U.S. patent application number 12/870432 was filed with the patent office on 2011-03-10 for ni based casting alloy and turbine casing.
This patent application is currently assigned to Hitachi, Ltd.. Invention is credited to Hiroyuki Doi, Shinya Imano, Jun Sato.
Application Number | 20110058977 12/870432 |
Document ID | / |
Family ID | 43027457 |
Filed Date | 2011-03-10 |
United States Patent
Application |
20110058977 |
Kind Code |
A1 |
Sato; Jun ; et al. |
March 10, 2011 |
NI BASED CASTING ALLOY AND TURBINE CASING
Abstract
A Ni based cast alloy consisting essentially of C: 0.01 to 0.2%
by weight, Si: 0.5 to 4.0% by weight, Cr: 14 to 22% by weight,
Mo+W: 4.0 to 10% by weight, B: 0.001 to 0.02% by weight, Co: up to
10% by weight, Al: up to 0.5% by weight, Ti: up to 0.5% by weight,
Nb: up to 5.0% by weight, Fe: up to 10% by weight, the balance
being Ni and incidental impurities, wherein a .gamma.' phase
precipitates in a matrix phase thereof.
Inventors: |
Sato; Jun; (Hitachi, JP)
; Imano; Shinya; (Hitachi, JP) ; Doi;
Hiroyuki; (Tokai, JP) |
Assignee: |
Hitachi, Ltd.
|
Family ID: |
43027457 |
Appl. No.: |
12/870432 |
Filed: |
August 27, 2010 |
Current U.S.
Class: |
420/448 ;
148/410; 420/442; 420/445; 420/449; 420/450; 420/451; 420/453;
420/454 |
Current CPC
Class: |
C22C 19/055 20130101;
C22C 19/056 20130101; C22C 19/051 20130101; C22F 1/10 20130101 |
Class at
Publication: |
420/448 ;
420/445; 420/450; 420/451; 420/442; 420/449; 420/453; 420/454;
148/410 |
International
Class: |
C22C 19/05 20060101
C22C019/05 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 4, 2009 |
JP |
2009-204246 |
Claims
1. A Ni based cast alloy having an as-cast structure and consisting
essentially of C: 0.01 to 0.2% by weight, Si: 0.5 to 4.0% by
weight, Cr: 14 to 22% by weight, Mo+W: 4.0 to 10% by weight, B:
0.001 to 0.02% by weight, Co: up to 10% by weight, Al: up to 0.5%
by weight, Ti: up to 0.5% by weight, Nb: up to 5.0% by weight, Fe:
up to 10% by weight, the balance being Ni and incidental
impurities, wherein a .gamma.' phase precipitates in a matrix phase
thereof.
2. A Ni based cast alloy having an as-cast structure and consisting
essentially of C: 0.01 to 0.2% by weight, Si: 0.5 to 4.0% by
weight, Cr: 14 to 22% by weight, Mo+W: 4.0 to 10% by weight, B:
0.001 to 0.02% by weight, the balance being Ni and incidental
impurities, wherein a .gamma.' phase precipitates in a matrix phase
thereof.
3. A Ni based cast alloy having an as-cast structure and consisting
essentially of C: 0.01 to 0.2% by weight, Si: 0.5 to 4.0% by
weight, Cr: 14 to 22% by weight, Co: 0.1 to 10% by weight, Al: 0.1
to 0.5% by weight, Ti: 0.1 to 0.5% by weight, Nb: 1.0 to 4.0% by
weight, Mo+W: 4.0 to 10% by weight, Fe: 0.1 to 10% by weight, B:
0.001 to 0.02% by weight, the balance being Ni and incidental
impurities, wherein a .gamma.' phase precipitates in a matrix phase
thereof.
4. A Ni based cast alloy having an as-cast structure and consisting
essentially of C: 0.05 to 0.15% by weight, Si: 1.0 to 3.5% by
weight, Cr: 15 to 20% by weight, Al: 0.1 to 0.5% by weight, Ti: 0.1
to 0.5% by weight, Nb: 1.0 to 4.0% by weight, Co: 1.0 to 5% by
weight, Fe: 1.0 to 5% by weight, Mo+W: 5.0 to 8% by weight, B:
0.002 to 0.01% by weight, the balance being Ni and incidental
impurities, wherein a .gamma.' phase precipitates in a matrix phase
thereof.
