U.S. patent application number 10/342617 was filed with the patent office on 2003-11-13 for ferrous articles sintered using a fluidized bed.
Invention is credited to Allard, Sylvain, Grau, Alphonso, Lavallee, Gregory.
Application Number | 20030211002 10/342617 |
Document ID | / |
Family ID | 23366798 |
Filed Date | 2003-11-13 |
United States Patent
Application |
20030211002 |
Kind Code |
A1 |
Grau, Alphonso ; et
al. |
November 13, 2003 |
Ferrous articles sintered using a fluidized bed
Abstract
A process for preparing a sintered article of a compacted
iron-based metallurgical powder. The green compact is sintered at a
closely held predetermined temperature in order to achieve desired
density and dimensional stability.
Inventors: |
Grau, Alphonso; (Montreal,
CA) ; Lavallee, Gregory; (Montreal, CA) ;
Allard, Sylvain; (Boucherville, CA) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
23366798 |
Appl. No.: |
10/342617 |
Filed: |
January 15, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60348140 |
Jan 15, 2002 |
|
|
|
Current U.S.
Class: |
419/58 |
Current CPC
Class: |
B22F 3/10 20130101; C22C
33/02 20130101; B22F 2998/10 20130101; B22F 2998/10 20130101; B22F
2999/00 20130101; B22F 3/1021 20130101; B22F 2202/15 20130101; B22F
2201/02 20130101; B22F 3/1007 20130101; B22F 3/10 20130101; B22F
3/10 20130101; B22F 2202/15 20130101; B22F 3/10 20130101; B22F
2201/013 20130101; B22F 3/02 20130101; B22F 2999/00 20130101; B22F
2999/00 20130101; B22F 2203/01 20130101; B22F 3/12 20130101; B22F
2999/00 20130101 |
Class at
Publication: |
419/58 |
International
Class: |
B22F 003/12 |
Claims
We claim:
1. A process for preparing a sintered article comprising the steps
of: selecting an iron-based P/M powder; pressing the P/M powder to
produce a green compact; and sintering the green compact at a
predetermined temperature of 1120-1250.degree. C. for up to 20
minutes, where the predetermined temperature is maintained
.+-.1.degree. C. for at least 5 minutes.
2. A process for preparing a sintered article comprising the steps
of: selecting an iron-based P/M powder; pressing the P/M powder to
produce a green compact; and sintering the green compact at a
predetermined temperature of 1120-1250.degree. C. for up to 20
minutes, in a fluidized bed, where the predetermined temperature is
maintained .+-.1.degree. C. for at least 5 minutes.
3. The process according to claim 1 or 2, wherein the predetermined
temperature is 1155.degree.-1165.degree. C.
4. The process according to claim 1 or 2, wherein the sintering
atmosphere consists of a mixture of nitrogen and hydrogen
5. The process according to claim 1 or 2, wherein the sintering
atmosphere consists of a mixture of nitrogen with from 5 to 25%
hydrogen.
6. The process according to claim 1 or 2, further comprising
initially heating the green compact to vaporize lubricant prior to
maintaining the predetermined temperature.
7. The process according to claim 1 or 2, further comprising
rapidly cooling the sintered article.
8. The process according to claim 1 or 2, wherein said P/M powder
comprises a ferrous powder.
9. The process according to claim 1 or 2, wherein said P/M powder
comprises an iron-graphite composite powder.
10. The process according to claim 1 or 2, wherein said P/M powder
comprises an iron-carbon-silicon alloy.
Description
[0001] This application is a continuation of prior provisional
application No. 60/348,140 filed Jan. 15, 2002.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to processes for making parts using
powder metallurgy manufacturing technology. In particular, this
invention relates to the sintering of structural parts having
tightly controlled mechanical and dimensional properties when using
a fluidized bed furnace.
[0004] 2. Related Background Art
[0005] The making and using of ferrous powders are well known, and
are described in considerable detail in Kirk-Othmer's Encyclopedia
of Chemical Technology, Third Edition, Volume 19, at pages 28-62.
