U.S. patent application number 10/208819 was filed with the patent office on 2003-12-25 for pre-alloyed iron based powder.
Invention is credited to Berg, Sigurd, Engstrom, Ulf.
Application Number | 20030233911 10/208819 |
Document ID | / |
Family ID | 20288190 |
Filed Date | 2003-12-25 |
United States Patent
Application |
20030233911 |
Kind Code |
A1 |
Engstrom, Ulf ; et
al. |
December 25, 2003 |
Pre-alloyed iron based powder
Abstract
The invention concerns a new pre alloyed steel powder comprising
in addition to iron and inevitable impurities, by wt %, 1.3-1.7% by
weight of Cr, 0.15-0.3% by weight of Mo, 0.09-0.3% by weight of Mn,
not larger than 0.01 by weight of C, not larger than 0.25% of
O.
Inventors: |
Engstrom, Ulf; (Johnstown,
PA) ; Berg, Sigurd; (Hoganas, SE) |
Correspondence
Address: |
Benton S. Duffett, Jr.
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
20288190 |
Appl. No.: |
10/208819 |
Filed: |
August 1, 2002 |
Current U.S.
Class: |
75/252 |
Current CPC
Class: |
B22F 2998/10 20130101;
C22C 38/20 20130101; C22C 38/04 20130101; B22F 2998/10 20130101;
C22C 38/22 20130101; C22C 33/0264 20130101; B22F 2998/10 20130101;
C22C 38/004 20130101; B22F 2998/10 20130101; B22F 1/0003 20130101;
B22F 9/082 20130101; B22F 9/082 20130101; C22C 33/0207 20130101;
B22F 3/02 20130101; B22F 3/02 20130101; B22F 3/10 20130101 |
Class at
Publication: |
75/252 |
International
Class: |
C22C 001/05 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 14, 2002 |
SE |
0201824-0 |
Claims
1. A pre-alloyed steel powder comprising, in addition to iron and
inevitable impurities, by wt %, 1.3-1.7% by weight of Cr 0.15-0.3%
by weight of Mo 0.09-0.3% by weight of Mn not larger than 0.01 by
weight of C not larger than 0.25% of O.
2. A alloy steel powder according to claim 1 further comprising Ti,
B, V and Nb as pre-alloyed elements.
3. A alloy steel powder according to claim 2 including, in % by
weight, 0.01-0.04 of Ti, 0.01-0.04 of B, 0.05-0.3 of V and not more
than 0.1 of Nb.
4. A alloy steel powder according to claim 1, 2 or 3 admixed with
at least one element selected from the group consisting of Cu and
Ni.
5. A alloy steel powder according to claim 1, 2 or 3 wherein
particles of at least one element selected from the group
consisting of Cu and Ni are bonded to the alloy steel powder.
6. A alloy steel powder according to claim 4 or 5, wherein the
amount of Cu is 0.5-3.0% by weight.
7. A alloy steel powder according to claim 4 or 5, wherein the
amount of Ni is 0.5-3.0%by weight.
8. Alloy steel powder according to any one of the preceding claims
admixed with graphite.
9. A alloy steel powder according to any one of the preceding
claims admixed with a lubricant.
10. A alloy steel powder according to claim 9 characterized in that
the lubricant is a cold compaction lubricant selected from the
group consisting of metal soaps and waxes.
11. A alloy steel powder according to any one of the preceding
claims comprising 1.35-1.65% by weight of Cr 0.17-0.27% by weight
of Mo 0.09-0.2% by weight of Mn not larger than 0.006 by weight of
c
Description
FIELD OF INVENTION
[0001] The present invention concerns a pre-alloyed iron based
powder. Particularly the invention concerns a pre-alloyed iron
based powder including small amounts of alloying elements which
permits a cost effectively manufacture of sintered parts for an
increasing P/M market.
BACKGROUND OF THE INVENTION
[0002] In industry the use of metal products manufactured by
compacting and sintering metal-powder compositions is becoming
increasingly widespread. A number of different products of varying
shapes and thickness are being produced and the quality
requirements are continuously raised at the same time as it is
desired to reduce the costs. This is particularly true for P/M
parts for the automotive market, which is an important market for
the P/M industry and for which cost is a major driving force.
