U.S. patent application number 14/987121 was filed with the patent office on 2016-04-28 for iron-based powder and composition thereof.
This patent application is currently assigned to HOGANAS AB (PUBL). The applicant listed for this patent is HOGANAS AB (PUBL). Invention is credited to Sigurd BERG, Ulf ENGSTROM, Caroline LARSSON.
Application Number | 20160114392 14/987121 |
Document ID | / |
Family ID | 40129967 |
Filed Date | 2016-04-28 |
United States Patent
Application |
20160114392 |
Kind Code |
A1 |
BERG; Sigurd ; et
al. |
April 28, 2016 |
IRON-BASED POWDER AND COMPOSITION THEREOF
Abstract
A water-atomized iron-based powder is provided that is
pre-alloyed with 0.75-1.1% by weight of Ni, 0.75-1.1% by weight of
Mo and up to 0.45% by weight of Mn, and further including 0.5-3.0%,
preferably 0.5-2.5% and most preferably 0.5-2.0% by weight of Cu,
and inevitable impurities, the balance being Fe. An alloyed
iron-based powder composition including a water-atomized iron-based
powder
Inventors: |
BERG; Sigurd; (Hoganas,
SE) ; ENGSTROM; Ulf; (Hoganas, SE) ; LARSSON;
Caroline; (Nyhamnslage, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HOGANAS AB (PUBL) |
Hoganas |
|
SE |
|
|
Assignee: |
HOGANAS AB (PUBL)
Hoganas
SE
|
Family ID: |
40129967 |
Appl. No.: |
14/987121 |
Filed: |
January 4, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12664139 |
Feb 16, 2010 |
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PCT/SE2008/050709 |
Jun 12, 2008 |
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14987121 |
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60943889 |
Jun 14, 2007 |
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Current U.S.
Class: |
419/39 ; 420/92;
75/252 |
Current CPC
Class: |
B22F 3/12 20130101; B22F
2201/01 20130101; B22F 1/0003 20130101; B22F 1/0085 20130101; B22F
9/08 20130101; B22F 9/04 20130101; B22F 2201/20 20130101; B22F
2201/10 20130101; B22F 1/0088 20130101; B22F 2201/20 20130101; B22F
3/12 20130101; B22F 2998/10 20130101; B22F 2201/01 20130101; B22F
2998/00 20130101; B22F 2998/00 20130101; C22C 38/04 20130101; Y10T
428/12181 20150115; B22F 2998/10 20130101; B22F 5/00 20130101; C22C
38/12 20130101; B22F 2009/0828 20130101; C22C 33/0264 20130101;
C22C 38/16 20130101; B22F 1/0059 20130101; B22F 3/16 20130101; C22C
38/08 20130101; C22C 38/002 20130101; B22F 2301/35 20130101 |
International
Class: |
B22F 3/16 20060101
B22F003/16; B22F 5/00 20060101 B22F005/00; C22C 38/00 20060101
C22C038/00; C22C 38/12 20060101 C22C038/12; C22C 38/08 20060101
C22C038/08; C22C 38/04 20060101 C22C038/04; B22F 1/00 20060101
B22F001/00; C22C 38/16 20060101 C22C038/16 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 14, 2007 |
SE |
0701446-7 |
Claims
1. A water-atomized iron-based powder pre-alloyed with Ni and Mo at
contents by weight-%: 0.75-1.1 Ni, 0.75-1.1 Mo, and Mn<0.45, the
iron-based powder further including 0.5-3.0% by weight of Cu and
inevitable impurities, the balance being Fe.
2. A water-atomized iron-based powder according to claim 1, wherein
the content of Mo is more than 0.8 weight-%.
3. A water-atomized iron-based powder according to claim 1, wherein
the content of Mn is less than 0.35 weight-%.
4. A water-atomized iron-based powder according to claim 1, wherein
at least a portion or the total amount of Cu is diffusion bonded to
the surface of the Ni- and Mo-alloyed Fe-powder.
