U.S. patent application number 12/965954 was filed with the patent office on 2011-04-21 for austenitic heat resistant alloy, heat resistant pressure member comprising the alloy, and method for manufacturing the same member.
This patent application is currently assigned to SUMITOMO METAL INDUSTRIES, LTD.. Invention is credited to Masaaki IGARASHI, Hirokazu OKADA, Hiroyuki SEMBA.
Application Number | 20110088819 12/965954 |
Document ID | / |
Family ID | 41434076 |
Filed Date | 2011-04-21 |
United States Patent
Application |
20110088819 |
Kind Code |
A1 |
SEMBA; Hiroyuki ; et
al. |
April 21, 2011 |
AUSTENITIC HEAT RESISTANT ALLOY, HEAT RESISTANT PRESSURE MEMBER
COMPRISING THE ALLOY, AND METHOD FOR MANUFACTURING THE SAME
MEMBER
Abstract
An austenitic heat resistant alloy, which comprises by mass
percent, C: over 0.02 to 0.15%, Si.ltoreq.2%, Mn.ltoreq.3%,
P.ltoreq.0.03%, S.ltoreq.0.01%, Cr: 28 to 38%, Ni: over 40 to 60%,
Co.ltoreq.20% (including 0%), W over 3 to 15%, Ti: 0.05 to 1.0%,
Zr: 0.005 to 0.2%, Al: 0.01 to 0.3%, N.ltoreq.0.02%, and
Mo<0.5%, with the balance being Fe and impurities, in which the
following formulas (1) to (3) are satisfied has high creep rupture
strength and high toughness after a long period of use at a high
temperature, and further it is excellent in hot workability. This
austenitic heat resistant alloy may contain a specific amount of
one or more elements selected from Nb, V, Hf, B, Mg, Ca, Y, La, Ce,
Nd, Sc, Ta, Re, Ir, Pd, Pt and Ag.
P.ltoreq.3/{200(Ti+8.5.times.Zr)} . . . (1),
1.35.times.Cr.ltoreq.Ni+Co.ltoreq.1.85.times.Cr . . . (2),
Al.gtoreq.1.5.times.Zr . . . (3).
Inventors: |
SEMBA; Hiroyuki; (Sanda-shi,
JP) ; OKADA; Hirokazu; (Kobe-shi, JP) ;
IGARASHI; Masaaki; (Sanda-shi, JP) |
Assignee: |
SUMITOMO METAL INDUSTRIES,
LTD.
Osaka
JP
|
Family ID: |
41434076 |
Appl. No.: |
12/965954 |
Filed: |
December 13, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/JP2009/060837 |
Jun 15, 2009 |
|
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12965954 |
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Current U.S.
Class: |
148/707 ;
148/442; 420/584.1; 420/586 |
Current CPC
Class: |
C22C 38/105 20130101;
C22F 1/10 20130101; C22C 19/055 20130101; C22C 38/001 20130101;
C22C 38/00 20130101; C22C 19/053 20130101; C22C 38/50 20130101;
C22F 1/00 20130101; C22C 38/08 20130101; C22C 38/18 20130101; C21D
6/002 20130101; C21D 6/02 20130101; C21D 8/00 20130101; C22C 38/28
20130101; C22C 38/02 20130101; C22C 38/40 20130101; C22C 38/44
20130101; C22C 38/04 20130101; C21D 2211/001 20130101; C22C 38/22
20130101; C22C 38/30 20130101; C22C 30/00 20130101; C22C 38/10
20130101; C22C 38/06 20130101 |
Class at
Publication: |
148/707 ;
420/586; 148/442; 420/584.1 |
International
Class: |
C22C 30/00 20060101
C22C030/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 16, 2008 |
JP |
2008-156352 |
Claims
1. An austenitic heat resistant alloy, which comprises by mass
percent, C: more than 0.02% to not more than 0.15%, Si: 2% or less,
Mn: 3% or less, P: 0.03% or less, S: 0.01% or less, Cr: 28 to 38%,
Ni: more than 40% to not more than 60%, W: more than 3% to not more
than 15%, Ti: 0.05 to 1.0%, Zr: 0.005 to 0.2%, Al: 0.01 to 0.3%, N:
0.02% or less, and Mo: less than 0.5%, with the balance being Fe
and impurities, in which the following formulas (1) to (3) are
satisfied: P.ltoreq.3/{200(Ti+8.5.times.Zr)} (1),
1.35.times.Cr.ltoreq.Ni.ltoreq.1.85.times.Cr (2),
Al.gtoreq.1.5.times.Zr (3); wherein each element symbol in the
equations (1) to (3) represents the content by mass % of the
element concerned.
2. An austenitic heat resistant alloy, which comprises by mass
percent, C: more than 0.02% to not more than 0.15%, Si: 2% or less,
Mn: 3% or less, P: 0.03% or less, S: 0.01% or less, Cr: 28 to 38%,
Ni: more than 40% to not more than 60%, Co: 20% or less, W: more
than 3% to not more than 15%, Ti: 0.05 to 1.0%, Zr: 0.005 to 0.2%,
Al: 0.01 to 0.3%, N: 0.02% or less, and Mo: less than 0.5%, with
the balance being Fe and impurities, in which the following
formulas (1), (3) and (4) are satisfied:
P.ltoreq.3/{200(Ti+8.5.times.Zr)} (1), Al.gtoreq.1.5.times.Zr (3),
1.35.times.Cr.ltoreq.Ni+Co.ltoreq.1.85.times.Cr (4); wherein each
element symbol in the equations (1), (3) and (4) represents the
content by mass % of the element concerned.
3. The austenitic heat resistant alloy according to claim 1, which
further contains, by mass percent, one or more elements of one or
more groups selected from the 1 to 3 groups listed below in lieu of
a part of Fe: 1 Nb: 1.0% or less, V: 1.5% or less, Hf: 1% or less
and B: 0.05% or less; 2 Mg: 0.05% or less, Ca: 0.05% or less, Y:
0.5% or less, La: 0.5% or less, Ce: 0.5% or less, Nd: 0.5% or less
and Sc: 0.5% or less; 3 Ta: 8% or less, Re: 8% or less, Ir: 5% or
less, Pd: 5% or less, Pt: 5% or less and Ag: 5% or less.
4. The austenitic heat resistant alloy according to claim 2, which
further contains, by mass percent, one or more elements of one or
more groups selected from the 1 to 3 groups listed below in lieu of
a part of Fe: 1 Nb: 1.0% or less, V: 1.5% or less, Hf: 1% or less
and B: 0.05% or less; 2 Mg: 0.05% or less, Ca: 0.05% or less, Y:
0.5% or less, La: 0.5% or less, Ce: 0.5% or less, Nd: 0.5% or less
and Sc: 0.5% or less; 3 Ta: 8% or less, Re: 8% or less, Ir: 5% or
less, Pd: 5% or less, Pt: 5% or less and Ag: 5% or less.
5. A heat resistant pressure member excellent in creep resistance
properties and structural stability in a high temperature range,
which is made from the austenitic heat resistant alloy according to
claim 1.
6. A heat resistant pressure member excellent in creep resistance
properties and structural stability in a high temperature range,
which is made from the austenitic heat resistant alloy according to
claim 2.
7. A heat resistant pressure member excellent in creep resistance
properties and structural stability in a high temperature range,
which is made from the austenitic heat resistant alloy according to
claim 3.
8. A heat resistant pressure member excellent in creep resistance
properties and structural stability in a high temperature range,
which is made from the austenitic heat resistant alloy according to
claim 4.
9. A method for manufacturing the heat resistant pressure member
excellent in creep resistance and structural stability in a high
temperature range, wherein the austenitic heat resistant alloy
according to claim 1 is treated in sequence by the following steps
(i), (ii) and (iii): step (i): heating to 1050 to 1250.degree. C.
at least once before final hot or cold working; step (ii): carrying
out a final hot or cold plastic working such that the reduction of
area is 10% or more; step (iii): carrying out a final heat
treatment in which cooling is performed after heating and holding
at a temperature in the range of 1100 to 1250.degree. C.
10. A method for manufacturing the heat resistant pressure member
excellent in creep resistance and structural stability in a high
temperature range, wherein the austenitic heat resistant alloy
according to claim 2 is treated in sequence by the following steps
(i), (ii) and (iii): step (i): heating to 1050 to 1250.degree. C.
at least once before final hot or cold working; step (ii): carrying
out a final hot or cold plastic working such that the reduction of
area is 10% or more; step (iii): carrying out a final heat
treatment in which cooling is performed after heating and holding
at a temperature in the range of 1100 to 1250.degree. C.
11. A method for manufacturing the heat resistant pressure member
excellent in creep resistance and structural stability in a high
temperature range, wherein the austenitic heat resistant alloy
according to claim 3 is treated in sequence by the following steps
(i), (ii) and (iii): step (i): heating to 1050 to 1250.degree. C.
at least once before final hot or cold working; step (ii): carrying
out a final hot or cold plastic working such that the reduction of
area is 10% or more; step (iii): carrying out a final heat
treatment in which cooling is performed after heating and holding
at a temperature in the range of 1100 to 1250.degree. C.
