U.S. patent application number 14/220280 was filed with the patent office on 2014-07-24 for cast product having alumina barrier layer.
This patent application is currently assigned to KUBOTA CORPORATION. The applicant listed for this patent is KUBOTA CORPORATION. Invention is credited to YOUHEI ENJO, KUNIHIDE HASHIMOTO, MAKOTO HINENO, SHINICHI URAMARU.
Application Number | 20140205802 14/220280 |
Document ID | / |
Family ID | 51207910 |
Filed Date | 2014-07-24 |
United States Patent
Application |
20140205802 |
Kind Code |
A1 |
ENJO; YOUHEI ; et
al. |
July 24, 2014 |
CAST PRODUCT HAVING ALUMINA BARRIER LAYER
Abstract
To provide a cast product having an alumina barrier layer and
method for producing the same. A cast product having an alumina
barrier layer of the present invention is a cast product in which
an alumina barrier layer containing Al.sub.2O.sub.3 is formed on
the surface of a cast body, and the cast body contains C: 0.3 mass
% to 0.7 mass %, Si: 0.1 mass % to 1.5 mass %, Mn: 0.1 mass % to 3
mass %, Cr: 15 mass % to 40 mass %, Ni: 20 mass % to 55 mass %, Al:
2 mass % to 4 mass %, rare earth element: 0.005 mass % to 0.4 mass
%, W: 0.5 mass % to 5 mass % and/or Mo: 0.1 mass % to 3 mass %, and
25 mass % or more of Fe in the remainder and an inevitable
impurity, and 80 mass % or more of the rare earth element is
La.
Inventors: |
ENJO; YOUHEI; (Hirakata-shi,
JP) ; HINENO; MAKOTO; (Kobe-shi, JP) ;
URAMARU; SHINICHI; (Hirakata-shi, JP) ; HASHIMOTO;
KUNIHIDE; (Ikoma-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KUBOTA CORPORATION |
Osaka |
|
JP |
|
|
Assignee: |
KUBOTA CORPORATION
Osaka
JP
|
Family ID: |
51207910 |
Appl. No.: |
14/220280 |
Filed: |
March 20, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2013/056240 |
Mar 7, 2013 |
|
|
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14220280 |
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Current U.S.
Class: |
428/141 ;
428/336; 428/472.2 |
Current CPC
Class: |
C22C 19/052 20130101;
Y10T 428/24355 20150115; C23C 8/02 20130101; B22D 21/00 20130101;
C21D 6/004 20130101; B23K 2101/34 20180801; C22C 38/04 20130101;
C22C 38/50 20130101; B22D 13/02 20130101; C22F 1/10 20130101; C23C
8/18 20130101; C22C 19/05 20130101; B23K 31/00 20130101; C21D 6/005
20130101; C22C 38/48 20130101; C23C 8/14 20130101; C22C 19/055
20130101; C22C 19/056 20130101; C21D 6/008 20130101; Y10T 428/265
20150115; C22C 38/02 20130101; C22C 38/44 20130101; C22C 38/005
20130101; C22C 38/06 20130101; C22C 38/54 20130101; B22D 29/00
20130101; C22C 30/00 20130101; B23K 9/23 20130101; C22C 38/40
20130101; B23K 2103/06 20180801; C21D 9/08 20130101; C22C 38/58
20130101; C22C 19/053 20130101 |
Class at
Publication: |
428/141 ;
428/472.2; 428/336 |
International
Class: |
C23C 8/10 20060101
C23C008/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 23, 2012 |
JP |
2012-067827 |
Mar 23, 2012 |
JP |
2012-067828 |
Mar 30, 2012 |
JP |
2012-078851 |
Jan 24, 2013 |
JP |
2013-010883 |
Claims
1. A cast product comprising a cast body having a surface formed
with an alumina barrier layer containing Al.sub.2O.sub.3, wherein
the cast body contains C: 0.3 mass % to 0.7 mass %, Si: 0.1 mass %
to 1.5 mass %, Mn: 0.1 mass % to 3 mass %, Cr: 15 mass % to 40 mass
%, Ni: 20 mass % to 55 mass %, Al: 2 mass % to 4 mass %, rare earth
element: 0.005 mass % to 0.4 mass %, W: 0.5 mass % to 5 mass %
and/or Mo: 0.1 mass % to 3 mass %, and 25 mass % or more of Fe in
the remainder and an inevitable impurity, and 80% or more of the
rare earth element is La.
2. The cast product according to claim 1, wherein the rare earth
element does not contain Ce.
3. The cast product according to claim 1, wherein the rare earth
element contains 0.1 mass % or less of Ce.
4. The cast product according to claim 1, wherein the cast body
further contains at least one kind selected from the group
consisting of Ti: 0.01 mass % to 0.6 mass %, Zr: 0.01 mass % to 0.6
mass % and Nb: 0.1 mass % to 3.0 mass %.
5. The cast product according to claim 2, wherein the cast body
further contains at least one kind selected from the group
consisting of Ti: 0.01 mass % to 0.6 mass %, Zr: 0.01 mass % to 0.6
mass % and Nb: 0.1 mass % to 3.0 mass %.
6. The cast product according to claim 3, wherein the cast body
further contains at least one kind selected from the group
consisting of Ti: 0.01 mass % to 0.6 mass %, Zr: 0.01 mass % to 0.6
mass % and Nb: 0.1 mass % to 3.0 mass %.
7. The cast product according to claim 1, wherein the cast body
further contains more than 0 mass % and 0.1 mass % or less of
B.
8. The cast product according to claim 2, wherein the cast body
further contains more than 0 mass % and 0.1 mass % or less of
B.
9. The cast product according to claim 3, wherein the cast body
further contains more than 0 mass % and 0.1 mass % or less of
B.
10. The cast product according to claim 4, wherein the cast body
further contains more than 0 mass % and 0.1 mass % or less of
B.
11. The cast product according to claim 5, wherein the cast body
further contains more than 0 mass % and 0.1 mass % or less of
B.
12. The cast product according to claim 6, wherein the cast body
further contains more than 0 mass % and 0.1 mass % or less of
B.
13. The cast product according to claim 1, wherein the alumina
barrier layer has a thickness of 0.05 .mu.m to 3 .mu.m.
14. The cast product according to claim 1, wherein the cast body
covered with the alumina barrier layer has a surface roughness (Ra)
of 0.05 .mu.m to 2.5 .mu.m.
15. The cast product according to claim 1, wherein the cast body is
produced by centrifugal casting.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a cast product having an
alumina barrier layer, and a method for producing the same.
[0003] 2. Description of Related Art
[0004] In heat resistant cast steel products such as reaction tubes
and decomposition tubes for production of ethylene, hearth rolls,
radiant tubes and metal resistant dusting members, austenite-based
heat resistant alloys that are excellent in high temperature
strength are used because they are exposed to a high temperature
atmosphere.
[0005] In these austenite-based heat resistant alloys, a metal
oxide layer is formed on the surface during use in a high
temperature atmosphere, and this oxide layer serves as a barrier to
protect the base material in the high temperature atmosphere.
[0006] On the other hand, when Cr oxides (mainly composed of
Cr.sub.2O.sub.3) are formed as such metal oxides, the function of
preventing entry of oxygen or carbon is insufficient because of
their poor tightness, and internal oxidization occurs in a high
temperature atmosphere, and thus the oxide film is bloated.
Further, these Cr oxides are easy to peel off during repeated
cycles of heating and cooling, and even when they do not eventually
peel off, oxygen or carbon from the external atmosphere can pass
through the film to disadvantageously lead internal oxidization or
cementation in the base material because the function of preventing
entry of oxygen or carbon is insufficient.
[0007] For addressing to this problem, it is proposed to form, on
the surface of a base material, an oxide layer mainly composed of
alumina (Al.sub.2O.sub.3) that is tight and less permeable to
oxygen and carbon, by containing larger quantity of Al compared
with that of general austenite-based heat resistant alloys (see,
for example, Japanese Patent Laid-open Publication No. 52-78612,
and Japanese Patent Laid-open Publication No. 57-39159).
[0008] However, since Al is a ferrite generating element, it will
deteriorate the ductility of the material, and decrease high
temperature strength when the content is too large. This ductility
decreasing tendency is observed, in particular, when the content of
Al exceeds 4%.
[0009] Therefore, the austenite-based heat resistant alloys in the
above patent documents have a disadvantage of causing deterioration
in ductility of a base material although they are expected to
improve the barrier function by Al.sub.2O.sub.3.
[0010] For addressing to this problem, WO 2010/113830 proposes a
cast product in which an alumina barrier layer containing
Al.sub.2O.sub.3 is formed on the inner surface of the cast body,
and Cr base particles having higher Cr concentration than the base
material matrix are dispersed at the boundary between the alumina
barrier layer and the cast body, by conducting an inner surface
process so that the surface roughness (Ra) of the cast body is 0.05
.mu.m to 2.5 .mu.m, followed by a heat treatment in an oxidizing
atmosphere, for providing a cast product capable of ensuring high
temperature stability of an alumina barrier layer without making
the Al content exceeding 4%, and capable of exerting excellent
barrier function in a high temperature atmosphere without
deteriorating the ductility of the material.