5. The Ni based cast alloy according to claim 1, wherein a .gamma.'
phase precipitates in a matrix phase thereof.
6. The Ni based cast alloy according to claim 2, wherein a .gamma.'
phase precipitates in a matrix phase thereof.
7. The Ni based cast alloy according to claim 3, wherein a .gamma.'
phase precipitates in a matrix phase thereof.
8. The Ni based cast ally according to claim 1, the alloy having
been subjected to heat treatment at 700 to 800.degree. C. to
precipitate Ni.sub.3Si type intermetallic compound.
9. The Ni based cast ally according to claim 1, the alloy having
been prepared by melting and casting in air or inert gas
atmosphere.
10. A high temperature part for a steam turbine made of the alloy
according to claim 1.
11. A high temperature part for a steam turbine made of the cast
alloy according to claim 2.
12. A high temperature part for a steam turbine made of the alloy
according to claim 3.
13. A steam turbine casing made of cast alloy according to claim
1.
14. A steam turbine casing made of cast alloy according to claim
2.
15. A steam turbine casing made of cast alloy according to claim 3.
Description
CLAIM OF PRIORITY
[0001] This application claims priority from Japanese patent
application serial No. 2009-204246, filed on Sep. 4, 2009, the
content of which is hereby incorporated by reference into this
application.
FIELD OF THE INVENTION
[0002] The present invention relates to a Ni based casting alloy
suitable for high temperature parts for steam turbines and a steam
turbine casing.
BACKGROUND OF THE INVENTION
[0003] In order to increase a power generation efficiency of a
steam turbine, an increase in temperature of steam is necessary. As
for materials that withstand high temperatures and high pressures,
ferritic steels such as Cr--Mo--V steels or 12Cr steels have been
utilized. The ferritic steels are excellent in high temperature
strength and productivity and they are of low cost. Therefore, they
have been utilized as forging materials for turbine rotors and
casting materials for turbine casings, etc. (Patent document Nos.
1, 2).
[0004] Ni based superalloys that have higher strength than the
conventional ferritic heat resisting steels have been utilized as
high temperature parts for gas turbines. The Ni based superalloys
have higher heat resisting temperature than the ferritic heat
resisting steels, and when they are utilized, it is expected to
obtain higher power generation efficiency.
[0005] The Ni based superalloys generally contain Al and/or Ti,
which precipitates an intermetallic compound phase of
Ni.sub.3(Al,Ti) type, called .gamma.' phase to thereby increase a
mechanical strength (patent document No. 3 etc). Since the .gamma.'
phase increases mechanical strength as temperatures increase, it is
suitable for strengthening phase for heat resisting materials.
However, these elements have a problem in production of the steel
because they tend to be oxidized during melting of the materials
for the steels. If Al and Ti are oxidized, a desired mechanical
strength is not obtained because of shortage of strengthening
elements in the alloys, and in addition, reliability of the alloys
decrease because of inclusion of the oxides as casting defects in
the alloys. Therefore, in a melting process for the Ni based
superalloys, such high technical melting process as electroslag
re-melting or vacuum arc re-melting have been essential (patent
document No. 4). The patent document No. 4 relates to a Ni--Fe
based alloy containing Al and Ti. However, the above technologies
cannot be applied to such large scale and complicated parts such as
turbine casings, and therefore, it was difficult to produce Ni
based casting alloys of high temperature parts with high mechanical
strength and high reliability.
[0006] If, in conventional Ni based alloys containing Cr, Mo+W and
B, Al and/or Ti is not added in order to avoid oxidation, a
sufficient mechanical strength is not obtained because the .gamma.'
phase for strengthening by precipitation does not exist. Therefore,
it is impossible to elevate temperatures of steam to obtain high
power generation efficiency.
