U.S. patent application number 10/584903 was filed with the patent office on 2008-02-14 for method for the manufacture of an austenitic product as well as the use thereof.
Invention is credited to Kenneth Goransson, Andreas Rosberg, Eva Witt.
Application Number | 20080038143 10/584903 |
Document ID | / |
Family ID | 30768915 |
Filed Date | 2008-02-14 |
United States Patent
Application |
20080038143 |
Kind Code |
A1 |
Witt; Eva ; et al. |
February 14, 2008 |
Method for the Manufacture of an Austenitic Product as Well as the
Use Thereof
Abstract
The present invention relates to a method to produce an
austenitic alloy by an austenitic stainless substrate alloy of low
Al content being coated with an alloy of higher Al content at a
temperature between 100.degree. C. and 600.degree. C., so that the
resulting product has an Al content of 4.5-12% by weight.
Inventors: |
Witt; Eva; (Sandviken,
SE) ; Goransson; Kenneth; (Gavle, SE) ;
Rosberg; Andreas; (Sandviken, SE) |
Correspondence
Address: |
DRINKER BIDDLE & REATH (DC)
1500 K STREET, N.W., SUITE 1100
WASHINGTON
DC
20005-1209
US
|
Family ID: |
30768915 |
Appl. No.: |
10/584903 |
Filed: |
December 15, 2004 |
PCT Filed: |
December 15, 2004 |
PCT NO: |
PCT/SE04/02017 |
371 Date: |
May 3, 2007 |
Current U.S.
Class: |
420/40 ;
148/688 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/04 20130101; C22C 38/02 20130101; C22C 38/50 20130101 |
Class at
Publication: |
420/40 ;
148/688 |
International
Class: |
C22C 38/18 20060101
C22C038/18; C22F 1/04 20060101 C22F001/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 30, 2003 |
SE |
0303608-4 |
Claims
1. Method to produce an austenitic alloy, wherein an austenitic
substrate alloy of low Al content is coated with at least one layer
of an alloy of higher Al content at a temperature between
100.degree. C. and 600.degree. C., so that the resulting product
has an Al content of 4.5-12% by weight.
2. Method to produce an austenitic alloy according to claim 1,
wherein a substrate alloy having the following composition (in % by
weight): 20-70% of Ni, 15-27% of Cr, 0-5% of Al, 0-4% of Mo and/or
W, 0-2% of Si, 0-3% of Mn, 0-2% of Nb, 0-0.5% of Ti, 0-0.1% of one
or more rare earth metals (REM) balance Fe and normally occurring
impurities is coated with at least one layer of a composition of
higher Al content.
3. Method for the manufacture of an austenitic alloy according to
claim 1, wherein at least one layer is aluminium.
4. Method for the manufacture of an austenitic alloy according to
claim 1, wherein at least one layer is an aluminium-based
alloy.
5. Method for the manufacture of an austenitic alloy according to
claim 1, in which the aluminium-based alloy is Al having 0.5 to 25%
by weight of Si.
6. Method for the manufacture of an austenitic alloy according to
claim 1, wherein the austenitic final product has the following
composition (in % by weight): 0-0.2% of C, 0-0.1% of N, 25-70% of
Ni, 15-25% of Cr, 4.5-12% of Al, 0-4% of Mo and/or W, 0-4% of Si,
0-3% of Mn, 0-2% of Nb, 0-0.5% of Ti, 0-0.5% of Y, Sc, Zr and/or
Hf, 0-0.2% of one or more rare earth metals (REM) such as, e.g.,
Ce, La, Sm, balance Fe and normally occurring impurities.
7. Austenitic alloy with an Al content of 4.5-12% by weight,
wherein it is manufacturable by the method according to claim
1.
8. Use of the method according to claim 1 for producing material to
be used in high temperature applications such as supporting
material in catalytic converters and resistive heating.
Description
[0001] The present invention relates to a method for the
manufacture of an austenitic product having elevated content of
aluminium, and the use thereof in applications where high
temperature resistance in the form of oxidation resistance and
improved mechanical properties are required.