[0011] The cast product in WO 2010/113830 is able to keep the
excellent oxidation resistance, cementation resistance, nitriding
resistance, corrosion resistance and so on for a long term in use
in a high temperature atmosphere owing to the presence of the
stable alumina barrier layer.
[0012] As a result of the study by the present inventors, it was
demonstrated that when the cast products having excellent oxidation
resistance, cementation resistance, nitriding resistance, corrosion
resistance and so on disclosed in WO 2010/113830 are exposed to
higher temperature, tensile ductility was deteriorated in some of
the cast products.
[0013] Accordingly, it is a first object of the present invention
to find the factor of deteriorating the high temperature tensile
ductility and to provide an austenite-based cast product having an
alumina barrier layer that is excellent in high temperature tensile
ductility.
[0014] Further, when the inner surface process to a cast body is
conducted by a skiving process which is a general finishing
process, scratches can arise on the surface of the cast body. Such
a scratch part has a surface property different from that of the
remaining part of the base material because a processing strain is
excessively added, and the surface roughness is roughened. As a
result, in the subsequent heat treatment step, Cr oxide is formed
on the superficial surface of the scratch part, and a mass of Al
oxide will be formed directly beneath the same.
[0015] As described above, since uniform film of Al.sub.2O.sub.3 is
not formed and Cr.sub.2O.sub.3 film is mainly formed in the scratch
part, high temperature corrosion is more likely to occur in the
scratch part when it is exposed to high temperature of about
1080.degree. C. or higher for a long time, because the base
material cannot be protected by oxidation film in the scratch part
in comparison with the base material part where Al.sub.2O.sub.3
film is uniformly formed.
[0016] For addressing to this problem, it is conceivable to conduct
polishing such as honing process for removing these scratches,
however, increase in process costs, and extension of production
period will be caused.
[0017] Further, when the cast product is a straight tube, and thus
has a small diameter or a large length, the polishing such as the
honing process cannot be effected over the entire length as
described above, and a part having large surface roughness can be
left. As a result, in these parts, desired Al.sub.2O.sub.3 film
cannot be formed in some cases.
[0018] A so-called U-shaped tube having a bent portion is produced
by bending a straight tube having previously subjected to a surface
treatment and a heat treatment by processing. However, the alumina
barrier layer formed on the surface of the straight tube can peel
off due to strain or the like occurring in the bent portion at the
time of bending the straight tube. This phenomenon is significantly
observed, in particular, on the ventral side, or the inner side of
the bent portion.
[0019] Accordingly, it is a second object of the present invention
to provide a cast product capable of forming a uniform alumina
barrier layer on the entire surface, and a method for producing the
same.
[0020] When a cast product formed with an alumina barrier layer is
prepared, and the obtained cast product is joined by welding,
residual stress and strain occur in a so-called heat influenced
part that is susceptible to the heat at the time of welding. As a
result, the preliminarily formed alumina barrier layer can partly
peel off.
[0021] For addressing to this problem, it is conceivable to form an
alumina barrier layer by conducting a heat treatment after joining
the cast products having subjected to a surface treatment by
welding, however, in this case, metal oxides mainly composed of Cr
oxides are formed, in particular, in the welded part, and an
alumina barrier layer having sufficient cementation resistance
cannot be formed.
[0022] In conventional arts, in contrast to the cast body formed
with an alumina barrier layer, a welded part not formed with an
alumina barrier layer allows entry of oxygen, carbon, nitrogen and
the like from the external atmosphere and cannot prevent oxidation,
carbonization and nitriding for a long term.
[0023] Accordingly, it is a third object of the present invention
to provide a cast product in which metal oxides mainly composed of
Cr oxides are not formed in a welded part, and an alumina barrier
layer is formed on the entire surface, and a method for producing
the same.
SUMMARY OF THE INVENTION
[0024] For solving the aforementioned first object, a cast product
having an alumina barrier layer according to the present invention
is a cast product including an alumina barrier layer containing
Al.sub.2O.sub.3 formed on the surface of a cast body, and the cast
body contains C: 0.3 mass % to 0.7 mass %, Si: 0.1 mass % to 1.5
mass %, Mn: 0.1 mass % to 3 mass %, Cr: 15 mass % to 40 mass %, Ni:
20 mass % to 55 mass %, Al: 2 mass % to 4 mass %, rare earth
element: 0.005 mass % to 0.4 mass %, W: 0.5 mass % to 5 mass %
and/or Mo: 0.1 mass % to 3 mass %, and 25 mass % or more of Fe, and
an inevitable impurity, and 80 mass % or more of the rare earth
element is La.
[0025] Further, for solving the aforementioned second object, a
method for producing a cast product having an alumina barrier layer
on the surface according to the present invention is a method for
producing a cast product for use in a high temperature atmosphere,
and the method including:
[0026] a step of conducting an acid treatment by an acid solution
containing a polyhydric alcohol liquid on the surface of a cast
body made of a heat resistant alloy containing 15 mass % or more of
Cr, 20 mass % or more of Ni, and 2 mass % to 4 mass % of Al,
and
[0027] a heat treatment step of conducting a heat treatment on the
cast body on which the acid treatment is conducted, to form an
alumina barrier layer containing Al.sub.2O.sub.3 on the
surface.
[0028] Further, for solving the aforementioned third object, a
method for producing a cast product according to the present
invention is a method for producing a cast product for use in a
high temperature atmosphere, obtainable by joining a first cast
body and a second cast body made of a heat resistant alloy
containing 15 mass % or more of Cr, 20 mass % or more of Ni, and 2
mass % to 4 mass % of Al by welding, and the method includes: the
step of joining the first cast body and the second cast body by
welding;
[0029] the step of conducting a surface treatment on the joined
welded part; and
[0030] the step of conducting a heat treatment on the welded part
having subjected to the surface treatment.
[0031] Further, for solving the aforementioned third object, the
cast product having an alumina barrier layer of the present
invention is a cast product for use in a high temperature
atmosphere, formed by joining a first cast body and a second cast
body made of a heat resistant alloy containing 15 mass % or more of
Cr, 20 mass % or more of Ni, and 2 mass % to 4 mass % of Al by
welding, and the welded part between the first cast body and the
second cast body is covered with an alumina barrier layer
containing Al.sub.2O.sub.3.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a surface photograph of specimen No. 201 which is
an inventive example in Example 2 after acid treatment and before
heat treatment;
[0033] FIG. 2 is a surface photograph of specimen No. 202 which is
an inventive example in Example 2 after acid treatment and before
heat treatment;
[0034] FIG. 3 is a surface photograph of specimen No. 203 which is
an inventive example in Example 2 after acid treatment and before
heat treatment;
[0035] FIG. 4 is a surface photograph of specimen No. 204 which is
an inventive example in Example 2 after acid treatment and before
heat treatment;
[0036] FIG. 5 is a surface photograph of specimen No. 205 which is
an inventive example in Example 2 after acid treatment and before
heat treatment;
[0037] FIG. 6 is a surface photograph of specimen No. 206 which is
an inventive example in Example 2 after acid treatment and before
heat treatment;
[0038] FIG. 7 is a surface photograph of specimen No. 207 which is
an inventive example in Example 2 after acid treatment and before
heat treatment;
[0039] FIG. 8 is a surface photograph of specimen No. 311 which is
a reference example in Example 2 after acid treatment and before
heat treatment;
[0040] FIG. 9 is a surface photograph of specimen No. 312 which is
a reference example in Example 2 after acid treatment and before
heat treatment;
[0041] FIG. 10 is a surface photograph of specimen No. 421 which is
a comparative example in Example 2 before heat treatment (without
acid treatment);
[0042] FIG. 11 is a surface photograph of specimen No. 201 which is
an inventive example in Example 2 after heat treatment;
[0043] FIG. 12 is a surface photograph of specimen No. 202 which is
an inventive example in Example 2 after heat treatment;
[0044] FIG. 13 is a surface photograph of specimen No. 203 which is
an inventive example in Example 2 after heat treatment;
[0045] FIG. 14 is a surface photograph of specimen No. 204 which is
an inventive example in Example 2 after heat treatment;
[0046] FIG. 15 is a surface photograph of specimen No. 205 which is
an inventive example in Example 2 after heat treatment;
[0047] FIG. 16 is a surface photograph of specimen No. 206 which is
an inventive example in Example 2 after heat treatment;
[0048] FIG. 17 is a surface photograph of specimen No. 207 which is
an inventive example in Example 2 after heat treatment;
[0049] FIG. 18 is a surface photograph of specimen No. 311 which is
a reference example in Example 2 after heat treatment;
[0050] FIG. 19 is a surface photograph of specimen No. 312 which is
a reference example in Example 2 after heat treatment;
[0051] FIG. 20 is a surface photograph of specimen No. 421 which is
a comparative example in Example 2 after heat treatment;
[0052] FIG. 21 is a section SEM photograph of specimen No. 201
which is an inventive example in Example 2;
[0053] FIG. 22 is a section SEM photograph of specimen No. 202
which is an inventive example in Example 2;
[0054] FIG. 23 is a section SEM photograph of specimen No. 203
which is an inventive example in Example 2;
[0055] FIG. 24 is a section SEM photograph of specimen No. 204
which is an inventive example in Example 2;
[0056] FIG. 25 is a section SEM photograph of specimen No. 205
which is an inventive example in Example 2;
[0057] FIG. 26 is a section SEM photograph of specimen No. 206
which is an inventive example in Example 2;
[0058] FIG. 27 is a section SEM photograph of specimen No. 207
which is an inventive example in Example 2;
[0059] FIG. 28 is a section SEM photograph of specimen No. 311
which is a reference example in Example 2;
[0060] FIG. 29 is a section SEM photograph of specimen No. 312
which is a reference example in Example 2;
[0061] FIG. 30 is a section SEM photograph of specimen No. 421
which is a comparative example in Example 2;
[0062] FIG. 31 is a section photograph taken by axially cutting
specimen tube No. 504 which is an inventive example in Example
3;
[0063] FIG. 32 is a section photograph taken by axially cutting
specimen tube No. 613 which is a comparative example in Example
3;
[0064] FIG. 33 is a section photograph of a specimen piece obtained
from specimen tube No. 504 which is an inventive example in Example
3, taken perpendicularly to a welded part;
[0065] FIG. 34 is a section photograph of a specimen piece obtained
from specimen tube No. 613 which is a comparative example in
Example 3, taken perpendicularly to a welded part;
[0066] FIG. 35 is a photograph of specimen tube No. 504 which is an
inventive example in Example 3 by section SEM analysis; and
[0067] FIG. 36 is a photograph of specimen tube No. 613 which is a
comparative example in Example 3 by section SEM analysis.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0068] In the following, three embodiments of the present invention
will be specifically described.