PRIOR ART
[0007] Patent document No. 1: Japanese patent laid-open
2006-22343
[0008] Patent document No. 2: Japanese patent laid-open
2007-92123
[0009] Patent document No. 3: Japanese patent laid-open
2000-169924
[0010] Patent document No. 4: Japanese patent laid-open
2006-118016
SUMMARY OF THE INVENTION
[0011] It is an object of the present invention to provide Ni based
cast alloys having the precipitated .gamma.' phase and high
mechanical strength, which can be produced by a low cost casting
process that is similar to that of the conventional heat resisting
steels.
[0012] According to one aspect of the Ni based casting alloy of the
present invention, the Ni based cast alloy has an as-cast structure
and consists essentially of C: 0.01 to 0.2% by weight, Si: 0.5 to
4.0% by weight, Cr: 14 to 22% by weight, Mo+W: 4.0 to 10% by
weight, B: 0.001 to 0.02% by weight, Co: up to 10% by weight, Al:
up to 0.5% by weight, Ti: up to 0.5% by weight, Nb: up to 5.0% by
weight, Fe: up to 10% by weight, the balance being Ni and
incidental impurities, wherein .gamma.' phase precipitates in a
matrix phase. The matrix phase in this specification means a
dominant part of an alloy structure, and the alloy structure of the
alloy means a group of different phases and grains constituting the
alloy.
[0013] According to one aspect of the present invention, the cast
alloy consisting essentially of C: 0.01 to 0.2% by weight, Si: 0.5
to 4.0% by weight, Cr: 14 to 22% by weight, Mo+4.0 to 10% by
weight, B: 0.001 to 0.02% by weight, the balance being Ni and
incidental impurities.
[0014] According to another aspect of the present invention, the
cast alloy consists essentially of C: 0.01 to 0.2% by weight, Si:
0.5 to 4.0% by weight, Cr: 14 to 22% by weight, Co: 0.1 to 10% by
weight, Al: 0.1 to 0.5% by weight, Ti: 0.1 to 0.5% by weight, Nb:
1.0 to 4.0% by weight, Mo+W: 4.0 to 10% by weight, Fe: 0.1 to 10%
by weight, B: 0.001 to 0.02% by weight, the balance being Ni and
incidental impurities.
[0015] According to still another aspect of the present invention,
the Ni based cast alloy consists essentially of C: 0.05 to 0.15% by
weight, Si: 1.0 to 3.5% by weight, Cr: 15 to 20% by weight, Al: 0.1
to 0.5% by weight, Ti: 0.1 to 0.5% by weight, Nb: 1.0 to 4.0% by
weight, Co: 1.0 to 5% by weight, Fe: 1.0 to 5% by weight, Mo+W: 5.0
to 8% by weight, B: 0.002 to 0.01% by weight, the balance being Ni
and incidental impurities.
[0016] These alloys precipitate the .gamma.' Ni.sub.3Si phase as
the strengthening phase by a suitable heat treatment, and the phase
can exist during in its service, to thereby obtain excellent high
temperature mechanical strength. Since there is no loss of
strengthening elements by oxidation even in conventional melting
process and no inclusion of oxides, reliability of the castings is
high, which are suitable for high temperature parts such as steam
turbine casings.
[0017] According to the above alloy compositions, it is possible to
provide high mechanical strength Ni based casting alloys, which can
be produced by low cost conventional melting process. In addition,
it is possible to produce steam turbine casting parts with high
reliability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a diagrammatic view of the alloy structure of the
present invention.
[0019] FIG. 2 is a graph showing creep rupture time of the alloys
of the examples.
[0020] FIG. 3 a graph showing creep rupture elongation of the
alloys of the examples.
[0021] FIG. 4 shows a cross sectional view of a steam valve for a
steam turbine to which the present invention is applied.
[0022] FIG. 5 shows a cross sectional view of a steam turbine rotor
to which the present invention is applied.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] The present inventors have investigated influences of
alloying elements on properties of Ni based alloys, and as a
result, they invented Ni based casting alloys suitable for steam
turbines. In the following, alloying elements and adding ranges
thereof are explained.
