Note: Descriptions are shown in the official language in which they were submitted.
1
Ferritic alloy
Technical field
The present disclosure further relates to use of the ferritic alloy and to
objects or coatings
manufactured thereof.
Background and introduction
Ferritic alloys, such as FeCrAl-alloys comprising chromium (Cr) levels of 15
to 25 wt%
and aluminium (Al) levels from 3 to 6 wt% are well known for their ability to
form
protective a-alumina (A1203), aluminium oxide, scales when exposed to
temperatures
between 900 and 1300 C. The lower limit of Al content to form and maintain the
alumina
scale varies with exposure conditions. However, the effect of a too low Al
level at higher
temperatures is that the selective oxidation of Al will fail and less stable
and less
protective scales based on chromium and iron will be formed.
It is commonly agreed that FeCrAl alloys will normally not form the protective
a-
alumina layer if exposed to temperatures below about 900 C. There have been
attempts
to optimize the compositions of FeCrAl alloys so that they will form the
protective a-
alumina at temperature below about 900 C. However, in general, these attempts
have not
been very successful because the diffusion of oxygen and aluminium to the
oxide-metal
interface will be relatively slow at lower temperatures and thereby the rate
of formation
of the alumina scale will be low, which means that there will be a risk of
severe corrosion
attacks and formation of less stable oxides.
Another problem arising at lower temperature, i.e. temperatures below 900 C,
is a long
term embrittlement phenomena arising from a low temperature miscibility gap
for Cr in
the FeCrAl alloy system. The miscibility gap exists for Cr levels above
approximately 12
wt% at 550 C. Recently, alloys with lower Cr levels of about 10 to 12 wt% Cr
have been
developed in order to avoid this phenomenon. This group of alloys has been
found to
work very well in molten lead at controlled and low pressure 02.
Date Regue/Date Received 2022-08-11
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EP 0 475 420 relates to a rapidly solidified ferritic alloy foil essentially
consisting of Cr,
Al, Si, about 1.5 to 3 wt % and REM (Y, Ce, La, Pr, Nd, the balance being Fe
and
impurities. The foil may further contain about 0.001 to 0.5 wt % of at least
one element
selected from the group consisting of Ti, Nb, Zr and V. The foil has a grain
size of not
more than about 10 gm. EP 075 420 discusses Si additions in order to improve
the flow
properties of the alloy melt but the success was limited due to reduced
ductility.
EP 0091 526 relates to thermal cyclic oxidation resistant and hot workable
alloys, more
particularly, to iron-chromium-aluminium alloys with rare earth additions. In
oxidation,
the alloys will produce a whisker-textured oxide that is desirable on
catalytic converter
surfaces. However, the obtained alloys did not provide a high temperature
resistance.
Hence, there is still a need to further improve the corrosion resistance of
ferritic alloys so
that they can be used in corrosive environments during high temperature
conditions. The
aspects of the present disclosure are to solve or at least reduce the above-
mentioned
problems.
Summary of the disclosure
The present disclosure therefore relates to a ferritic alloy, which will
provide a
combination of good oxidation resistance and an excellent ductility,
comprising the
following composition in weight% (wt%):
0.01 to 0.1;
N: 0.001 - 0.1;
0: 0.2;
Cr 4 to 15;
Al 2 to 6;
Si 0.5 to 3;
Mn: <0.4;
Mo + W < 4;
Y <1.0;
Sc, Ce, and/or La < 0.2;
Date Regue/Date Received 2022-08-11
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Zr < 0.40;
RE < LO;
balance Fe and normal occurring impurities and also fulfilling the following
equation has
to be fulfilled:
0.014 (Al + 0.5Si)(Cr + 10Si + 0.1) 0.022.
Thus, there exists a relationship between the content of Cr and Si and Al in
the alloy
according to the present disclosure, which if fulfilled will provide an alloy
having
excellent oxidation resistance and ductility and also a reduced brittleness in
combination
with increased high temperature corrosion resistance.