First Embodiment
[0069] The first embodiment according to the present invention
provides a cast product in which an alumina barrier layer
containing Al.sub.2O.sub.3 is formed on a surface of a cast body,
and the cast body contains C: 0.3 mass % to 0.7 mass %, Si: 0.1
mass % to 1.5 mass %, Mn: 0.1 mass % to 3 mass %, Cr: 15 mass % to
40 mass %, Ni: 20 mass % to 55 mass %, Al: 2 mass % to 4 mass %,
rare earth element: 0.005 mass % to 0.4 mass %, W: 0.5 mass % to 5
mass % and/or Mo: 0.1 mass % to 3 mass %, and 25 mass % or more of
Fe in the remainder and an inevitable impurity, and 80 mass % or
more of the rare earth element is occupied by La. In this
description, "%" is "mass %" unless otherwise specified.
<Description of Reason for Component Limitation>
[0070] C: 0.3% to 0.7%
[0071] C has a function of improving the castability and increasing
the high temperature creep rupture strength. Therefore, C is
contained in at least 0.3%. However, when the content is too large,
primary carbide of Cr.sub.7C.sub.3 tends to be widely formed, and
migration of Al that forms the alumina barrier layer is suppressed,
so that the supply of Al to the surface part of the cast body is
insufficient, and the alumina barrier layer is locally fragmented
and the continuity of the alumina barrier layer is impaired.
Further, since secondary carbide excessively precipitates,
ductility and toughness are deteriorated. For this reason, the
upper limit is set at 0.7%. The content of C is more preferably
0.4% to 0.5%.
[0072] Si: 0.1% to 1.5%
[0073] Si is contained in at least 0.1% as a deoxidant for molten
alloy, for increasing the fluidity of molten alloy, and the upper
limit is 1.5% because too large content will lead deterioration in
high temperature creep rupture strength. The upper limit of Si is
more preferably 1.0%.
[0074] Mn: 0.1% to 3%
[0075] Mn is contained in at least 0.1% as a deoxidant for molten
alloy, for immobilizing S in molten metal, and the upper limit is
3% because too large content will lead deterioration in high
temperature creep rupture strength. The upper limit of Mn is
preferably 1.6%.
[0076] Cr: 15% to 40%
[0077] Cr is contained in 15% or more for the purpose of
contribution to improvement in high temperature strength and
repeated oxidation resistance. However, the upper limit is 40%
because too large content will lead deterioration in high
temperature creep rupture strength. Cr occupies more preferably 20%
to 30%.
[0078] Ni: 20% to 55%
[0079] Ni is an element required for ensuring repeated oxidation
resistance and stability of metal tissue. When the content of Ni is
small, the content of Fe is relatively large, so that Cr--Fe--Mn
oxide is more likely to be generated on the surface of the cast
body, and hence generation of an alumina barrier layer is
inhibited. For this reason, Ni is contained in 20% or more.
However, since the effect of increasing the amount is no longer
obtained with the content of Ni exceeding 55%, the upper limit is
55%. Ni is contained more preferably in 28% to 45%.
[0080] Al: 2% to 4%
[0081] Al is an element that is effective for improving the
cementation resistance and the caulking resistance. In the present
invention, it is an essential element for generating an alumina
barrier layer on the surface of a cast body. Therefore, Al is
contained in at least 2% or more. However, when the content exceeds
4%, the ductility will be deteriorated as described above, and
hence the upper limit is specified at 4% in the first embodiment of
the present invention. The content of Al is more preferably 2.5% to
3.8%.
[0082] Rare Earth Element: 0.005% to 0.4%, Wherein 80% or More of
the Same is La.
[0083] Rare earth elements mean seventeen elements including
fifteen elements in the lanthanum series from La to Lu in the
periodic table, as well as Y and Sc, and 80% or more of the rare
earth element contained in the heat resistant alloy in the first
embodiment of the present invention is La. By containing La in 80%
or more, it is possible to increase the generation amount of Ni--La
compounds such as Ni.sub.2La and Ni.sub.3La having excellent high
temperature tensile ductility, and in particular, high temperature
tensile ductility at 1100.degree. C. or higher.
[0084] Rare earth elements have the ability of immobilizing S and
the ability of immobilizing oxide film by rare earth oxide, and are
contained in 0.005% or more for contributing to facilitation for
generation and stabilization of an alumina barrier layer. The upper
limit is 0.4% because too large amount will deteriorate the
ductility and toughness.
[0085] Further, the content of Ce in the rare earth element is
desirably 0.1% or less. By controlling the Ce content, it is
possible to reduce the generation amount of Ce compounds such as
Ni.sub.2Ce and Ni.sub.3Ce that cause high temperature brittleness,
and to increase the high temperature tensile ductility. More
preferably, the rare earth element does not contain Ce and is
composed of only La.
[0086] W: 0.5% to 5% and/or Mo: 0.1% to 3%
[0087] W and Mo solid-solved in the matrix and reinforce the
austenite phase of the matrix, thereby improving the creep rupture
strength. For exertion of this effect, at least either of W and Mo
is contained, and in the case of W, 0.5% or more is contained, and
in the case of Mo, 0.1% or more is contained.
[0088] However, too large W and Mo contents will lead decrease in
ductility and deterioration in cementation resistance, and W and Mo
have a function of inhibiting generation of an alumina barrier
layer by suppressing migration of Al by being solid-solved in the
matrix because they have large atomic radii. Further, likewise the
case where the content of C is large, primary carbides of (Cr, W,
Mo).sub.7C.sub.3 are likely to be formed widely, and migration of
Al that forms an alumina barrier layer is suppressed, so that the
supply of Al to the surface part of the cast body is insufficient,
and the alumina barrier layer is locally fragmented and the
continuity of the alumina barrier layer is more likely to be
impaired. Since W and Mo have large atomic radii, they are
solid-solved in the matrix and have the effect of preventing
generation of an alumina barrier layer by suppressing migration of
Al or Cr.
[0089] Therefore, W is 5% or less, and Mo is 3% or less. More
preferably, W is 0.5% to 3%, and Mo is 2% or less.
[0090] At Least One of Ti: 0.01% to 0.6%, Zr: 0.01% to 0.6% and Nb:
0.1% to 3.0%
[0091] Since Ti, Zr and Nb are elements that are easy to form
carbide, and are less likely to be solid-solved in the matrix
compared with W and Mo, something special function on formation of
an alumina barrier layer is not recognized, however, it has a
function of improving the creep rupture strength. At least one of
Ti, Zr and Nb may be contained as is necessary. The content is
0.01% or more for Ti and Zr, and 0.1% or more for Nb.
[0092] However, excess addition will cause deterioration in the
ductility. Nb also deteriorates the peeling resistance of the
alumina barrier layer. Therefore, the upper limit is 0.6% for Ti
and Zr, and 3.0% for Nb. Preferably, the upper limit is 0.3% for Ti
and Zr, and 1.5% for Nb.
[0093] B: 0.1% or Less
[0094] B may be contained as is necessary because it has a function
of reinforcing grain boundaries of a cast body. The amount of B, if
added, should be more than 0 to 0.1% or less because too large
content will deteriorate the creep rupture strength. The content of
B is more preferably more than 0.01% and 0.1% or less.