Another factor of importance is the possibility of recycling scrap
from the automotive industry and to consider the effect on the
environment. Known alloying systems which have gained wide
acceptance within this field have frequently included alloying
elements such as Ni and Cu. However, nickel is a strong allergen
and is also considered to have other detrimental medical effects. A
problem with copper is that, during recycling of scrap used for
steel manufacture, copper is accumulated. In many steel qualities
copper is however not suitable and scrap without copper or with a
minimum of copper would be required. Iron-based powders having low
amounts of alloying elements without nickel and copper are
previously known from e.g. the U.S. Pat. Nos. 4,266,974, 5,605,559,
5,666,634 and 6,348,080 (Arvidsson)
[0003] The purpose of the invention according to the U.S. Pat. No.
4,266,974 is to provide a powder satisfying the demands of high
compressibility and moldability of the powder and good
heat-treatment properties, such as carburising, hardenability, in
the sintered body. The most important step in the production of the
steel alloy powder produced according to this patent is the
reduction annealing step (col. 5 line 15).
[0004] The U.S. Pat. Nos. 5,605,559 and 5,666,634 both concern
steel powders including Cr, Mo and Mn. The alloy steel powder
according to the U.S. Pat. No. 5,605,559 comprises, by wt %, about
0.5-2% of Cr, not greater than about 0.08% of Mn, about 0.1-0.6% of
Mo, about 0.05-0.5% of V, not greater than about 0.015 of S, not
greater than about 0.2% of O, and the balance being Fe and
incidental impurities. The U.S. Pat. No. 5,666,634 discloses that
the effective amounts should be between 0.5 and 3% by weight of
chromium, 0.1 and 2% by weight of molybdenum and at most 0.08% by
weight of manganese.
[0005] A serious drawback when using the inventions disclosed in
these U.S. Pat. Nos. 5,605,559 and 5,666,634 is that cheap scrap
cannot be used as this scrap normally includes more than 0.08% by
weight of manganese. In this context the U.S. Pat. No. 5,605,559
teaches that "when Mn content exceeds about 0.08 wt % oxide is
produced on the surface of alloy steel powders such that
compressibility is lowered and hardenability increased beyond the
required level. . . . . Mn content is preferably not greater than
about 0.06%wt." (col 3 47-53). This teaching is repeated in the
U.S. Pat. No. 5,666,634 disclosing that "a specific treatment is
used in order to reduce the Mn content to a level not larger than
0.08% by weight during the course of the steel making" (col. 3 line
40-44). Another problem is that nothing is taught about the
reduction annealing and the possibility to obtain the low oxygen
and carbon content in water-atomised iron powders including
elements sensitive to oxidation, such as chromium, manganese. The
only information given in this respect seems to be in example 1,
which discloses that a final reduction has to be performed.
Furthermore the U.S. Pat. No. 5,666,634 refers to a Japanese Patent
Laid-open No. 4-165002 which concerns an alloy steel powder
including in addition to Cr also Mn, Nb and V. This alloy powder
may also include Mo in amounts above 0.5% by weight. According to
the investigations referred to in the U.S. Pat. No. 5,666,634, it
was found that this Cr-based alloy steel powder is disadvantageous
due to the existence of the carbides and nitrides which act as
sites of fracture in the sintered body.
[0006] The possibility of using powders from scrap is disclosed in
the U.S. Pat. No. 6,348,080 which discloses a water-atomised,
annealed iron-based powder comprising, by weight %, Cr 2.5-3.5, Mo
0.3-0.7, Mn 0.09-0.3, O<0.2, C<0.01 the balance being iron
and, an amount of not more than 1%, inevitable impurities. This
patent also discloses a method of preparing such a powder.
Additionally the U.S. Pat. No. 6,261,514 discloses the possibility
of obtaining sintered products having high tensile strength and
high impact strength if powders having this composition is warm
compacted and sintered at a temperature >1220.degree. C.