5. A water-atomized iron-based powder according to claim 4, wherein
all of the Cu is diffusion bonded to the surface of the Ni- and
Mo-alloyed Fe-powder.
6. A water-atomized iron-based powder according to claim 1, wherein
at least portion of the total amount of Cu is bonded to the surface
of the Ni- and Mo-alloyed Fe-powder by means of a binding
agent.
7. A water-atomized iron-based powder according to claim 6, wherein
all of the Cu is bonded to the surface of the Ni- and Mo-alloyed
Fe-powder by means of a binding agent.
8. A water-atomized iron-based powder according to claim 1, wherein
at least a portion or the total amount of Cu is admixed to the Ni-
and Mo-alloyed Fe-powder.
9. A water-atomized iron-based powder according to claim 8, wherein
all of the Cu is admixed to the Ni- and Mo-alloyed Fe-powder.
10. A water-atomized iron-based powder according to claim 1,
wherein the content of C in the Ni- and Mo-alloyed Fe-powder is at
most 0.02 weight-%.
11. A water-atomized iron-based powder according to claim 1,
wherein the content of 0 in the Ni- and Mo-alloyed Fe-powder is at
most 0.25 weight-%.
12. An alloyed iron-based powder composition comprising a
water-atomized iron-based powder according to claim 1, graphite in
an amount of 0.4-0.9 weight-%, lubricants and optionally other
additives.
13. An alloyed iron-based powder composition containing a
water-atomized iron-based powder according to claim 1, graphite in
an amount of 0.4-0.9 weight-%, lubricants and optionally other
additives wherein at least one of graphite, lubricants and
optionally other elements are bonded to the surface of Ni- and
Mo-alloyed Fe-powder.
14. Method for producing a component comprising: a. providing a
powder metallurgical composition according to claim 12, b.
compacting the powder metallurgical composition; and c. sintering
the compacted powder metallurgical composition in a reducing or
neutral atmosphere, at an atmospheric pressure or below, and at a
temperature above 1000.degree. C.
15. Method according to claim 14 wherein in b) the compaction
pressure is up to 2000 MPa.
16. Method according to claim 14 wherein in c) the sintering
temperature is performed at a temperature range of 1000.degree. C.
to 1400.degree. C.
17. A sintered component produced from the alloyed iron-based
powder composition according to claim 11.
18. A water-atomized iron-based powder according to claim 2,
wherein the content of Mn is less than 0.35 weight-%.
19. A water-atomized iron-based powder according to claim 2,
wherein at least a portion or the total amount of Cu is diffusion
bonded to the surface of the Ni- and Mo-alloyed Fe-powder.
20. A water-atomized iron-based powder according to claim 3,
wherein at least a portion or the total amount of Cu is diffusion
bonded to the surface of the Ni- and Mo-alloyed Fe-powder.
21. Method for producing a component comprising: a. providing a
powder metallurgical composition according to claim 13, b.
compacting the powder metallurgical composition; and c. sintering
the compacted powder metallurgical composition in a reducing or
neutral atmosphere, at an atmospheric pressure or below, and at a
temperature above 1000.degree. C.
22. Method according to claim 15 wherein in c) the sintering
temperature is performed at a temperature range of 1000.degree. C.
to 1400.degree. C.
23. A sintered component produced from the alloyed iron-based
powder composition according to claim 12.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S.
application Ser. No. 12/664,139, filed on Feb. 16, 2010, which is a
U.S. national stage application of International Application No.
PCT/SE2008/050709, filed on Jun. 12, 2008, which claims the benefit
of U.S. Provisional Application No. 60/943,889, filed on Jun. 14,
2007, and which claims the benefit of Swedish Application No.
0701446-7, filed on Jun. 14, 2007. The entire contents of each of
U.S. application Ser. No. 12/664,139, International Application No.