12. A method for manufacturing the heat resistant pressure member
excellent in creep resistance and structural stability in a high
temperature range, wherein the austenitic heat resistant alloy
according to claim 4 is treated in sequence by the following steps
(i), (ii) and (iii): step (i): heating to 1050 to 1250.degree. C.
at least once before final hot or cold working; step (ii): carrying
out a final hot or cold plastic working such that the reduction of
area is 10% or more; step (iii): carrying out a final heat
treatment in which cooling is performed after heating and holding
at a temperature in the range of 1100 to 1250.degree. C.
Description
[0001] This application is a continuation of the international
application PCT/JP2009/060837 field on Jun. 15, 2009, the entire
content of which is herein incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to an austenitic heat
resistant alloy, which has a high temperature strength far higher
than that of a conventional heat resistant alloy, and is excellent
in toughness after a long period of use, and also excellent in hot
workability, and relates to a heat resistant pressure member
comprising the said alloy, and also a method for manufacturing the
same member. More particularly, the present invention relates to an
austenitic heat resistant alloy which contains 28 to 38 mass % of
Cr, which is excellent in high temperature strength, especially
creep rupture strength, and is excellent in toughness after a long
period of use due to high structural stability. Further it has
remarkably improved hot workability, especially high temperature
ductility at 1150.degree. C. or higher, being used as a pipe
material, a plate material for a heat resistant pressure member, a
bar material, forgings, and the like for a boiler for power
generation, a plant for chemical industry, and the like, and
relates to a heat resistant pressure member comprising the said
alloy, and a method for manufacturing the same member.
BACKGROUND ART
[0003] Conventionally, for a boiler used in a high temperature
environment, a chemical plant, and the like, a so-called "18-8 type
austenitic stainless steel" such as SUS 304H, SUS 316H, SUS 321H,
SUS 347H, and the like has been used as an equipment material.
[0004] However, in recent years, the conditions under which this
equipment was used in a high temperature environment have become
extremely severe, and therefore the performance requirements of
material to be used have become stringent; under these
circumstances, the above-described 18-8 type austenitic stainless
steel, having been used conventionally, has become remarkably
insufficient in high temperature strength, especially creep rupture
strength. Therefore, in order to solve the said problem, an
austenitic stainless steel, with improved creep rupture strength,
has been developed by containing proper amounts of various
elements.
[0005] On the other hand, nowadays, in the field of a boiler for
thermal power generation, for example, a project is underway to
raise steam temperature, which has conventionally been about
600.degree. C. at the most, to 700.degree. C. or higher. In this
case, the temperature of a member to be used far exceeds
700.degree. C., and therefore, even if the above-described newly
developed austenitic stainless steel is used, the creep rupture
strength and corrosion resistance are insufficient.
[0006] Generally, in order to improve the corrosion resistance, it
is effective to increase the content of Cr in the steel. However,
in the case where the Cr content is increased, for example, as seen
in SUS 310S which contains about 25 mass % of Cr, the creep rupture
strength at 600 to 800.degree. C. rather becomes lower than that of
18-8 type stainless steels, and the toughness is deteriorated due
to the precipitation of u phase. Further, even if the Cr content is
increased, about 25 mass % of Cr cannot provide sufficient
corrosion resistance in a severe corrosive environment.
[0007] Thus, the Patent Documents 1 to 7 disclose heat resistant
alloys in which the contents of Cr and Ni are increased, and
moreover one or more kinds of Mo and W are contained in order to
improve the creep rupture strength as high temperature
strength.
[0008] Further, in order to meet the increasingly stringent
requirements for high temperature strength characteristics,
especially the requirements for creep rupture strength, the Patent
Document 8 discloses a heat resistant alloy which contains, by mass
%, 28 to 38% of Cr and 30 to 50% of Ni, and the Patent Documents 9
to 14 disclose heat resistant alloys which contain, by mass %, 28
to 38% of Cr and 35 to 60% of Ni. For all of the heat resistant
alloys proposed in the Patent Documents 8 to 14, the creep rupture
strength is further improved by utilizing the precipitation of
.alpha.-Cr phase of a body-centered cubic structure consisting
mainly of Cr.
CITATION LIST
Patent Document
[0009] Patent Document 1: JP 60-100640 A [0010] Patent Document 2:
JP 61-174350 A [0011] Patent Document 3: JP 61-276948 A [0012]
Patent Document 4: JP 62-63654 A [0013] Patent Document 5: JP
64-55352 A [0014] Patent Document 6: JP 2-200756 A [0015] Patent
Document 7: JP 3-264641 A [0016] Patent Document 8: JP 7-34166 A
[0017] Patent Document 9: JP 7-70681 A [0018] Patent Document 10:
JP 7-216511 A [0019] Patent Document 11: JP 7-331390 A [0020]
Patent Document 12: JP 8-127848 A [0021] Patent Document 13: JP
8-218140 A [0022] Patent Document 14: JP 10-96038 A
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0023] The heat resistant alloys disclosed in the Patent Documents
1 to 7 cannot necessarily obtain sufficiently high creep rupture
strength in a severe environment in which the steam temperature is
700.degree. C. or higher.
[0024] Also, it cannot be said that the heat resistant alloys
disclosed in the Patent Documents 8 to 14 are sufficient in creep
rupture strength that has been required to be high in recent years.
Further, the heat resistant alloys disclosed in the Patent
Documents 8 to 14 are sometimes insufficient in toughness after a
long period of use depending on the alloy composition thereof.
Moreover, regarding these heat resistant alloys, it has been
desired to further improve the hot workability, especially the hot
workability on the high temperature side of 1150.degree. C. or
higher. The reason for this is that in a case where a seamless
steel pipe is manufactured by using a material having a poor hot
workability, the seamless steel pipe is often manufactured by the
hot extrusion process, and if the hot workability on the high
temperature side of 1150.degree. C. or higher is insufficient, the
internal temperature of the material becomes higher than the
heating temperature due to a work heat generation, so that defects,
such as two-piece cracks and scabs, are formed. If the hot
workability on the high temperature side of 1150.degree. C. or
higher is insufficient, in a piercing process using a piercing mill
of, for example, a Mannesmann-mandrel mill system, the
above-described defects are formed in the same way.
[0025] In view of the above-mentioned state of affairs, the
objective of the present invention is to provide an austenitic heat
resistant alloy containing 28 to 38 mass % of Cr, which has high
temperature strength, especially creep rupture strength, which is
far higher than that of the conventional heat resistant alloys,
especially the heat resistant alloys disclosed in the Patent
Documents 8 to 14. It has high toughness because the structural
stability is excellent even after a long period of use at a high
temperature, and further it has remarkably improved hot
workability, especially high temperature ductility at 1150.degree.
C. or higher.
Means for Solving the Problems
[0026] The present inventors examined the creep rupture strength,
structural stability in a long period of use, hot workability, and
the like by using various heat resistant alloys containing, by mass
%, 28 to 38% of Cr and more than 40% to not more than 60% of Ni as
base components and capable of utilizing precipitation
strengthening of the .alpha.-Cr phase. As a result, the present
inventors obtained the following findings (a) to (g).
[0027] (a) If a proper amount of W is contained, an Fe.sub.2W type
Laves phase and/or an Fe.sub.7W.sub.6 type .mu. phase precipitate,
and therefore the creep rupture strength is significantly
improved.
[0028] (b) In the case where 28 to 38% of Cr is contained, and if W
can be dissolved into the precipitated .alpha.-Cr phase, the
growing and coarsening of the .alpha.-Cr phase during a long period
of use at a high temperature are restrained, a sudden decrease in
creep rupture strength on the long time side does not occur.
[0029] (c) Conventionally, it has generally been thought that Mo
and W have equivalent operational advantages; however, in the case
where Mo is compositely contained in an alloy containing W and 28
to 38% of Cr, the .sigma. phase sometimes precipitates on the long
time side. Therefore, the creep rupture strength, ductility, and
toughness may decrease.
[0030] (d) By properly controlling the content of Ni, which is an
austenite stabilizing element, with respect to the Cr content, the
precipitation of the .sigma. phase during a long period of use at a
high temperature can be restrained stably and reliably, and
moreover, the optimum amount of .alpha.-Cr phase can be
precipitated. In the case where the alloy compositely contains Co,
by controlling the contents of Ni and Co with respect to the Cr
content so that the sum of the contents of Ni and Co (that is,
"Ni+Co") is proper, the precipitation of the .sigma. phase, during
a long period of use at a high temperature, can be restrained
stably and reliably, and moreover, the optimum amount of .alpha.-Cr
phase can be precipitated.
[0031] (e) Zr, which has generally been known as a "grain boundary
strengthening element", is competent for improving the creep
rupture strength in the case of the heat resistant alloy capable of
utilizing the precipitation strengthening of .alpha.-Cr phase.
Further, by properly controlling the content of Al in accordance
with the content of Zr, the creep rupture strength is significantly
improved.
[0032] (f) Ti also improves the creep rupture strength of the heat
resistant alloy capable of utilizing the precipitation
strengthening of .alpha.-Cr phase. By containing Ti compositely
with Zr, the precipitation of .alpha.-Cr phase is further promoted,
whereby the creep rupture strength can be further enhanced.
[0033] (g) Since Ti and Zr lower the melting point of the heat
resistant alloy, the hot workability, especially the hot
workability on the high temperature side of 1150.degree. C. or
higher, decreases, and further the high temperature crack
resistance at the time of welding may decrease. However, by
properly controlling the content of P in accordance with the
contents of Ti and Zr, the hot workability on the high temperature
side of 1150.degree. C. or higher can be improved stably and
reliably while a high creep rupture strength is maintained.