(1) C: Carbon solid-dissolves into a matrix phase to increase a
tensile strength at high temperatures, and forms carbides such as
MC, M.sub.23C.sub.6 to strengthen grain boundaries. These effects
becomes when 0.01% by weight of carbon is contained. If the amount
exceeds 0.2% by weight, coarse eutectic carbides precipitate to
lower ductility of the alloys. Thus, 0.2% by weight is an upper
limit. An amount of 0.05 to 0.15% by weight is a preferable range.
(2) Si: Si has been known as an effective element for deoxidizing
and casting performance. In the present invention, silicon is added
as a strengthening element. An excess amount of silicon lowers a
melting point, and forms undesirable phase. In the present
invention, after detailed investigations of influences of elements,
it is possible to add a larger amount of silicon than the
conventional alloys by balancing the elements. In order to
precipitate Ni3Si as the strengthening phase, 0.5% by weight of
silicon is necessary, but if the amount exceeds 4% by weight,
segregation at solidification becomes large to thereby lower
strength at grain boundaries. A preferable amount range is 1.0 to
3.5% by weight. (3) Cr: Chromium increases anti-oxidation property
and high temperature anti-corrosion property by forming dense oxide
film made of Cr.sub.2O.sub.3 on the surface of the alloy. At least
14% by weight of Cr is necessary for the high temperature parts. If
the amount exceeds 22% by weight, a a phase precipitates to
decrease ductility and rupture ductility. A preferable range is 15
to 20% by weight. (4) Mo, W: Molybdenum and tungsten strengthen the
matrix phase by solid-solution strengthening. In order to obtain
sufficient strengthening, a total amount of them should be 4% by
weight or more, but if the total amount exceeds 10% by weight, the
elements may accelerate formation of hard and brittle intermetallic
compound phase and may deteriorate ductility at high temperatures.
A preferable total amount range is 6 to 9% by weight. (5) B: A
small amount of boron strengthens grain boundaries and improves
creep strength. An excess amount of B precipitates undesirable
phases and lowers melting point, which may be a cause of partial
melting. An amount of B should be 0.001 to 0.02% by weight. A
preferable range is 0.001 to 0.02% by weight. (6) Co: Cobalt
improves high temperature strength by solid-dissolving into the
matrix phase to thereby substitute with Ni and contributes to
improvement of high temperature anti-corrosion property. In the
alloy composition of the present invention, 0.1% by weight or more
is necessary for the above properties. An excess amount assists
precipitation of undesirable phases such as the .sigma. phase or
.mu. phase, and an upper limit is 10% by weight. (7) Al: In
conventional Ni based alloys, Al has been added to form Ni.sub.3Al
phase as a strengthening element. In the present invention, Al
contributes to strengthening of the Ni.sub.3Si phase. However,
since Al is an active element, Al is heavily oxidized during casing
in air to deteriorate reliability of the products. An upper limit
of Al is 0.5% by weight, accordingly. A preferable range is 0.1 to
0.4% by weight. (8) Ti: Titanium, similarly to Al, stabilizes and
strengthens the .gamma.' phase. Since Ti is also an active element,
an upper limit is 0.5% by weight. A preferable range is 0.1 to 0.4%
by weight. (9) Nb: Niobium contributes to strengthening of the
.gamma.' phase, similarly to Al and Ti. Since Nb is less oxidative
than Al and Ti, 5% by weight as an upper limit is acceptable. If an
excess amount is added, Ni.sub.3Nb is formed to deteriorate
stability of the alloy structure for a long time. (10) Fe: Iron
contributes to solid-solution strengthening by substituting with
Ni. From the view point of economy, it is preferable to add iron as
much as possible, but Fe is relatively poor in stabilizing the
.gamma.' phase, compared with Ni. Thus, an upper limit is 10% by
weight. A preferable range is 1.0 to 5.00% by weight.
[0024] Table 1 shows alloy compositions of the example Nos. 1 to 8
and comparative example alloy Nos. 1 to 5.
[0025] 10 Kgs of each of the alloys was melted in air, and the
molten metal was casted in a sand mold to produce a columnar cast
ingots with a diameter of 100 mm. The resulting ingots were
subjected to heat treatment at 1150.degree. C. for 30 minutes, and
720.degree. C. for 24 hours. Thereafter, alloy structures of the
ingots were observed, and the ingots were subjected to high
temperature creep tests.