BACKGROUND OF THE INVENTION
[0002] With increasing application temperatures for steel materials
in various high-temperature applications, today's ferritic
materials have, more precisely ferritic FeCrAl materials,
occasionally turned out to be mechanically weak in order to resist
the stresses that arise upon use at high temperatures in the form
of fast changes of temperature, gas flows varying in temperature,
direction and/or composition, and mechanical stresses, such as,
e.g., vibrations.
[0003] EP-B1-1235682 discloses the use of an austenitic nickel base
or cobalt base alloy that is coated with aluminium or aluminium
alloy and rolled to finished dimension. In such a way, by means of
a heat treatment at temperatures above 600.degree. C., foil of a
min. thickness of approx. 50 .mu.m can be manufactured, which has
been tested to approximately 1100.degree. C. At 1100.degree. C.,
the mass of the sample increased by up to 7.6% after 400 h. The
disadvantage is, in comparison with conventional FeCrAl material,
the relatively high oxidation rate, which is regarded to be the
decisive factor for the service life of the catalyst supporting
material.
[0004] Austenitic alloys generally have higher mechanical strength
at the same high temperatures than ferritic alloys. Austenitic
materials of high aluminium content have a considerable improved
oxidation resistance in comparison with austenitic materials of
lower aluminium contents by virtue of the material having a very
low ductility at typical hot-working temperatures, i.e., at 750 to
1200.degree. C.
[0005] A metallic material for catalysts with elevated working
temperatures and in increased mechanical load has to meet the
following requirements: an improved mechanical strength in relation
to materials used today, as is disclosed in U.S. Pat. No. 5,578,265
and furthermore considerably better oxidation resistance than the
austenitic, high-strength materials disclosed in EP-B1-1235682. In
order to allow the use thereof as supporting material in a
catalyst, a foil material of a thickness of 50 .mu.m should not
increase in weight more than 6% after 400 h of oxidation in air at
1100.degree. C., preferably the increase in weight should be below
4%. An Al alloyed austenitic steel or nickel or cobalt base alloy
of more than 4.5% of Al can be expected to have sufficiently
satisfactory mechanical properties and may, for a foil having a
thickness of 50 .mu.m, during certain circumstances attain an
increase in weight corresponding to 6% after 400 h at 1100.degree.
C., but by virtue of utmost limited hot workability at Al contents
above 4.5% by weight, such products are not possible to produce as
thin strips by means of conventional methods. Furthermore, the
oxidation resistance of such materials are still inferior compared
to ferritic materials used today.
[0006] Therefore, there is a need for a material that is more
heat-resistant and oxidation-resistant and has a higher mechanical
strength than those of today. Further, though, this material has to
have satisfactory or better manufacturing properties than the
materials known hitherto. This is the case for all different
product forms that are used at the above-described conditions, such
as strip, foil, wire, sheet-metal plate and tube.
SUMMARY OF THE INVENTION
[0007] It is therefore an object of the present invention to
provide a method where, by an austenitic substrate alloy of low Al
content being coated with an aluminium composition of higher Al
content at a temperature between 100.degree. C. and 600.degree. C.,
the resulting product has an Al content of 4.5-12% by weight,
preferably 5.5-12% by weight.
[0008] It is an additional object of the present invention to
provide an austenitic alloy material for use in high-temperature
applications, manufacturable by said method.
BRIEF DESCRIPTION OF THE FIGURES
[0009] FIG. 1 shows results of hot ductility testing, so-called
Gleeble testing, the reduction of area to fracture being measured
as a function of the test temperature.
[0010] FIG. 2 shows the oxidation rate in air at 1000.degree. C. of
examples C and D in comparison with comparison example 1.
[0011] FIG. 3 shows the oxidation rate in air at 1100.degree. C. of
example C and comparison example 1.
[0012] FIG. 4 shows the oxidation rate in air at 1100.degree. C. of
examples C, E and F as well as of comparison example 1 having the
thicknesses of 50 .mu.m and 3 mm, and comparison example 3.
[0013] FIG. 5 shows the content of aluminium in an Al-coated
material after annealing times of different length at 1050.degree.
C., plotted as a function of the distance from the surface.
[0014] FIG. 6 shows the micro structure in an Al-coated and
annealed material after 50 min annealing at 1150.degree. C. in Ar
gas, wherein 0=Fe--Cr rich layer, 1=Ni and Al rich layer,
2=diffusion zone, 3=composition of the substrate material.
