U.S. patent application number 17/020919 was filed with the patent office on 2020-12-31 for ferritic alloy.
This patent application is currently assigned to Sandvik Intellectual Property AB. The applicant listed for this patent is Sandvik Intellectual Property AB. Invention is credited to Jesper EJENSTAM, Susanne HELLSTROM SELIN, Bo JONSSON, Fernando RAVE, Peter SZAKALOS.
Application Number | 20200407831 17/020919 |
Document ID | / |
Family ID | 1000005087074 |
Filed Date | 2020-12-31 |
United States Patent
Application |
20200407831 |
Kind Code |
A1 |
RAVE; Fernando ; et
al. |
December 31, 2020 |
FERRITIC ALLOY
Abstract
A ferritic alloy comprising in weight %: C: 0.01-0.1; N:
0.001-0.1; O: .ltoreq.0.2; B: .ltoreq.0.01; Cr: 9.0-13.0; Al:
2.5-8.0; Si: .ltoreq.0.5; Mn: .ltoreq.0.4; Y: .ltoreq.2.2;
Sc+Ce+La: .ltoreq.0.2; Mo+W: .ltoreq.4.0; Ti: .ltoreq.1.7; Zr:
.ltoreq.3.3; Nb: .ltoreq.3.3; V: .ltoreq.1.8; Hf+Ta+Th:
.ltoreq.6.5; the balance being Fe and unavoidable impurities,
wherein, the amounts of Ti+Zr+Nb+V+Hf+Ta+Th and C, N and O are
balanced such that: at % Ti + at % Zr + at % Nb + at % V + at % Hf
+ at % Ta + at % Th - x * at % O - at % N at % C .gtoreq. 1
##EQU00001## wherein x is 0.5 unless the content of Y is more than
or equal to 0.01 wt % then x is 0.67.
Inventors: |
RAVE; Fernando; (Vasteras,
SE) ; SZAKALOS; Peter; (Stockholm, SE) ;
JONSSON; Bo; (Vasteras, SE) ; EJENSTAM; Jesper;
(Solna, SE) ; HELLSTROM SELIN; Susanne; (Vasteras,
SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sandvik Intellectual Property AB |
Sandviken |
|
SE |
|
|
Assignee: |
Sandvik Intellectual Property
AB
Sandviken
SE
|
Family ID: |
1000005087074 |
Appl. No.: |
17/020919 |
Filed: |
September 15, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15534566 |
Jun 9, 2017 |
10815554 |
|
|
PCT/EP2015/079457 |
Dec 11, 2015 |
|
|
|
17020919 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 33/0285 20130101;
C22C 38/04 20130101; C22C 1/02 20130101; C22C 38/002 20130101; C22C
38/02 20130101; C22C 38/22 20130101; C22C 38/24 20130101; Y02E
30/30 20130101; C22C 38/001 20130101; C22C 38/28 20130101; G21C
15/28 20130101; C22C 38/005 20130101; C22C 38/26 20130101; C22C
38/06 20130101; C22C 38/32 20130101 |
International
Class: |
C22C 38/28 20060101
C22C038/28; G21C 15/28 20060101 G21C015/28; C22C 38/00 20060101
C22C038/00; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/06 20060101 C22C038/06; C22C 38/22 20060101
C22C038/22; C22C 38/24 20060101 C22C038/24; C22C 38/26 20060101
C22C038/26; C22C 1/02 20060101 C22C001/02; C22C 33/02 20060101
C22C033/02; C22C 38/32 20060101 C22C038/32 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 11, 2014 |
EP |
14197362.8 |
Claims
1. A ferritic alloy comprising in weight %: C: 0.01-0.1; N:
0.001-0.1; B: 0.01; Cr: 9.0-11.5; Al: 2.5-8.0; Si: <0.5; Mn:
<0.4; Sc+Ce+La: .ltoreq.0.2; Mo+W: .ltoreq.4.0; Ti: 0.02-1.3; Y
0-1.2; Zr: 0-2.4; Nb: 0-2.4; V: <1.8; Hf+Ta+Th: <6.5; the
balance being Fe and unavoidable impurities, wherein the amounts of
Ti+Zr+Nb+V+Hf+Ta+Th and C, N and O are balanced such that: 1.2
.ltoreq. at % Ti + at % Zr + at % Nb + at % V + at % Hf + at % Ta +
at % Th - x * at % O - at % N at % C .ltoreq. 2.0 ##EQU00008##
wherein x is 0.5 unless the content of Y is more than or equal to
0.01 wt % then x is 0.67, and wherein the ferritic alloy includes
dispersoids of one or more of ZrC, NbC, Nb.sub.2C, HfC, VC, ThC,
TaC, or combinations thereof.
2. The ferritic alloy according to claim 1, wherein the ferritic
alloy comprises no added wt % Sc+Ce+La or no added wt % Ce+La.
3. The ferritic alloy according to claim 1, wherein the ferritic
alloy comprises Cr in the range of from 9.0 to 11.0 wt %.
4. The ferritic alloy according to claim 1, wherein N is in the
range of from 0.001-0.08 wt %.
5. The ferritic alloy according to claim 4, wherein N is in the
range of from 0.001-0.05 wt %.
6. The ferritic alloy according to claim 1, wherein 0 is in the
range of from 0.001-0.1 wt %.
7. The ferritic alloy according to claim 6, wherein 0 is in the
range of from 0.001-0.08 wt %.
8. The ferritic alloy according to claim 1, wherein C is in the
range of from 0.03-0.04 wt %.
9. The ferritic alloy according to claim 1, wherein Y is in the
range of from 0-1.0 wt %.
10. The ferritic alloy according to claim 1, wherein Y is in the
range of from 0.01-1.2 wt %.
11. The ferritic alloy according to claim 1, wherein Zr+Nb is in
the range of 0.04-4.6 wt %.
12. The ferritic alloy according to claim 11, wherein Zr+Nb is in
the range of 0.04-1.7 wt %.
13. The ferritic alloy according to claim 1, wherein Ti+Nb is in
the range of 0.04-3.1 wt %.
14. The ferritic alloy according to claim 13, wherein Ti+Nb is in
the range of 0.04-1.7 wt %.
15. The ferritic alloy according to claim 1, wherein Ti+Zr is in
the range of 0.04-3.1 wt %.