Ferrous powders can be made by discharging molten iron metal from a
furnace into a tundish where, after passing through refractory
nozzles, the molten iron is subjected to granulation by horizontal
water jets. The granulated iron is then dried and reduced to a
powder, which is subsequently annealed to remove oxygen and carbon.
A pure iron cake is recovered and then crushed back to a
powder.
[0006] Ferrous powders have many applications, such as powder
metallurgy (P/M) part fabrication, welding electrode coatings,
flame cutting and scarfing. For P/M applications, the iron powder
is often blended with selected additives such as lubricants,
binders and alloying agents. A ferrous P/M part is formed by
injecting iron or steel powder into a die cavity shaped to some
specific configuration, applying pressure to form a compact,
sintering the compact, and then finishing the sintered compact to
the desired specifications.
[0007] Other ferrous powders which can desirably be utilized in
connection with this invention include the iron-graphite composite
powders described in allowed U.S. Ser. No. 09/609,115, filed Jun.
30, 2000 which produce a malleable iron compact. The P/M powders
and processes taught and claimed in the commonly-assigned U.S. Pat.
Nos. 4,927,461, 5,069,714 and 5,682,591, and allowed Ser. No.
09/609,115 are hereby incorporated by reference.
[0008] There are many instances of sintering non metallic materials
and metallic products including powders, green compacts or other
pressed iron powder structural parts. Commonly-assigned U.S. Pat.
No. 5,876,481 (the P/M powders and processes taught and claimed in
which are also hereby incorporated by reference) teaches
sinter-hardening process by carefully controlling the rate of
change of heating temperatures. Similarly, U.S. Pat. Nos.
3,249,662, and 5,796,018 refer to sintering in a fluid bed. The
'662 patent relates to ceramic articles and the '018 patent relates
to ferrous powder. Use of fluid beds is also mentioned in U.S. Pat.
Nos. 4,317,676, 4,410,373, 4,415,527, 4,693,682, 5,271,891,
5,584,910, 5,745,834, 5,620,751 and 6,030,434.
[0009] These methods, however, were never used to sinter ferrous
green P/M compacts. Therefore the efficiency of the fluidized bed
process for maintaining dimensional stability were not revealed in
the prior art.
SUMMARY OF THE INVENTION
[0010] It would be desirable to provide a P/M process that provides
the advantages of producing sintered articles with excellent
dimensional stability. Accordingly, the invention relates to a
general method to improve the dimensional control and consistency
of mechanical properties of parts produced from powder
metallurgy.
[0011] The present inventions have discovered that the sintering
step of P/M processing unexpectedly critical to achieving improved
dimensional tolerances. In particular, the present inventors have
discovered it is necessary to strictly maintain sintering
temperature during the sintering process to improve the dimensional
control and consistency of mechanical properties. Conventional
furnaces, due to their large temperature gradients across the width
of their belts, radiant heat shadowing effects, and the complex
relationship between belt speed, muffle temperature profile and
part temperature, do not have the capability to sinter parts with
today's necessary degree of dimensional control capability.
[0012] These objects and others are attained by sintering iron or
steel parts at a tightly controlled temperature for a predetermined
period of time. This produces parts with a minimum of variation one
to another in dimensional and mechanical performance
properties.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 illustrates varying transverse rupture mechanical
strengths obtained for a high carbon powder sintered at various
temperatures in accordance with the present invention.