DESCRIPTION OF THE INVENTION
[0015] These objects are attained by means of an austenitic product
that is manufactured by coating an austenitic substrate alloy with
the following composition (in % by weight): 20-70% of Ni, 15-27% of
Cr, 0-5% of Al, 0-4% of Mo and/or W, 0-2% of Si, 0-3% of Mn, 0-2%
of Nb, 0-0/5% of Y, Zr and/or Hf, 0-0.5% of Ti, 0-0.1% of one or
more rare earth metals (REM) such as, e.g., Ce, La, Sm, 0-0.2% of
C, 0-0.1% of N, balance Fe and normally occurring impurities, with
an aluminium composition such as aluminium or an aluminium-based
alloy such as is described below.
[0016] A preferred composition of the substrate material is (in %
by weight) 25-70% of Ni, 18-25% of Cr, 1-4 % of Al, 0-4 % of Mo
and/or W, 0-2% of Si, 0-3% of Mn, 0-2% of Nb, 0-0.5% of Y, Zr
and/or Hf, 0-0.5% of Ti, 0-0.1% of one or more rare earth metals
(REM) such as, e.g., Ce, La, Sm, 0-0.1% of C, 0-0.05% of N, balance
Fe and normally occurring impurities.
[0017] By a two-stage process, the content of aluminium of the
final product as well as its mechanical properties and oxidation
resistance can be optimized independently of each other.
[0018] After coating the substrate material with aluminium or an
aluminium-based alloy, the final alloy has a composition consisting
of (in % by weight) 25-70% of Ni, 15-25% of Cr, 4.5-12% of Al, 0-4%
of Mo and/or W, 0-4% of Si, 0-3% of Mn, 0-2% of Nb, 0-0.5% of Ti,
0-0.5% of Y, Sc, Zr and/or Hf, 0-0.2% of one or more rare earth
metals (REM) such as, e.g., Ce, La, Sm, 0-0.2% of C, 0-0.1% of N,
balance Fe and normally occurring impurities.
[0019] The austenitic substrate material has in itself a good
high-temperature strength, which is increased by the presence of
precipitations of Ni (Nb, Al) and, if required, also by Mo and/or W
in solid solution. Additionally increased mechanical stability and
resistance to grain growth may be given by the presence of
precipitations of carbides and/or nitrides of any one or some of
the elements Ti, Nb, Zr, Hf.
[0020] Carbon in solid solution or as carbides contributes to an
increased mechanical strength at high temperatures. Simultaneously,
higher contents of carbon in the substrate material imply
deteriorated properties upon cold working. Therefore, the maximal
content of carbon in the substrate should be limited to 0.2% by
weight.
[0021] Nitrogen in solid solution or as nitrides contributes to an
increased mechanical strength at high temperatures. Simultaneously,
higher contents of nitrogen in the substrate material imply that
embrittling aluminium nitride may be formed in the production of
the substrate or after coating with aluminium or an aluminium-based
alloy. Therefore, the maximal content of nitrogen in the substrate
should be limited to 0.1% by weight.
[0022] The austenitic alloy manufactured according to the invention
is used in a coated and not heat-treated state or after a diffusion
annealing. The most favourable compositions for the substrate alloy
are obtained if it contains 1-4% by weight of Al. This content of
aluminium gives the finished alloy an improved oxidation resistance
and an improved production economy without entailing an increased
risk of production disturbances in comparison with the manufacture
of a material of low content of aluminium. After coating with
aluminium or an aluminium-based alloy, the material should in total
contain more than 4.5% by weight of Al.
[0023] According to the invention, the coating with aluminium or an
aluminium-based alloy should take place within a temperature range
of the substrate that is lower than the melting point of the
aluminium, i.e., at a temperature between 100.degree. C. and
600.degree. C., preferably 150.degree. C.-450.degree. C.
[0024] Addition of Zr and/or Hf and REM and/or Y and/or Sc gives an
increased resistance to peeling and flaking of the formed oxide.
The finished product's contents of said elements may be supplied by
addition in the substrate alloy and/or in the aluminium-based alloy
that are used in the coating.