[0007] The present inventors have now unexpectedly found that more
narrow ranges of the alloying elements, especially chromium, will
give unexpected improvements as regards the possibilities of
annealing and sintering the powders in comparison with the powders
disclosed in the U.S. Pat. No. 6,348,080.
[0008] Additionally, when comparing green bodies prepared from
these known powders with green bodies prepared from the new powders
according to the present invention it was found that the green
strength of compacted bodies prepared from the new powders are
distinguished by an unexpectedly high green strength. This is
particularly true when die wall lubrication is used. Green strength
is one of the most important physical properties of green parts.
The importance of this property increases as P/M parts increase in
size and geometry becomes more complex. Green strength increases
with increasing compact density and is influenced by type and
amount of lubricant admixed to the powder. The green strength is
also influenced by the type of powder used. A high green strength
is required in order to prevent compacts from cracking during the
ejection from the compacting tool and prevent them from getting
damaged during the handling and the transport between the press and
the sintering furnace. Presently used compacts having a relatively
high green strength are advantageously prepared from sponge iron
powders whereas difficulties have been met as regards the
preparation of compacts of atomised powders in spite of the fact
that an atomised powder is more compressible and hence gives a
higher green density.
OBJECTS OF THE INVENTION
[0009] An object is to provide a new pre-alloyed powder including
low amounts of alloying elements.
[0010] A second object is to provide a pre-alloyed powder which is
essentially free from nickel and copper.
[0011] A third object is to provide a pre-alloyed powder which can
be compacted at both ambient temperature and at elevated
temperature to high green strength at moderate compaction
pressures.
[0012] A further object is to provide a pre-alloyed powder which
can be produced from cheap scrap.
[0013] Still another object is to provide a new pre-alloyed powder
which can be cost effectively compacted and sintered in industrial
scale.
SUMMARY OF THE INVENTION
[0014] According to the present invention these objects are
achieved by a pre alloyed steel powder comprising,
[0015] 1.3-1.7% by weight of Cr
[0016] 0.15-0.3%by weight of Mo
[0017] 0.09-0.3% by weight of Mn
[0018] not larger than 0.01 by weight of C
[0019] not larger than 0.25% of O
[0020] and the balance being inevitable impurities and Fe.
[0021] According to a more preferred embodiment of the invention
the powder has the composition 1.35-1.65% by weight of Cr
0.17-0.27% by weight of Mo, 0.09-0.2% by weight of Mn not larger
than 0.006 by weight of C.
DETAILED DESCRIPTION OF THE INVENTION
[0022] Amount of Cr
[0023] The component Cr is a suitable alloying element in steel
powders, since it provides sintered products having an improved
hardenability but not significantly increased ferrite hardness. To
obtain a sufficient strength after sintering and still maintain a
good compressibility a Cr range of 1.3 to 1.7 is suitable. A higher
chromium content decreases the compressibility and also increases
the risk of forming unwanted carbides. A lower content decreases
the hardenability.
[0024] Amount of Mn
[0025] The component Mn improves the strength of steel by improving
hardenability and through solution hardening. However, if the
amount of Mn exceeds 0.3%, the ferrite hardness will increase
through solid solution hardening. If the amount of Mn is less than
0.08 it is not possible to use cheap scrap that normally has an Mn
content above 0.08%, unless a specific treatment for the reduction
of Mn during the course of the steel manufacturing is carried out
(see above). Thus, the preferred amount of Mn according to the
present invention is 0.09-0.3%. In combination with C contents
below 0.01% this Mn interval gives the most interesting
results.
[0026] Amount of Mo
[0027] The component Mo serves to improve the strength of steel
through the improvement of hardenability and also through solution
and precipitation hardening. To the given chemical composition the
Mo addition in the range of 0.15 to 0.3 is sufficient to move the
perlite noose in the CCT-diagram below to the right making it
possible to form a bainitic structure at normally used cooling
rates.
[0028] C Amount
[0029] The reason why C in the alloy steel powder is not larger
than 0.01% is that C is an element which serves to harden the
ferrite matrix through formation of a solid solution as penetrated
in the steel. If the C content exceeds 0.01% by weight, the powder
is hardened considerably, which results in a too poor
compressibility for a powder intended for commercial use.