Further, the high temperature crack resistance at the time of
welding can be improved.
[0034] The present invention has been accomplished on the basis of
the above-described findings. The main points of the present
invention are austenitic heat resistant alloys shown in the
following (1) to (3), a heat resistant pressure member shown in the
(4), and a method for manufacturing a heat resistant pressure
member shown in the (5).
[0035] (1) An austenitic heat resistant alloy, which comprises by
mass percent, C: more than 0.02% to not more than 0.15%, Si: 2% or
less, Mn: 3% or less, P: 0.03% or less, S: 0.01% or less, Cr: 28 to
38%, Ni: more than 40% to not more than 60%, W: more than 3% to not
more than 15%, Ti: 0.05 to 1.0%, Zr: 0.005 to 0.2%, Al: 0.01 to
0.3%, N: 0.02% or less, and Mo: less than 0.5%, with the balance
being Fe and impurities, in which the following formulas (1) to (3)
are satisfied:
P.ltoreq.3/{200(Ti+8.5.times.Zr)} (1),
1.35.times.Cr.ltoreq.Ni.ltoreq.1.85.times.Cr (2),
Al.gtoreq.1.5.times.Zr (3);
wherein each element symbol in the equations (1) to (3) represents
the content by mass % of the element concerned.
[0036] (2) An austenitic heat resistant alloy, which comprises by
mass percent, C: more than 0.02% to not more than 0.15%, Si: 2% or
less, Mn: 3% or less, P: 0.03% or less, S: 0.01% or less, Cr: 28 to
38%, Ni: more than 40% to not more than 60%, Co: 20% or less, W:
more than 3% to not more than 15%, Ti: 0.05 to 1.0%, Zr: 0.005 to
0.2%, Al: 0.01 to 0.3%, N: 0.02% or less, and Mo: less than 0.5%,
with the balance being Fe and impurities, in which the following
formulas (1), (3) and (4) are satisfied:
P.ltoreq.3/{200(Ti+8.5.times.Zr)} (1),
Al.gtoreq.1.5.times.Zr (3),
1.35.times.Cr.ltoreq.Ni+Co.ltoreq.1.85.times.Cr (4);
wherein each element symbol in the equations (1), (3) and (4)
represents the content by mass % of the element concerned.
[0037] (3) The austenitic heat resistant alloy according to the
above (1) or (2), which further contains, by mass percent, one or
more elements of one or more groups selected from the 1 to 3 groups
listed below in lieu of a part of Fe:
[0038] 1 Nb: 1.0% or less, V: 1.5% or less, Hf: 1% or less and B:
0.05% or less;
[0039] 2 Mg: 0.05% or less, Ca: 0.05% or less, Y: 0.5% or less, La:
0.5% or less, Ce: 0.5% or less, Nd: 0.5% or less and Sc: 0.5% or
less;
[0040] 3 Ta: 8% or less, Re: 8% or less, Ir: 5% or less, Pd: 5% or
less, Pt: 5% or less and Ag: 5% or less.
[0041] (4) A heat resistant pressure member excellent in creep
resistance properties and structural stability in a high
temperature range, which is made from the austenitic heat resistant
alloy according to any one of the above (1) to (3).
[0042] (5) A method for manufacturing the heat resistant pressure
member excellent in creep resistance and structural stability in a
high temperature range according to the above (4), wherein the
austenitic heat resistant alloy according to any one of the above
(1) to (3) is treated in sequence by the following steps (i), (ii)
and
[0043] step (i): heating to 1050 to 1250.degree. C. at least once
before final hot or cold working;
[0044] step (ii): carrying out a final hot or cold plastic working
such that the reduction of area is 10% or more;
[0045] step (iii): carrying out a final heat treatment in which
cooling is performed after heating and holding at a temperature in
the range of 1100 to 1250.degree. C.
[0046] The term "impurities" so referred to in the phrase "the
balance being Fe and impurities" indicates those impurities which
come from ores and scraps as raw materials, environments, and so on
in the industrial production of alloys. Also, the "high temperature
range" is a temperature range in which creep deformation occurs,
and means a temperature range of 600.degree. C. or higher in the
alloy of the present invention, and about 600 to 900.degree. C.
considering the upper limit in terms of strength.
EFFECTS OF THE INVENTION
[0047] The austenitic heat resistant alloy according to the present
invention, has high temperature strength, especially creep rupture
strength, higher than that of the conventional heat resistant
alloys, and also has high toughness because the structural
stability is excellent even after a long period of use at a high
temperature. Further it is excellent in hot workability, especially
high temperature ductility at 1150.degree. C. or higher. Therefore,
this austenitic heat resistant alloy can be suitably used as a pipe
material, a plate material for a heat resistant pressure member, a
bar material, forgings, and the like for a boiler for power
generation, a plant for chemical industry and so on.
MODES FOR CARRYING OUT THE INVENTION
[0048] Hereunder, the requirements of the present invention are
described in detail. In the following description, the symbol "%"
for the content of each element means "% by mass".
[0049] (A) Austenitic Heat Resistant Alloy
[0050] C: more than 0.02% to not more than 0.15%
[0051] C (carbon) forms carbides which have an effect of ensuring
tensile strength and creep rupture strength that are necessary when
the alloy is used in a high temperature environment. In order to
obtain this effect, a content of C more than 0.02% is necessary.
However, even if an amount of more than 0.15% of C is contained,
the amount of undissolved carbides after the solution heat
treatment merely increases; C does not contribute to the
improvement in high temperature strength, and other mechanical
properties such as toughness and the weldability are deteriorated.
Therefore, the content of C is set to more than 0.02% to not more
than 0.15%. The preferable content range of C is more than 0.03% to
not more than 0.13%, and the further preferable range thereof is
more than 0.05% to not more than 0.12%.
[0052] Si: 2% or less
[0053] Si (silicon) is added as a deoxidizing element. Si also is
an element effective in raising oxidation resistance, steam
oxidation resistance and so on. However, if the Si content
increases and especially exceeds 2%, the formation of intermetallic
compounds such as the a phase is promoted, so that the structural
stability at high temperatures is deteriorated, and the toughness
and ductility decrease. Further, the weldability and hot
workability are deteriorated. Therefore, the content of Si is set
to 2% or less. In the case where much importance is attached to the
toughness and ductility, the content of Si is preferably set to 1%
or less. In the case where the deoxidizing action has been ensured
by any other element, it is not necessary to regulate the lower
limit of the Si content.
[0054] In a case where much importance is attached to the
deoxidizing action, oxidation resistance, steam oxidation
resistance, and the like, the content of Si is preferably 0.05% or
more, further preferably 0.1% or more.
[0055] Mn: 3% or less
[0056] Like Si, Mn (manganese) has a deoxidizing effect. Mn also
has the effect of fixing S, which is inevitably contained in the
alloy, as sulfides, and therefore Mn does improve the hot
workability. However, if the Mn content exceeds 3%, the
precipitation of intermetallic compounds, such as the .sigma. phase
is promoted, so that the structural stability and the mechanical
properties, such as high temperature strength, are deteriorated.
Therefore, the content of Mn is set to 3% or less.
[0057] It is not necessary to regulate the lower limit of the Mn
content; however in the case where much importance is attached to
the action for improving hot workability, the content of Mn is
preferably set to 0.1% or more. The content of Mn is further
preferably set to 0.2 to 2%, still further preferably set to 0.2 to
1.5%.
[0058] P: 0.03% or less
[0059] P (phosphorus) is inevitably incorporated in the alloy as an
impurity, and deteriorates the hot workability. In particular, if
the content of P exceeds 0.03%, the hot workability deteriorates
remarkably. Therefore, the content of P is set to 0.03% or
less.
[0060] In addition to being limited to 0.03% or less, the content
of P must satisfy the following formula:
P.ltoreq.3/{200(Ti+8.5.times.Zr)} (1).
[0061] S: 0.01% or less
[0062] Like P, S (sulfur) is inevitably incorporated in the alloy
as an impurity, and deteriorates the hot workability. In
particular, if the content of S exceeds 0.01%, the remarkable
deterioration of hot workability occurs. Therefore, the content of
S is set to 0.01% or less.
[0063] In the case where it is desired to ensure excellent hot
workability, the content of S is preferably set to 0.005% or less,
further preferably set to 0.003% or less.
[0064] Cr: 28 to 38%
[0065] Cr (chromium) has the effect of improving the corrosion
resistance such as oxidation resistance, steam oxidation
resistance, and high temperature corrosion resistance. Further, in
the present invention, Cr is an element that is essential in
precipitating as .alpha.-Cr phase which enhances the creep rupture
strength. However, if the content of Cr is less than 28%, these
effects cannot be obtained. On the other hand, if the Cr content
increases and especially exceeds 38%, the hot workability is
deteriorated, and further the structural stability is impaired by
the precipitation of a phase and the like. Therefore, the content
of Cr is set to 28 to 38%. An amount more than 30% of Cr content is
preferable.
[0066] Ni: more than 40% to not more than 60%
[0067] Ni (nickel) is an element that is essential in ensuring a
stable austenitic microstructure. In the present invention, since
28 to 38% of Cr is contained, in order to restrain the
precipitation of the .sigma. phase and to stably precipitate
.alpha.-Cr phase, a content of Ni more than 40% is necessary.