[0025] Certain compositions of the alloy according to the invention
could be manufactured by conventional metallurgy. However, unlike
this, in production by means of the process according to the
present invention, a material can be obtained, the microstructure
of which is controlled and the oxidation properties and mechanical
properties of which are optimal. It is an additional advantage of
the process according to the present invention that the total
content of aluminium of the final product is not limited by the
embrittling effect that contents of aluminium above approx. 4.5% by
weight may give upon later cold and/or hot working. Furthermore,
the method to coat a substrate material with aluminium or an
aluminium-based alloy according to the invention gives a final
product, the contents of which of, e.g., Mo, C, Nb can be
considerably higher than in a conventionally manufactured material
without the presence of said elements resulting in any noticeable
deterioration of the oxidation properties.
[0026] The proper coating of the substrate alloy with aluminium or
an aluminium-based alloy may be effected by processes such as,
e.g., dipping in melt, electrolytic coating, rolling together
strips of aluminium or an aluminium alloy from a gas phase by
so-called CVD or PVD technique. The coating with aluminium or
aluminium-based alloy can be carried out after the substrate alloy
has been rolled or in another way been machined to desired product
dimension. During this process, a diffusion annealing may be
carried out in order to provide a homogenization of the material
and then plastic machining in one or more steps may be carried out
in order to provide the final product. Plastic machining, such as,
e.g., rolling or drawing may also be effected directly on a coated
product of larger dimensions than the desired final dimension. In
this case, the plastic machining may be followed by annealing.
[0027] The content of aluminium in the final product can be varied
by means of different factors: the thickness of the substrate
material in relation to the thickness of the coating, the content
of aluminium in the substrate material as well as the content of
aluminium of the coating.
[0028] However, as has been described above, the total content of
aluminium in the finished product always has to be at least 4.5% by
weight in order to secure sufficient properties. The product may be
used in the form of an annealed, homogeneous material or a laminate
or a material having a concentration gradient of Al with the Al
content being higher at the surface than in the centre of the
material.
[0029] Depending on the coating process used, various compositions
of the applied Al alloy are more suitable than others. The
aluminium alloy contains 0-25% of Si and/or 0-2% by weight of one
or more of the elements Ce, La, Sc, Y, Zr, Hf and/or 0-5% by weight
of one or more of the elements Mg, Ti, Cr, Mn, Fe, Ni, Co and/or
0-1% by weight of one or more of the elements B, Ge, preferably the
aluminium alloy should contain at least 90% of Al, 0-10% of Si
and/or 0-2% by weight of one or more of the elements Ce, La, Sc, Y,
Zr, Hf, more preferably the aluminium alloy should contain at least
95% of Al, 0-5% of Si and/or 0-2% by weight of one or more of the
elements Ce, La, Sc, Y, Zr, Hf.
EMBODIMENT EXAMPLES
[0030] In the following, it is shown how the requirements on
strength and oxidation resistance are met by an austenitic Al
alloyed material manufactured according to the method described in
the present invention. Furthermore, it is shown that a material
manufactured according to the same method is superior to a material
that has the same composition but has been manufactured according
to conventional methods, in respect of high-temperature strength,
oxidation resistance and workability.
Example 1
[0031] Table 1 indicates examples of compositions of examined
alloys. The alloys according to examples A and B as well as the
Comparative examples 1, 2 and 3 were manufactured in the
conventional way by pyrometallurgy and hot working.
TABLE-US-00001 TABLE 1 Compositions of investigated alloys Exemple
Nr. A (359) B (357) Comparison 1 Comparison 2 C 0.052 0.021 0.01
0.019 N 0.016 0.032 0.01 0.020 Ni 32.35 17.61 0.3 33.18 Al 2.95
1.68 5.3 5.43 Cr 21.83 21.81 20 21.50 Nb 0.01 0.61 0.01 <0.01 Mo
0.02 0.02 0.01 <0.01 Zr 0.07 <0.002 0.01 <0.005 REM
<0.005 0.026 0.03 0.042 Ti 0.16 <0.005 0.01 <0.005 Si 0.15
0.14 0.3 0.16 Mn 0.12 0.10 0.3 0.11
[0032] Comparison example 1 is an alloy that today is used as
supporting material in catalytic converters and that has acceptable
oxidation resistance for this use. Comparison example 2 is an
austenitic alloy of high Al content, manufactured by conventional
methods. The yield upon hot working of said alloy was only
approximately 10%, i.e., 90% of the material had such internal
defects in the form of, e.g., cracks that it could not be used for
further working.