[0026] Among the heat treatments, the first one was a solution heat
treatment, which makes non-uniform cast structure uniform. The
higher the temperature, the better the result is obtained; however,
in order to avoid partial melting or coarsening of the structure,
the heat treatment at 1100 to 1200.degree. C. is preferable. A heat
treatment after the solution heat treatment is carried out for
precipitating a strengthening phase. Though a temperature for the
second heat treatment may be chosen based on materials or use
conditions of components, an amount of precipitation of the
strengthening phase is too small if the temperature is higher than
800.degree. C., but on the other hand, precipitation is hard to
take place if the temperature is lower than 700.degree. C.
Therefore, the temperature for precipitating the strengthening
phase is preferably 700 to 800.degree. C.
TABLE-US-00001 TABLE 1 Alloying elements No. Alloy Ni C Si Cr Mo W
B Co Al Ti Nb Fe 1 Ex. 1 Bal. 0.05 1.6 18.0 2.0 4.0 0.005 -- -- --
-- -- 2 Ex. 2 Bal. 0.04 2.7 16.0 4.0 2.5 0.005 -- -- -- -- -- 3 Ex.
3 Bal. 0.05 3.6 16.0 -- 5.0 0.004 -- -- -- -- -- 4 Ex. 4 Bal. 0.05
3.0 18.0 8.0 -- 0.004 2.0 0.2 0.2 -- 2.5 5 Ex. 5 Bal. 0.05 3.0 18.0
5.0 2.5 0.004 2.0 0.2 0.2 3.0 2.5 6 Ex. 6 Bal. 0.05 2.5 20.0 5.0
2.5 0.004 -- 0.2 0.1 5.0 5.0 7 Ex. 7 Bal. 0.1 1.6 18.0 5.0 -- 0.002
5.0 0.4 -- 4.0 5.0 8 Ex. 8 Bal. 0.1 3.0 20.0 3.0 3.0 0.002 8.0 --
0.2 -- 5.0 9 Com. Bal. 0.05 0.5 18.0 8.0 -- 0.004 5.0 -- -- -- --
Ex. 1 10 Com. Bal. 0.05 4.5 20.0 8.0 -- 0.004 -- -- -- 5.0 10.0 Ex.
1 11 Com. Bal. 0.05 2.5 16.0 2.0 4.0 0.004 10.0 0.5 2.0 4.0 2.0 Ex.
1 12 Com. Bal. 0.05 2.5 18.0 8.0 -- 0.004 2.0 1.5 -- 2.0 -- Ex. 1
13 Com. Bal. 0.05 0.1 22.0 9.0 -- 0.004 0.5 0.2 0.2 4.0 2.5 Ex.
1
[0027] FIG. 1 shows a diagrammatic view of the alloy structures of
example alloy Nos. 1 to 8. In the inventive alloys, the .gamma.'
phase for strengthening precipitates dispersedly and a small amount
of carbides precipitate at grain boundaries. The structure is
similar to the conventional .gamma.' precipitation strengthening
type Ni based alloys. This shows an effect of Si addition.
[0028] On the other hand, in comparative example alloy No. 1, since
an amount of Si is small, and since no Al and T are added, the
.gamma.' phase did not precipitate. In the comparative alloy No. 2,
since a sufficient amount of Si was added, the .gamma.' phase
precipitated, but large precipitation of the .gamma.' phase was
observed at the grain boundaries and boundaries of dendrites. In
comparative alloy No. 3, though Al and Ti were added in addition to
Si, it was observed that oxides formed during casting were included
in the alloy. The comparative alloy No. 4 is the same. The
comparative example alloy No. 5 corresponds to alloy 625, which has
been commercially available on the market. Though inclusion of
oxides was not observed since amounts of Al and Ti were small,
alloy materials that have been subjected to holding at high
temperatures such as creep tests, precipitation of Ni.sub.3Nb was
observed.
[0029] Kinds of precipitates and evaluation results of soundness of
the alloy structures are shown in Table 2.