The present disclosure also relates to an object and/or a coating comprising
the ferritic
alloy according to the present disclosure. Additionally, the present
disclosure also relates
to the use of the ferritic alloy as defined hereinabove or hereinafter for
manufacturing an
object and/or a coating.
Brief description of the figures
Figure la and Figure lb disclose the phases in Fe-10%Cr-5%Al vs. Si level
(figure
la) and Fe-20%Cr-5%Al vs. Si level (figure lb). The
diagram has been made by using Database TCFE7 and
Thermocalc software.
Figures 2a to e disclose polished sections of two alloys according
to the
present disclosure compared to three reference alloys after
exposure to 50 times 1 hour cycles at 850 C exposed to
biomass (wood pellets) ash containing large amounts of
potassium.
Detailed description of the disclosure
As already stated above, the present disclosure provides a ferritic alloy
comprising in
weight% (wt%):
0.01 to 0.1;
Date Regue/Date Received 2022-08-11
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N: 0.001 - 0.1;
0:
Cr 4 to 15;
Al 2 to 6;
Si 0.5 to 3;
Mn: <0.4;
Mo + W < 4;
< LO;
Sc, Ce, and/or La < 0.2;
Zr <0.40;
RE <1.0;
balance Fe and normal occurring impurities and also fulfilling the following
equation has
to be fulfilled:
0.014 5 (Al + 0.5Si)(Cr + 10Si + 0.1) 5 0.022.
It has surprisingly been found that an alloy as defined hereinabove or
hereinafter, i.e.
containing the alloying elements and in the ranges mentioned herein,
unexpectedly will
form a protective surface layer containing aluminium rich oxide even at
chromium levels
as low as 4 wt%. This is very important both for the workability and for the
long term
phase stability of the alloy as the undesirable brittle a-phase, after
exposure for long time
in the herein mentioned temperature range, will be reduced or even avoided.
Thus, the
interaction between Si and Al and Cr will enhance the formation of a stable
and
continuous protective surface layer containing aluminium rich oxide, and by
using the
above equation, it will be possible to add Si and still obtain a ferritic
alloy which will be
possible both to produce and to form into different objects. The inventor has
surprisingly
found that if the amounts of Si and Al and Cr are balanced so that the
following condition
is fulfilled (all the numbers of the elements are in weight fractions):
0.014 5 (Al + 0.5Si)(Cr + 1051 + 0.1) 5 0.022,
the obtained alloy will have a combination of excellent oxidation resistance
and
workability and formability within the Cr range of the present disclosure.
According to
one embodiment, 0.0155 (Al + 0.5Si)(Cr + 10Si + 0.1) 50.021, such as 0.0165
Date Regue/Date Received 2022-08-11
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(Al + 0.5SO(Cr +10Si + 0.1) 0.020, such as 0.017 (Al + 0.55i)(Cr + 10Si +
0.1) 0.019.
The ferritic alloy of the present disclosure is especially useful at
temperatures below
about 900 C since a protective surface layer containing aluminium rich oxide
will be
formed on an object and/or a coating made of said alloy, which will prevent
corrosion,
oxidation and embrittlement of the object and/or the coating. Furthermore, the
present
ferritic alloy may provide protection against corrosion, oxidation and
embrittlement at
temperatures as low as 400 C as a protective surface layer containing
aluminium rich
oxide will be formed on the surface of the object and/or coating manufactured
thereof.
Additionally, the alloy according to the present disclosure will also work
excellent at
temperatures up to about 1100 C and it will show a reduced tendency for long-
term
embrittlement in the temperature range of 400 to 600 C.
The present alloy may be used in the form of a coating. Additionally, an
object may also
comprise the present alloy. According to the present disclosure, the term
"coating" is
intended to refer to embodiments in which the ferritic alloy according to the
present
disclosure is present in form of a layer exposed to a corrosive environment
that is in
contact with a base material, regardless of the means and methods to
accomplish it, and
regardless of the relative thickness relation between the layer and the base
material.