However, if the content of Ni becomes excessive and especially
exceeds 60%, depending on the content of Cr, the .alpha.-Cr phase
does not precipitate sufficiently, and the economic efficiency is
damaged. Therefore, the content of Ni is set to more than 40% to
not more than 60%.
[0068] In addition to being limited to more than 40% to not more
than 60%, the content of Ni must satisfy the following formula:
1.35.times.Cr.ltoreq.Ni.ltoreq.1.85.times.Cr (2),
or, in the case where the later-described amount of Co is
compositely contained, the content of Ni must satisfy the following
formula:
1.35.times.Cr.ltoreq.Ni+Co.ltoreq.1.85.times.Cr (4).
[0069] W: more than 3% to not more than 15%
[0070] W (tungsten) is a very important element that not only
contributes to the improvement in creep rupture strength as a solid
solution strengthening element by dissolving into the matrix but
also significantly improves the creep rupture strength by
precipitating as an Fe.sub.2W type Laves phase or an
Fe.sub.7W.sub.6 type .mu. phase. Further, in the present invention,
since 28 to 38% of Cr is contained, W dissolves into the
precipitated .alpha.-Cr phase, restraining the growing and
coarsening of .alpha.-Cr phase during a long period of use at a
high temperature, and inhibiting a sudden decrease in creep rupture
strength on the long time side. However, if the content of W is 3%
or less, the above-described effects cannot be obtained. On the
other hand, even if an amount more than 15% of W is contained, the
effects saturate and only the cost increases, and moreover, the
structural stability and hot workability are deteriorated.
Therefore, the content of W is set to more than 3% to not more than
15%. The content of W is preferably set to more than 3% to not more
than 13%. In the case where much importance is further attached to
the effect of improving the creep rupture strength, the content of
W is further preferably set to more than 6% to not more than
13%.
[0071] Ti: 0.05 to 1.0%
[0072] Ti (titanium) is an important element that promotes the
precipitation of .alpha.-Cr phase and thereby enhances the creep
rupture strength. In particular, by containing Ti compositely with
the later-described amount of Zr, the precipitation of .alpha.-Cr
phase is further promoted, so that the creep rupture strength can
further be enhanced. However, if the content of Ti is less than
0.05%, sufficient effects cannot be obtained. On the other hand, if
the content of Ti exceeds 1.0%, the hot workability deteriorates.
Therefore, the content of Ti is set to 0.05 to 1.0%. The content of
Ti is preferably set to 0.1 to 0.9%, further preferably set to 0.2
to 0.9%. The still further preferable upper limit of the content of
Ti is 0.5%.
[0073] In addition to being limited to 0.05 to 1.0%, the content of
Ti must satisfy the following formula:
P.ltoreq.3/{200(Ti+8.5.times.Zr)} (1).
[0074] Zr: 0.005 to 0.2%
[0075] Like Ti, Zr (zirconium) is an important element that
promotes the precipitation of .alpha.-Cr phase and thereby enhances
the creep rupture strength. In particular, by containing Zr
compositely with the above-described amount of Ti, the
precipitation of .alpha.-Cr phase is further promoted, so that the
creep rupture strength can further be enhanced. However, if the
content of Zr is less than 0.005%, sufficient effects cannot be
obtained. On the other hand, if the content of Zr exceeds 0.2%, the
hot workability deteriorates. Therefore, the content of Zr is set
to 0.005 to 0.2%. The content of Zr is preferably set to 0.01 to
0.1% and more preferably set to 0.01 to 0.05%.
[0076] In addition to being limited to 0.005 to 0.2%, the content
of Zr must satisfy the following two formulas:
P.ltoreq.3/{200(Ti+8.5.times.Zr)} (1),
Al.gtoreq.1.5.times.Zr (3).
[0077] Al: 0.01 to 0.3%
[0078] Al (aluminum) is an element having the effect of
deoxidizing, and in order to obtain the said effect, the content of
Al should be 0.01% or more. In the case where much Al is contained,
the creep rupture strength can be enhanced by the precipitation of
Y' phase. In the present invention, however, since the proper
amounts of W, Ti and Zr are contained, and the creep rupture
strength can be enhanced dramatically by the composite
precipitation strengthening due to .alpha.-Cr phase, Laves phase,
and the like, the strengthening due to Y' phase is not necessary.
Moreover, in the case where the content of Al exceeds 0.3%, the hot
workability, ductility, and toughness may be deteriorated.
Therefore, attaching much importance to hot workability, ductility,
and toughness, the content of Al is set to 0.01 to 0.3%.
[0079] In addition to being limited to 0.01 to 0.3%, the content of
Al must satisfy the following formula:
Al.gtoreq.1.5.times.Zr (3).
[0080] N: 0.02% or less
[0081] In the present invention in which Zr and Ti are contained as
essential elements to promote the precipitation of .alpha.-Cr
phase, N (nitrogen), which is an element contained inevitably in
the ordinary melting method, must be decreased in content as far as
possible to avoid the consumption of Zr and Ti caused by the
formation of ZrN and TiN. However, an extreme decrease in N content
lowers the economic efficiency because of the necessity of the
special melting method and high purity raw material. Therefore, the
content of N is set to 0.02% or less. The content of N is
preferably 0.015% or less.
[0082] Mo: less than 0.5%
[0083] Mo (molybdenum) has conventionally been thought to be an
element that dissolves into the matrix and contributes to the
improvement in creep rupture strength as a solid solution
strengthening element and that has the action equivalent to that of
W. However, by the studies of the present inventors, it turned out
that in the case where Mo is compositely contained in the alloy
containing the above-described amounts of W and Cr, the .sigma.
phase may precipitate on the long time side, and therefore the
creep rupture strength, ductility, and toughness may deteriorate.
Consequently, the content of Mo is preferably as low as possible,
and so, the content thereof is set to less than 0.5%. The content
of Mo is further preferably limited to less than 0.2%.
[0084] One austenitic heat resistant alloy of the present invention
comprises the above-described elements with the balance being Fe
and impurities. Another austenitic heat resistant alloy of the
present invention contains Co in the amount described below in
addition to the above-described elements.
[0085] Co: 20% or less
[0086] Like Ni, Co (cobalt) is an element that has the effect of
stabilizing the austenitic microstructure. Co also contributes to
the improvement in creep rupture strength. And therefore, Co may be
contained to obtain the above-described effects. However, even if
the content of Co exceeds 20%, the above-described effects saturate
and the cost increases, and moreover the hot workability is also
deteriorated. Therefore, in the case where Co is contained, the
content of Co is set to 20% or less. The upper limit of the Co
content is preferably set to 15%. On the other hand, in order to
ensure the above-described effects of stabilizing the austenitic
microstructure and of improving the creep rupture strength due to
the Co, the lower limit of the Co content is preferably set to
0.05% and more preferably set to 0.5%.
[0087] In the case where Co is contained, in addition to being
limited to 20% or less, the content of Co must satisfy the
following formula:
1.35.times.Cr.ltoreq.Ni+Co.ltoreq.1.85.times.Cr (4).
[0088] Another austenitic heat resistant alloy of the present
invention further contains, in addition to the above-described
elements of C to Mo or in addition to the above-described elements
of C to Co, one or more elements of one or more groups selected
from the 1 to 3 groups listed below in lieu of a part of Fe:
[0089] 1 Nb: 1.0% or less, V: 1.5% or less, Hf: 1% or less, and B:
0.05% or less;
[0090] 2 Mg: 0.05% or less, Ca: 0.05% or less, Y: 0.5% or less, La:
0.5% or less, Ce: 0.5% or less, Nd: 0.5% or less, and Sc: 0.5% or
less;
[0091] 3 Ta: 8% or less, Re: 8% or less, Ir: 5% or less, Pd: 5% or
less, Pt: 5% or less, and Ag: 5% or less.
[0092] Hereunder, the above-mentioned elements will be
explained.
[0093] Each of Nb, V, Hf and B being elements of the 1 group, has
the effects of enhancing the high temperature strength and creep
rupture strength. Therefore, in the case where it is desired to
obtain the enhanced high temperature strength and creep rupture
strength, these elements are added positively, and one or more
elements among them may be contained in the range described
below.
[0094] Nb: 1.0% or less
[0095] Nb (niobium) has the effects of enhancing the high
temperature strength and creep rupture strength by forming
carbo-nitrides and also it improves the ductility by making the
grains fine. Therefore, in order to obtain these effects, Nb may be
contained. However, if the content of Nb exceeds 1.0%, the hot
workability and toughness are deteriorated. Therefore, in the case
where Nb is contained, the content of Nb is set to 1.0% or less.
The upper limit of the Nb content is preferably set to 0.9%. On the
other hand, in order to ensure the above-described effects of
enhancing the high temperature strength, creep rupture strength,
and ductility due to Nb, the lower limit of the Nb content is
preferably set to 0.05% and further preferably set to 0.1%.