[0095] Fe: 25% or More
[0096] Diffusing speeds of Al in Fe, Ni and Cr are expected to
increase as the sizes of the atoms decrease. Therefore, by
increasing Fe which is a smaller atom, and reducing the amount of
Cr, it is possible to increase diffusion of Al in the alloy, to
facilitate migration of Al, and to promote generation of film of
Al.sub.2O.sub.3. Further, by reducing Cr, it is possible to inhibit
generation of Cr oxide.
[0097] For this reason, Fe is contained in 25% or more. More
preferably, Fe is contained in 30% or more.
[0098] Inevitable Impurity
[0099] P, S and other impurities that are inevitably contained
during melt production of alloy may exist within the range normally
allowed for this type of alloy material.
<Cast Body>
[0100] The cast body forming the cast product of the first
embodiment of the present invention is casted to have the
aforementioned composition by producing molten metal of the
aforementioned element composition, followed by centrifugal
casting, stationary casting and so on.
[0101] The obtainable cast body may have a shape suited for the
intended use.
[0102] The first embodiment of the present invention is
particularly suited for a cast body produced by centrifugal
casting. By applying the centrifugal casting, fine metal tissues
grow radially with an orientation as the cooling by a mold
progresses, and alloy tissues where Al is easy to migrate can be
obtained. As a result, in the heat treatment as will be described
later, it is possible to obtain a cast product formed with film
which is an alumina barrier layer having a smaller thickness than a
conventional one, but having excellent strength even under a
repetitively-heated environment in the later-described heat
treatment.
[0103] As the cast product produced by centrifugal casting, a tube,
in particular, a reaction tube used under a high temperature
environment can be exemplified.
[0104] The cast body is subjected to a heating treatment in an
oxidizing atmosphere after it is surface-treated in an objective
site that is to be in contact with the high temperature atmosphere
during use of the product, and the surface roughness of the site is
adjusted.
<Surface Treatment>
[0105] As a surface treatment, a polishing treatment can be
exemplified. The surface treatment is preferably conducted on the
entire objective site that is to be in contact with a high
temperature atmosphere during use of the product.
[0106] The surface treatment may be conducted so that the surface
roughness (Ra) of the objective site is 0.05 .mu.m to 2.5 .mu.m.
More desirably, the surface roughness (Ra) is 0.5 .mu.m to 2.0
.mu.m. When the surface roughness (Ra) is less than 0.05 .mu.m, Cr
is oxidized dominantly to Al, whereas when it is 0.05 .mu.m or
more, generation of Cr oxide scale can be suppressed, and an
alumina barrier layer can be formed more preferably by the
subsequent heat treatment. It is expected that Cr oxide scale is
more likely to be generated due to the residual processing strain
when it is 2.5 .mu.m or more. At this time, by adjusting the
surface roughness by the surface treatment, it is possible to
remove the residual stress and strain in the heat-influenced part
concurrently.
[0107] When the surface treatment is conducted by a polishing
treatment, it is desired to conduct finish polishing with the use
of #240 to #1200 after conducting paper polishing with the use of
#12 to #220.
<Heat Treatment>
[0108] After conducting the surface treatment, a heat treatment is
conducted in the following conditions.
[0109] The heat treatment is carried out by conducting a heating
treatment in an oxidizing atmosphere.
[0110] The oxidizing atmosphere means an oxidizing environment in
which oxidizing gas containing 20% by volume or more of oxygen or
steam or CO.sub.2 is mixed. The heating treatment is conducted at a
temperature of 900.degree. C. or higher, preferably 1000.degree. C.
or higher, and more preferably 1050.degree. C. or higher, and the
heating time is 1 hour or more.
<Cast Product>
[0111] As described above, by sequentially conducting the surface
treatment and the heat treatment on the cast body, it is possible
to obtain a cast product in which an alumina barrier layer
containing Al.sub.2O.sub.3 is stably formed on the surface of the
cast body.
<Alumina Barrier Layer>
[0112] The alumina barrier layer containing Al.sub.2O.sub.3 formed
in the cast product of the present invention is highly tight and
functions as a barrier for preventing external oxygen, carbon and
nitrogen from entering the base material. In the first embodiment
according to the present invention, the surface treatment is
conducted in the site that is to be in contact with a high
temperature atmosphere during use of the product to adjust the
surface roughness of the site, and then the site is subjected to
the heating treatment in an oxidizing atmosphere, and thus
Al.sub.2O.sub.3 can be continuously formed as an alumina barrier
layer on the surface of the cast product.
[0113] The thickness of the alumina barrier layer formed on the
cast body is preferably 0.05 .mu.m or more and 3 .mu.m or less for
effectively exerting the barrier function. When the thickness of
the alumina barrier layer is less than 0.05 .mu.m, the cementation
resistance may be deteriorated, whereas when it exceeds 3 .mu.m,
peeling of the alumina barrier layer may be likely to advance due
to the influence of difference in heat expansion coefficient
between the base material and the film.
[0114] For avoiding the influence, the thickness of the alumina
barrier layer is more preferably 0.1 .mu.m or more and 2.5 .mu.m or
less. On the other hand, when the film thickness has variation,
peeling of the film can advance as the temperature widely changes.
Therefore, the thickness of the alumina barrier layer is desirably
0.5 .mu.m or more and 1.5 .mu.m or less, and most desirably about 1
.mu.m on average.
[0115] When the surface of the cast product of the first embodiment
of the present invention is observed by SEM/EDX, Cr oxide scale
formed on a part of the alumina barrier layer is sometimes
observed. This is attributed to that Cr oxide scale formed inside
the alumina barrier layer is pushed up to the surface of the
product by Al.sub.2O.sub.3. However, it is preferred that the oxide
scale is as little as possible, and it is preferred that the oxide
scale occupies less than 20 area % of the product surface, so that
Al.sub.2O.sub.3 occupies 80 area % or more.
<Regarding La>
[0116] The cast product of the first embodiment according to the
present invention is able to increase the tensile ductility at high
temperature (concretely 1100.degree. C. or higher) as much as
possible as will be described later in Example 1 by making the
content of La in the rare earth element 80%.
[0117] This is because the melting temperatures of Ni--La compounds
are higher than the melting temperatures of Ni--Ce compounds, and
high temperature embrittlement of a La-added material occurs in a
temperature zone higher than 1200.degree. C. More concretely, while
melting points of Ni.sub.2Ce and Ni.sub.3Ce are respectively
1000.degree. C. and 1180.degree. C., the melting points of
Ni.sub.2La and Ni.sub.3La are respectively 1100.degree. C. and
1240.degree. C.
[0118] Therefore, for use as a reaction tube, in particular, it is
effective to contain La in 80% or more as the rare earth element
that will not be embrittled in the use temperature region of Ce
(about 1100.degree. C.).
[0119] In the first embodiment according to the present invention,
80% or more of La is contained in the rare earth element by
controlling the content of Ce, and for the Ce added material and
the La added material, a repetitive oxidation test in an
atmospheric air under furnace cooling at 1050.degree. C. for a
retention time of 10 hours was conducted, and difference in peeling
resistance of Al.sub.2O.sub.3 was little observed.
[0120] Further, sensitivity to cracking (susceptibility to
cracking) was evaluated for the case containing La in 80% or more
by controlling the content of Ce in the rare earth element by a
bead placement test (crack sensitivity test: see The Japan Welding
Engineering Society web site
http://www-it.jwes.or.jp/qa/details.jsp?pg_no=0100080100 for
reference), to reveal that there is little influence.
[0121] The first embodiment according to the present invention is
suitable as a cast product that is excellent in high temperature
tensile ductility and is capable of effectively preventing oxygen,
carbon, nitrogen and the like from entering from the external
atmosphere by the alumina barrier layer.
Second Embodiment
[0122] In the second embodiment according to the present invention,
a cast product formed with a so-called "alumina barrier layer"
containing Al.sub.2O.sub.3 on the surface is obtained by conducting
a surface treatment by an acid treatment on a heat resistant alloy
containing 15% or more of Cr and 20% or more of Ni, and 2% to 4% of
Al, and then conducting a heat treatment.
[0123] Influences of the components contained in the cast product
are described in the part of <Description of reason for
component limitation> in the first embodiment.
[0124] The contents of the components contained in the cast product
of the second embodiment are as follows.
[0125] Cr: 15% or More
[0126] Cr is contained in 15% or more, and the upper limit is 40%.
The content of Cr is more desirably 20% to 30%.
[0127] Ni: 20% or More
[0128] Ni is contained in at least 20% or more. Since the effect of
increasing the amount is not obtained when Ni is contained in more
than 55%, the upper limit is 55%. The content of Ni is more
preferably 28% to 45%.
[0129] Al: 2% to 4%
[0130] Al is contained in at least 2% or more, and the upper limit
is defined as 4%. The content of Al is more desirably 2.5% to
3.8%.
[0131] Besides these, the following components are preferably
contained.
[0132] C: 0.3% to 0.7%
[0133] C is contained in at least 0.3%, and the upper limit is
0.7%. The content of C is more desirably 0.4% to 0.5%.