PCT/SE2008/050709, U.S. Provisional Application No. 60/943,889, and
Swedish Application No. 0701446-7 are hereby incorporated herein by
reference in their entirety.
FIELD OF THE INVENTION
[0002] The present invention concerns an alloyed iron-based powder
as well as an alloyed iron-based powder composition comprising the
alloyed iron-based powder, graphite, lubricants and eventually
other additives. The composition is designed for a cost effective
production of pressed and sintered components having good
mechanical properties.
BACKGROUND OF THE INVENTION
[0003] In industries the use of metal products manufactured by
compacting and sintering metal powder compositions is becoming
increasingly widespread. A number of different products of varying
shape and thickness are being produced and the quality requirements
are continuously raised at the same time as it is desired to reduce
the cost. This is particular true for P/M parts for the automotive
market, which is an important market for the P/M industry. In the
P/M industry alloying elements such as Mo, Ni and Cu have commonly
been used for improving the properties of pressed and sintered
components. However, these alloying elements are costly and it
would therefore be desirable if the contents of these alloying
elements could be kept as low as possible while maintaining
sufficient properties of the pressed and sintered component.
[0004] In order to achieve high strength of a pressed and sintered
component the hardenability of the material is essential. A cost
effective way of hardening a P/M component is the so called sinter
hardening method where the component is hardened directly after
sintering during the cooling step. By carefully choosing the
alloying elements, and content of the elements, sinter hardening
may be achieved at cooling rates normally applied in conventional
sintering furnaces.
[0005] Another factor of importance when producing pressed and
sintered components is the variation of dimensions between
different sintered parts which shall be as small as possible in
order to avoid costly machining after sintering. Furthermore, it is
desirable that the dimensional change, between the component in the
green stage, i.e. after pressing, and the component after it has
been sintered, is low and that the influence of variations in
carbon content of the dimensional change is a low as possible in
order to avoid introduction of stresses and possible distortion of
the components as this also will lead to costly machining. This is
of special importance for materials having high hardness and
strength as machining costs increases with increasing hardness and
strength.
[0006] Another important factor is the possibility of recycling
scrap from the automotive industry at preparation of the melt to be
atomized which has great environmental impact. In this respect the
possibility of accepting contents of up to 0.3% Mn in the alloyed
iron-based powder is critical as such levels of Mn is common in
recycled steel scrap.
[0007] Iron-based powders alloyed with Ni, Mo and Cu are widely
used as alloying elements and known from a variety of patent
applications. As an example, U.S. Pat. No. 6,068,813 to Semel,
reveals a powder composition comprising a pre-alloyed iron and
molybdenum powder having a content of 0.10-2.0 weight % of
molybdenum, admixed with a copper containing powder and a nickel
containing powder, whereby the copper containing powder and the
nickel containing powder are bonded to the iron-molybdenum powder
by means of a binding agent. The powder composition containing
0.5-4.0% by weight of copper and 0.5-8.0% by weight of nickel. The
iron-based powder used in the examples have a content of Mo of
0.56% by weight, a content of Ni of 1.75% or 4.00% by weight and a
Cu content of 1.5% by weight.
[0008] Another example in the patent literature concerning
pre-alloyed powders containing Ni, Mo and Mn, which may be mixed
with Cu-powder is U.S. Pat. No. 4,069,044 to Mocarski. This patent
discloses a method of making a powder, the powder being suitable
for producing powder-forged articles. Results from tests of forged
components according to a preferred composition containing
0.4-0.65% of Mo and Ni are reported. The patent also mention a
variation containing pre-alloyed iron-based powder with 0.2-1.0%
Ni, 0.2-0.8% Mo and 0.25-0.6% of Mn admixed with graphite and Cu--
or Cu containing powders giving a composition containing 0.2-2.1%
Cu to be compacted, suitable sintered at 2250-2350.degree. F., and
hot forged. However, no test results are shown for Ni contents
above 0.60 wt %, neither for Mo contents above 0.65 wt %.