16. The ferritic alloy according to claim 1, wherein Al is in the
range of from 3.0-7.0 wt %.
17. The ferritic alloy according to claim 16, wherein Al is in the
range of from 4.0-6.0 wt %.
18. The ferritic alloy according to claim 1, wherein Zr is in the
range of from 0.3-1.4 wt %.
19. The ferritic alloy according to claim 1, wherein Nb is in the
range of 0.08-1.4 wt %.
20. The ferritic alloy according to claim 1, wherein Zr is in the
range of from 0.1-0.9 wt % and wherein Nb is in the range of
0.08-1.4 wt %
21. The ferritic alloy according to claim 1, wherein the ferritic
alloy includes dispersoids, wherein the dispersoids are in the form
of a cluster including a TiC dispersoid core enclosed by
dispersoids of ZrC, NbC, Nb.sub.2C, HfC, VC, ThC, TaC, or
combinations thereof.
22. The ferritic alloy according to claim 1, wherein Ti is in the
range of from 0.07-0.09 wt %.
23. The ferritic alloy according to claim 1, wherein Ti is in the
range of from 0.07-0.09 wt % and C is in the range of from 0.03 to
0.04 wt. %.
24. A method of improving corrosion resistance, comprising: forming
an object from the ferritic alloy according to claim 1, wherein, in
operation, the object is exposed to a temperature range of
300-800.degree. C.
25. The method of claim 24, wherein the object is selected from the
group consisting of a boiler and a furnace of a heat generating
plant or of an energy generating plant.
26. The method of claim 24, wherein the object is selected from the
group consisting of an overlay welding, a spray coating and a
composite tube.
27. A ferritic alloy comprising in weight %: C: 0.01-0.1; N:
0.001-0.05; O: 0.001-0.1; B: .ltoreq.0.01; Cr: 9.0-11.0; Al:
4.0-6.0; Si: <0.5; Mn: <0.4; Sc+Ce+La: .ltoreq.0.2; Mo+W:
.ltoreq.4.0; Ti: 0.02-1.3; Y 0-1.2; Zr: 0.3-1.4; Nb: 0.08-1.4; V:
<1.8; Hf+Ta+Th: <6.5; the balance being Fe and unavoidable
impurities, wherein at least one of Zr+Nb is in the range of
0.08-2.8 wt %, Ti+Nb is in the range of 0.12-1.7 wt %, and Ti+Zr is
2.0 wt %, wherein the amounts of Ti+Zr+Nb+V+Hf+Ta+Th and C, N and O
are balanced such that: 1.2 .ltoreq. at % Ti + at % Zr + at % Nb +
at % V + at % Hf + at % Ta + at % Th - x * at % O - at % N at % C
.ltoreq. 2.0 ##EQU00009## wherein x is 0.5 unless the content of Y
is more than or equal to 0.01 wt % then x is 0.67, and wherein the
ferritic alloy includes dispersoids of one or more of ZrC, NbC,
Nb.sub.2C, HfC, VC, ThC, TaC, or combinations thereof.
Description
RELATED APPLICATION DATA
[0001] This application is a continuation application of U.S.
application Ser. No. 15/534,566, filed Jun. 9, 2017, which is a
.sctn. 371 National Stage Application of PCT International
Application No. PCT/EP2015/079457, filed Dec. 11, 2015, claiming
priority of EP Application No. 14197362.8, filed Dec. 11, 2014, the
entire contents of each are incorporated herein by reference.
TECHNICAL FIELD
[0002] The present disclosure relates to a ferritic alloy
(FeCrAl-alloy) and to the use of the ferritic alloy in the
temperature range of 300-800.degree. C.
BACKGROUND ART
[0003] In so-called lead-cooled fast reactor (LFR) systems liquid
lead is used as a cooling agent. Liquid lead and lead bismuth
eutectic (LBE) provides the possibility of passive cooling and thus
contributes in making nuclear power safe. However, liquid lead is
highly corrosive and put high demands on the corrosion resistance
of the construction materials used in the LFR systems.
[0004] FeCrAl-alloys have recently been proposed as a candidate
material for use in lead-cooled fast reactors due to the good
oxidation properties of these alloys, which is a result of the
formation of a layer of alumina (Al.sub.2O.sub.3) on the alloy
surfaces. FeCrAl-alloys are based on aluminum, chromium and iron
and due to their oxidation resistance and good creep resistance
these alloys are commonly used in heating elements and wires at
temperatures at around and in excess of 1000.degree. C. However,
lead-cooled fast reactors operate in the temperature interval of
400-600.degree. C. and in this temperature range, commercial
FeCrAl-alloys that typically contains around 15-20 wt % Cr suffer
from .alpha.-.alpha.' phase separation due to a miscibility gap
that is present in the Fe--Cr system. The .alpha.-.alpha.' phase
separation results in embrittlement of the FeCrAl-alloy and this
makes the commercial FeCrAl-alloys unsuitable as construction
materials in the temperature range used in (LFR) systems.
[0005] FeCrAl compositions have been tested in liquid lead or
liquid lead-bismuth-eutectic solution. In a study by Weisenburger
et al, it is shown that model FeCrAl-alloys containing .gtoreq.12.5
wt % Cr and .gtoreq.6 wt % Al were able to form thin protective
alumina scales in the temperature interval of 400-600.degree. C.
[Weisenburger, Jianu, Doyle, Bruns, Fetzer, Heinzel, DelGiacco, An,
Muller, "Oxide scales formed on Fe--Cr--Al-based model alloys
exposed to oxygen containing molten lead", Journal of Nuclear
Materials 437 (2013) 282-292]. Another work by Lim et al, showed
that a Fe-13Cr-4Al alloy would be able to form protective alumina
from 500.degree. C. [Lim, Hwang, Kim, "Design of alumina forming
FeCrAl steels for lead or lead-bismuth cooled fast reactors",
Journal of Nuclear Materials 441 (2013) 650-660].
[0006] Korean patent KR 10-1210531 shows a FeCrAl-alloy which is
intended for use in nuclear power applications.