[0134] Si: Exceeding 0.1% and 1.5% or Less
[0135] Si is contained in at least 0.1%, and is contained in the
upper limit of 1.5%. The content of Si is desirably 1.0% or
less.
[0136] Mn: 0.1% to 3.0% or Less
[0137] Mn is contained in the upper limit of 3.0%. The content of
Mn is more desirably 1.6% or less.
[0138] Rare Earth Element: 0.005% to 0.4%
[0139] The rare earth element contained in the heat resistant alloy
of the second embodiment according to the present invention is
preferably at least one kind selected from the group consisting of
Ce, La and Nd.
[0140] When generation of the alumina barrier layer is conducted by
a heating treatment in an oxidizing atmosphere at high temperature,
the rare earth element is contained in 0.005% or more, and the
upper limit of 0.4%.
[0141] W: 0.5% to 5% and/or Mo: 0.1% to 3%
[0142] As to W and Mo, at least one of W and Mo is contained, and
when W is contained, the content is 0.5% or more, whereas when Mo
is contained, the content is 0.1% or more.
[0143] W is 3% or less, and Mo is 2% or less. Even when both
elements are contained, the total content is preferably 3% or
less.
[0144] At Least One of Ti: 0.01% to 0.6%, Zr: 0.01% to 0.6% and Nb:
0.1% to 3.0%
[0145] As to Ti, Zr and Nb, Ti and Zr are contained in 0.01% or
more, and Nb is contained in 0.1% or more.
[0146] However, the upper limit is 0.6% for Ti and Zr, and 3.0% for
Nb.
[0147] B: 0.1% or Less
[0148] B may be contained as is necessary. Even when it added, the
amount is more than 0% and 0.1% or less. The content of B is more
preferably more than 0.01% and 0.1% or less.
[0149] The heat resistant alloy forming the cast body of the second
embodiment according to the present invention contains the above
components, and the remainder of Fe, and for increasing diffusion
of Al and promoting generation of film of Al.sub.2O.sub.3, it
preferably contains 25% or more of Fe. Further, the remainder of Fe
may contain P, S and other impurities that are inevitably contained
at the time of casting into an alloy, within the ranges that are
generally allowable for this kind of alloy material.
<Cast Body>
[0150] The cast body forming the cast product of the second
embodiment according to the present invention is casted to have the
aforementioned composition by producing molten metal, and followed
by mold centrifugal casting, stationary casting or the like.
[0151] As a shape of the cast body, a straight tube, and a U-shaped
tube with a bent portion formed by bending a straight tube, etc.
are exemplified. As to a straight tube, the one having such inner
diameter or length for which surface treatment by a polishing
process or the like is difficult to be effected, is particularly
suited, and as such a cast body, for example, a straight tube
having an inner diameter of 40 mm or less and/or a length of 3000
mm or more can be exemplified. Further, so-called finishing
processes such as an inner surface process and an inner surface
honing may also be conducted as is necessary.
[0152] The aforementioned cast body is subjected to a surface
treatment by an acid treatment for the objective site where it is
to be in contact with a high temperature atmosphere during use of
the product, and thus the surface roughness in the site is
adjusted, and then subjected to a heating treatment in an oxidizing
atmosphere.
<Surface Treatment (Acid Treatment)>
[0153] The surface treatment is an acid treatment by an acid
solution containing a polyhydric alcohol liquid. The surface
treatment by an acid treatment is preferably conducted for the
entire objective site where it is to be in contact with a high
temperature atmosphere during use of the product. As for the part
finished by a polishing process or the like in a straight tube or a
U-shaped tube, the acid treatment may be conducted only in the part
out of reach of the polishing process, or in and around the bent
portion of the U-shaped tube.
[0154] The acid treatment may be conducted so that the surface
roughness (Ra) of the objective site is 0.05 .mu.m to 2.5 .mu.m.
More desirably, the surface roughness (Ra) is 0.5 .mu.m to 1.0
.mu.m. As a result, it is possible to suppress generation of Cr
oxide scale, and to form the alumina barrier layer containing
Al.sub.2O.sub.3 by the subsequent heat treatment more
preferably.
[0155] The acid treatment can be achieved by dipping the objective
site in an acid solution containing a polyhydric alcohol liquid for
a predetermined time, or by applying an acid solution containing a
polyhydric alcohol liquid to the objective site. The corrosive
liquid adhered to the objective site after the acid treatment is
desirably washed out by water washing or the like.
[0156] As the acid solution, glyceregia liquid (nitric
acid:hydrochloric acid:glycerol=1:3:1) and glycol liquid (nitric
acid:hydrochloric acid:ethyleneglycol=1:3:1) are exemplified. As
shown in the examples as will be described later, by the acid
treatment only by strong acid such as aqua regia (nitric
acid:hydrochloric acid=1:3), the surface layer is rough and
Al.sub.2O.sub.3 is difficult to be formed.
[0157] As the polyhydric alcohol liquid, polyhydric alcohols such
as glycerol and glycol are exemplified. In conducting the acid
treatment, the oxidizing power is too strong only by strong acid,
and the surface of the objective site will be corroded too much to
reversely make the surface rough. For this reason, a polyhydric
alcohol liquid is added to the acid solution. By adding the
polyhydric alcohol liquid, it is possible to control or suppress
the degree of oxidation or corrosion in the objective site by the
acid solution, and to adjust the surface roughness. By using the
polyhydric alcohol liquid, it is possible to suppress the oxidizing
force and to facilitate adjustment of the surface roughness in
comparison with the case of using monohydric alcohol.
<Heat Treatment>
[0158] On the cast body having subjected to the acid treatment, a
heat treatment is conducted in the same condition as described in
the part <Heat treatment> in the first embodiment.
<Cast Product>
[0159] As described above, by sequentially conducting the heat
treatment after the acid treatment, it is possible to obtain a cast
product in which the alumina barrier layer is stably formed over
the entire objective site.
<Alumina Barrier Layer>
[0160] In the second embodiment according to the present invention,
as described above, by subjecting the cast body to a surface
treatment by an acid treatment in a site that is to be in contact
with a high temperature atmosphere during use of the product, to
adjust the surface roughness of the site, and then subjecting the
site to a heating treatment in an oxidizing atmosphere,
Al.sub.2O.sub.3 is formed continuously as an alumina barrier layer
on the surface of the cast product. As a result, it is possible to
form an alumina barrier layer over the entire surface of the
objective site of the cast body.
[0161] The thickness of the alumina barrier layer formed on the
cast body is formed into 0.05 .mu.m or more and 3 .mu.m or less for
effectively exerting a barrier function, and is preferably about 1
.mu.m on average. More desirably, the thickness of the alumina
barrier layer is 0.5 .mu.m or more and 1.5 .mu.m or less.
[0162] Regarding a cast body having the aforementioned composition
of Cr--Ni--Al heat resistant alloy, when a straight tube having
small diameter and/or large length, and thus having a site for
which a surface treatment by a polishing process of the like cannot
be effected, is heated in an oxidizing atmosphere, the surface
roughness is large and an alumina barrier layer is not formed in
the site for which the surface treatment cannot be effected.
Therefore, it is influenced by oxidization, cementation and so on
from such a site.
[0163] Regarding a U-shaped tube formed by bending a straight tube,
when a bending process is effected after forming an alumina barrier
layer by subjecting a straight tube to a surface treatment and a
heat treatment, the barrier layer formed on the surface of the
straight tube, particularly on the ventral side of the bent portion
can peel off due to strain or the like arising in the bent
portion.
[0164] Further, Cr oxide scale based on Cr.sub.2O.sub.3 is
dispersedly formed on the surface of the cast body, and is easy to
peel off as described above, and at the time of peeling off, the
alumina barrier layer beneath the same can peel off together.
[0165] In light of this, in the second embodiment according to the
present invention, by subjecting the objective site of the cast
body to a surface treatment by an acid treatment, to adjust the
surface roughness as described above, it is possible to form the
alumina barrier layer stably over the entire objective site.
[0166] In the cast product of the second embodiment according to
the present invention, it is preferred that Cr oxide scale
scattered on the alumina barrier layer is less than 20 area % of
the product surface and the alumina barrier layer occupies 80 area
% or more when the surface of the product is examined by
SEM/EDX.
[0167] Also it is desired that the alumina barrier layer covers 50%
or more of the bent portion, and has a thickness of 0.05 .mu.m or
more by subjecting a straight tube to a bending process before acid
treatment, and then conducting a treatment by acid containing the
polyhydric alcohol liquid.
Third Embodiment
[0168] The third embodiment according to the present invention
obtains a cast product in which a welded part is formed by a
so-called "alumina barrier layer" containing Al.sub.2O.sub.3, by
joining a first cast body and a second cast body made of a heat
resistant alloy containing 15% or more of Cr and 20% or more of Ni,
and 2 to 4% of Al by welding, and subjecting a welded part between
the welded first cast body and the second cast body to a surface
treatment, and then subjecting the welded part to a heat
treatment.
[0169] The influence of the component contained in the cast product
is described in the part of <Description of reason for component
limitation> in the first embodiment.
[0170] The contents of the components contained in the cast product
of the third embodiment are identical to those in the second
embodiment.