[0096] V: 1.5% or less
[0097] V (vanadium) has the effects of enhancing the high
temperature strength and creep rupture strength by forming
carbo-nitrides. Therefore, in order to obtain these effects, V may
be contained. However, if the content of V exceeds 1.5%, the high
temperature corrosion resistance is deteriorated, and further the
ductility and toughness are decreased due to the precipitation of
brittle phase. Therefore, in the case where V is contained, the
content of V is set to 1.5% or less. The upper limit of the V
content is preferably set to 1%. On the other hand, in order to
ensure the above-described effects of enhancing the high
temperature strength and creep rupture strength due to V, the lower
limit of the V content is preferably set to 0.02% and more
preferably set to 0.04%.
[0098] Hf: 1% or less
[0099] Hf (hafnium) contributes to precipitation strengthening as a
carbo-nitride and has the effects of enhancing the high temperature
strength and creep rupture strength. Therefore, in order to obtain
these effects, Hf may be contained. However, if the content of Hf
exceeds 1%, the workability and weldability are impaired.
Therefore, in the case where Hf is contained, the content of Hf is
set to 1% or less. The upper limit of the Hf content is preferably
set to 0.8% and more preferably set to 0.5%. On the other hand, in
order to ensure the above-described effects of enhancing the high
temperature strength and creep rupture strength due to Hf, the
lower limit of the Hf content is preferably set to 0.01% and
further preferably set to 0.02%.
[0100] B: 0.05% or less
[0101] B (boron) exists at grain boundaries as a single form or it
exists in carbo-nitrides. B has the effects of enhancing the high
temperature strength and creep rupture strength by restraining a
grain boundary slip caused by grain boundary strengthening during
the use at a high temperature and also by promoting the fine
dispersing precipitation of carbo-nitrides. However, if the content
of B exceeds 0.05%, the weldability is deteriorated. Therefore, in
the case where B is contained, the content of B is set to 0.05% or
less. The upper limit of the B content is preferably set to 0.01%
and more preferably set to 0.005%. On the other hand, in order to
ensure the above-described effects of enhancing the high
temperature strength and creep rupture strength due to B, the lower
limit of the B content is preferably set to 0.0005% and further
preferably set to 0.001%.
[0102] The upper limit of the sum of the contents of the
above-described elements from Nb to B may be 3.55%. The upper limit
of the sum of contents thereof is further preferably 2.5%.
[0103] Each of Mg, Ca, Y, La, Ce, Nd and Sc being elements of the 2
group, has the effect of improving the hot workability by fixing S
as sulfides. Therefore, in the case where it is desired to obtain
further excellent hot workability, these elements are added
positively, and one or more elements among them may be contained in
the range described below.
[0104] Mg: 0.05% or less
[0105] Mg (magnesium) has the effect of improving the hot
workability by fixing S, which is contained inevitably in the
alloy, as sulfides. Therefore, in order to obtain this effect, Mg
may be contained. However, if the content of Mg exceeds 0.05%, the
cleanliness of the alloy is deteriorated, and the hot workability
and ductility are contrarily impaired. Therefore, in the case where
Mg is contained, the content of Mg is set to 0.05% or less. The
upper limit of the Mg content is preferably set to 0.02% and more
preferably set to 0.01%. On the other hand, in order to ensure the
above-described effect of improving the hot workability due to Mg,
the lower limit of the Mg content is preferably set to 0.0005% and
further preferably set to 0.001%.
[0106] Ca: 0.05% or less
[0107] Ca (calcium) has the effect of improving the hot workability
by fixing S, which inhibits the hot workability, as sulfides.
Therefore, in order to obtain this effect, Ca may be contained,
however, if the content of Ca exceeds 0.05%, the cleanliness of the
alloy is deteriorated, and the hot workability and ductility are
contrarily impaired. Therefore, in the case where Ca is contained,
the content of Ca is set to 0.05% or less. The upper limit of the
Ca content is preferably set to 0.02% and more preferably set to
0.01%. On the other hand, in order to ensure the above-described
effect of improving the hot workability due to Ca, the lower limit
of the Ca content is preferably set to 0.0005% and further
preferably set to 0.001%.
[0108] Y: 0.5% or less
[0109] Y (yttrium) has the effect of improving the hot workability
by fixing S as sulfides. Y also has the effect of improving the
adhesiveness of a Cr.sub.2O.sub.3 protective film on the alloy
surface, especially improving the oxidation resistance at the time
of repeated oxidation, and further Y has the effects of enhancing
the creep rupture strength and creep rupture ductility by
contributing to grain boundary strengthening. However, if the
content of Y exceeds 0.5%, the amounts of inclusions, such as
oxides increase, so that the workability and weldability are
impaired. Therefore, in the case where Y is contained, the content
of Y is set to 0.5% or less. The upper limit of the Y content is
preferably set to 0.3% and further preferably set to 0.15%. On the
other hand, in order to ensure the above-described effects due to
Y, the lower limit of the Y content is preferably set to 0.0005%.
The lower limit of the Y content is more preferably 0.001% and
still more preferably 0.002%.
[0110] La: 0.5% or less
[0111] La (lanthanum) has the effect of improving the hot
workability by fixing S as sulfides. La also has the effect of
improving the adhesiveness of a Cr.sub.2O.sub.3 protective film on
the alloy surface, especially improving the oxidation resistance at
the time of repeated oxidation, and further La has the effects of
enhancing the creep rupture strength and creep rupture ductility by
contributing to grain boundary strengthening. However, if the
content of La exceeds 0.5%, the amounts of inclusions, such as
oxides increase, so that the workability and weldability are
impaired. Therefore, in the case where La is contained, the content
of La is set to 0.5% or less. The upper limit of the La content is
preferably set to 0.3% and further preferably set to 0.15%. On the
other hand, in order to ensure the above-described effects due to
La, the lower limit of the La content is preferably set to 0.0005%.
The lower limit of the La content is more preferably 0.001% and
still more preferably 0.002%.
[0112] Ce: 0.5% or less
[0113] Ce (cerium) also has the effect of improving the hot
workability by fixing S as sulfides. In addition, Ce has the effect
of improving the adhesiveness of a Cr.sub.2O.sub.3 protective film
on the alloy surface, especially improving the oxidation resistance
at the time of repeated oxidation, and further Ce has the effects
of enhancing the creep rupture strength and creep rupture ductility
by contributing to grain boundary strengthening. However, if the
content of Ce exceeds 0.5%, the amounts of inclusions, such as
oxides increase, so that the workability and weldability are
impaired. Therefore, in the case where Ce is contained, the content
of Ce is set to 0.5% or less. The upper limit of the Ce content is
preferably set to 0.3% and further preferably set to 0.15%. On the
other hand, in order to ensure the above-described effects due to
Ce, the lower limit of the Ce content is preferably set to 0.0005%.
The lower limit of the Ce content is more preferably 0.001% and
still more preferably 0.002%.
[0114] Nd: 0.5% or less
[0115] Nd (neodymium) has the effect of improving the hot
workability by fixing S as sulfides. Nd also has the effect of
improving the adhesiveness of a Cr.sub.2O.sub.3 protective film on
the alloy surface, especially improving the oxidation resistance at
the time of repeated oxidation, and further Nd has the effects of
enhancing the creep rupture strength and creep rupture ductility by
contributing to grain boundary strengthening. However, if the
content of Nd exceeds 0.5%, the amounts of inclusions, such as
oxides increase, so that the workability and weldability are
impaired. Therefore, in the case where Nd is contained, the content
of Nd is set to 0.5% or less. The upper limit of the Nd content is
preferably set to 0.3% and further preferably set to 0.15%. On the
other hand, in order to ensure the above-described effects due to
Nd, the lower limit of the Nd content is preferably set to 0.0005%.
The lower limit of the Nd content is more preferably 0.001% and
still more preferably 0.002%.
[0116] Sc: 0.5% or less
[0117] Sc (scandium) also has the effect of improving the hot
workability by fixing S as sulfides. In addition, Sc has the effect
of improving the adhesiveness of a Cr.sub.2O.sub.3 protective film
on the alloy surface, especially improving the oxidation resistance
at the time of repeated oxidation, and further Sc has the effects
of enhancing the creep rupture strength and creep rupture ductility
by contributing to grain boundary strengthening. However, if the
content of Sc exceeds 0.5%, the amounts of inclusions, such as
oxides increase, so that the workability and weldability are
impaired. Therefore, in the case where Sc is contained, the content
of Sc is set to 0.5% or less. The upper limit of the Sc content is
preferably set to 0.3% and further preferably set to 0.15%. On the
other hand, in order to ensure the above-described effects due to
Sc, the lower limit of the Sc content is preferably set to 0.0005%.
The lower limit of the Sc content is more preferably 0.001% and
still more preferably 0.002%.
[0118] The upper limit of the sum of contents of the
above-described elements from Mg to Sc may be 2.6%. The upper limit
of the sum of contents thereof is further preferably 1.5%.
[0119] Each of Ta, Re, Ir, Pr, Pt and Ag being elements of the (3)
group, has the effect of solid solution strengthening by dissolving
into the austenite, which is the matrix. Therefore, in a case where
it is desired to obtain far higher strength by the solid solution
strengthening action, these elements are added positively, and one
or more elements among them may be contained in the range described
below.
[0120] Ta: 8% or less
[0121] Ta (tantalum) has the effects of enhancing the high
temperature strength and creep rupture strength by dissolving into
the austenite, which is the matrix, and by forming carbo-nitrides.
Therefore, in order to obtain theses effects, Ta may be contained.