[0007] FeCrAl-alloys may also comprise reactive element (RE), which
are elements with high affinity to oxygen and carbon. At high
temperatures, approx. 1000.degree. C., additions of reactive
element such as Y, Zr and Hf improve the alloys oxidation
properties by balancing the outward diffusion of metal ions and the
inward diffusion of oxygen, which leads to a balanced oxide growth
and thus a reduction of mechanical stresses and/or porosity in the
oxide scale. Studies have been made on the influence of Zr- and Ti
additions on the long-term (10,000 h) corrosion resistance of
Fe-10Cr-6Al alloys in liquid lead at 550.degree. C. [Ejenstam et
al, "Oxidation studies of Fe10CrAl-RE alloys exposed to Pb at
550.degree. C. for 10,000 h", Journal of Nuclear Materials 443
(2013) 161-170].
[0008] However, although the research disclosed above has
contributed to improvements of the properties of FeCrAl-alloys,
there is still a need to increase the corrosion resistance of these
alloys further.
SUMMARY
[0009] Thus, it is an aspect of the present disclosure to provide a
FeCrAl-alloy having improved corrosion resistance. In particular,
it is an aspect of the present disclosure to provide a FeCrAl-alloy
having very good corrosion resistance at temperatures in the range
of 300-800.degree. C. in highly corrosive environments. Moreover,
it is an aspect of the present disclosure to provide a FeCrAl-alloy
which is resistance to corrosion in liquid lead alloys at
temperatures in the range of 300-800.degree. C. A further aspect of
the present disclosure is to provide a FeCrAl-alloy which is
suitable as construction material in applications such as boilers,
furnaces and heat and energy generating plants and processes. Other
suitable applications include materials for overlay welding, spray
coating or composite tubes. In particular, it is an aspect of the
present disclosure is to provide a FeCrAl-alloy which is suitable
as construction material in lead-cooled fast reactor system at a
temperature range of 300-800.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic drawing showing the purpose of
balancing RE and carbon.
[0011] FIGS. 2a and 2b are micrographs showing chromium-rich
carbide formed in a comparative alloy.
[0012] FIG. 3a is a SEM image and FIG. 3b is a micrograph thereof
showing oxidation behavior in comparative alloys.
[0013] FIG. 4a is a SEM image and FIG. 4b is a micrograph thereof
showing oxidation behavior in an alloy according to a first
alternative of the disclosure.
[0014] FIG. 5a is a SEM image and FIG. 5b is a micrograph thereof
showing oxidation behavior in an alloy according to a second
alternative of the disclosure.
DETAILED DESCRIPTION
[0015] The present disclosure therefore relates to a ferritic
(FeCrAl)-alloy comprising in weight % (wt %):
[0016] C: 0.01-0.1;
[0017] N: 0.001-0.1;
[0018] O: .ltoreq.0.2;
[0019] B: 0.01;
[0020] Cr: 9-13;
[0021] Al: 2.5-8;
[0022] Si: .ltoreq.0.5;
[0023] Mn: .ltoreq.0.4;
[0024] Y: .ltoreq.2.2;
[0025] Sc+Ce+La: .ltoreq.0.2;
[0026] Mo+W: .ltoreq.4.0;
[0027] Ti: .ltoreq.1.7;
[0028] Zr: .ltoreq.3.3;
[0029] Nb: .ltoreq.3.3;
[0030] V: .ltoreq.1.8;
[0031] Hf+Ta+Th: .ltoreq.6.5;
the balance being Fe and unavoidable impurities, wherein, the
amounts of Ti+Zr+Nb+Hf+V+Ta+Th and C, N and O are balanced such
that:
at % Ti + at % Zr + at % Nb + at % V + at % Hf + at % Ta + at % Th
- x * at % O - at % N at % C .gtoreq. 1 ##EQU00002##
wherein x is 0.5 unless the content of Y is more than or equal to
0.01 wt % then x is 0.67.
[0032] The present disclosure also relates to a ferritic alloy
comprising in weight %:
[0033] C: 0.01-0.1;
[0034] N: 0.001-0.1;
[0035] O: .ltoreq.0.2;
[0036] B: .ltoreq.0.01;
[0037] Cr: .ltoreq.9-11.5;
[0038] Al: .ltoreq.2.5-8;
[0039] Si: .ltoreq.0.5;
[0040] Mn: .ltoreq.0.4;
[0041] Y: .ltoreq.2.2;
[0042] Sc+Ce+La: .ltoreq.0.2;
[0043] Mo+W: .ltoreq.4.0;
[0044] Ti: .ltoreq.1.7;
[0045] Zr: .ltoreq.3.3;
[0046] Nb: .ltoreq.3.3;
[0047] V: .ltoreq.1.8;
[0048] Hf+Ta+Th: .ltoreq.6.5;
the balance being Fe and unavoidable impurities, wherein, the
amounts of Ti+Zr+Nb+Hf+V+Ta+Th and C, N and O are balanced such
that:
at % Ti + at % Zr + at % Nb + at % V + at % Hf + at % Ta + at % Th
- x * at % O - at % N at % C .gtoreq. 1 ##EQU00003##
wherein x is 0.5 unless the content of Y is more than or equal to
0.01 wt % then x is 0.67.
[0049] The present disclosure also relates to a ferritic alloy
comprising in weight %:
[0050] C: 0.01-0.1;
[0051] N: 0.001-0.1;
[0052] O: .ltoreq.0.2;
[0053] B: .ltoreq.0.01;
[0054] Cr: 9-11;
[0055] Al: 2.5-8;
[0056] Si: .ltoreq.0.5;
[0057] Mn: .ltoreq.0.4;
[0058] Y: .ltoreq.2.2;
[0059] Sc+Ce+La: .ltoreq.0.2;
[0060] Mo+W: .ltoreq.4.0;
[0061] Ti: .ltoreq.1.7;
[0062] Zr: .ltoreq.3.3;
[0063] Nb: .ltoreq.3.3;
[0064] V: .ltoreq.1.8;
[0065] Hf+Ta+Th: .ltoreq.6.5;
the balance being Fe and unavoidable impurities, wherein, the
amounts of Ti+Zr+Nb+Hf+V+Ta+Th and C, N and O are balanced such
that:
at % Ti + at % Zr + at % Nb + at % V + at % Hf + at % Ta + at % Th
- x * at % O - at % N at % C .gtoreq. 1 ##EQU00004##
wherein x is 0.5 unless the content of Y is more than or equal to
0.01 wt % then x is 0.67.