<Cast Body>
[0171] The first cast body and the second cast body forming the
cast product of the third embodiment according to the present
invention are casted to have the aforementioned composition by
producing molten metal, followed by centrifugal casting, stationary
casting or the like.
[0172] The obtained first cast body and second cast body may be
joined by welding into a shape suited for the intended use.
[0173] Before conducting welding, edge preparation or the like may
be conducted as is necessary.
[0174] In the third embodiment according to the present invention,
the welding method and the composition of the welding electrode
used at the time of welding are not limited, and as a method
capable of welding the cast bodies of the present invention, TIG
welding and arc welding can be recited.
[0175] In the cast bodies joined by welding, a welded part
including a heat influenced part and a molten metal part is formed
in the joint portion regardless of whether a previous surface
treatment is conducted. Residual stress and strain arise in this
heat influenced part, and Cr migrates along the strain line of the
heat influenced part, and Cr oxide is likely to be generated
dominantly, and Al.sub.2O.sub.3 is difficult to be generated.
[0176] In such a welded part, even if a heating treatment is
conducted in a subsequent step, it is impossible to sufficiently
form Al.sub.2O.sub.3 that forms the alumina barrier layer.
[0177] For this reason, in the third embodiment according to the
present invention, after joining the cast bodies by welding, a
surface treatment is conducted on an objective site that is to be
in contact with a high temperature atmosphere during use of the
product, to adjust the surface roughness of the site, and then a
heating treatment is conducted in an oxidizing atmosphere.
<Surface Treatment>
[0178] As a surface treatment, a polishing treatment can be
exemplified. It is desired that the surface treatment is conducted
on the entire objective site that is to be in contact with a high
temperature atmosphere during use of the product. It is not
necessary to treat the entire objective site concurrently, and the
part other than the welded part may be previously subjected to a
surface treatment or the like to adjust the surface roughness, and
the surface treatment may be conducted only on the welded part or
only on and around the welded part.
[0179] The surface treatment may be conducted so that the surface
roughness (Ra) of the objective site is 0.05 .mu.m to 2.5 .mu.m.
More desirably, the surface roughness (Ra) is 0.5 .mu.m to 1.0
.mu.m. The influence of the surface roughness (Ra) is described in
the part <Surface treatment> in the first embodiment.
[0180] When the surface treatment is conducted by a polishing
treatment, it is desired to conduct finish polishing with the use
of #240 to #1200 after conducting paper polishing with the use of
#12 to #220.
[0181] In the case of an acid treatment, the surface treatment can
be achieved by dipping the objective site in a corrosive liquid for
a predetermined time, or by applying a corrosive liquid. The acid
used in the acid treatment may contain alcohol besides acid. The
corrosive liquid adhered to the objective site after the acid
treatment is desirably washed out by water washing or the like.
<Heat Treatment>
[0182] After conducting the surface treatment on the welded cast
bodies, a heat treatment is conducted in the same condition as
described in the part <Heat treatment> in the first
embodiment.
<Cast Product>
[0183] As described above, by sequentially conducting the welding,
the surface treatment and the heat treatment on the welded part, it
is possible to obtain a cast product in which an alumina barrier
layer is stably formed in the welded part including the heat
influenced part and the molten metal part of the cast bodies
arising by the welding.
<Alumina Barrier Layer>
[0184] In the third embodiment according to the present invention,
after joining cast bodies of intended use by welding as described
above, a site that is to be in contact with a high temperature
atmosphere during use of the product is subjected to a surface
treatment, and the surface roughness of the site is adjusted, and
then the site is subjected to a heating treatment in an oxidizing
atmosphere, and thereby Al.sub.2O.sub.3 is continuously formed as
an alumina barrier layer on the surface continuing across the
welded part of the cast product. As a result, the alumina barrier
layer is formed not only on the surface of the cast bodies, but
also in the welded part including a heat influenced part arising in
the abutting surface of the cast bodies by the welding.
[0185] The thickness of the alumina barrier layer formed on the
cast bodies is 0.05 .mu.m or more and 3 .mu.m or less for effective
exertion of barrier function, and is preferably about 1 .mu.m on
average. More desirably, the thickness of the alumina barrier layer
is 0.5 .mu.m or more and 1.5 .mu.m or less.
[0186] In the cast body having the aforementioned composition of
Cr--Ni--Al heat resistant alloy, an alumina barrier layer will not
be formed particularly in the welded part having large surface
roughness when the heating treatment is conducted in an oxidizing
atmosphere without conducting a surface treatment after conducting
the welding. Therefore, it is influenced by oxidation, cementation
or the like from the welded part.
[0187] Further, Cr oxide scale based on Cr.sub.2O.sub.3 is
dispersedly formed on the superficial surface of the cast bodies,
and is easy to peel off as described above, and at the time of
peeling off, the alumina barrier layer beneath the same can peel
off together.
[0188] In light of this, in the third embodiment according to the
present invention, by adjusting the surface roughness by a surface
treatment of a cast product after joining the cast bodies by
welding and before formation of an alumina barrier layer by a heat
treatment as described above, it is possible to form an alumina
barrier layer stably in a welded part including a heat influenced
part of the cast bodies arising by welding.
[0189] In the cast product of the third embodiment according to the
present invention, it is preferred that Cr oxide scale scattered on
the alumina barrier layer is less than 20 area % of the product
surface and the alumina barrier layer occupies 80 area % or more
when the surface of the product is examined by SEM/EDX.
Example 1
[0190] Molten metal was produced by atmospheric melting in a high
frequency induction melting furnace, and specimen tubes having the
alloy chemical compositions as shown in Table 1 below (outer
diameter 59 mm, thickness 8 mm, length 3000 mm) were casted by
centrifugal casting. Specimen Nos. 11 to 23 are inventive examples,
and specimen Nos. 101 to 105 are comparative examples.
[0191] More concretely, the comparative examples include specimen
Nos. 101 to 104 which are comparative examples containing a larger
amount of Ce than La in comparison with the alloy chemical
composition of the present invention, and specimen No. 105 which is
a comparative example in which the content of La is less than 80%
with respect to the total amount of La and Ce.
TABLE-US-00001 TABLE 1 MASS % NO. C Si Mn Cr Ni Al Ce La W Mo Ti Nb
B INVENTIVE 11 0.43 0.39 0.8 23.1 32.0 3.0 0.15 1.56 0.12 EXAMPLE
INVENTIVE 12 0.41 0.26 0.4 23.8 34.4 3.1 0.11 0.95 1.3 EXAMPLE
INVENTIVE 13 0.35 0.33 0.6 24.1 34.7 3.5 0.01 0.89 EXAMPLE
INVENTIVE 14 0.43 0.51 0.4 24.9 34.3 3.8 0.04 1.1 0.11 EXAMPLE
INVENTIVE 15 0.46 0.47 0.9 24.8 35.1 2.7 0.20 1.18 EXAMPLE
INVENTIVE 16 0.46 0.41 0.5 23.5 34.6 3.9 0.07 0.9 0.06 EXAMPLE
INVENTIVE 17 0.33 0.12 0.17 25.0 33.4 3.9 0.02 0.09 0.83 0.12
EXAMPLE INVENTIVE 18 0.34 0.46 0.16 24.7 32.9 3.5 0.03 0.12 2.8
0.19 EXAMPLE INVENTIVE 19 0.46 0.49 0.9 24.3 43.6 2.1 0.16 1.5 0.12
EXAMPLE INVENTIVE 20 0.43 0.62 1.1 22.1 29.3 3.4 0.05 0.31 0.6 0.5
EXAMPLE INVENTIVE 21 0.38 0.38 0.5 26.3 36.7 2.0 0.33 1.6 0.29
EXAMPLE INVENTIVE 22 0.31 0.49 0.32 24.5 40.1 2.7 0.01 0.24 2.1 0.5
0.8 EXAMPLE INVENTIVE 23 0.41 0.33 0.7 23.8 31.5 2.6 0.05 0.21 2.8
0.04 EXAMPLE COMPARATIVE 101 0.37 0.42 0.7 24.4 33.2 2.8 0.12 0.05
2.82 0.12 EXAMPLE COMPARATIVE 102 0.45 0.56 0.6 23.8 29.7 3.8 0.12
0.03 1.5 0.21 EXAMPLE COMPARATIVE 103 0.45 1.43 1.3 22.9 34.7 2.4
0.19 0.05 3.15 0.23 EXAMPLE COMPARATIVE 104 0.46 0.54 0.28 23.9
29.7 3.7 0.11 0.02 1.51 0.2 EXAMPLE COMPARATIVE 105 0.41 0.23 0.9
26.4 38.4 3.0 0.07 0.18 0.9 1.34 EXAMPLE
<Surface Treatment>
[0192] For these specimen tubes, a skiving process which is rough
processing, and a surface treatment by paper polishing were
conducted on the inner surface of the tubes, and the surface
roughness (Ra) was adjusted to 1.0 .mu.m.
<Heat Treatment>
[0193] After the surface treatment, for all specimen tubes, a
treatment of heating in atmospheric air (oxygen about 21%), at
1000.degree. C. for 10 hours, and cooling the furnace after the
heating was conducted.