Hence, examples of this but not limited to is a PVD coating, a cladding or a
compound or
composite material. The aim of the alloy is that is should protect the
material underneath
from both corrosion and oxidation. Examples, but not limited to, of suitable
objects is a
compound tube, a tube, a boiler, a gas turbine component and a steam turbine
component.
Other examples include a superheater, a water wall in a power plant, a
component in a
vessel or a heat exchanger (for example for reforming or other processing of
hydrocarbons or gases containing CO/CO2), a component used in connection with
industrial heat treatment of steel and aluminium, powder metallurgy processes,
gas and
electric heating elements.
Date Regue/Date Received 2022-08-11
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Furthermore, the alloy according to the disclosure is suitable to be used in
environments
having corrosive conditions. Examples of such environments include but are not
limited
exposure to salts, liquid lead and other metals, exposures to ash or high
carbon content
deposits, combustion atmospheres, atmospheres with low p02 and/or high N2
and/or high
carbon activity environments.
Additionally, the present ferritic alloy may be manufactured by using normally
occurring
solidification rates ranging from conventional metallurgy to rapid
solidification. The
present alloy will also be suitable for manufacturing all types of objects
both forged and
extruded, such as a wire, a strip, a bar and a plate. The amount of hot and
cold plastic
deformation as well as grain structure and grain size will, as the person
skilled in the art
know vary between the forms of the objects and the production route.
The functions and effects of essential alloying elements of the alloy defined
hereinabove
and hereinafter will be presented in the following paragraphs. The listing of
functions and
effects of the respective alloying elements is not to be seen as complete as
there may be
further functions and effects of said alloying elements.
Carbon (C)
Carbon may be present as an unavoidable impurity resulting from the production
process.
Carbon may also be included in the ferritic alloy as defined hereinabove or
hereinafter to
increase strength by precipitation hardening. To have a noticeable effect on
the strength in
the alloy, carbon should be present in an amount of at least 0.01 wt%. At too
high levels,
carbon may result in difficulties to form the material and also a negative
effect on the
corrosion resistance. Therefore, the maximum amount of carbon is 0.1 wt%. For
example,
the content of carbon is 0.02 ¨0.09 wt%, such as 0.02 ¨0.08 wt%, such as 0.02
¨0.07 wt%
such as 0.02 ¨ 0.06 wt% such as 0.02 ¨0.05 wt%, such as 0.01 ¨0.04 wt%.
Nitrogen (N)
Nitrogen may be present as an unavoidable impurity resulting from the
production process.
Nitrogen may also be included in the ferritic alloy as defined hereinabove or
hereinafter to
Date Regue/Date Received 2022-08-11
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increase strength by precipitation hardening, in particular when a powder
metallurgical
process route is applied. At too high levels, nitrogen may result in
difficulties to form the
alloy and also have a negative effect on the corrosion resistance. Therefore,
the maximum
amount of nitrogen is 0.1 wt%. Suitable ranges of nitrogen are for example
0.001 ¨ 0.08
wt%, such as 0.001 ¨ 0.05 wt%, such as 0.001 ¨0.04 wt%, such as 0.001 ¨ 0.03
wt%, such
as 0.001 ¨ 0.02 wt%.
Oxygen (0)
Oxygen may exist in the alloy as defined hereinabove or hereinafter as an
impurity resulting
from the production process. In that case, the amount of oxygen may be up to
0.02 wt%,
such as up to 0.005 wt%. If oxygen is added deliberately to provide strength
by dispersion
strengthening, as when manufacturing the alloy through a powder metallurgical
process
route, the alloy as defined hereinabove or hereinafter, comprises up to or
equal to 0.2 wt%
oxygen.
Chromium (Cr)
Chromium is present in the present alloy primarily as a matrix solid solution
element.