[0066] The present disclosure further relates to a ferritic alloy
comprising in weight %:
[0067] C: 0.01-0.1;
[0068] N: 0.001-0.1;
[0069] O: .ltoreq.0.2;
[0070] B: .ltoreq.0.01;
[0071] Cr: 9-13;
[0072] Al: 2.5-8;
[0073] Si: .ltoreq.0.5;
[0074] Mn: .ltoreq.0.4;
[0075] Y: .ltoreq.2.2;
[0076] Mo+W: .ltoreq.4.0;
[0077] Sc: .ltoreq.0.2;
[0078] Ti: .ltoreq.1.7;
[0079] Zr: .ltoreq.3.3;
[0080] Nb: .ltoreq.3.3;
[0081] V: .ltoreq.1.8;
[0082] Hf+Ta+Th: .ltoreq.6.5;
the balance being Fe and unavoidable impurities, wherein, the
amounts of Ti+Zr+Nb+Hf+V+Ta+Th and C, N and O are balanced such
that:
at % Ti + at % Zr + at % Nb + at % V + at % Hf + at % Ta + at % Th
- x * at % O - at % N at % C .gtoreq. 1 ##EQU00005##
wherein x is 0.5 unless the content of Y is more than or equal to
0.01 wt % then x is 0.67.
[0083] The present disclosure further relates to a ferritic alloy
comprising in weight %:
[0084] C: 0.01-0.1;
[0085] N: 0.001-0.1;
[0086] O: .ltoreq.0.2;
[0087] B: .ltoreq.0.01;
[0088] Cr: 9-13;
[0089] Al: 2.5-8;
[0090] Si: .ltoreq.0.5;
[0091] Mn: .ltoreq.0.4;
[0092] Y: .ltoreq.2.2;
[0093] Mo+W: .ltoreq.4.0;
[0094] Ti: .ltoreq.1.7;
[0095] Zr: .ltoreq.3.3;
[0096] Nb: .ltoreq.3.3;
[0097] V: .ltoreq.1.8;
[0098] Hf+Ta+Th: .ltoreq.6.5;
the balance being Fe and unavoidable impurities, wherein, the
amounts of Ti+Zr+Nb+Hf+V+Ta+Th and C, N and O are balanced such
that:
at % Ti + at % Zr + at % Nb + at % V + at % Hf + at % Ta + at % Th
- x * at % O - at % N at % C .gtoreq. 1 ##EQU00006##
wherein x is 0.5 unless the content of Y is more than or equal to
0.01 wt % then x is 0.67.
[0099] The alloy of the present disclosure further comprises
chromium and reactive elements such as Ti, Zr, Nb, Hf, V, Ta and
Th. The reactive elements may be present individually in the alloy
or in any combination. As will be explained more in detail below,
these elements are added to the alloy for improving the formation
of the protective Al.sub.2O.sub.3 layer or its properties.
FeCrAl-alloys typically contain carbon. Carbon may have been added
deliberately, for example to increase strength, or may exist in the
alloy as an incidental impurity resulting from the production
process, for instance. These reactive elements, such as Ti, Zr, Nb,
Hf, V, Ta and Th, are also strong carbide formers, i.e. their
affinity to carbon is high and thus carbon present in the alloy
will be drawn to chromium or the reactive elements and form
carbides.
[0100] Through experiments, the inventors have surprisingly found
that the oxidation resistance of a FeCrAl-alloy is greatly improved
when the amounts, in atom percent, of the reactive elements are
balanced with the amount, in atom percent, of carbon in the alloy
such that the quotient between reactive elements and carbon is
equal to or greater than 1.
[0101] The mechanism behind the disclosure is more easily
understood when a corrosion study is made in an alloy in which
there is a deficit of reactive elements in relation to carbon.
[0102] FIG. 1 shows schematically a FeCrAl-alloy in which the
quotient between reactive elements and carbon is close to or less
than 1.
[0103] During solidification after casting of the alloy, the
reactive elements (RE) initially form carbides and nitrides in the
bulk of the alloy.
[0104] According to the disclosure, the amounts, in atom percent,
of the reactive elements are balanced with the amount, in atom
percent, of carbon in the alloy such that the quotient between
reactive elements available to form carbides and available carbon
is equal to or greater than 1. This will result in that at least an
equal amount of atoms of the available reactive elements and
available carbon atoms will be present in the alloy as defined
hereinabove or hereinafter. Since the reactive elements are the
strongest carbide formers in the alloy, stronger than chromium and
the other alloy constituents, all free carbon will be drawn to the
available reactive elements and form carbides therewith. Hence,
there will be no free carbon left in the alloy to form
chromium-rich carbides as long as the content of available reactive
elements is sufficient.
[0105] An additional positive effect is that the formed carbides of
the reactive elements are homogenously distributed throughout the
bulk of FeCrAl-alloy as defined hereinabove or hereinafter and thus
contribute to the mechanical strength of the alloy by dispersion
hardening.
[0106] According to one embodiment of the FeCrAl alloy as defined
hereinabove or hereinafter, the quotient between available reactive
elements and carbon is greater than or equal to 1 i.e. there is a
surplus of reactive elements in the alloy. The reason therefore is
that some reactive elements will be consumed in the process of
formation of the protective Al.sub.2O.sub.3 layer and contribute to
increased adhesion between the Al.sub.2O.sub.3 layer and the alloy
surface. By balancing the amount of reactive elements so that there
is a surplus thereof after reacting with carbon and nitrogen, the
formation of even minute amounts of chromium rich carbides are
avoided.
[0107] According to another embodiment, the lowest quotient for the
FeCrAl alloy as defined hereinabove or hereinafter is at least or
equal to 1.1. This quotient means that there will be at least
enough reactive elements present in the alloy after formation of
the Al.sub.2O.sub.3 layer to consume all free carbon.
[0108] The highest possible quotient between the reactive elements
and carbon is determined by the stability of the intermetallics
which the reactive elements may form in the alloy. According to
embodiment of the present FeCrAl alloy, the highest quotient may be
2.3 or lower.
[0109] The quotient between reactive elements and carbon may also
be 1.1-2.2, such as 1.2-2.0.