<High Temperature Ductility Test>
[0194] A tensile test piece was prepared from a specimen tube in
conformance with JIS Z 2201, and a ductility test was conducted.
Concretely, the test piece was processed to have a parallel part
diameter of 10 mm and a parallel part length of 50 mm, and
ductility test was conducted according to the metal material
tensile test method of JIS G 0567. The test was conducted at
1100.degree. C.
[0195] The results of respective tests described above are shown in
Table 2.
TABLE-US-00002 TABLE 2 TEST TENSILE ELONGA- TEMPERATURE STRENGTH
TION NO. (.degree. C.) (Mpa) (%) INVENTIVE 11 1100 59.1 30.0
EXAMPLE INVENTIVE 12 1100 60.2 23.1 EXAMPLE INVENTIVE 13 1100 52.4
35.6 EXAMPLE INVENTIVE 14 1100 58.7 40.1 EXAMPLE INVENTIVE 15 1100
57.7 34.0 EXAMPLE INVENTIVE 16 1100 58.0 32.5 EXAMPLE INVENTIVE 17
1100 49.8 37.4 EXAMPLE INVENTIVE 18 1100 51.2 30.2 EXAMPLE
INVENTIVE 19 1100 57.0 36.8 EXAMPLE INVENTIVE 20 1100 55.0 27.8
EXAMPLE INVENTIVE 21 1100 56.4 23.7 EXAMPLE INVENTIVE 22 1100 51.2
24.2 EXAMPLE INVENTIVE 23 1100 53.2 21.3 EXAMPLE COMPARATIVE 101
1100 52.1 3.3 EXAMPLE COMPARATIVE 102 1100 53.4 3.3 EXAMPLE
COMPARATIVE 103 1100 29.7 1.0 EXAMPLE COMPARATIVE 104 1100 56.0 4.2
EXAMPLE COMPARATIVE 105 1100 53.8 4.1 EXAMPLE
<Discussion of Test Results>
[0196] Regarding tensile strength, Table 2 reveals that specimen
Nos. 11 to No. 23 which are inventive examples are almost
comparable with specimen Nos. 101 to No. 105 which are comparative
examples.
[0197] Regarding elongation (high temperature tensile ductility),
the inventive example is about 10 times the comparative
example.
[0198] Excellent elongation (high temperature tensile ductility) in
specimen Nos. 11 to No. 23 which are inventive examples is
attributed to the fact that the generation amount of Ni--La
compounds such as Ni.sub.2La and Ni.sub.3La having excellent high
temperature tensile ductility can be increased by making the
content of La in the rare earth element 80% or more.
[0199] On the other hand, poor elongation (high temperature tensile
ductility) in specimens 101 to 105 which are comparative examples
is attributed to the fact that the generation amount of Ni--Ce
compounds such as Ni.sub.2Ce and Ni.sub.3Ce is large due to a high
content of Ce in the rare earth element, namely a content of La of
less than 80%, and this causes high temperature brittleness.
[0200] Regarding the inventive examples, specimen No. 18 containing
0.12% of La and 0.03% of Ce as the rare earth element shows
elongation (high temperature tensile ductility) comparable with
those in other inventive examples. This is because the generation
amount of the Ni--Ce compounds can be controlled by making Ce 0.1%
or less.
[0201] In both of the inventive examples and comparative examples,
film thickness and the area percentage of the alumina barrier layer
were excellent, and for inventive examples, when the specimen piece
is plated with Ni, and covered with stainless sheet thereon, and
further coated with resin thereon, and then a section SEM analysis
was conducted, it was found that a preferred alumina barrier layer
of 0.05 .mu.m or more and 3 .mu.m or less was formed in any
examples.
[0202] As shown in the above example, the cast product of the
present invention is not only able to form a uniform alumina
barrier layer on the entire surface of the cast body, and
effectively prevent oxygen, carbon, nitrogen and the like from
entering from external atmosphere, but also has excellent high
temperature tensile ductility.
[0203] In the above example, the specimen tubes were produced by
centrifugal casting, however, similar results can be obtained by
stationary casting.
Example 2
[0204] Molten metal was produced by atmospheric melting in a high
frequency induction melting furnace, and a specimen tube (outer
diameter 59 mm, thickness 8 mm, length 3000 mm) containing C: 0.4
mass %, Si: 1.3 mass %, Mn: 1.1 mass %, Cr: 24.3 mass %, Ni: 34.7
mass %, Al: 3.36 mass %, rare earth element: 0.25 mass %, W: 2.9
mass %, Ti: 0.12 mass %, and the remainder of Fe and an inevitable
impurity was casted by mold centrifugal casting. Specimen Nos. 201
to 209 are inventive examples, specimen Nos. 311 to 312 are
reference examples, and specimen No. 421 is a comparative
example.
[0205] For every specimen tube, a skiving process was conducted on
the inner surface, and the surface roughness (Ra) was adjusted to
0.6 .mu.m.
<Acid Treatment>
[0206] As shown in Table 3, specimen Nos. 201 to No. 209 which are
inventive examples were dipped in an acid solution containing a
polyhydric alcohol liquid for 3 minutes or for 10 minutes.
[0207] Specimen Nos. 311 and No. 312 which are reference examples
were dipped in an acid solution not containing a polyhydric alcohol
liquid in a similar manner.
[0208] The inventive examples and reference examples subjected to
an acid treatment were washed with water after the acid
treatment.
[0209] Specimen No. 421 which is a comparative example was not
subjected to an acid treatment.
TABLE-US-00003 TABLE 3 MIXING RATIO (% BY VOLUME) ALUMINA SPEC-
HYDRO- POLY- SURFACE FILM FILM AREA IMEN ACID NITRIC CHLORIC HYDRIC
DIPPING ROUGHNESS THICKNESS PERCENTAGE NO. SOLUTION ACID ACID
ALCOHOL TIME Ra (.mu.m) (.mu.m) (%) 201 INVENTIVE GLYCEREGIA 22.5
67.5 10 3 0.74 0.1 75 EXAMPLE LIQUID MINUTES 202 INVENTIVE 22.5
67.5 10 10 0.72 0.4 78.3 EXAMPLE MINUTES 203 INVENTIVE 20 60 20 2
0.56 0.7 91.7 EXAMPLE MINUTES 204 INVENTIVE 20 60 20 10 0.57 0.9
86.2 EXAMPLE MINUTES 205 INVENTIVE 15 45 40 3 0.45 0.6 93.2 EXAMPLE
MINUTES 206 INVENTIVE 15 45 40 10 0.42 0.6 89.7 EXAMPLE MINUTES 207
INVENTIVE 10 30 60 3 0.52 0.5 72.4 EXAMPLE MINUTES 208 INVENTIVE
GLYCOL 20 60 20 3 0.53 0.8 90.2 EXAMPLE LIQUID MINUTES 209
INVENTIVE 20 60 20 10 0.51 0.9 84.7 EXAMPLE MINUTES 311 REFERENCE
AQUA REGIA 75 25 0 2 1.07 N N EXAMPLE MINUTES 312 REFERENCE 75 25 0
10 1.12 N N EXAMPLE MINUTES 421 COMPARATIVE WITHOUT 0 0 0 0 0.6 0.7
63.1 EXAMPLE ACID MINUTE TREATMENT
<Surface Roughness (Ra)>
[0210] From each of the specimen tubes, a specimen piece of 20 mm
wide.times.30 mm long was cut out, and surface roughness (Ra) of
the inner surface of each specimen piece was measured. Also, for
specimen Nos. 201 to No. 207, No. 311, No. 312 and No. 421, a
surface photograph of the inner surface of the specimen piece was
taken.
[0211] Measurement results of surface roughness (Ra) are shown in
Table 3, and surface photographs of specimen pieces are shown in
FIG. 1 to FIG. 10.
[0212] Table 3 reveals that in specimen Nos. 201 to No. 209 which
are inventive examples, surface roughness (Ra) is adjusted within
the range of 0.42 .mu.m to 0.74 .mu.m by conduction of the acid
treatment. Also, FIG. 1 to FIG. 7 which are surface photographs
reveal that every specimen piece has surface glaze, and the scratch
occurring by the skiving process is smoothed by the acid treatment
by the acid solution containing a polyhydric alcohol liquid.
[0213] In specimen Nos. 201 and No. 202, surface roughness (Ra) is
increased in comparison with specimen No. 421 which is a
comparative example not subjected to an acid treatment. However,
comparison between the surface photographs of specimen Nos. 201 and
No. 202 (FIG. 1 and FIG. 2) and the surface photograph of specimen
No. 421 which is a comparative example (FIG. 10) reveals that
although a large number of scratches by skiving process are
observed in the vertical direction in specimen No. 421, most of
such scratches disappear in specimen Nos. 201 and No. 202.
[0214] Specimen No. 311 and No. 312 which are reference examples
dipped in an acid solution (aqua regia) not containing a polyhydric
alcohol liquid have surface roughness (Ra) exceeding 1.0 .mu.m, and
the surface photographs thereof (FIG. 8 and FIG. 9) reveal that the
surface do not have glaze. This is because the surface is
excessively corroded by corrosion by the treatment only with strong
acid, and asperity occurs adversely.
