Note: Descriptions are shown in the official language in which they were submitted.
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Method for the production of a heat resistant alloy with good resistance to
high-
temperature oxidation
The present invention relates to a method for producing an alloy with good
resistance to
high-temperature oxidation and good thermal stability.
Very frequently, because of their good resistance to oxidation and their good
thermal
stability, stainless steels and nickel based alloys that contain aluminum are
used in
industrial furnaces and terotechnology, in motor vehicle exhaust systems, as
well as for
alloys used in resistors, or heat conductors. If, for reasons of economy,
thinner wall
thicknesses, andlor high temperatures andlor component loads are selected, the
aluminum contents between 1-3% that are found in typical nickel based alloys,
as
defined by Material Numbers 2.4633 or 2.4851 (DIN Material Numbers), are
inadequate
for forming a protective layer of aluminum oxide over a protracted period of
time. The
chromium oxides that form as a result of aluminum depletion entail the danger
that they
will evaporate at temperatures above 1000°C and contaminate the
annealing material.
Metallic materials based on iron-chromium-aluminum, as a described by Material
Number DIN 1.4767, are used, for example, as carrier foils in metal exhaust
gas
catalyzers or as electrical heat conductors.
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Usually, these iron based alloys contain approximately 20% chromium, 5%
aluminum,
and additives of zirconium, titanium, and rare earth metals (lanthanoids) as
described,
for example, in DE-C 3706415, which improve the adhesion of the oxide layer
and thus
the required resistance to oxidation at elevated temperatures of up to
1200°C.
At present, metal foils that are 50-70 ~cm thick are used as carriers for
automobile
exhaust gas catalyzers. In response to ever-increasing environmental concerns,
the
thickness of the foils is constantly decreasing. To the same extent that the
thickness of
the foils is being reduced, demands for increased thermal stability are being
imposed,
and these cannot be satisfied even with the alloys that are described in EP-A
0516097.
DE-C 19524234 describes a ductile nickel based alloy with 25-30% chromium, 8-
11
iron, and 2.4-3.0% aluminum. This material is characterised by a high level of
thermal
stability and long-time rupture strength at temperatures of up to 1200"C.
Foils that are
thinner than 50 ~,m, which are intended for use as automobile exhaust gas
catalyzers,
taking into account the present level of development, can only be produced in
this alloy
under difficult conditions and at great cost.
The iron based alloys that have been described, and which contain
approximately 20%
chromium, 5% aluminum, and additives of zirconium, titanium, and rare earth
metals,
are distinguished by outstanding resistance to high-temperature oxidation;
however,
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because of their ferritic structure they frequently do not have the thermal
stability that is
required for many high-temperature applications. Nickel-based alloys as
described, for
example, by DIN Material Number 2.4851, possess good heat resistance combined
with good thermal stability because of their austenitic microstructure. In
comparison to
the above described ferritic materials, relative to resistance to high-
temperature
oxidation, these alloys exhibit poor behaviour since an increase in the
aluminum
content in nickel based ductile alloys to more than 4% has not been possible
up to now
because of the reforming problems connected with a high aluminum contents. But
it is
precisely the combination of thermal stability and good resistance to high-
temperature
oxidation that is so urgently required for automobile exhaust catalysts, in
furnace and
terotechnology, and in waste gas plants, in order to control process
parameters.
GB-A 1, 116, 377 describes a composite material in which an AI-2024 alloy,
which is
optionally coated with a 7072 alloy, is bonded to a sheet of an austenitic
alloy by rolling.
The austenitic alloy is to have 8-10% nickel and 14-8% chromium. The aluminum
content should amount to 0.75-1.5%, the carbon content should amount to a
maximum
of 0.09%, the manganese content should amount to a maximum of 1.6%, the
sulphur
content should be at a maximum of 0.03%, and the chromium content should
amount to
a maximum of 1.0%. Prior to the rolling process, the AI-2024 alloy, optionally
configured as a composite, is heated to 482°C for 10 minutes. After the
rolling process,
it is annealed at 493°C for 20 minutes and subsequently cooled in cold
water.
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EP-A 0 511 699 and DE-A 196 52 399 describes ferritic alloys coated with
aluminum,
as well as reforming and heat treatment, although these are not usable on
austenitic
alloys.
Finally, US-A 4,535,034 describes an austenitic alloy of the following
composition:
maximum 0.7% carbon, maximum 3% silicon, maximum 2% manganese, 10-40%
nickel, 9-30% chromium, 2-8% aluminum, and the remainder iron, in addition to
impurities caused by the production process. The alloy is in the form of sheet
metal
that is coated with aluminum and then subjected to heat treatment. Reforming
is not
described, and neither is it possible, since intermetallic phases are formed
in the
boundary layers. It is preferred that this composite material be used in major
terotechnology projects.
Thus, it is the objective of the present invention to describe a method and an
alloy with
which components with dimensions of less than 50 ~,m can be manufactured
without
any increased outlay, and which has a high level of resistance to high-
temperature
oxidation, and thermal stability of greater than 50 MPa at up to
1000°C. The alloy is
intended for use in a wide variety of applications.