[0009] For sinter hardening applications there exists a lot of
commercially available powders such as Ancorsteel 737 SH, available
from Hoeganaes Corp., NJ, US, and Atomet 4701, available from
Quebec Metal Powders, Canada. The mentioned iron-based powders are
alloyed with Mo, Ni and Mn and ATOMET 4701 is additionally alloyed
with Cr. Ancorsteel 737 SH is a pre-alloyed steel powder having a
chemical composition of 0.42% Mn, 1.25% Mo, 1.40% Ni. The chemical
composition of Atomet 4701 is 0.45% Mn, 1.00% Mo, 0.9% Ni and 0.45%
Cr.
OBJECTS OF THE INVENTION
[0010] It is an object of the invention to provide a new iron-based
powder and/or powder composition thereof, having low contents of
Mo, Ni and Cu.
[0011] Further objects of the invention are: [0012] provide a new
iron-based powder and/or powder composition thereof, suitable for
producing compacted and sinter hardened components. [0013] provide
a new iron-based powder and/or powder composition thereof, suitable
for producing sintered products having low dimensional change
between green stage and sintered stage. [0014] provide a new
iron-based powder and/or powder composition thereof, where the
influence from variations in carbon content on the dimensional
change is as low as possible. [0015] provide a new iron-based
powder and/or powder composition thereof, which iron-based alloyed
powder comprises Mn up to 0.45 weight-% allowing the iron-based
alloyed powder to be produced from cheap scrap.
SUMMARY
[0016] At least one of the above mentioned objects and/or problems
are met by providing an iron-based powder being pre-alloyed with
0.75-1.1 wt % (% by weight) Mo, preferably more than 0.8 wt % Mo,
0.75-1.1 wt % Ni, up to 0.45 wt % Mn and inevitable impurities.
[0017] The iron-based powder having at most 0.25 wt % of oxygen,
preferably at most 0.20 wt % 0 and most preferably at most 0.15 wt
% 0. The iron-based powder furthermore having 0.5-2.5 wt % Cu
present as: 1) diffusion bonded to the surface of the pre-alloyed
iron-based powder, and/or 2) bonded by means of a binding agent to
the surface of the pre-alloyed iron-based powder, and/or 3) admixed
with the iron-based powder. Further a powder composition thereof
containing the iron-based powder, graphite, lubricants and
optionally machinability enhancing agents
[0018] The content of graphite is preferably in the range of
0.4-0.9% by weight of the powder composition, more preferably in
the range of 0.5-0.9 wt % and the content of lubricant is
preferably in the range of 0.05-1.0% by weight of the powder
composition.
[0019] In the preferred embodiment Cu is diffusion bonded to the
surface of the pre-alloyed iron-based powder.
[0020] According to an embodiment of the invention at least one of
graphite, lubricants and machinability improving agents are bonded
to the surface of the pre-alloyed iron-based powder.
DETAILED DESCRIPTION OF THE INVENTION
Preparation of the Alloyed Iron-Based Powder
[0021] The alloyed iron-based powder of the invention can be
readily produced by subjecting a steel melt prepared to have the
above-defined composition of the alloying elements Ni, Mo and Mn to
any known water atomising method.
Amount of Mo
[0022] Mo serves to improve the strength of steel through
improvement of the hardenability and also through solution and
precipitation hardening. It has been found that to ensure that
enough amount of martensite is formed at normal cooling rates the
amount of Mo should be in the range of 0.75-1.1% by weight.
However, preferably the content of Mo is more than 0.8 wt %, more
preferably more than 0.85 wt % to ensure that enough amount of
martensite is formed at normal cooling rates.
Amount of Ni
[0023] Ni is added to P/M steel to increase strength and ductility.
Ni addition increases also the hardenability of the steel. Addition
of Ni less than 0.75 wt % will have an insufficient influence on
the mechanical properties whereas additions above 1.1 wt % will not
add any further improvements to the intended use of the steel.