[0033] The alloys according to examples A and B have compositions
that are suitable to be used as substrate materials in a coating
process where a thin layer of aluminium or an aluminium-based alloy
is deposited on said substrate. From the alloys according to
Examples A and B as well as comparison example 1, 50 .mu.m thick
strips were manufactured via hot rolling and cold rolling. The
yield in the production of the alloys in examples A and B was the
same as comparison example 1.
[0034] In order to avoid formation of aluminium nitride, the
content of nitrogen in the substrate materials is low. In order to
limit this tendency further, Ti, Nb and/or Zr and/or Hf were added.
Addition of these elements results in the formation of nitrides
that are more stable than AIN, which entails a reduced formation of
the latter. Furthermore, the compositions are chosen in order to
enable efficient production of thin strips of the substrate
material. For instance, the content of carbon is below 0.10%, which
allows satisfactory material yields in cold working processes. By
the relatively high Al content in the substrates, the necessary
amount of Al that has to be deposited on the substrate is decreased
with the purpose of achieving sufficient Al content in the finished
product.
[0035] In table 2, it is shown that the substrate alloys have a
very good high-temperature strength; e.g., at 700.degree. C. the
ultimate strength of the alloys according to examples A and B is up
to 3 times larger than of the conventional material in comparison
example 1, and at this temperature the yield point in tension is
2.8 to 5 times larger than of comparison example 1. At 900.degree.
C., the yield point in tension of the alloy according to examples A
and B is approximately 5 times larger of for comparison example 1,
while the ultimate strength is at least 3.5 times higher than of
comparison example 1.
TABLE-US-00002 TABLE 2 Results of tensile testing at different
temperatures Room temperature 700.degree. C. 900.degree. C.
Examples Rp.sub.0,2 Rm A5 Rp.sub.0,2 Rm A5 Rp.sub.0,2 Rm A5 A 275
625 39 254 441 22 98 146 99 B 239 546 39 141 337 44 108 150 78 E
166 Comp. 480 670 25 50 140 90 20 40 150 example 1 Comp. 437 814 25
340 511 -- 91 151 55 example 2
[0036] Thus, the two substrate alloys used in Examples A and B meet
the requirements of sufficient mechanical strength and
manufacturability as thin foil.
[0037] FIG. 1 shows results of hot ductility testing, so-called
Gleeble testing, where the reduction of area to fracture is
measured as a function of the test temperature, for the alloys
according to Examples A and B and for comparison example 2. In
order to be able to hot-work an alloy in practice, the reduction of
area to fracture should exceed 40%. In order to obtain a
reproducibly high yield in hot working operations, the average
reduction of area to fracture should be at least 70% at a
temperature difference of 100.degree. C.
[0038] From FIG. 1, it is clearly seen that the alloys according to
Examples A and B can be manufactured via hot rolling and/or
forging, while the alloy according to comparison example 2 cannot
be manufactured with sufficient yield by means of conventional
methods. One consequence of this is that, in order to be able to
obtain the good oxidation resistance that an alloy according to
Comparison example 2 can be expected to have, the necessary Al
content has to be added after the alloy has been produced in the
form of a thin strip. The alloys according to Examples A and B are
both sufficiently ductile at room temperature and at elevated
temperatures in order to be able to be cold-rolled to very thin
strips with satisfactory productivity, which is seen in that they
could be manufactured without problems down to material thicknesses
of 50 .mu.m, and are thereby good candidate materials to be used as
substrate materials for a coating with aluminium or an
aluminium-based alloy Al.
Example 2
[0039] The alloys in examples C and D were manufactured by coating
the two surfaces of cold-rolled, 50 .mu.m thick, strips of the
alloy according to examples A and B, respectively, by vaporization
or sputtering with Al in such an amount that the total Al content
corresponded to 5.5-6% (see table 3). The coating was effected by a
certain heating of the substrate material, however not to such a
high temperature that melted Al was present on the substrate. The
coating with Al or Al alloy according to the invention should
accordingly be effected within a temperature range of the substrate
of 100.degree. C.-660.degree. C., preferably in the temperature
range of 150-450.degree. C.