[0030] The amount of C in the sintered product is determined by the
amount of graphite powder mixed with the alloy steel powder of the
invention. Typically the amount of graphite added to the powders is
between 0.15 and 0.9% by weight.
[0031] O Amount
[0032] The amount of O should not exceed 0.25% by weight. When O
content exceeds about 0.25 wt %, oxides are formed with Cr and V
which reduce strength and compressibility. O content is preferably
limited to not greater than about 0.2 wt % and more preferably to
not greater than about 0.15 wt %.
[0033] Other Elements
[0034] Other elements which may be included in the pre-alloyed
powder are Ti, B, V and Nb. Ti, V and Nb can form carbides which
will give precipitation hardening effects. B has the same effect as
carbon, a solution hardening effect, and can form borides with Ti,
Nb and V giving a precipitation hardening effect. The amounts of
these elements are preferably, in % by weight, 0.01-0.04 of Ti,
0.01-0.04 of B, 0.05-0.3 of V and not more than 0.1 of Nb.
[0035] Ni and/or Cu may be admixed with the new powder.
Alternatively particles of Cu and/or Ni may be adhered to the
particles of the new powder by using a bonding agent. The addition
of Ni and/or Cu permits sinter hardening in conventional sintering
furnaces. Additive amounts of these alloys are limited to about
0.5-3 wt % or Ni and about 0.5-3 wt % of Cu.
[0036] Lubricant
[0037] A lubricant, is also normally mixed with the powder
composition. The presently most interesting application of the new
powder seems to be for the production of sintered parts compacted
at ambient temperature (=cold compaction). The new powder is then
mixed with cold compaction lubricants such as waxes e.g. ethylene
bisstearamide, metal soaps etc. such as zinc stearate in amounts up
to 1% by weight of the iron-based powder.
[0038] Compaction and Sintering
[0039] After compaction at ambient or elevated temperature at
pressures normally between 400 and 800 MPa sintering may be
performed as high temperature sintering e.g. sintering at
temperatures above 1200.degree. C. as high temperature sintering is
beneficial for chromium containing materials. However also low
temperature, i.e. temperatures below about 1220.degree. C.,
preferably below 1200.degree. C. or even below about 1150.degree.
C. sintering is sometimes preferable. The sintering times may be
comparatively short, i.e. below 1 h, such as 45 minutes. Usually
the sintering time is about 30 minutes.
[0040] Depending on i.a. the composition of the iron powder and the
amount of graphite added, cooling rates typical for belt furnaces,
i.e. 0.5-2.degree. C./s lead to fully bainitic structures as
disclosed in FIG. 1. Such a bainitic structure is desirable for a
good combination of strength and toughness. FIG. 2 shows the
tensile strength as a function of carbon content at a cooling rate
of 0.8.degree. C./s.
[0041] A cooling rate below 0.5.degree. C./s results in the
formation of perlite and cooling rates exceeding 3.degree. C./s
result in martensite formation.
[0042] Sinterhardening
[0043] Sinter hardening is process which might be a powerful tool
in reducing the costs. New types of sintering furnaces allow low
alloy steel parts to be sintered with neutral carbon potential
(without decarburization or carburization) and then to be hardened
in a rapid cooling zone. The heat treatment is achieved by high
speed circulation of a water cooled protective gas in the rapid
cooling zone of the furnace with cooling rates of up to 7.degree.
C./sec achievable between 900.degree. C. and 400.degree. C. This
results in a homogeneous martensitic structure in the PM steels. In
order to take advantage of the advantages of the sinter hardening
the selection of alloying system is of outmost importance.
[0044] It has now been found that at a cooling rate above 7.degree.
C./s the new powders--if including about 0.6% by weight of
carbon--have a transition to martensite. This indicates the
possibility of using the new material for sinterhardening
applications. For sinter hardening of the new powder in
conventional mesh belt sintering furnaces equipped with a rapid
cooling unit giving a cooling rate of 1-5.degree. C./s addition of
Cu and/or Ni has to be used. As indicated above suitable amounts of
copper and nickel which may be used in combination with the new
powder are 0.5-3%.