Amount of Mn
[0024] Mn improves the strength of the steel by improving
hardenability and through solution hardening. However if the amount
of Mn becomes too high the ferrite hardness will increase through
solution hardening, leading to lower compressibility of the powder.
Amounts of Mn up to 0.45 wt % can be accepted as the decrease of
the compressibility will be almost negligible, preferably the
amount of Mn is lower than 0.35 wt %. If the amount of Mn is less
than 0.08% it is not possible to use cheap recycled material that
normally has a Mn content above 0.08%, unless a specific treatment
for the reduction of Mn during the course of the steel
manufacturing is carried out. Thus, the preferred amount of Mn
according to the present invention is 0.09-0.45%
C Amount
[0025] The reason why C in the alloyed iron-based powder is not
larger than 0.02 wt %, preferably not larger than 0.01 wt %, is
that C is an element, which serves to harden the ferrite matrix
through interstitial solid solution hardening. If the C content
exceeds 0.02% by weight, the powder is hardened considerably, which
results in a too poor compressibility.
O Amount
[0026] The amount of O should not exceed 0.25% by weight, O content
is preferably limited to 0.2% by weight and most preferably to
0.15% by weight.
Inevitable Impurities
[0027] The total amount of inevitable impurities in the alloyed
iron-based powder should not exceed totally 0.5% by weight.
Amount of Cu
[0028] Particulate Cu is often used in P/M industry as copper
particles melts before the sintering temperature is reached thus
increasing the diffusion rate and creating sintering necks by
wetting. Addition of Cu will also increase the strength of the
component. Preferably copper is bonded to the iron-based powder to
avoid segregation in the composition which may lead to uneven
distribution of copper and varying properties in component, but it
would also be possible admixing Cu with the iron-based powder. Any
known method of diffusion annealing Cu-particles or Cu-oxide
particles to the iron-based powder may be applied as well as
bonding Cu-particles to the iron-base powder by an organic binder.
The amount of Cu should be between 0.5-3.0% by weight, preferably
between 0.5-2.5% by weight, more preferably 0.5-2.0 wt %.
Graphite
[0029] Graphite is normally added to a P/M composition in order to
improve the mechanical properties. Graphite also acts a reducing
agent decreasing the amount of oxides in the sintered body further
increasing the mechanical properties. The amount of C in the
sintered product is determined by amount of graphite powder added
to the alloyed iron-based powder composition. In order to reach
sufficient properties of the sintered component the amount of
graphite should be 0.4-0.9% by weight of the composition,
preferably 0.5-0.9 wt %.
Lubricant
[0030] A lubricant may also be added to the alloyed iron-based
powder composition to be compacted. Representative examples of
lubricants used at ambient temperatures are Kenolube.RTM.,
ethylene-bis-stearamide (EBS), metal stearates such as Zn-stearate,
fatty acid derivates such as oleic amide, glyceryl stearate and
polethylene wax.
[0031] Representative examples of lubricants used at elevated
temperatures (high temperature lubricants) are polyamides, amide
oligomers, polyesters. The amount of lubricants added is normally
up to 1% by weight of the composition.
Other Additives
[0032] Other additives which optionally may be used according to
the invention include hard phase materials, machinability improving
agents and flow enhancing agents.
Compaction and Sintering
[0033] Compaction may be performed in an uniaxially pressing
operation at ambient or elevated temperature at pressures up to
2000 MPa although normally the pressure varies between 400 and 800
MPa.
[0034] After compaction, sintering of the obtained component is
performed at a temperature of about 1000.degree. C. to about
1400.degree. C. Sintering in the temperature range of 1050.degree.
C. to 1200.degree. C. leads to a cost effective manufacture of high
performance components.
[0035] The invention is further illustrated by the following
non-limiting examples.
Example
[0036] This example illustrates that high tensile strength, at the
same level as a material having higher content of the alloying
elements Cu, Ni and Mo can be obtained for components produced from
P/M compositions according to the invention.