However, if the content of Ta exceeds 8%, the workability and
mechanical properties are impaired. Therefore, in the case where Ta
is contained, the content of Ta is set to 8% or less. The upper
limit of the Ta content is preferably set to 7% and more preferably
set to 6%. On the other hand, in order to ensure the
above-described effects due to Ta, the lower limit of the Ta
content is preferably set to 0.01%. The lower limit of the Ta
content is more preferably 0.1% and still more preferably 0.5%.
[0122] Re: 8% or less
[0123] Re (rhenium) has the effects of enhancing the high
temperature strength and creep rupture strength by dissolving into
the austenite, which is the matrix. Therefore, in order to obtain
theses effects, Re may be contained. However, if the Re content
exceeds 8%, the workability and mechanical properties are impaired.
Therefore, in the case where Re is contained, the content of Re is
set to 8% or less. The upper limit of the Re content is preferably
set to 7% and more preferably set to 6%. On the other hand, in
order to ensure the above-described effects due to Re, the lower
limit of the Re content is preferably set to 0.01%. The lower limit
of the Re content is more preferably 0.1% and still more preferably
0.5%.
[0124] Ir: 5% or less
[0125] Ir (iridium) has the effects of enhancing the high
temperature strength and creep rupture strength by dissolving into
the austenite, which is the matrix, and by forming fine
intermetallic compounds according to the content. Therefore, in
order to obtain theses effects, Ir may be contained. However, if
the Ir content exceeds 5%, the workability and mechanical
properties are impaired. Therefore, in the case where Ir is
contained, the content of Ir is set to 5% or less. The upper limit
of the Ir content is preferably set to 4% and more preferably set
to 3%. On the other hand, in order to ensure the above-described
effects due to Ir, the lower limit of the Ir content is preferably
set to 0.01%. The lower limit of the Ir content is more preferably
0.05% and still more preferably 0.1%.
[0126] Pd: 5% or less
[0127] Pd (palladium) has the effects of enhancing the high
temperature strength and creep rupture strength by dissolving into
the austenite, which is the matrix, and by forming fine
intermetallic compounds according to the content. Therefore, in
order to obtain theses effects, Pd may be contained. However, if
the Pd content exceeds 5%, the workability and mechanical
properties are impaired. Therefore, in the case where Pd is
contained, the content of Pd is set to 5% or less. The upper limit
of the Pd content is preferably set to 4% and more preferably set
to 3%. On the other hand, in order to ensure the above-described
effects due to Pd, the lower limit of the Pd content is preferably
set to 0.01%. The lower limit of the Pd content is more preferably
0.05% and still more preferably 0.1%.
[0128] Pt: 5% or less
[0129] Pt (platinum) also has the effects of enhancing the high
temperature strength and creep rupture strength by dissolving into
the austenite, which is the matrix, and by forming fine
intermetallic compounds according to the content. Therefore, in
order to obtain theses effects, Pt may be contained. However, if
the Pt content exceeds 5%, the workability and mechanical
properties are impaired. Therefore, in the case where Pt is
contained, the content of Pt is set to 5% or less. The upper limit
of the Pt content is preferably set to 4% and more preferably set
to 3%. On the other hand, in order to ensure the above-described
effects due to Pt, the lower limit of the Pt content is preferably
set to 0.01%. The lower limit of the Pt content is more preferably
0.05% and still more preferably 0.1%.
[0130] Ag: 5% or less
[0131] Ag (silver) has the effects of enhancing the high
temperature strength and creep rupture strength by dissolving into
the austenite, which is the matrix, and by forming fine
intermetallic compounds according to the content. Therefore, in
order to obtain theses effects, Ag may be contained. However, if
the Ag content exceeds 5%, the workability and mechanical
properties are impaired. Therefore, in the case where Ag is
contained, the content of Ag is set to 5% or less. The upper limit
of the Ag content is preferably set to 4% and more preferably set
to 3%. On the other hand, in order to ensure the above-described
effects due to Ag, the lower limit of the Ag content is preferably
set to 0.01%. The lower limit of the Ag content is more preferably
0.05% and still more preferably 0.1%.
[0132] The sum of contents of the above-described elements from Ta
to Ag is preferably 10% or less. The upper limit of the sum of
contents thereof is further preferably 8%.
P.ltoreq.3/{200(Ti+8.5.times.Zr)}
[0133] Regarding the austenitic heat resistant alloy of the present
invention, the contents of Ti, Zr and P each must be in an
already-described range, and also must satisfy the following
formula:
P.ltoreq.3/{200(Ti+8.5.times.Zr)} (1).
The reason for this is as follows. Since Ti and Zr lower the
melting point of the heat resistant alloy, and P deteriorates the
hot workability, in the case where the contents of Ti, Zr and P are
in the already-described ranges respectively but do not satisfy the
above formula (1), the hot workability, especially the hot
workability on the high temperature side of 1150.degree. C. or
higher deteriorates, and further the high temperature crack
resistance at the time of welding may deteriorate. However, if the
contents of Ti, Zr and P satisfy the above-described formula (1),
the hot workability on the high temperature side of 1150.degree. C.
or higher can be improved stably and reliably, while the high creep
rupture strength is maintained, and further, the high temperature
crack resistance at the time of welding can also be improved.
1.35.times.Cr.ltoreq.Ni.ltoreq.1.85.times.Cr, or
1.35.times.Cr.ltoreq.Ni+Co.ltoreq.1.85.times.Cr
[0134] In a case where the content of Ni is in the
already-described range and satisfies the following formula,
1.35.times.Cr.ltoreq.Ni.ltoreq.1.85.times.Cr (2)
in relation to the Cr content, or, in a case where Co is
compositely contained and the contents of both Ni and Co are in the
already-described range and satisfy the following formula,
1.35.times.Cr.ltoreq.Ni+Co.ltoreq.1.85.times.Cr (4)
in relation to the Cr content, the precipitation of a phase during
a long period of use at a high temperature can be restrained stably
and reliably, and moreover, the optimum amount of the .alpha.-Cr
phase can be precipitated. Therefore, the austenitic heat resistant
alloy of the present invention is regulated to satisfy the formula
(2) or formula (4).
Al.gtoreq.1.5.times.Zr
[0135] Regarding the austenitic heat resistant alloy of the present
invention, the content of Al and Zr must be in the
already-described range, and also must satisfy the following
formula:
Al.gtoreq.1.5.times.Zr (3).
The reason for this is that in a case where the contents of Al and
Zr do not satisfy formula (3), though being in the
already-described range, in some cases, the action of Zr for
promoting the precipitation of the .alpha.-Cr phase to enhance the
creep rupture strength cannot be ensured sufficiently. However, if
the contents of Al and Zr satisfy formula (3), the action of Zr for
promoting the precipitation of the .alpha.-Cr phase to enhance the
creep rupture strength can be performed stably and reliably.
[0136] As described above, the austenitic heat resistant alloy of
the present invention is excellent in creep resistance properties
and structural stability. Therefore, if this austenitic heat
resistant alloy is used as a starting material, a heat resistant
pressure member excellent in creep resistance and structural
stability in a high temperature range in accordance with the
present invention, can be obtained easily. The austenitic heat
resistant alloy of the present invention used as the starting
material for the heat resistant pressure member of the present
invention may be melted and cast in the same way as that of the
ordinary austenitic alloy.
[0137] (B) Method for Manufacturing a Heat Resistant Pressure
Member
[0138] Next, a preferred method for manufacturing the heat
resistant pressure member, which is made from the austenitic heat
resistant alloy of the present invention is explained. This
manufacturing method has the feature of including the
before-described steps (i), (ii) and (iii) performed in
sequence.
[0139] Step (i): heating to 1050 to 1250.degree. C. at least once
before final hot or cold working
[0140] In the method in accordance with the present invention, it
is necessary to dissolve the precipitates in the alloy which
precipitated during the working sufficiently, by heating at least
once before the final hot or cold working. However, in the case
where the heating temperature is lower than 1050.degree. C.,
undissolved carbo-nitrides and/or oxides, which contain Ti and B,
come to exist stably in the heated alloy. As a result, the
existence thereof results in the accumulation of nonuniform strain
in the next step (ii), and makes the recrystallization nonuniform
in the final heat treatment of the step (iii). Moreover, the said
undissolved carbo-nitrides and oxides themselves hinder a uniform
recrystallization. On the other hand, if the heating is performed
at a temperature more than 1250.degree. C., high temperature
intergranular fracture and lowering of ductility may be caused.
Therefore, in the preferred method of the present invention,
heating to 1050 to 1250.degree. C. is performed at least once
before the final hot or cold working. The preferable lower limit of
the heating temperature is 1150.degree. C., and the preferable
upper limit thereof is 1230.degree. C.
[0141] Step (ii): carrying out a final hot or cold plastic working
such that the reduction of area is 10% or more
[0142] The plastic working in step (ii) is carried out to give
strains for promoting recrystallization in the next final heat
treatment. In the case where the reduction of area is less than 10%
in this working, a strain necessary for recrystallization cannot be
obtained. Therefore, the plastic working is carried out so that the
reduction of area is 10% or more. The preferable lower limit of the
reduction of area is 20%. Since a larger reduction of area is
better, the upper limit thereof is not defined; however, the
maximum value thereof in the ordinary working is about 90%. This
working step is a step that determines the size of product.