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This objective has been achieved by a method for producing an alloy with a
high level
of resistance to high-temperature oxidation and by a high level of thermal
stability, a
base material of an austenitic, thermally stable nickel based alloy or an
austenitic cobalt
based alloy or an austenitic stainless steel with good reforming properties
being coated
on one or both sides with a layer of aluminum or an aluminum alloy and this
composite
material, formed from the basic material and the aluminum coating, which has
good
adhesive properties being brought to its end dimensions by reforming, with or
without
the intermediate annealing, a base material of the following analysis (in mass-
%) being
used:
Nickel Chromium Carbon Aluminum Iron
20 - 80 10 - 35 0.01 - 0.4 < 4 Remainder
+ the usual tramp elements
this basic material that is used containing at least one of the following
maximal additives
(in mass-%):
5% cobalt, 10% molybdenum, 4% tungsten, 4% niobium, 5% tantalum, 4% silicon,
3%
titanium, 5% copper.
On the other hand, this objective has been achieved with a method far
producing an
alloy with good resistance to high-temperature oxidation, and a high level of
thermal
stability, a base material of an austenitic, thermally stable nickel based
alloy, an
austenitic cobalt based alloy, or austenitic stainless steel with good
reforming properties
a
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being coated on one or both sides with a layer of aluminum or an aluminum
alloy, and
this composite material, formed from the base material and the aluminum
coating,
which has good adhesive properties, being brought to its end dimensions, with
or
without intermediate annealing, when a base material of the following analysis
(in mass-
%) is used:
Nickel Chromium Carbon Aluminum Iron
20 - 80 10 - 35 0.01 - 0.4 < 4 Remainder
+ the usual tramp elements
when the base material that is used contains of least one of the following
maximal
additives (in mass-%):
20% cobalt, 28% molybdenum, 11 % tungsten, 5% niobium, 12% tantalum, 4%
silicon,
5% titanium, 5% copper, 2.5% zirconium.
Advantageous developments of the object of the present invention as set out in
the
primary claims are described in the associated secondary claims.
The object of the present invention refers to a method for producing a multi-
layer
composite material, in which a base material of an austenitic, thermally
stable nickel
based alloy or cobalt based alloy or stainless steel with good reforming
properties is
coated on one or both sides with a layer of aluminum or an aluminum alloy, and
this
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composite material, which is formed from the base material and the aluminum
coating,
which has good adhesive properties, is brought to its end dimensions, with or
without
intermediate annealing, and then homogenized at a temperature of greater than
600°C.
The homogenization can be carried out at the intermediate or end dimensions or
in a
subsequent step in the process, depending on the demands that are made on the
end
product.
Most surprisingly, an homogenous material with good thermal stability and a
high level
of resistance to high-temperature oxidation resistance, which is simple to
work, can be
produced by this method.
This objective has been achieved by an alloy with a high level of resistance
to high-
temperature oxidation and a high level of thermal stability with a base
material
consisting of the following (in mass-%)
Nickel: 20-80%
Chromium: 10-35%
Carbon: 0.01-0.4%
Aluminum: < 4%
Iron: remainder
which includes impurities caused by production conditions, with the least one
of the
following maximal additives (in mass-%):
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Cobalt: 5%
Molybdenum: 10%
Tungsten: 4%
Niobium: 4%
Tantalum: 5%
Silicon: 4%
Titanium: 3%
Copper: 5%
this base material being coated on one or both sides with a layer of aluminum
or an
aluminum alloy.
As an alternative, this objective can be achieved by an alloy with a high
level resistance
to high-temperature oxidation and a high level of thermal stability, with the
base
material consisting of the following (in mass-%)
Nickel: 20-80%
Chromium: 10-35%
Carbon: 0.01-0.4%
Aluminum < 4%
Iron: remainder
which includes impurities caused by production conditions, with the least one
of the
following maximal additives (in mass-%):
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Cobalt: 20%
Molybdenum: 28%
Tungsten: 11
Niobium: 5%
Tantalum: 12%
Silicon: 4%
Titanium: 5%
Copper: 5%
Zirconium: 2.5%
this base material being coated on one or both sides with a layer of aluminum
or an
aluminum alloy.
The preferred areas of application for the object of the present invention are
as follows:
-catalyst carrier foils
-heat conductors or material for resistors
-components in industrial furnaces or terotechnology
-exhaust gas systems used in motor vehicles.
A number of examples that document the good material properties of the object
of the
present invention are set out below.
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Example 1
The base material is of the following composition:
Ni Cr C Mn Si AI Ti Fe
31.5 20.1 0.02 0.4 0.4 0.2 0.4 Remainder
The base material was cast as a block, heated to form an ingot, and then
processed to
form a 3.5-mm thick, hot-rolled strip. It was then cold rolled to its
thickness of 0.6 mm,
soft annealed, and then coated with a layer of 4.7 mass-% aluminum by roll
bonding.
The coated composite material could then be rolled out to a 50 ~,m thick foil
without
further heat treatment. After homogenization at a temperature above
600°C, this
resulted in a homogenous material with a thermal stability of 60 MPa at
1000°C.
Resistance to high-temperature oxidation was tested after the material had
been kept at
1100 degrees Celsius. After 400 hours, the mass of the sample changed by less
than
7.6%.
Example 2
The base material was of the following composition:
Ni Cr C Mn Si AI Ti Fe
30.5 20.1 0.04 0.4 0.5 0.3 0.4 Remainder
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The base material was cast as a block, heated to form an ingot, and then
processed to
form a 3.5-mm thick, hot-rolled strip. It was then cold rolled to its
thickness of 0.6 mm,
soft annealed, and then coated with a layer of 4.7 mass-% aluminum by roll
bonding.
The coated composite material could then be rolled out to a 50 ~cm thick foil
without
further heat treatment.
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