composition:
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~ 7 7~ ~ ~
Carbon About 0.5 to 1.25%
Manganese About 1.0% maximum
Silicon Abou~ 1.0% maximum -
Chromium About 12% to 27%
Molybdenum About 0.75% maximum
Iron Essentially balance
The following is a specific example of the method
of the present invention for producing a heat treatable
stainless steel 440C steel:
EXAMPLE N0. 7
1. A heat of steel corresponding to an AISI 430
stainless powder was water atomized and screened into a
~100 mesh powder metal~ having a composition correspond-
ing to the desired 440C stainless steel except for car-
bon content. The powder metal had the following analysis:
Carbon - .02%
Manganese - .17%
- Silicon - .98%
Chromium - 16.2%
Iron - essentially balance
2. The powder metal is blended with 1.00% by weight
natural graphite to achieve carbon content to form the
desired heat treatable stainless steel composition. A
molybdenum disulfide grease is used as a die wall lubri-
cant. The powder metal is then cold ~ompacted in a
closed die at 7022 kg per sq. cm. into blanks 8.89 cm. high.
3, The lubrican~ was burned off between 427C to
649C for one hour in nitrogen.
4. The blanks were then sintered in a vacuum at
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982C for 10 minutes and then at 1205C for 60 minutes,
with a partial pressure of nitrogen of 500 microns The
blanks were then cooled in nitrogen atmosphere.
5. Thereafter the sintered blanks were hardened
by heating to 1010C, holding for 30 minu~es at tem-
perature, rapidly cooling to room temperature followed
by heating to a temperature between 150C and 205Cl
holding for 120 minutes and cooling to room temperature.
The properties of the sintered blanks included:
Density 6.1 gm/cc 79% of theoretical
density
Particle Hardness Rc 58
The heat hardenable, high carbon st~inless steel
according to the present invention is suitable for
cutlery and other purposes.
EXAMPLE N0. 8
Compacted blanks were prepared and the lubricant
burned off in like manner as set forth in Example No. 7
above.
The blanks were then vacuum s;ntered at 1310C,
20 jU5~ above the solidus ~emperature, with a partial
pressure of nitrogenO
High density products were produced with good
dimenslonal control.
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~77~48
Non-Ferrous AlloYs
The process according to the present invention may
be applied to high carbon non-ferrous base alloys which
have the necessary alloying components to form the hard
carbides of such elements as chromium, vanadium, tungsten
and molybdenum. Iron also will form hard carbides and
may be a desired alloying element of an essential.ly
non~ferrous base alloy.
One alloy which may be made by the present method
is a nickel-chromium alloy known commercially as Eatonite
10 and having a final composition as follows:
Carbon About 2 0 to 2.75%
Manga~ese About .025%
Silicon About 1.5% maximum
Chromium About 27 to 31%
Nickel About 37 to 41%
Iron About 7% maximum
Tungsten About 14 to 16%
Cobalt About 9 to 11%
EXAMPLE NO 9
1. A heat of alloy known as Eatonite but with
carbon omitted was water atomized and screened into a
-100 mesh powder metal having the following composition:
Carbon .02%
Manganese . 024%
Silicon 0.86%
Chromium . 29.8%
Nickel 39 3%
Iron 4.1%
Tungsten 15.1%
Cobalt 10.4%
rr~ P~
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2, The powder was blended with 2,5% by weight
natural graphite and 1% lubricant such as Acrawax "C".
The powder metal was cold compacted in a closed die at
8425 kg per sq. cm. The powder metal was compacted into
blanks of 2.54 cm. in diameter and 0.95 mm thick.
3. The lubricant was burned off at 538C for 60
minutes in nitrogen.
4, The blanks were sintered at 1238C for 120
minutes.
The properties of the blanks included:
Particle Hardness Rc 47 to 53
Density 7.22 gm/cc, 81%
theoretical density
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