Chromium promotes the formation of the aluminium oxide layer on the alloy
through the
so-called third element effect, i.e. by formation of chromium oxide in the
transient
oxidation stage. Chromium shall be present in the alloy as defined hereinabove
or
hereinafter in an amount of at least 4 wt% to fulfill this purpose. In the
present inventive
alloy, Cr also enhances the susceptibility to form brittle a phase and Cr3Si.
This effect
emerges at around 12 wt% and is enhanced at levels above 15 wt%, therefore the
limit of
Cr is 15 wt%. Also from oxidation point of view, higher levels than 15 wt%
will result in
an undesirable contribution of Cr into the protective oxide scales. According
to one
embodiment, the content of Cr is 5 to 13 wt%, such as 5 to 12 wt%, such as 6
to 12 wt%,
such as 7 to 11 wt%, such as 8 to 10 wt%.
Aluminium (Al)
Aluminium is an important element in the alloy as defined hereinabove or
hereinafter.
Aluminium, when exposed to oxygen at high temperature, will form the dense and
thin
Date Regue/Date Received 2022-08-11
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oxide, A1203, through selective oxidation, which will protect the underlying
alloy surface
from further oxidation. The amount of aluminium should be at least 2 wt% to
ensure that a
protective surface layer containing aluminium rich oxide is formed and also to
ensure that
sufficient aluminium is present to heal the protective surface layer when
damaged.
However, aluminium has a negative impact on the formability and high amounts
of
aluminium may result in the formation of cracks in the alloy during mechanical
working
thereof. Consequently, the amount of aluminium should not exceed 6 wt%. For
example,
aluminium may be 3 ¨5 wt%, such as 2.5 ¨4.5 wt%, such as 3 to 4 wt%.
Silicon (Si)
In commercial FeCrAl alloys, silicon is often present in levels of up to 0.4
wt%. In ferritic
alloys as defined hereinabove or hereinafter, Si will play a very important
role as it has
been found to have a great effect on improving the oxidation and corrosion
resistance. The
upper limit of Si is set by the loss of workability in hot and cold condition
and increasing
susceptibility to formation of brittle Cr3Si and cs phase during long term
exposure.
Additions of Si therefore have to be performed in relation to the content of
Al and Cr. The
amount of Si is therefore between 0.5 to 3 wt%, such as 1 to 3 wt%, such as 1
to 2.5 wt%,
such as 1.5 to 2.5 wt%.
Manganese (Mn)
Manganese may be present as an impurity in the alloy as defined hereinabove or
hereinafter
up to 0.4 wt%, such as from 0 to 0.3 wt%.
Yttrium (Y)
In melt metallurgy, yttrium may be added in an amount up to 0.3 wt% to improve
the
adherence of the protective surface layer. Furthermore, in powder metallurgy,
if yttrium
is added to create a dispersion of together with oxygen and/or nitrogen, the
yttrium
content is in an amount of at least 0.04 wt%, in order to accomplish the
desired dispersion
hardening effect by oxides and/or nitrides. The maximum amount of yttrium in
dispersion
hardened alloys in the form of oxygen containing Y compounds may be up to 1.0
wt%.
Date Regue/Date Received 2022-08-11
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Scandium (Sc), Cerium (Ce) and Lanthanum (La)
Scandium, Cerium, and Lanthanum are interchangeable elements and may be added
individually or in combination in a total amount of up to 0.2 wt% to improve
oxidation
properties, self-healing of the aluminium oxide (A1203) layer or the adhesion
between the
alloy and the Al2O3 layer.
Molybdenum (Mo) and Tungsten (W)
Both molybdenum and tungsten have positive effects on the hot-strength of the
alloy as
defined hereinabove or hereinafter. Mo has also a positive effect on the wet
corrosion
properties. They may be added individually or in combination in an amount up
to 4.0 wt%,
such as from 0 to 2.0 wt%.
Reactive elements (RE)
Per definition, the reactive elements are highly reactive with carbon,
nitrogen and oxygen.