[0110] The ferritic alloy as defined hereinabove or hereinafter may
be manufactured by conventional steel manufacturing methods, so
called melt metallurgy, including: melting in an inductive furnace,
ladle refining of the melt by casting in moulds followed by hot
rolling. The ferritic alloy as defined hereinabove or hereinafter
may also be manufactured by powder metallurgy, in this case
including the steps of producing metal powder by atomizing,
compacting the powder and followed by sintering or Hot Isostatic
Pressing.
[0111] The constituents of the alloy as defined hereinabove or
hereinafter will in the following be described in detail.
[0112] The balance in FeCrAl 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
FeCrAl alloy.
[0113] When the term ".ltoreq." is 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.
[0114] Carbon (C)
[0115] Carbon is included in the FeCrAl alloy as defined
hereinabove or hereinafter to increase strength by precipitation
hardening. Carbon may also be present as an unavoidable impurity
resulting from the production process. To achieve sufficient
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 a negative effect on the
corrosion resistance. Therefore, the maximum amount of carbon is
0.1 wt % in the alloy as defined hereinabove or hereinafter. For
example 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 %.
[0116] Nitrogen (N)
[0117] Nitrogen may be included in the FeCrAl alloy as defined
hereinabove or hereinafter to increase strength by precipitation
hardening. Nitrogen may also be present as an unavoidable impurity
resulting from the production process. At too high levels, nitrogen
may result in difficulties to form the material and may have a
negative effect on the corrosion resistance. Therefore, the maximum
amount of nitrogen is 0.1 wt % in the FeCrAl alloy as defined
hereinabove or hereinafter. To achieve sufficient precipitation
hardening in melt metallurgy, nitrogen should be at least 0.001 wt
%, examples of suitable ranges of nitrogen is 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 %, such as
0.001-0.01 wt %. In powder metallurgy, the nitrogen content may be
at least 0.01 wt %. For example, in powder metallurgy nitrogen is
0.01-0.1 wt %, such as 0.01-0.08 wt %.
[0118] Oxygen (O)
[0119] Oxygen may exist in the ferritic alloy as defined
hereinabove or hereinafter as an impurity resulting from the
production process in amounts up to 0.02 wt %, such as up to 0.005
wt %. In melt metallurgy, oxygen may be 0.001-0.08 wt %, such as
0.001-0.05 wt %, such as 0.001-0.02 wt %. In powder metallurgy,
oxygen may be deliberately added to achieve a precipitation
hardening effect. The alloy as defined hereinabove or hereinafter
then comprises up to 0.2 wt % oxygen, such as 0.01-0.2 wt % oxygen,
such as 0.01-0.1 wt %, such as 0.01-0.08 wt %.
[0120] Chromium (Cr)
[0121] Chromium promotes the formation of the Al.sub.2O.sub.3 layer
on the alloy as defined hereinabove or hereinafter 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 9 wt %. However, as the alloy as defined hereinabove or
hereinafter is intended for use in the temperature range of
300-800.degree. C., chromium should not exceed 13 wt % in order to
avoid the miscibility gap in the Fe--Cr system where
.alpha.-.alpha.' phase separation results in embrittlement of the
FeCrAl-alloy. For example, chromium may be 9-13 wt %, such as 9-12
wt %, such as 9-11 wt %, such as 9-10 wt %. According to one
specific embodiment, Cr is in the range of 9-11 wt %, such as 9 to
10 wt %. According to yet one specific embodiment, Cr is in the
range of 9 to 11.5 wt %.
[0122] Aluminum (Al)
[0123] Aluminum is an important element in the alloy as defined
hereinabove or hereinafter as aluminum, when exposed to oxygen at
high temperature, will form the dense and thin oxide
Al.sub.2O.sub.3, which will protect the underlying alloy surface
from further oxidation. The amount of aluminum should be at least
2.5 wt % to ensure that the Al.sub.2O.sub.3 layer is formed and
that sufficient aluminum is present to heal the Al.sub.2O.sub.3
layer when damaged. However, aluminum has a negative impact on the
formability of the alloy and the amount of aluminum should not
exceed 8 wt % in the alloy as defined hereinabove or hereinafter.
For example, aluminum may be 3-7 wt %, such as 3-5 wt %, such as
3.5-6 wt %, such as 4 to 6 wt %.
[0124] Silicon (Si)
[0125] Silicon may be present as an impurity in the alloy as
defined hereinabove or hereinafter up to 0.5 wt %, such as from 0
to 0.4 wt %
[0126] Manganese (Mn)
[0127] 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 %
[0128] Yttrium (Y)
[0129] Yttrium may be added in an amount up to 0.8 wt % to improve
the adherence of the Al.sub.2O.sub.3 layer, such as amounts up to
0.3 wt %, such as up to 0.1 wt %. However, if yttrium is added to
the ferritic alloy as defined hereinabove or hereinafter in amounts
of more than or equal to 0.01 wt %, then the formation of yttrium
oxides will affect the quotient and then x is 0.67. Furthermore,
when using powder metallurgy, if yttrium is added, the yttrium
content is present in an amount of at least 0.01 wt %, in order to
accomplish a desired dispersion hardening effect by oxides and/or
nitrides. The maximum amount of yttrium in dispersion hardened
alloys may be up to 2.2 wt %, such as up to 1.2 wt %, such as up to
1 wt %. Examples on suitable ranges are: 0.01 to 1.2 wt %, 0.01 to
1 wt % and 0.04 to 1.
[0130] Scandium (Sc), Cerium (Ce) and Lanthanum (La)
[0131] 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 Al.sub.2O.sub.3 layer or the adhesion between the alloy and the
Al.sub.2O.sub.3 layer. According to one specific embodiment, the
ferritic alloy as defined hereinabove or hereinafter, does not
comprise any added Sc, Ce and La, i.e. zero wt % purposely added
Sc, Ce and/or La. According to yet one specific embodiment, the
ferritic alloy as defined hereinabove or hereinafter, does not
comprise any added Ce and/or La.
[0132] Molybdenum (Mo) and Tungsten (W)
[0133] Both molybdenum and tungsten have positive effects on the
hot-strength of the alloy as defined hereinabove or hereinafter and
may be added individually or in combination in an amount up to 4.0
wt %, such as from 0 to 2.0 wt %.