[0143] In the case where the final working after heating is a hot
working, the finish temperature of the hot working is preferably
set to 1000.degree. C. or higher in order to avoid nonuniform
deformation in the temperature range in which carbides precipitate.
Moreover, the cooling condition after working is not subject to any
special restriction; however, after the finish of the hot working,
in order to restrain the precipitation of coarse carbo-nitrides, it
is desirable to perform cooling at the highest possible cooling
rate of 0.25.degree. C./s or higher in the temperature range down
to 500.degree. C.
[0144] In the case where the working after heating is a cold
working, the cold working may be performed once as the final
working or may be performed a number of times. In the case where
the cold working is performed a number of times, a cold working is
performed after intermediate heat treatment, and the heat treatment
temperature in the step (i) and the reduction of area of cold
working in the step (ii) have only to be satisfied in the final
cold working and in the previous intermediate heat treatment.
[0145] Step carrying out a final heat treatment in which cooling is
performed after heating and holding at a temperature in the range
of 1100 to 1250.degree. C.
[0146] If the heating temperature of this heat treatment is lower
than 1100.degree. C., a sufficient recrystallization does not
occur. Moreover, grains become depressed working microstructures,
so that the creep strength decreases. On the other hand, if heating
is performed to a temperature more than 1250.degree. C., high
temperature intergranular fracture and lowering of ductility may be
caused, and therefore, the temperature of the final product heat
treatment is 1100 to 1250.degree. C. The preferable heat treatment
temperature is a temperature 10.degree. C. or more higher than the
heating temperature in the step (i).
[0147] The heat resistant pressure member of the present invention
need not be made of a fine grain microstructure from the viewpoint
of corrosion resistance. When it is desired to make the heat
resistant pressure member a fine grain microstructure, the final
heat treatment has only to be performed at a temperature of
10.degree. C. or lower than the hot working finish temperature or
at a temperature of 10.degree. C. or lower than the above-described
intermediate heat treatment temperature. After this final heat
treatment, in order to restrain the precipitation of coarse
carbo-nitrides, cooling is preferably performed at the highest
possible cooling rate of 1.degree. C./s or higher.
[0148] The following examples illustrate the present invention more
specifically. These examples are, however, by no means limited to
the scope of the present invention.
EXAMPLES
[0149] Austenitic alloys 1 to 17 and A to K, having the chemical
compositions shown in Table 1, were melted by using a
high-frequency vacuum melting furnace and cast to form 17 kg ingots
each having an outside diameter of 100 mm.
[0150] The alloys 1 to 17 shown in Table 1 are alloys whose
chemical compositions fall within the range regulated by the
present invention. On the other hand, the alloys A to K are alloys
of comparative examples whose chemical composition are out of the
range regulated by the present invention. Both of the alloys G and
H are alloys in which the individual contents of Ni and Co are
within the range regulated by the present invention, the value of
"Ni+Co" does not satisfy the said formula (4). The alloy I is an
alloy whose Al content of 0.03% is within the range of "0.01 to
0.3%" which is regulated by the present invention; but the said
content of Al does not satisfy the formula (3). The alloy K is an
alloy whose P content of 0.009% is within the range of "0.03 or
less" which is regulated by the present invention; however the said
content of P does not satisfy the formula (1).
TABLE-US-00001 TABLE 1 Table 1 Chemical composition (% by mass)
Balance: Fe and impurities Alloy C Si Mn P S Cr Ni Co Ni + Co Mo W
Ti Al 1 0.057 0.43 1.05 0.011 0.003 29.2 47.8 -- 47.8 -- 4.3 0.43
0.12 2 0.059 0.41 1.07 0.008 0.002 31.3 50.2 -- 50.2 -- 7.9 0.72
0.16 3 0.056 0.44 1.11 0.005 0.002 31.0 53.4 -- 53.4 -- 11.6 0.53
0.21 4 0.062 0.89 0.95 0.012 0.003 35.2 56.5 -- 56.5 -- 6.8 0.71
0.08 5 0.059 0.37 1.20 0.006 0.002 30.4 43.4 7.2 50.6 -- 8.1 0.82
0.25 6 0.060 0.43 1.08 0.011 0.001 35.8 42.4 14.5 66.9 -- 10.5 0.65
0.14 7 0.055 0.41 0.41 0.008 0.003 30.5 50.3 -- 50.3 -- 7.6 0.74
0.13 8 0.061 0.38 1.85 0.012 0.002 30.2 51.0 -- 51.0 -- 8.5 0.55
0.14 9 0.055 0.50 1.02 0.025 0.0004 34.8 54.9 -- 54.9 -- 6.6 0.19
0.11 10 0.089 0.39 1.08 0.010 0.002 29.7 50.3 -- 50.3 -- 7.8 0.59
0.12 11 0.058 0.42 1.05 0.006 0.002 30.4 50.1 -- 50.1 -- 8.0 0.81
0.17 12 0.134 0.41 1.12 0.005 0.003 30.8 50.5 -- 50.5 -- 7.5 0.74
0.20 18 0.074 0.44 1.17 0.011 0.001 35.7 58.1 -- 58.1 -- 6.8 0.77
0.10 14 0.056 1.23 0.34 0.010 0.002 30.2 51.2 -- 51.2 -- 5.4 0.70
012 15 0.059 0.50 1.52 0.010 0.002 30.5 50.7 -- 50.7 -- 7.1 0.85
0.15 16 0.061 0.45 1.10 0.008 0.001 30.9 50.6 -- 50.6 -- 8.1 0.75
0.14 17 0.058 0.47 1.08 0.007 0.002 31.2 50.4 -- 50.4 -- 7.7 0.70
0.16 A 0.061 0.40 1.01 0.007 0.002 31.0 49.9 -- 49.9 -- 8.0 0.75
0.16 B 0.057 0.43 1.06 0.007 0.002 31.2 50.1 -- 50.1 -- 8.1 * --
0.14 C 0.060 0.44 1.07 0.010 0.003 30.1 48.1 -- 48.1 -- * 2.7.sup.
0.45 0.14 D 0.060 0.41 1.01 0.010 0.002 31.5 49.9 -- 49.9 -- 8.0
0.74 0.15 E 0.062 0.41 1.10 0.008 0.003 31.1 50.5 -- 50.5 * 2.5 *
-- 0.75 0.14 F 0.061 0.47 0.99 0.010 0.002 31.0 50.4 -- 50.4 * 2.2
3.4 0.72 0.16 G 0.057 0.37 1.18 0.008 0.003 32.0 40.2 2.4 *
42.6.sup. -- 7.5 0.78 0.13 H 0.059 0.39 1.15 0.006 0.002 29.2 52.1
7.3 * 59.4.sup. -- 8.1 0.84 0.25 I 0.060 0.43 1.07 0.009 0.003 31.1
50.5 -- 50.5 -- 8.2 0.74 * 0.03.sup. J 0.062 0.43 1.10 0.011 0.002
31.0 50.7 -- 50.7 -- 7.8 0.71 * 0.64.sup. K 0.061 0.38 1.17 *
0.009.sup. 0.002 30.2 44.5 7.6 52.1 -- 7.8 0.89 0.25 Chemical
composition (% by mass) Balance: Fe and impurities Alloy N Zr
Others value of f1 value of f2 value of f3 1 0.009 0.04 -- 0.019
1.637 0.060 2 0.013 0.05 -- 0.018 1.604 0.075 3 0.011 0.13 -- 0.009
1.723 0.195 4 0.012 0.02 B: 0.0063 0.017 1.605 0.030 5 0.008 0.13
-- 0.008 1.664 0.195 6 0.014 0.03 B: 0.0041 0.017 1.589 0.045 7
0.012 0.02 V: 0.78, Nb: 0.32, B: 0.0026 0.016 1.649 0.030 8 0.011
0.04 B: 0.0033, Mg: 0.0023, Ca: 0.0028 0.017 1.689 0.060 9 0.010
0.02 B: 0.0043, Y: 0.02, La: 0.03 0.042 1.578 0.030 10 0.012 0.03
Nd: 0.03 0.018 1.694 0.045 11 0.005 0.03 Ce: 0.03, Sc: 0.05 0.014
1.648 0.045 12 0.007 0.05 Hf: 0.28, Re: 1.2 0.013 1.640 0.075 18
0.005 0.02 Ta: 1.3 0.016 1.627 0.030 14 0.012 0.03 Ir: 1.2, Ag: 1.5
0.016 1.695 0.045 15 0.008 0.02 Pd: 1.1, Pt: 1.0 0.015 1.662 0.030
16 0.013 0.04 Ca: 0.0035, Ta: 3.8 0.014 1.638 0.060 17 0.012 0.03
B: 0.0031, Mg: 0.0041, Re: 2.4 0.016 1.615 0.045 A 0.013 * -- --
0.020 1.610 -- B 0.013 0.06 -- 0.029 1.606 0.090 C 0.010 0.04 --
0.019 1.598 0.060 D * 0.024.sup. 0.05 -- 0.013 1.584 0.075 E 0.012
0.04 -- 0.014 1.624 0.060 F 0.013 0.05 -- 0.013 1.626 0.075 G 0.007
0.04 -- 0.013 1.331 0.060 H 0.008 0.12 -- 0.008 2.034 0.180 I 0.014
0.05 -- 0.013 1.624 0.075 J 0.013 0.04 -- 0.014 1.635 0.060 K 0.008
0.15 -- 0.007 1.725 0.225 f1 = 3/{200(Ti + 8.5 .times. Zr)}, f2 =
(Ni + Co)/Cr, f3 = 1.5 .times. Zr The mark * indicates falling
outside the conditions regulated by the present invention.