TABLE-US-00002 TABLE 2 Creep Creep rupture Rupture Soundness of
time elongation No. Alloy Alloy Structure structure (h) (%) 1 Ex. 1
.gamma.' and carbides .largecircle. 468 35 2 Ex. 2 .gamma.' and
carbides .largecircle. 553 33 3 Ex. 3 .gamma.' and carbides
.largecircle. 701 25 4 Ex. 4 .gamma.' and carbides .largecircle.
635 27 5 Ex. 5 .gamma.' and carbides .largecircle. 820 26 6 Ex. 6
.gamma.'and carbides .largecircle. 612 32 7 Ex. 7 .gamma.' and
carbides .largecircle. 605 32 8 Ex. 8 .gamma.' and carbides
.largecircle. 688 31 9 Com. Ex. 1 -- .DELTA. 165 41 10 Com. Ex. 2
.gamma.' and carbides .DELTA. 305 10 11 Com. Ex. 3 .gamma.' and
carbides, X 184 8 oxides 12 Com. Ex. 4 .gamma.' and carbides, X 206
7 oxides 13 Com. Ex. 5 .gamma.' and carbides, .DELTA. 410 27
Ni.sub.3Nb phase
[0030] FIGS. 2 and 3 show creep rupture time and creep rupture
elongation of the alloys shown in Table 1. The creep test was
conducted at 700.degree. C. under a load of 333 MPa. Every
inventive alloy exhibited superior creep rupture time to the
conventional alloy (Comparative example alloy No. 5). Addition of
Si precipitated the .gamma.' phase to thereby improve high
temperature strength. As to the high temperature ductility, 25% or
more of elongation was observed.
[0031] The comparative example alloy No. 1 contained small amounts
of strengthening elements and no .gamma.' phase exists. Therefore,
it has low creep rupture strength. In the comparative example alloy
No. 2, which contained a large amount of Si, it has higher creep
rupture strength than that of the comparative example alloy No. 1,
but it has a low creep elongation. This is because large
precipitates existed at grain boundaries and dendrite boundaries,
which means the amount of Si was excess.
[0032] In the comparative example alloy Nos. 3 and 4, there was
observed inclusion of oxides. Rupture cracks were found wherein the
ruptures started at included oxides so that the creep rupture time
and creep rupture elongation were quite low. Accordingly, active
amounts of Al and Ti should be made small to improve
characteristics of the alloys for the present invention. Since the
amounts of Al and Ti in the comparative alloy No. 5 are controlled
to small amounts, deterioration of characteristics due to oxidation
was not observed, but Ni.sub.3Nb precipitated as the time goes at
high temperatures. Therefore, the example alloy of the present
invention showed excellent structure stability by virtue of Si.
[0033] The alloys of the present invention are applied to high
temperature components such as a casing for a rotor or a steam
valve of a steam turbine.
[0034] FIG. 4 shows a cross sectional view of a steam valve
comprising a valve casing 1, a valve body 2, a valve sheet 3, a
piping 4, a sleeve 5, a shaft bush 6 and a valve shaft 7. The alloy
of the present invention is applied to the valve casing 1, valve
body 2 and valve sheet 3, which are produced by casting. These
components that have as-cast structures having .gamma.' precipitate
in the matrix phase are subjected to proper heat treatments before
assembling. Detailed descriptions of the steam valve are omitted
because the structure and functions of the components are well
known in the art.
[0035] FIG. 5 shows a cross sectional view of a steam turbine rotor
comprising nozzles 14, 15, blades 16, 17, inner casings 18, 20, 21,
outer casings 19, 22, flange and elbow 25, a steam inlet 28, a
rotor shaft 33, a nozzle box 38 and a journal 43. The alloy of the
present invention is applied to the inner casings 18, 20, 21 and
the outer casings 19, 22, which are produced by casting. These
components as-cast structures having .gamma.' precipitate in the
matrix phase are subjected to proper heat treatments before
assembling. Detailed descriptions of the steam turbine rotor are
omitted because the structure and functions of the components are
well known in the art.
* * * * *