[0134] Reactive Elements (RE)
[0135] Per definition, the reactive elements are highly reactive
with carbon, nitrogen and oxygen. Titanium (Ti), Zirconium (Zr),
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 as defined hereinabove or hereinafter by balancing the
diffusion of metal ions and oxygen, which will govern the kinetics
of the oxide growth process. The maximum amounts of respective
reactive element will depend mainly on tendency of the element to
form adverse intermetallic phases. Yttrium is usually considered to
be a reactive element but is in the present disclosure discussed in
a separated paragraph and is also not a part of the quotient
(except that it will affect which number x should have) because it
will not form as strong carbides as the other reactive elements
mentioned above.
[0136] Therefore, the maximum amount of titanium is 1.7 wt % in the
alloy as defined hereinabove or hereinafter, such as 0.02-1.7 wt %.
In melt metallurgy, suitable amounts of titanium are 0.02-1.3 wt %,
such as 0.02-0.98 wt %, such as 0.02-0.85 wt %, such as 0.04-0.75
wt %. In powder metallurgy, suitable amounts of titanium are
0.02-1.3 wt %, such as 0.04-0.75 wt %, such as 0.05-0.75 wt %.
[0137] The maximum amount of zirconium and niobium is 3.3 wt % in
the alloy as defined hereinabove or hereinafter respectively. The
amounts of zirconium and niobium may be of from 0.04-3.3 wt %
respectively. In melt metallurgy, zirconium may be 0.04-2.4 wt %,
such as 0.04-1.9 wt %, such as 0.04-1.6 wt %, such as 0.08-1.4 wt
%, such as 0.1 to 0.9 wt %, such as 0.1 to 0.6 wt %. In powder
metallurgy, zirconium may be 0.04-2.4 wt %, such as 0.08-1.4 wt %,
such as 0.3-1.4 wt %, such as 0.1 to 0.9 wt %, such as 0.1 to 0.6.
In melt metallurgy, niobium may be 0.04-2.4 wt %, such as 0.04-1.9
wt %, such as 0.04-1.6 wt %, such as 0.08-1.4 wt %. In powder
metallurgy, niobium may be 0.04-2.4 wt %, such as 0.08-1.4 wt %,
such as 0.08-1.2 wt %.
[0138] As an example, an alloy may include Ti+Zr in an amount of
0.04-3.1 wt %. An alloy for suitable melt metallurgy may include
Ti+Zr in amounts of 0.06-2.0 wt %, such as 0.12-1.7 wt %. An alloy
suitable for powder metallurgy may include Ti+Zr in amounts of
0.04-2.3 wt %, such as 0.06-2.0 wt %.
[0139] Other Examples are:
[0140] An alloy may include Ti+Nb in amounts of 0.04-3.1 wt %. An
alloy suitable for melt metallurgy may include Ti+Nb in amounts of
0.06-2.0 wt %, such as 0.12-1.7 wt %. An alloy suitable for powder
metallurgy may include Ti+Nb in amounts of 0.04-2.3 wt %, such as
0.06-2.0 wt %.
[0141] An alloy may include Zr+Nb in amounts of 0.04-4.6 wt %. An
alloy for melt metallurgy suitable may include Zr+Nb in amounts of
0.08-2.8 wt %, such as 0.16-2.5 wt %. An alloy for suitable powder
metallurgy may include Zr+Nb in amounts of 0.04-3.3 wt %, such as
0.08-2.8 wt %.
[0142] The maximum amount of vanadium is 1.8 wt %;
[0143] Hafnium, tantalum and thorium are interchangeable elements
and may be added to the alloy as defined hereinabove or hereinafter
individually or in combination in a total amount of up to 6.5 wt
%.
[0144] Iron (Fe) and unavoidable impurities make up the balance in
the alloy as defined hereinabove or hereinafter.
[0145] The quotient between available reactive elements and oxygen,
nitrogen and carbon:
at % Ti + at % Zr + at % Nb + at % V + at % Hf + at % Ta + at % Th
- x * at % O - at % N at % C ##EQU00007##
[0146] In the alloy as defined hereinabove or hereinafter, the
amount of each reactive element is balanced to the amount of carbon
with regards to the type of carbides that are thermodynamically
stable under the conditions that prevail during the manufacturing
of the alloy.
[0147] Under the thermodynamic conditions that prevail during the
manufacturing of the alloy, the reactive elements form the
following carbides:
Ti+C.revreaction.TiC
Nb+C.revreaction.NbC
Zr+C.revreaction.ZrC
Hf+C.revreaction.HfC
V+C.revreaction.VC
[0148] Under some circumstances the reactive elements Nb and V may
also form lower carbides:
2V+C.revreaction.V.sub.2C
2Nb+C.revreaction.Nb.sub.2C
However, at the thermodynamic conditions that prevail in the alloy
during the manufacturing process, these carbides have a low
stability and may therefore be neglected in the quotient.
[0149] Further, in addition to the content of carbon, the
FeCrAl-alloy may also contain nitrogen and/or oxygen, therefore
these two elements must also be considered in the quotient. As
described above, nitrogen and oxygen may exist in the alloy as
impurities or may have been added deliberately in order to improve
the properties of the FeCrAl-alloy. For example, when the
FeCrAl-alloy is used in powder metallurgical applications, the
content of nitrogen and oxygen in the FeCrAl alloy may be
substantial. When nitrogen and oxygen are present, some of the
reactive elements are consumed in the form of nitrides and oxides.
This will result in that less reactive elements are left to react
with the free carbon to form stable carbides which in turn will
have an impact on the protective oxide formation.
[0150] Examples of thermodynamically stable nitrides of the
reactive elements in the alloy are: TiN, ZrN, HfN, VN, TaN and ThN
and examples of thermodynamically stable oxides of the reactive
elements in the alloy are: TiO.sub.2, ZrO.sub.2, HfO2,
Y.sub.2O.sub.3 and ThO.sub.2.
[0151] Thus, to compensate for the reactive elements consumed by
forming stable nitrides and oxides, the amount of nitrogen and of
oxygen in the alloy is deducted from the amount of the reactive
elements in the quotient.