[0014] FIG. 2 illustrates the temperature control maintained in a
fluidized bed furnace during sintering
DETAILED DESCRIPTION OF THE INVENTION
[0015] Currently, metal powders and more particularly iron and
steel powdered metal parts, are pressed to shape in a die at
relatively high compaction pressures to create a green part. P/M
parts are first formed by injecting the metal powder into a die
cavity shaped to some desired configuration, and applying pressure
to form a compact. These compacts are then sintered. Sintering at
temperatures typically in the range of 1100 to 1250.degree. C. for
a controlled period of time increases the strength of the bond
between particles. Where alloys such as graphite (0-0.8%) and
copper (0-2.5%) are present, this sintering results in the
diffusion of the alloys throughout the metal matrix. When higher
carbon levels are present, liquid phase sintering may occur in
where a liquid phase forms between particles. The end result of
this sintering process is an increase in mechanical properties of
the part, an increase in part density and a change in the dimension
of the parts themselves (a growth of +0.2% to a shrinkage in liquid
phase sintering of -4.0%). Because of the inherent problems of
controlling the temperature precisely in conventional radiant tube
muffle furnaces, significant variation in dimensional and physical
part properties occur.
[0016] Essentially any ferrous powder having a maximum particle
size less than about 300 microns can be used in the composition of
this invention. Typical iron powders are the Atomet.RTM. iron
powders manufactured by Quebec Metal Powders Limited of Tracy,
Quebec, Canada.
[0017] The iron powder of this invention may also be an
iron-carbon-silicon alloy comprising about 2% to about 4.5% by
weight carbon and about 0.05% to about 2.5% by weight silicon.
Preferably, the composite powder comprises about 3% to about 4% by
weight carbon and about 0.1% to about 2% by weight silicon. In one
preferred embodiment, the composite powder comprises about 3% to
about 4% by weight carbon and about 0.3% to about 2% by weight
silicon. Exemplary iron-graphite composite powders according to
this invention, having a microstructure comprised of carbon
clusters embedded in a ferrous matrix, comprise about 3.2% to about
3.7% by weight carbon and about 0.8% to about 1.3% by weight
silicon. The composite iron powder and/or resulting sintered
articles of this invention may also contain at least one other
alloying element conventionally used in the art.
[0018] In the present invention, compacted parts are introduced
into a batch or continuous fluid bed for a controlled period of
time at a particular temperature or range of temperatures. In its
preferred embodiment, this invention involves the placement of
parts in a multi layer part container where each part is held in
place on a ceramic or other type of high temperature resistant
fixture. The fixtures are then introduced into a furnace with a
tightly controlled temperature, preferably under a tightly
controlled atmosphere. Fluidized beds have traditionally been used
for batch heat treating of parts but not for the sintering of metal
powder parts. Fluidized beds are commercially available, e.g., from
Procedyne Corp., Newark, N.J.
[0019] Fluidized beds in their simple form consist of a retort
filled with an inert aggregate media through which heat is
introduced in some manner, preferably through the walls and by
preheating the fluidizing gas. The bed is fluidized through the
introduction of a controlled volume of gas through the bottom. The
continuous stirring and mixing that occurs as a result of the
fluidization results in a isotherm condition throughout the bed and
almost instantaneous heat transfer to any part introduced in the
bed.
[0020] The fluidized bed can operate in a continuous fashion in a
rectangular configuration, with parts introduced at one end
conveyed through the bed at a controlled rate of speed and then
removed at the other. Nitrogen is the prime fluidizing gas with
preferably at least 10% hydrogen added to achieve the best part
properties. Hydrogen at elevated levels can be used as
required.
[0021] A steady state temperature profile is created from one end
of the bed to the other and controlled by the rate and temperature
of fluidizing gas that is introduced along the length of the bed as
well as the amount of heat that is introduced to the bed through
the metal shell or retort. The bed itself is composed of aluminum
oxide ca.-80 mesh.
[0022] As the parts move through the bed they are initially rapidly
heated to approximately 600.degree. C. where they are held for the
vaporization and removal of lubricant (<1% by weight within the
green part). Delubing generally takes about 10 minutes. As the
parts continue through the bed they are then rapidly heated to the
final sintering temperature in the range of 1120 to 1160.degree. C.
and held at this constant temperature .+-.1.degree. C. for 5 to 15
minutes, as evidenced in FIG. 2. At the end of the cycle they are
rapidly cooled to 100.degree. C. at which time the fixtures exit
the bed and the parts are removed.