[0037] An alloyed iron-based powder having a content of 0.9% by
weight of Mo, 0.9% by weight of Ni and 0.25% by weight of Mn was
produced by subjecting a steel melt to water atomization. Annealing
of the raw water atomized powder was conducted in a laboratory
furnace at a temperature of 960.degree. C. in an atmosphere of
moist hydrogen. Further, to the annealed powder were added
different amount of cuprous oxide, giving powders having contents
of 1%, 2% and 3% by weight of diffusion bonded copper respectively.
The diffusion bonding or annealing was carried out in a laboratory
furnace at 830.degree. C. in an atmosphere of dry hydrogen. The
annealed powders were crushed, milled and sieved and the resulting
powder having 95% of the particles less than about 180 p.m.
[0038] A first reference composition, composition nr 10, was based
on the iron-based powder Ancorsteel 737, available from Hoeganaes
Corp. NJ, US admixed with 2 wt % copper powder and 0.75%
graphite.
[0039] Three further reference compositions, compositions 11-13,
were based on a pre-alloyed powder iron-based powder having a
content of 0.6% Mo, 0.45% Ni, and 0.3% Mn admixed with 2% copper
powder and graphite of 0.65%, 0.75%, and 0.85% respectively.
[0040] Powder compositions according to the invention and reference
material were prepared by adding different amounts of graphite and
0.8% by weight of an EBS lubricant. Table 1 shows the different
compositions.
TABLE-US-00001 TABLE 1 Tested Compositions Mo-content, Ni-content
Mn-content, Cu-content Graphite. wt % of wt % of wt % of wt % of wt
% of Composition No powder powder powder powder composition 1 0.9
0.9 0.25 1 0.65 2 0.9 0.9 0.25 1 0.75 3 0.9 0.9 0.25 1 0.85 4 0.9
0.9 0.25 2 0.65 5 0.9 0.9 0.25 2 0.75 6 0.9 0.9 0.25 2 0.85 7 0.9
0.9 0.25 3 0.65 8 0.9 0.9 0.25 3 0.75 9 0.9 0.9 0.25 3 0.85 10
[ref] 1.25 1.40 0.42 2.1 (mixed) 0.75 Ancorsteel 737 11 [ref] 0.6
0.45 0.30 2 0.65 12 [ref] 0.6 0.45 0.30 2 0.75 13 [ref] 0.6 0.45
0.30 2 0.85
[0041] Tensile test bars according to SS-EN 10002-1 were produced
by compacting the compositions at a compaction pressure of 600 MPa.
The samples were sintered in a laboratory belt furnace at sintering
temperature of 1120.degree. C. for 30 minutes in an atmosphere of
90% N.sub.2/10% H.sub.2.
[0042] In order to study the influence of the cooling rate half of
the number of samples were subjected to forced cooling after
sintering at a cooling rate of 2.degree. C./second followed by
tempering at 200.degree. C. for 60 minutes, while the other half
was subjected to normal cooling rate at about 0.8.degree.
C./second. Table 2 shows the results corresponding to the normal
cooling rate and table 3 shows the results corresponding to the
forced cooling rate.
Results
[0043] The dimensional change between compacted and sintered
samples were measured as well as the tensile strength, according to
SS-EN 10002-1, and the micro Vickers hardness at a load of 10 grams
according to EN ISO6507-1 were measured.