[0152] The amount of oxygen in the quotient must be multiplied with
a weighting factor, "x", which is selected from 0.5 or 0.67. The
value of the weighing factor depends on which type of oxide is
formed, i.e. it depends on which elements are comprised in the
alloy and also which oxide is the most thermodynamically stable at
the conditions that prevail during manufacturing. As the most
commonly formed oxide is a dioxide, the weighing factor may be
selected to about 0.5. However, if the element yttrium is present
in the alloy (more than or equal to 0.01 wt %), then the most
stable oxide formed is a tri-oxide, such as Y.sub.2O.sub.3 or
AlYO.sub.3 and then x is 0.67.
[0153] In the quotient, the reactive elements, carbon and if
applicable, nitrogen and oxygen, are balanced on the basis of the
atom percentage of respective element because it is important to
ensure that for each reactive element, sufficient amounts are added
in order to match the number of free carbon atoms in the alloy so
that the desired carbide is formed.
[0154] In operation, the ferritic alloy as defined hereinabove or
hereinafter will form and therefore comprise a surface oxide layer.
The surface oxide layer will comprise an outer layer of
(Al.sub.2Fe)O.sub.4-oxide, a middle layer of Cr.sub.2O.sub.3 oxide
and an inner layer of Al.sub.2O.sub.3.
[0155] The ferritic alloy as defined hereinabove or hereinafter may
also comprise dispersoids of ZrC and/or NbC and/or Nb.sub.2C and/or
HfC and/or VC and/or ThC and/or TaC. The dispersoids may be in the
form of cluster comprising a TiC dispersoid core and enclosed by
dispersoids of ZrC and/or NbC and/or Nb.sub.2C and/or HfC and/or VC
and/or ThC and/or TaC.
Detailed Description of Examples
[0156] The disclosure will be described by the following
non-limited example:
[0157] The example aims at investigating Fe-10Cr-4Al alloys, and
specifically to investigate the influence of different reactive
elements (RE) on the long-term (8,760 h) corrosion resistance to
liquid lead at 550.degree. C. In addition, a short-term (1,000 h)
test at 450.degree. C. was conducted for comparative reasons.
[0158] The influence of additions of the reactive elements (RE):
Zr, Nb and Y were selected for the example. Nine experimental
alloys, with varying RE content, were produced in a vacuum
induction furnace. Samples were hot-rolled to 8.times.1 mm strips
and homogenized at 1050.degree. C. for 5 minutes after each step.
The analyzed chemical compositions for all studied alloys are
presented in table 1.
TABLE-US-00001 TABLE 1 Chemical composition of studied alloys (wt
%) RE RE/C Alloy Fe Cr Al Si Mn C Ti Zr Nb Y (at. %) Zr-0.1 Bal.
10.12 3.98 0.12 0.11 0.04 0.08 0.11 -- -- 0.89 Zr-0.2 Bal. 10.15
3.95 0.13 0.11 0.03 0.09 0.21 -- -- 1.67 Zr-0.4 Bal. 10.20 4.06
0.12 0.12 0.03 0.07 0.39 -- -- 2.30 Nb-0.4 Bal. 10.20 4.10 0.15
0.07 0.03 0.11 -- 0.46 -- 2.94 Nb-0.8 Bal. 10.17 4.12 0.12 0.12
0.03 0.08 -- 0.9 -- 4.95 Nb--0.8C Bal. 10.12 4.16 0.12 0.12 0.1
0.06 -- 0.85 -- 1.32 Y-002* Bal. 10.26 4.24 0.07 0.12 0.03 0.07 --
-- 0.02 0.70 Y-0.1* Bal. 10.21 4.14 0.12 0.13 0.03 0.07 -- -- 0.09
0.87 Y-0.2* Bal. 10.12 4.05 0.12 0.11 0.03 0.08 -- -- 0.19 1.45
*Even though the quotient according to the definition herein does
not comprise Y, Y was included in quotient for the last three
alloys because of comparison. However, as will be shown and which
is also discussed in the present disclosure, yttrium will be acting
different compared to the RE.
[0159] The contents of RE and carbon was varied in the nine
samples, such that some samples had a deficit of RE in comparison
to the amount of carbon (samples Zr-0.1, Y-0.02, Y-0.1), in some
samples the amounts of RE and carbon was in balance (samples
Zr-0.2, Nb-0.8C, Y-0.2) and in some samples RE was in excess in
comparison to carbon (samples Zr-0.4, Nb-0.4, Nb-0.8).
[0160] Coupons measuring 30.times.8.times.1 mm were prepared of
each alloy for the oxidation study. The surfaces were polished to a
#800 grit finish using SiC abrasive papers, after which the coupons
sonicated in ethanol and subsequently placed in alumina crucibles
filled with 2 mm 99.9% (metal base) lead shots. The oxidation test
was carried out in a tube furnace, where the crucibles were placed
inside sealed quartz tubes. The dissolved oxygen content in the
liquid lead was controlled by means of a flowing
Ar--H.sub.2-H.sub.2O gas mixture. H.sub.2/H.sub.2O ratios of 1.3
and 0.2 were used to achieve a dissolved oxygen concentration of
10.sup.-7 wt % in the liquid lead at 550.degree. C. and 450.degree.
C. respectively. After finishing the oxidation tests, 1000 h at
450.degree. C. and 8760 h at 550.degree. C., the samples were air
cooled and cleaned from residual lead in a (1:1) solution of acetic
acid and hydrogen peroxide. Transmission electron microscopy (TEM)
samples were prepared through the standard lift-out using a FEI
quanta 3D field emission scanning electron microscope (FEG-SEM).
The TEM evaluation was carried out using a JEOL JEM-2100F FEG TEM.
Energy dispersive spectroscopy (EDS) elemental analysis was made
using an Oxford instruments 80 mm.sup.2 X-Max.sup.N silicon drift
detector (SDD). SEM samples were prepared by molding the oxidized
sample into a conductive resin followed by fine polishing down to a
final 0.25 .mu.m diamond step. A Zeiss Leo 1530 FEG-SEM an Oxford
50 mm.sup.2 X-Max SDD EDS were used for general characterization.
Thermodynamic modeling was carried out using ThermoCalc running the
TCFE7 and SSOL4 databases.
[0161] Results from the Investigations
[0162] The results from the 8,760 h oxidation test at 550.degree.
C. showed clear differences in oxidation properties with respect to
various RE additions.