[0151] Thus the obtained ingot was heated to 1180.degree. C., and
then was hot forged so that the finish temperature was 1050.degree.
C. to form a plate material having a thickness of 15 mm. After the
hot forging, the plate material was air cooled.
[0152] From a middle portion in the thickness direction of the 15
mm thick plate material obtained by the above-mentioned hot
forging, a round bar tensile test specimen, having a diameter of 10
mm and a length of 130 mm, was produced by machining the plate
material in parallel to the longitudinal direction, and the tensile
test specimen was used to evaluate the high temperature
ductility.
[0153] That is to say, the said round bar tensile test specimen was
heated to 1200.degree. C. and was held for 3 minutes, and then a
high speed tensile test was conducted at a strain rate of 10/s in
order to determine the reduction of area from the fracture surface
after testing. It was found that if the reduction of area is 60% or
more, no major problem occurred, even if hot working, such as hot
extrusion is performed at that temperature. Therefore, the
reduction of area of "60% or more" was made the criterion of
excellent hot workability.
[0154] Moreover, using the 15 mm thick plate material obtained by
the said hot forging, a softening heat treatment was performed at
1100.degree. C., and then the plate material was cold rolled so
that the thickness thereof becomes 10 mm, and further, the cold
rolled plate material was water cooled after being held at
1200.degree. C. for 30 minutes.
[0155] Using a part of the above-described 10 mm thick plate
material water cooled after being held at 1200.degree. C. for 30
minutes, and from a middle portion in the thickness direction of
the part, a round bar tensile test specimen, having a diameter of 6
mm and a gage length of 30 mm, was produced by machining the part
in parallel to the longitudinal direction; the tensile test
specimen was used to conduct a creep rupture test.
[0156] That is to say, by using the above-described test specimen,
the creep rupture test was conducted in the air of 700.degree. C.,
750.degree. C. and 800.degree. C., and by generalizing the obtained
rupture strength using the Larson-Miller parameter method, the
rupture strength at 700.degree. C. in 10,000 hours was
determined.
[0157] Furthermore, the remainder of the 10 mm thick plate material
water cooled after being held at 1200.degree. C. for 30 minutes was
subjected to an aging treatment in which the test specimen was held
at 750.degree. C. for 5000 hours, and then was water cooled.
[0158] From a middle portion in the thickness direction of the 10
mm thick plate material water cooled after an aging treatment, a
V-notch test specimen having a width of 5 mm, a height of 10 mm,
and a length of 55 mm, specified in JIS Z 2242 (2005) was produced
in parallel to the longitudinal direction, and a Charpy impact test
at 0.degree. C. was conducted on the test specimen in order to
measure the impact value and evaluate the toughness.
[0159] The results of the above-described tests are summarized in
Table 2.
TABLE-US-00002 TABLE 2 Creep rupture strength Reduction at
700.degree. C. .times. Charpy of area Test 10000 h impact value at
1200.degree. C. No. Alloy (MPa) (J/cm.sup.2) (%) Note 1 1 158.4
63.4 86.5 Inventive 2 2 165.2 55.6 80.4 example 3 3 168.5 48.3 85.2
4 4 170.1 57.4 80.8 5 5 169.3 51.2 71.5 6 6 172.2 41.3 72.6 7 7
169.5 53.5 81.0 8 8 164.1 56.2 88.7 9 9 155.3 59.0 92.5 10 10 163.5
57.8 90.1 11 11 166.4 55.9 85.6 12 12 165.0 56.0 81.2 13 13 171.2
57.6 81.1 14 14 165.4 58.5 81.9 15 15 167.2 53.7 71.8 16 16 168.3
54.8 86.2 17 17 167.9 55.0 87.1 18 *A 142.5 55.8 80.8 Comparative
19 *B 135.1 55.1 85.3 example 20 *C 148.9 63.9 86.2 21 *D 151.5
51.6 78.8 22 *E 141.1 11.5 80.6 23 *F 143.5 13.4 81.0 24 *G 148.5
15.2 79.8 25 *H 139.6 52.8 71.1 26 *I 151.9 51.9 81.2 27 *J 164.9
24.8 52.3 28 *K 169.0 50.7 50.2 The mark * indicates falling
outside the conditions regulated by the present invention.
[0160] From Table 2, regarding the test Nos. 1 to 17 using the
alloys 1 to 17, which are the inventive examples, it is apparent
that all of the creep rupture strength, toughness after aging, and
hot workability are excellent.
[0161] In contrast, regarding the test Nos. 18 to 28 using the
alloys A to K, which are the comparative examples deviating from
the conditions regulated by the present invention, at least one of
the creep rupture strength, toughness after aging, and hot
workability is poorer than that of the above-mentioned test Nos. 1
to 17, being the inventive examples
[0162] That is to say, in the case of test No. 18, the chemical
composition of the alloy A is almost equivalent to that of the
alloy 2, used in the test No. 2. However, the said alloy A does not
contain Zr, and therefore the creep rupture strength is low.
[0163] In the case of test No. 19, the chemical composition of the
alloy B is almost equivalent to that of the alloy 2, used in the
test No. 2. However, the said alloy B does not contain Ti, and
therefore the creep rupture strength is low.
[0164] In the case of test No. 20, the chemical composition of the
alloy C is almost equivalent to that of the alloy 1, used in the
test No. 1. However, the W content of the said alloy C is "2.7%",
which is lower than the value regulated by the present invention,
and therefore the creep rupture strength is low.
[0165] In the case of test No. 21, the chemical composition of the
alloy D is almost equivalent to that of the alloy 2, used in the
test No. 2. However, the N content of the said alloy D is "0.024%",
which is higher than the value regulated by the present invention,
and therefore the creep rupture strength is low.
[0166] In the case of test No. 22, the chemical composition of the
alloy E is almost equivalent to that of the alloy 2, used in the
test No. 2. However the said alloy E does not contain W, and
moreover the Mo content thereof is "2.5%", which is higher than the
value regulated by the present invention. Therefore, the creep
rupture strength is low, and further the Charpy impact value after
aging is remarkably low, so that the toughness is poor.
[0167] In the case of test No. 23, if the operational advantage of
W is about a half of that of Mo, that is to say, if the W content
corresponds to about a half of the Mo content, as being said
conventionally, the alloy F is an alloy which is equivalent to the
alloy 2, used in the test No. 2. However, the Mo content of the
said alloy F is "2.2%", which exceeds the value regulated by the
present invention. Therefore, the creep rupture strength is low,
and further the Charpy impact value after aging is remarkably low,
so that the toughness is poor.
[0168] In the case of test No. 24, the chemical composition of the
alloy G is almost equivalent to that of the alloy 5, used in the
test No. 5. However the sum of the Ni content and the Co content,
that is to say, the value of "Ni+Co" of the said alloy G is lower
than "1.35.times.Cr" and does not satisfy the formula (4).
Therefore, the creep rupture strength is low, and moreover the
Charpy impact value after aging is remarkably low, so that the
toughness is poor.
[0169] In the case of test No. 25, the chemical composition of the
alloy H is almost equivalent to that of the alloy 5, used in the
test No. 5. However, the sum of the Ni content and the Co content,
that is to say, the value of "Ni+Co" of the said alloy H is higher
than "1.85.times.Cr" and does not satisfy the formula (4).
Therefore, the creep rupture strength is low.
[0170] In the case of test No. 26, the chemical composition of the
alloy I is almost equivalent to that of the alloy 2, used in the
test No. 2. However, the Al content of the said alloy I is lower
than "1.5.times.Zr" and does not satisfy the formula (3).
Therefore, the creep rupture strength is low.
[0171] In the case of test No. 27, the chemical composition of the
alloy J is almost equivalent to that of the alloy 2, used in test
No. 2. However, the Al content of the said alloy J is "0.64%",
which is higher than the value regulated by the present invention.
Therefore, the Charpy impact value after aging is remarkably low,
so that the toughness is poor. Moreover the reduction of area at
1200.degree. C. does not reach 60%, so that the hot workability is
low.
[0172] In the case of test No. 28, the chemical composition of the
alloy K is almost equivalent to that of the alloy 5, used in the
test No. 5. However, the P content of the said alloy K exceeds
"3/{200(Ti+8.5.times.Zr)}" and does not satisfy the formula (1).
Therefore, the reduction of area at 1200.degree. C. is 50.2%, so
that the hot workability is remarkably low.
INDUSTRIAL APPLICABILITY
[0173] The austenitic heat resistant alloy according to the present
invention, has high temperature strength, especially creep rupture
strength, higher than that of the conventional heat resistant
alloys, and also has high toughness because the structural
stability is excellent even after a long period of use at a high
temperature. Further it is excellent in hot workability, especially
high temperature ductility at 1150.degree. C. or higher. Therefore,
this austenitic heat resistant alloy can be suitably used as a pipe
material, a plate material for a heat resistant pressure member, a
bar material, forgings, and the like for a boiler for power
generation, a plant for chemical industry and so on.
* * * * *