TABLE-US-00003 TABLE 3 Coating test Coated Thickness thickness
Desired Measured before of total Al coating Substrate coating
coated Al content thickness alloy [.mu.m] alloy [.mu.m] [%] [.mu.m]
Example C Example A 50 2.5 6.0 2.7 Example D Example B 50 3.5
6.0
[0040] The alloy according to comparison example 2, which has
approximately the same total composition as the alloy in example C,
could, as has been mentioned previously, be forged, but only with a
very low material yield. Thus, the limited hot ductility entails
that this alloy hardly can be manufactured in the form of thin
strips. However, the same alloy has, as is seen in table 2, a very
good heat resistance; e.g., the ultimate strength at both
700.degree. C. and 900.degree. C. is 3 to 4 times larger than of
the conventional material in comparison example 1, and the yield
point in tension is more than 4 times as large at both test
temperatures.
Example 3
[0041] The thickness of the Al layer obtained on 50 .mu.m thick
strip according to example C was measured by GDOES (Glow Discharge
Optical Emission Spectroscopy), a method that enables accurate
measuring of compositions and thicknesses of thin surface layers.
The analysis showed that the sample had a total Al content of 5-6%
by weight. These samples were oxidized in air at 1000.degree. C.
for up to 620 h. The results are shown in FIG. 2. The alloy
according to Example C has an oxidation resistance that is
comparable with the conventionally manufactured Fe--Cr--Al alloy of
the same thickness (comparison example 1) and has a significantly
better oxidation resistance than the alloy according to example D.
After 400 h, the alloy according to example C has increased 2.3% in
weight, while the alloy according to example D increased 5 % in
weight. After the same time, comparison example 1 has increased
approx. 2.2% in weight.
[0042] The alloy according to Example C was oxidation tested at
1100.degree. C. together with comparison example 1, which is shown
in FIG. 3. The two materials were tested in the form of foil of a
thickness of 50 .mu.m. After up to 300 h of test time, the two
materials are equally good. After 400 h of testing, both the alloy
according to Example C and comparison example 1 have increased less
than 6% in weight: the alloy according to example C by 5.9% and
comparison example 1 by 4.3%. Thus, the alloy according to Example
C meets the requirement of sufficient oxidation resistance for use
in catalytic converters, maximum 6% increase in weight in 50 .mu.m
thickness upon oxidation in 400 h at 1100.degree. C.
Example 4
[0043] Examples E and F are the alloys according to examples C and
D, respectively, that have been annealed at 1200.degree. C. for 20
min with the purpose of providing an equalising of the Al content
in the material (see table 4). The ductility of the material was
assessed by means of a bending test where the smallest bending
radius that the material could be bent to without fracturing was
determined (see table 4). The narrowest radius that the material
was tested at was 0.38 mm. None of the materials exhibited any
damage after this bending. The radius is smaller than the one used
in the production of catalytic converters. Thereby, strips
manufactured according to the invention have a fully sufficient
ductility to allow the use thereof in catalytic converters. At
900.degree. C., example E has an ultimate strength of 166 MPa (see
table 2), which is more than four times larger than the material
according to comparison example 1 used at present, and furthermore
somewhat higher than a conventionally manufactured material
according to comparison example 2, having a similar composition as
alloys that have been manufactured in accordance with the
invention.
TABLE-US-00004 TABLE 4 Results of test of coated and
diffusion-annealed samples Smallest bending radius without Examples
Composition Diffusion annealing fracturing [mm] E The same as 20
min/1200.degree. C. in H.sub.2 0.38 example C F The same as 20
min/1200.degree. C. in H.sub.2 0.38 example D
Example 5
[0044] The alloys according to Examples E and F were oxidation
tested at 1100.degree. C. together with the alloy of Example C
according to the invention as well as comparison examples 1 and 2.
The results are shown in FIG. 4. Comparison example 2 was tested in
the form of an approx. 3 mm thick plate while Examples C, E and F
were tested in the form of 50 .mu.m thin foil. The alloy in
comparison example 1 was tested in two different states: in the
form of an approx. 3 mm thick plate extracted from a hot-rolled
strip, as well as in the form of a foil of a thickness of 50 .mu.m.