[0163] The three alloys (Zr-0.1, Y-0.02 and Y-0.1) which had a
deficit of RE in comparison to carbon formed significant amounts of
Cr-rich carbides. FIG. 2a shows a Cr-carbide (1) close to an
Al-rich oxide scale (2) formed on the surface (3) of the sample. In
FIG. 2a, the Cr-rich carbide may be detected as a white shape
inside the encircled area. FIG. 2b is a TEM Cr EDS map of the
chrome-rich carbide area (1) encircled in FIG. 2a. Here the shape
of the chrome-rich area is clearly visible.
[0164] The examples showed that nearly all Cr-carbides were formed
in contact with the Al-rich oxide at the sample surfaces. This may
be explained in that aluminum suppresses carbide formation, i.e.
stabilizes graphite. It seems thus likely that the Cr-carbide
nucleation is promoted at the Al-depleted metal-oxide phase
boundary. Thus, the protective Al-oxide had not been formed
properly.
[0165] Furthermore, the three alloys, Zr-0.1, Y-0.02 and Y-0.1, all
showed poor oxidation properties in the oxidation tests. The poor
results were consistent both at 550.degree. C. and at 450.degree.
C., this shows that it is important to select the correct quotient
as described hereinabove or hereinafter.
[0166] FIG. 3a shows a SEM image of a cross-section of a sample
taken from the Zr-0.1 alloy after oxidation at a temperature of
550.degree. C. It is clearly visible in the image an irregularly
shaped mixed oxide, which has grown into the bulk of the alloy. The
high amount of chromium-rich surface carbides in the metal-oxide
interface of the Zr-0.1 sample seemingly leads to a pitting type of
accelerated oxidation, displaying inward growing mixed metal oxides
measuring up to about 5 .mu.m.
[0167] At lower temperatures, where a low Cr-content is needed to
avoid .alpha.-.alpha.' phase-separation, the presence of
chromium-rich surface carbides resulted in formation of
non-protective oxide scales. This was confirmed by the shorter
(1000 h) oxidation test at 450.degree. C. The same three alloys
that contained carbides close to the surface (Zr-0.1, Y-0.02,
Y-0.1) and that had a deficit of RE in comparison to carbon were
completely covered with a three-layered oxide structure, consisting
of an outward growing Fe.sub.3O.sub.4 scale and an inward growing
FeCrAl mixed oxide, under which an internal oxidation zone was
seen. FIG. 3b shows a SEM micrograph of the Y-0.02 sample having
the above described structure of oxide layers. The total depth of
the corrosion attacks on the Y-0.02 sample was measured to 3-4
.mu.m.
[0168] Balanced Alloys
[0169] Hence, by balancing the C and RE content, the corrosion
performance of FeCrAl-alloy was improved. Three alloys in the
study, Zr-0.2, Y-0.2 and Nb-0.8C, contained RE in near balance with
respect to the C-content, displayed significantly different
oxidation behavior at 550.degree. C.
[0170] Zr-0.2
[0171] The Zr-0.2 alloy showed no signs of oxidation attacks. A TEM
evaluation was carried out to study the surface of the Zr-0.2
sample and it showed the presence of a thin, approximately 100 nm
thick oxide that had formed during the 8,760 h exposure in liquid
lead, see FIG. 4b. The oxide was divided into three layers, an
inward growing Al.sub.2O.sub.3 layer and an outward growing FeAl
mixed oxide, delimited by a thin Cr-rich oxide layer. FIG. 4b is a
TEM EDS line scan showing the chemical composition of the layer in
FIG. 3a as a function of the distance from the surface of the
layer.
[0172] At 450.degree. C., the Zr-0.2 alloy displayed favorable
oxidation properties, i.e. a protective oxide layer was formed, see
FIG. 4a. The thin oxide that was formed on its surfaces was
investigated by means of TEM and was measured to about 40 nm.
Similar to the TEM results at 550.degree. C., the oxide formed at
450.degree. C. was divided into three zones, an inner layer solely
enriched in Al, an outer part rich in Fe, and an intermediate layer
rich in Cr (FIG. 4b. FIG. 5b is a TEM EDS line scan showing the
chemical composition of the layer in FIG. 5a as a function of the
distance from the surface of the layer.
[0173] Nb-0.8C
[0174] The Nb-0.8C alloy showed no oxidation attacks at 550.degree.
C.
[0175] Y-0.2
[0176] Localized oxidation pits were found on the Y-0.2 alloy after
treatment at 550.degree. C. despite the slight excess of Y in
relation to C. This may be explained by the relatively weak
stability of yttrium carbides compared to those of Zr and Nb. The
microstructure characterization found Y-rich precipitates, which
were enriched in C, O, S and in particular Fe.
[0177] Large Excess Ratio Alloys
[0178] The samples having a large excess of RE compared to C, (i.e.
samples Zr-0.4, Nb-0.4, Nb-0.8) showed poor oxidation properties at
both 550.degree. C. and 450.degree. C. The Zr-0.4 alloy, showed
pitting type oxidation attack at 550.degree. C. and Fe and Zr-rich
phases measuring up to 2 .mu.m was found throughout the matrix. In
addition, the oxide pits on the same alloy were enriched in Zr.
[0179] The alloys Zr-0.4 and the Y-0.2 preformed in a similar
manner at 450.degree. C. The impact of the excess of Y and Zr did
not result in a marked decrease in oxidation properties at
450.degree. C., as was the case at 550.degree. C. However, the
slower reaction kinetics at 450.degree. C. in comparison to
550.degree. C. and a shorter exposure time may have masked the
result.
[0180] After treatment at 550.degree. C., the Nb-0.8 alloy was
almost entirely covered with oxidation pits, which measured up to 5
.mu.m. These precipitates were preferably found to decorate the
alloy grain boundaries, similar to the behavior of laves phases,
but were also found inside the grains of the Nb-0.8-alloy.
CONCLUSIONS
[0181] It has been shown that best oxidation resistance is achieved
when the RE-additions are in balance with the carbon content of the
alloy, i.e. the RE is in slight excess. A deficit of RE in respect
of carbon will lead to the formation of chromium rich carbides
close to the surface of the alloy which in turn will lead to poor
oxidation properties and reduced pitting corrosion resistance.
Further, over-doping of RE will lead to the formation of
intermetallics or laves phase, which will also decrease the
oxidation resistance of the alloy.
* * * * *