Titanium (Ti), Niobium (Nb), Vanadium (V), Hafnium (Hf), Tantalum (Ta) and
Thorium
(Th) are reactive elements in the sense that they have high affinity to
carbon, thus being
strong carbide formers. These elements are added in order to improve the
oxidation
properties of the alloy. The total amount of the elements is up to 1.0 wt%
such as 0.4 wt%,
such as up to 0.15.
The maximum amounts of respective reactive element will depend mainly on
tendency of
the element to form adverse intermetallic phases.
Zirconium (Zr)
Zirconium is often referred to as a reactive element as since it is very
reactive towards
oxygen, nitrogen and carbon. In the present alloy, it has been found that Zr
has a double
role as it will be present in the protective surface layer containing
aluminium rich oxide
thereby improving the oxidation resistance and it will also form carbides and
nitrides.
Thus, in order to achieve the best properties of the protective surface layer
containing
aluminium rich oxide, it is advantageous to include Zr in the alloy.
Date Regue/Date Received 2022-08-11
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However, Zr-levels above 0.40 wt% will have an effect on the oxidation due to
the
formation of Zr rich intermetallic inclusions and levels below 0.05 wt% will
be too small
to fulfill the dual purpose, regardless of the C and N content. Thus, the
range of Zr is
between 0.05 to 0.40 wt%, such as 0.10 to 0.35.
Furthermore, it has also been found that the relationship between Zr and N and
C may be
important in order to achieve even better oxidation resistance of the
protective surface
layer, i.e. the alumina scale. Thus, the inventor has surprisingly found that
if Zr is added
to the alloy and the alloy also comprises N and C and if the following
condition (the
element content given in weight%) is fulfilled:
4,7C+4N
-0.15 < Zr < 0.15, such as -0.15 < Zr 4,7C+4N < 0.10, such as -0.05<
Zr -
0,62 - 0,62 -4,7C+4N
< 0.10, the obtained alloy will achieve a good oxidation resistance.
0,62
The balance in the ferritic alloy as defined hereinabove or hereinafter is Fe
and
unavoidable impurities. Examples of unavoidable impurities are elements and
compounds
which have not been added on purpose, but cannot be fully avoided as they
normally
occur as impurities in e.g. the material used for manufacturing the ferritic
alloy.
Figure la and Figure lb shows that higher Cr in a Si-containing ferritic alloy
is prone to
form Si3Cr inclusions and at 20% Cr also to promote undesirable brittle a-
phase after
exposure for long time in the focus temperature area. Although diagrams are
only shown
for two Cr levels, 10 and 20%, the trend of embrittling phases increasing with
higher Cr
is clearly demonstrated Note the absence of a-phase at 10% Cr and the
increasing amount
of Cr3Si phase at higher Si content at both Cr levels. Hence, these figures
show that there
will be problems when using Cr levels around 20%.
When the terms "<" or "less than or equal to" are used in the following
context: "element
< number", the skilled person knows that the lower limit of the range is 0 wt%
unless
another number is specifically stated. Further, the undefined article "a" does
not exclude
a plurality.
Date Regue/Date Received 2022-08-11
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The present disclosure is further illustrated by the following non-limiting
examples.
Examples
Test melts were produced in a vacuum melting furnace. The compositions of the
test
melts are shown in table 1.
The obtained samples were hot rolled and machined to flat rods with a cross
section of 2
x 10 mm. They were then cut into 20 mm long coupons and ground with SiC paper
to 800
mesh for exposure to air and combustion conditions. Some of the rods were cut
to 200
mm long x 3 x12 mm rods for tensile testing at room temperature in a
Zwick/Roell Z100
tensile test apparatus.
The results from exposure and tensile tests are shown in table 1.
The samples were tested for yield and rupture stress as well as elongation to
rupture in a
standard tensile test machine and the result giving >3% elongation to rupture
is
designated "x" in "Workable" column of the table. The "x" therefore designates
an alloy
that is easily hot rolled and that shows ductile behavior at room temperature.