[0045] FIGS. 3 and 4 disclose that a martensitic structure is
obtained when the inventive powder in combination with 2% of Cu and
0.5% graphite is sinter hardenened with cooling rates of 4-5 and
higher.
[0046] Preparation of the New Powder.
[0047] The alloy steel powder of the invention can be readily
produced by subjecting ingot steel prepared to have the
above-defined composition of alloying elements to any known
water-atomising method. It is preferred that the water-atomised
powder is pre-pared in such a way that, before annealing, the
water-atomised powder has a weight ratio O:C between 1 and 4,
preferably between 1.5 and 3.5 and most, preferably between 2 and
3, and a carbon content between 0.1 and 0.9% by weight. For the
further processing according to the present invention this
water-atomised powder could be annealed according to methods
described in PCT/SE97/01292 (which is hereby incorporated by
reference)
[0048] Another process which can be used for the preparation of low
oxygen, low carbon iron-based powders including low amounts of
easily oxidised alloying elements is disclosed in the co-pending
Swedish application 9800153-0.
[0049] A distinguishing feature which has been observed concerning
the appearance of the annealed powder particles is that the
particle shape is slightly more irregular compared with the
particle shape of water atomised plain iron powder.
[0050] The invention is further illustrated by the following
non-limiting examples.
[0051] The green densities given in table 1 were obtained with a
powder known from the U.S. Pat. No. 6,348,080 including 3% by
weight of Cr, 0.5% by weight of Mo and 0.11% by weight of Mn.
1TABLE 1 Die wall Internal lubrication Compaction Lubrication
(g/cm.sup.3) Pressure Green Density Green Density (MPa)
(g/cm.sup.3) 0.8% ZnStearate 0.6 Advawax .TM. 400 6.43 6.52 6.65
600 6.93 6.96 7.07 800 7.25 7.17 7.24
[0052] From the following table 2 the corresponding results
obtained with a powder according to the present invention. The
powder consisted of 1.5% by weight of Cr, 0.2% by weight of Mo and
0.11% by weight of Mn.
2TABLE 2 Die wall Internal lubrication Compaction Lubrication
(g/cm.sup.3) Pressure Green Density Green Density (MPa)
(g/cm.sup.3) 0.8% ZnStearate 0.6% Advawax .TM. 400 6.55 6.61 6.75
600 7.04 7.02 7.17 800 7.32 7.21 7.38
[0053] From a comparison of the results listed in the tables 1 and
2 it can be seen that higher green densities are obtained with the
new powder.
[0054] The following tables 3 and 4 disclose the corresponding
green strengths for the known and new powders, respectively. The
green strength which is obtained especially when the new powder is
compacted in a lubricated die is remarkably higher than when the
previously known powder was used.
3TABLE 3 Die wall Compaction Lubrication Internal lubrication
Pressure Green Strength Green Strength (MPa) (MPa) (MPa) 0.8%
ZnStearate 0.6% Advawax .TM. 400 11.08 8.76 20.32 600 19.92 13.46
28.98 800 27.40 15.25 27.64
[0055]
4TABLE 4 Die wall Compaction Lubrication Internal lubrication
Pressure Green Strength Green Strength (MPa) (MPa) (MPa) 0.8%
ZnStearate 0.6% Advawax .TM. 400 21.5 11.3 19.3 600 38.2 17.3 29.5
800 53.9 18.8 32.2
Example 2
[0056] The following Table 5 discloses mechanical properties of
products prepared by sinterhardening (cooling rate 2.5.degree.
C./s) of the powder according to the invention in admixture with
copper. The cooling rate was The properties of the material
including only 1% by weight of Cu is as good as the standard
material FL 4608 according to USMPIF standard including 2% of
Cu.
5TABLE 5 Added Added TS YS Cu (%) graphite(%) (MPa) (MPa) HRC A(%)
2 0.6 894 854 31 0.27 2 0.8 791 743 34 0.21 1 0.6 892 779 29 0.37 1
0.8 738 35 0.19
* * * * *