The results are summarized in table 5.
[0045] It is evident from FIG. 4 and table 5 that the oxidation
rate of the thin foil of comparison example 1 is smaller than that
of the thick plate. This effect may be explained by the fact that
the thin foil easily can be deformed and thereby absorb the
difference in thermal expansion between the protective oxide and
the metal. Thereby, it is avoided that the oxide fractures upon
cooling and heating, an effect that otherwise means that
unprotected metal being exposed to oxidation. The relatively thick
plate cannot be deformed in the same way, and that sample will
thereby be more sensitive to heating and cooling.
TABLE-US-00005 TABLE 5 Comparison between increase in weight at
1100.degree. C. in 50 .mu.m thick strips and 3 mm thick plates and
between coated and coated + diffusion- annealed samples (Increase
of mass (g/m.sup.2) Test time state 1)/(Increase State 1 State 2
[h] of mass (g/m.sup.2) state 2) Comparison Comparison 400 0.62
example 1, 50 .mu.m example 1, thickness 3 mm thickness Example C
(50 .mu.m Comparison 400 0.40 thickness) example 3, 3 mm thickness
Example E (50 .mu.m Comparison 400 0.13 thickness) example 3, 3 mm
thickness Example F (50 .mu.m Comparison 220 0.31 thickness)
example 3, 3 mm thickness Example E Example C 400 0.32
[0046] The alloys according to Examples C and E have almost the
same composition as the alloy in comparison example 2, and also
here, a similar effect of different sample thickness would be
expected, as for comparison example 1. However, the relative
improvement in oxidation resistance with decreasing sample
thickness of the alloy according to the invention is considerably
larger than it is of comparison example 1 (see table 5). This may
be regarded to be a highly unexpected and valuable effect of the
method according to the invention.
[0047] Furthermore, the diffusion annealing that differs between
Examples C and E has turned out to give an unexpectedly large
additional improvement of the oxidation resistance (see table
5).
[0048] To start with, the alloy according to Example F has equally
good oxidation resistance as Example C or comparison example 1 in
the form of foil. Testing was interrupted after 220 h for the alloy
according to Example F. However, comparison between the increase in
weight up to 220 h at 1100.degree. C. of examples E and F shows
that the alloy according to Example E has the most suitable
combination of composition and way of production as regards
oxidation resistance.
Example 6
[0049] A 50 .mu.m thick strip of the alloy according to example A
was coated with Al by means of vaporization. Various samples were
annealed for different times at 1050.degree. C. in Ar gas.
Concentration profiles of Al in the material were determined by
GDOES. The results are shown in FIG. 5. It is clear that an Al
enriched area is left near the surface of the strip also after 8 h
of heat treatment. This area seems only to be consumed slowly by Al
diffusion inwardly in the strip.
Example 7
[0050] A 50 .mu.m thick strip of the alloy according to example A
was coated with Al by means of vaporization. A sample was annealed
for 50 min at 1150.degree. C. in Ar gas. The micro structure was
analysed by means of SEM (scanning electron microscopy). FIG. 6
shows the area closest to the sample surface. Farthest out, a
Fe--Cr rich layer (cf. "0" in FIG. 6) is formed, inside this, a Ni
and Al enriched area (cf. "1" in FIG. 6). Layer "2" corresponds to
a diffusion zone of slowly decreasing Al content with increasing
distance from the surface. In layer "3" in FIG. 6, the composition
is the same as in the substrate material.
Examples of Application
Supporting Material for Catalytic Conversion
[0051] Catalytic conversion is since a number of years a
requirement in most industrialised countries. The catalytically
active material is carried mechanically by a supporting material.
The requirements on the supporting material are, among other
things, that it should have a large surface, withstand temperature
variations and have sufficient mechanical strength and oxidation
resistance at the operating temperature of the catalytic
converter.
[0052] Two main types of supporting materials are used today:
ceramic and metallic. The ceramic supporting materials, which
frequently are manufactured from cordierit, are not affected by
oxidation, however their brittleness means that the resistance to
impacts and other mechanical stresses as well as to temperature
variations such as fast changes of temperature is very limited.