[0023] The sintered article thus formed may then be subjected to
post-sintering treatments, e.g., heat-treatment (such as quenching
and tempering, and the like), coining, forging and cutting or
machining, to produce a final article.
[0024] The Examples which follow are intended as an illustration of
certain preferred embodiments of the invention, and no limitation
of the invention is implied.
REFERENCE EXAMPLE 1
[0025] An iron powder was produced by water-atomization of a liquid
iron containing 0.94% silicon and 3.29% carbon. The water-atomized
iron powder was then thoroughly dried. Five samples of the powder
were consecutively heated in a Lindberg tubular furnace under a
vacuum atmosphere (less than approximately 30 mm Hg) at a
temperature of 1020.degree. C., maintained at that temperature for
three hours, then cooled in a stepwise process for approximately 4
hours. The samples were cooled from 1020.degree. C. to
approximately 760.degree. C. and were maintained at that
temperature for approximately 1.25 hours, cooled to approximately
730.degree. C. and maintained at that temperature for approximately
1.25 hours, then cooled to approximately 700.degree. C. and
maintained at that temperature for approximately 1.5 hours. The
samples were thereafter cooled to room temperature. The degree of
graphitization of the powder was determined by Computerized Image
Analysis using conventional procedures. The five iron-graphite
composite samples had an average graphite volume of approximately
10%.
EXAMPLE 1
[0026] A part was pressed conventionally using a sample of the
water-atomized iron powder described in Reference Example 1. The
green compact was sintered at 1150.degree. C. .+-.1.degree. C. for
5 to 15 minutes under a nitrogen atmosphere with 20% hydrogen.
[0027] Following sintering, the part was tested for transverse
rupture strength (TRS) and was determined to have a TRS of 115,000
pounds per square inch (PSI) or 115 KSI.
EXAMPLE 2
[0028] A part was pressed conventionally using a sample of the
water-atomized iron powder described in Reference Example 1. The
green compact was sintered at 1153.degree. C. .+-.1.degree. C. for
5 to 15 minutes under a nitrogen atmosphere with 20% hydrogen.
[0029] Following sintering, the part was tested for transverse
rupture strength and was determined to have a TRS of 134 KSI.
EXAMPLE 3
[0030] A part was pressed conventionally using a sample of the
water-atomized iron powder described in Reference Example 1. The
green compact was sintered at 1156.degree. C. .+-.1.degree. C. for
5 to 15 minutes under a nitrogen atmosphere with 20% hydrogen.
[0031] Following sintering, the part was tested for transverse
rupture strength and was determined to have a TRS of 180 KSI.
EXAMPLE 4
[0032] A part was pressed conventionally using a sample of the
water-atomized iron powder described in Reference Example 1. The
green compact was sintered at 1158.degree. C. .+-.1.degree. C. for
5 to 15 minutes under a nitrogen atmosphere with 20% hydrogen.
[0033] Following sintering, the part was tested for transverse
rupture strength and was determined to have a TRS of 170 KSI.
EXAMPLE 5
[0034] A part was pressed conventionally using a sample of the
water-atomized iron powder described in Reference Example 1. The
green compact was sintered at 1160.degree. C. .+-.1.degree. C. for
5 to 15 minutes under a nitrogen atmosphere with 20% hydrogen.
[0035] Following sintering, the part was tested for transverse
rupture strength and was determined to have a TRS of 168 KSI.
CONCLUSION
[0036] As a result of Examples 1-5, it is seen that a 6 degree
change in sintering temperature resulted in a 57% increase in TRS.
Moreover, it was determined that parts sintered at 1156.degree.
C..+-.1.degree. C. exhibited 3.5% shrinkage and were fully dense.
In contrast, parts sintered at 1130-1140.degree. C..+-.1.degree. C.
retained some porosity but exhibited only 1.5% shrinkage.
[0037] Other variations or modifications, which will be obvious to
those skilled in the art through routine experimentation, are
within the scope and teachings of this invention. This invention is
not to be limited except as set forth in the following claims.
* * * * *