In the
"Oxidation" column, the "x" designates that the alloy forms a protective
alumina rich
oxide scale at 950 C in air and at 850 C with biomass ash deposit.
Table 1 ¨ Composition of the melts and the results of testing workability and
oxidation
an (x) designates a value between 3 and 6% elongation.
Composition/ Cr Al Si C N Zr Workable Oxidation
Melt-number
4785 5.2 4.0 0.03 0.020 0.012
0.296 x No
Comparative
4784 5.2 6.0 0.02 0.025 0.012
0.297 x No
Comparative
Date Regue/Date Received 2022-08-11
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4783 5.2 3.9 1.96 0.021 0.010 0.292 x X
(disclosure)
4782 10.0 2.0 0.02 0.025 0.014 0.273 x No
Comparative
4781 10.0 3.0 0.03 0.025 0.021 0.296 x No
Comparative
4780 10.1 4.0 0.02 0.021 0.015 0.296 x No
Comparative
4779 10.1 4.0 1.91 0.022 0.013 0.296 x X
(disclosure)
4778 10.2 5.9 0.11 0.018 0.012 0.294 x No
Comparative
4777 20.0 4.0 0.02 0.018 0.020 0.295 Failed in No
Comparative rolling
4776 20.1 4.0 0.04 0.014 0.296 x No
Comparative
4774 20.2 5.1 0.05 0.014 0.009 <0.01 x No
Comparative
4773 19.7 4.8 0.02 0.004 <0.01 <0.01 x No
Comparative
4772 12.2 3.6 2.5 0.003 <0.01 0.237 Failed in No
comparative rolling
4799 20.0 2.8 1.87 0.023 0.017 0.281 x No
Comparative
4800 14.9 3.0 1.9 0.022 0.013 0.296
(disclosure)
4855 10.1 3.8 1.96 0.019 0.012 0.279
(disclosure)
4856 10.0 5.0 2.0 0.015 0.012 0.285 Failed in No
Comparative rolling
Date Recue/Date Received 2022-08-11
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4857 10.0 3.1 1.97 0.025 0.015
0.297
(disclosure)
4858 14.7 3.9 2.01 0.022 0.015
0.292
(disclosure)
4859 12.1 4.0 2 0.024 0.014 0.289 X
(disclosure)
4860 12.0 3.1 1.98 0.016 0.014
0.284 .. X
(disclosure)
4861 10.0 4.0 1.99 0.015 0.015
0.29 X
(disclosure)
Thus, as can be seen from the table above, the alloys of the present
disclosure shows
good workability and good oxidation performance.
Figures 2 a) to e) disclose samples which are polished sections of of the
present
disclosure (figures 2a) 4783 and 2b) 4779) compared to three comparative
alloys after
exposure to 50 times 1 hour cycles at 850 C exposed to biomass (wood pellets)
ash
containing large amounts of potassium. The micrographs are taken in a JEOL FEG
SEM
at 1000 times magnification and show a clear advantage in behavior between the
alloys of
the present disclosure and reference materials. As can be seen, on the alloys
of present
disclosure, a 3-4 gm thin and protective alumina scale (aluminium oxide layer)
has been
formed, whereas a thicker and less protective chromia (chromium oxide) rich
scale is
formed on the stainless steel (2c - 11Ni, 21Cr, N, Ce, Fe bal.) and Ni-base
alloy (2e -
Inconel 625: 58Ni, 21Cr, 0.4A1, 0.5Si, Mo, Nb, Fe), and a relatively porous
and not as
protective alumina scale forms on the comparative FeCrAl alloy (alloy 4776)
(figure 2d -
20Cr, 5A1, 0.04 Si, Fe bal).
As can be seen from figures 2a - e, the addition of Si, Al and Cr according to
the ranges
according to the present disclosure will promote alumina scale formation at Al
levels as
low as about 2 wt% and at chromium levels as low as 5 wt%.
Date Regue/Date Received 2022-08-11