Today, metallic supporting materials generally are based on thin
strips of ferritic Fe--Cr--Al alloys with additions of small
amounts of reactive elements such as rare earth metals (REM) or Zr
or Hf. In order to give the monolith a maximum active surface, the
supporting material should be as thin as possible, usually between
10 .mu.m and 200 .mu.m. Today, a common strip thickness is 50
.mu.m, but considerably enhanced efficiency of the catalytic
converter by virtue of an increased surface/volume-ratio and/or
decreased fall of pressure over the catalytic converter can be
expected upon a reduction of the strip thickness to 30 .mu.m or 20
.mu.m. The high ductility of the metal gives a good resistance to
mechanical and thermal fatigue. Aluminium in contents above approx.
4.5% by weight gives, together with the reactive elements, the
material the possibility of forming a thin, protective, aluminium
oxide upon heating. Furthermore, the reactive elements make the
oxide getting a considerably reduced tendency to peel, i.e., come
loose from the metal upon cooling or mechanical deformation.
However, conventional Fe--Cr--Al alloys have a large disadvantage:
they are mechanically very weak at high temperatures, and therefore
tend to be greatly deformed also upon small stresses by virtue of,
e.g., acceleration, changes of pressure, mechanical impacts or
changes of temperature.
[0053] The invention is not limited to products in small
dimensions, such as thin strips or thin wire. Since an austenitic
material having a content of aluminium that is larger than 4.5% by
weight cannot be produced with sufficient productivity and material
yield by hot working, it is valuable to be able to manufacture such
an alloy in thicker dimensions by the coating method described in
the present invention. This may be effected, e.g., by manufacturing
a product in the form of, e.g., sheet-metal plate, strip, foil or a
seamless tube, in a substrate alloy, and then said product is
coated, on one or both surfaces with an aluminium alloy in such an
amount that the total content of aluminium of the material exceeds
4.5% by weight. For instance, a seamless tube having the
composition according to example A may be manufactured by means of
conventional methods to the following dimensions: outer diameter
60.33 mm, wall thickness 3.91 mm. In order to be able to achieve a
total content of aluminium of at least 4.5% by weight in such a
tube, it needs to be coated with aluminium on the inner and outer
surface with a thickness of at least 0.1 mm. Such an amount may be
applied to the surfaces of the tube by conventional methods, e.g.,
by dipping in a melt of an aluminium alloy. If a homogeneous
material is desired, a longer heat treatment at high temperature is
required, suitably at least 1000.degree. C. Therefore, the finished
product should suitably be manufactured in a partly homogenized
form, where the material has an aluminium gradient that increases
towards the surfaces, e.g., by a heat treatment where the material
slowly is heated to 1100.degree. C. and is heat-treated at this
temperature for between 5 min and 10 h, depending on the desired
aluminium distribution. It is evident to a person skilled in the
art that, if this product should be possible to be manufactured
with satisfactory productivity, the content of aluminium in the
substrate material should be as high as possible, without causing
production disturbances in the manufacture of the substrate. In
this case, a suitable content of aluminium in the substrate
material is 2-4% by weight. This method can be used to manufacture
a finished product or to manufacture a starting material for
continued plastic machining at low temperature, e.g., a tubular
blank for pilgrim step rolling.
Resistive Heating
[0054] In industrial furnaces and in consumer goods including
resistive heating, such as hotplates, radiant heaters, flat irons,
ovens, toasters, hairdryers, tumble-dryers, drying cupboards,
electric kettles, car seat heaters, underfloor heating equipment,
radiators and other similar products, there is also a need for
using strip, wire or foil having the above-described properties.
Availability of a material having this product specification
results in the development of more efficient heat sources having
longer service life and/or higher operation temperature and
efficiency.
Further Applications
[0055] The alloy produced according to the invention may also be
used in other high temperature applications, such as applications
requiring a high oxidation resistance and good mechanical
properties. For example, it could be used in heat exchangers or as
protective plates. Also, it could be used in other environments
such as in reducing atmosphere. In this latter case it could be
advantageous in some cases to pre-oxidise the product before use in
order to assure a stable and dense Al-containing oxide on the
surface.
* * * * *