TABLE-US-00002 TABLE 2 Results From Measurements of Dimensional
Change, Tensile Tests and Hardness Tests Samples Subjected to
Normal Cooling Rate C- O- Dimensional Tensile Hard- content content
change, strength, ness, Composition No (wt %) (wt %) (%) (MPa) HV10
1. (1 wt % Cu) 0.65 0.011 -0.18 661 196 2. (1 wt % Cu) 0.73 0.012
-0.17 655 199 3. (1 wt % Cu) 0.83 0.011 -0.16 694 227 4. (2 wt %
Cu) 0.59 0.009 0.01 836 264 5. (2 wt % Cu) 0.71 0.010 0.00 778 319
6. (2 wt % Cu) 0.78 0.011 -0.02 631 395 7. (3 wt % Cu) 0.65 0.012
0.27 860 351 8. (3 wt % Cu) 0.71 0.011 0.21 696 356 9. (3 wt % Cu)
0.83 0.012 0.11 625 367 10 [ref] 0.71 0.014 0.12 723 411 11 [ref]
0.64 0.009 0.31 732 291 12 [ref] 0.72 0.010 0.32 739 332 13 [ref]
0.80 0.011 0.32 711 339
TABLE-US-00003 TABLE 3 Results From Measurements of Dimensional
Change, Tensile Tests and Hardness Tests Samples Subjected to
Forced Cooling (Sinter Hardened) Rate C- O- Dimensional Tensile
Hard- content content change, strength, ness, Composition No (wt %)
(wt %) (%) (MPa) HV10 1. (1 wt % Cu) 0.64 0.031 -0.06 1061 389 2.
(1 wt % Cu) 0.75 0.034 -0.05 1040 406 3. (1 wt % Cu) 0.82 0.029
-0.08 998 400 4. (2 wt % Cu) 0.65 0.033 0.11 1109 372 5. (2 wt %
Cu) 0.76 0.034 0.07 1036 386 6. (2 wt % Cu) 0.83 0.029 0.03 953 388
7. (3 wt % Cu) 0.63 0.030 0.33 1019 355 8. (3 wt % Cu) 0.75 0.030
0.21 993 372 9. (3 wt % Cu) 0.83 0.029 0.08 954 375 10 [ref] 0.74
0.032 0.14 980 394 11 [ref] 0.64 0.025 0.32 789 329 12 [ref] 0.73
0.024 0.32 801 359 13 [ref] 0.82 0.027 0.33 794 370
[0044] Table 2 and 3 shows that tensile strength and hardness
values, both for sinter hardened samples and samples cooled at
normal cooling rates, for samples produced from the compositions
1-9 reach the same level as samples produced from reference
composition 10 having higher contents of costly alloying elements
such as Ni and Mo.
[0045] Regarding the Cu-content, which also is desired to be kept
as low as possible due to high copper prices; it can be seen that
the dimensional change both in amount and in variance due to
variations of the carbon content, are much higher for compositions
7-9 having a Cu-content of 3 wt %, than for compositions 1-3 having
a Cu-content of 1 wt % as well as compositions 4-6 having a
Cu-content of 2 wt %. Therefore according to the invention the
copper content should preferably be at most 3 wt %, more preferably
at most 2.5 wt %, more preferably at most 2.0 wt %.
[0046] Regarding compositions 1-3 the amount of the Dimensional
change during normal cooling rate are higher than the reference
composition 10, however the variance due to carbon content is very
low why these results are also comparably good. During forced
cooling rate, however, the amount of dimensional change is low as
well as its variance.
[0047] Regarding compositions 4-6 the amount of the Dimensional
change during normal cooling is almost zero and the variance due to
carbon content is also very low. During forced cooling rate, the
amount of dimensional change is somewhat higher, but still lower
than the reference composition 10. The variance is also somewhat
higher but since the amount is comparably low this is not an
important issue.
[0048] Regarding the reference compositions 11, 12 and 13 it can be
noticed that a lower tensile strength is obtained, especially for
the samples subjected to forced cooling. Further the dimensional
change is comparably high in relation to the compositions according
to the invention.
Dimensional Change
[0049] The dimensional change between compacted and sintered
samples should be less than +-0.35%, preferably less than +-0.3%,
more preferably less than 0.2%.
Tensile Strength
[0050] Preferably the tensile strength should be above 900 MPa,
more preferably above 920 MPa, when subjected to fast cooling and
tempering.
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