[0215] From the above, it can be found that by the treatment by
acid containing a polyhydric alcohol liquid, surface roughness (Ra)
is appropriately adjusted, and a specimen tube with no scratch is
obtained.
<Heat Treatment>
[0216] For the specimen tubes having subjected to the surface
treatment, a treatment of heating in atmospheric air (oxygen about
21%) at 1050.degree. C. for 10 hours, and furnace cooling after the
heating was conducted.
<Surface Measurement>
[0217] For each specimen piece after conduction of the heat
treatment, thickness (.mu.m) of the formed alumina barrier layer
and area percentage (%) of the Al.sub.2O.sub.3 film in the surface
of the test piece were measured. The measurement results are
described in the aforementioned Table 3.
[0218] Thickness of an alumina barrier layer was measured by a SEM
(scanning electron microscope). The specimen in which an alumina
barrier layer was not generated, and the specimen in which the site
having a thickness of less than 0.5 .mu.m (including the thickness
of zero) appeared intermittently in a part of the alumina barrier
layer, are marked with the character N (No) in Table 3.
[0219] Further, as to the area percentage of Al.sub.2O.sub.3 film
in the surface of the test piece, distribution condition of Al was
measured by surface analysis for a region of 1.35 mm.times.1 mm on
the surface of the test piece by using a SEM/EDX measurement
tester, and the distribution quantity was converted to an area
percentage.
[0220] Further, for specimen Nos. 201 to No. 207, No. 311, No. 312
and No. 421 on which heat treatment was conducted, surface
photographing of the inner surface of the specimen piece and
section SEM analysis were conducted. In conducting the section SEM
analysis, a specimen piece was plated with Ni, covered with a
stainless sheet, and further coated with resin thereon.
[0221] FIG. 11 to FIG. 17 are surface photographs and FIG. 21 to
FIG. 27 are section SEM photographs of specimen Nos. 201 to No.
207, and FIG. 18 to FIG. 20 are surface photographs and FIGS. 28 to
30 are section SEM photographs of specimen Nos. 311, No. 312 and
No. 421.
[0222] Table 3 reveals that in any of specimen Nos. 201 to No. 209
which are inventive examples, the film thickness is 0.1 .mu.m to
0.9 .mu.m, and a desired alumina barrier layer is formed. Also FIG.
11 to FIG. 17, and FIG. 21 to FIG. 27 reveal that a uniform alumina
barrier layer is formed on the entire surface. This is because
surface roughness (Ra) is adjusted by the acid solution containing
a polyhydric alcohol liquid, and scratches and the like by skiving
process are also smoothed.
[0223] Comparing between inventive examples, in specimen Nos. 201
and No. 202 in which the polyhydric alcohol liquid is 10%, and in
specimen No. 207 in which the polyhydric alcohol liquid is 60%, the
area percentage of the film is less than 80%, and is somewhat
inferior to other inventive examples.
[0224] The low area percentage of each film in specimen Nos. 201
and No. 202 in which the polyhydric alcohol liquid is 10% is
attributable to the fact that the oxidizing power cannot be
sufficiently adjusted by the polyhydric alcohol liquid as a result
of increase in the acid solution, and asperity is formed by
corrosion, and thus surface roughness (Ra) is increased in
comparison with other inventive examples.
[0225] The low area percentage of film in specimen No. 207 in which
the polyhydric alcohol liquid is 60% is attributable to the fact
that the oxidizing power of the acid solution is decreased as a
result of increase in the polyhydric alcohol liquid, and sufficient
adjustment of surface roughness (Ra) by corrosion cannot be
achieved.
[0226] These suggest that the polyhydric alcohol liquid contained
in the acid solution is preferably more than 10% and 40% or
less.
[0227] On the other hand, in specimen Nos. 311 and No. 312 which
are reference examples, formation of a film is little observed as
shown in Table 3, FIG. 18 and FIG. 19. This is because as shown in
FIG. 28 and FIG. 29, as a result of conducting an acid treatment
only by strong acid, the surface of the base material is rough, and
formation of an alumina barrier layer is inhibited.
[0228] As to specimen No. 421 which is a comparative example, as
shown in Table 3, FIG. 20 and FIG. 30, surface roughness (Ra) is
preferable, and film is formed, however, it can be realized that
the formed film is not continuous. This is because formation of the
alumina barrier layer is inhibited by scratches occurring by a
skiving process.
[0229] As shown in the above examples, since the cast product of
the present invention has high ductility and a uniform alumina
barrier layer generated on the surface of the cast body, it is
resistant to peeling even when it is exposed to repeated heating
and cooling cycles, and since the alumina barrier layer is tight,
excellent repetitive oxidation resistance is exerted in use in a
high temperature atmosphere, and entry of oxygen, carbon, nitrogen
and the like from the external atmosphere is effectively prevented,
and excellent repetitive oxidation resistance at high temperature,
cementation resistance, nitriding resistance, corrosion resistance
and so on can be maintained for a long term.
[0230] The present invention may be applied to cast products for
which a horning process or the like cannot be effected, such as a
long cast product and a cast product subjected to a bending
process, and as a result, a preferable alumina barrier layer can be
formed.
Example 3
[0231] Molten metal was produced by atmospheric melting in a high
frequency induction melting furnace, and respectively two tube
bodies (outer diameter 59 mm, thickness 8 mm, length 3000 mm)
having alloy chemical compositions as recited in Table 4 below were
casted by mold centrifugal casting, and edge preparation was
conducted in one side of the tube bodies, and tube bodies having
the same composition and are to be a pair were joined by abutment
welding.
[0232] In Table 4, "REM" indicates a rare earth element.
[0233] The obtained specimen tubes include specimen tubes No. 501
to No. 508 which are examples of the present invention, and
specimen tubes No. 611 to No. 613 which are comparative examples.
More concretely, the comparative examples include specimen tube No.
611 which is a comparative example containing more Al than that in
the alloy chemical composition of the present invention, specimen
tube No. 612 which is a comparative example containing more Ni than
that in the alloy chemical composition of the present invention,
and specimen tube No. 613 which is a comparative example having the
alloy chemical composition falling within the present invention but
not subjected to a surface treatment in the welded part.
TABLE-US-00004 TABLE 4 ALLOY CHEMICAL COMPOSITION (REMAINDER OF Fe
AND INEVITABLE IMPURITY) SURFACE SPECIMEN (MASS %) ROUGHNESS NO. C
Si Mn Cr Ni Al REM W Mo Ti Zr Nb B (Ra) 501 0.4 1.5 1.2 25.2 35.0
3.1 0.22 3.0 0.09 0.11 0.9 502 0.26 1.4 1.2 23.8 44.4 3.5 0.13 2.1
1.6 0.07 503 0.41 1.5 1.1 23.9 33.4 2.9 0.19 2.9 0.12 2.40 504 0.48
1.4 0.2 23.5 34.6 3.0 0.17 1.54 0.12 0.68 505 0.45 1.3 1.2 25.4
34.8 2.7 0.23 2.7 0.11 506 0.44 1.2 1.2 17.5 69 3.4 0.33 3.5 0.05
0.13 507 0.34 0.7 1.2 25.0 45.4 2.8 0.10 1.5 2 2.9 508 0.38 0.5 0.2
23.9 33.9 3.3 0.23 2.7 0.09 0.03 611 0.37 1.3 1 24.4 33.9 5.6 0.3
3.1 0.11 612 0.40 1.3 0.9 25.4 12.1 3.0 0.29 2.9 0.12 613 0.40 0.4
0.2 23.8 32.5 3.1 0.17 2.4 6.2
<Surface Treatment>
[0234] For these specimen tubes, skiving which is rough processing
was conducted in the region extending about 20 mm to 40 mm in the
width direction centered at the welded part in the inner side of
the tube.
[0235] Further, for specimen tubes No. 501 to No. 508, No. 611 and
No. 612 (namely, other than Specimen tube No. 613), a surface
treatment by paper polishing was conducted.
[0236] Surface roughness (Ra) in a welded part of each specimen
tube is shown in Table 4.
<Visual Observation Before Heat Treatment>
[0237] For specimen tube No. 504 which is an inventive example, and
specimen tube No. 613 which is a comparative example, photographs
of the specimen tubes cut along the axial direction are
respectively shown in FIG. 31 and FIG. 32.
[0238] Comparison between FIG. 31 and FIG. 32 reveal that specimen
No. 504 which is an inventive example has glaze in the welded part,
and has reduced asperity of the welded part as a result of the
surface treatment.
<Heat Treatment>
[0239] After the surface treatment, for every specimen tube, a
treatment of heating in atmospheric air (oxygen about 21%), at
1000.degree. C. for 10 hours, and cooling the furnace after the
heating was conducted.
<Surface Measurement>
[0240] For each test tube having subjected to the aforementioned
treatments, a specimen piece of 20 mm wide.times.30 mm long
including a welded part was cut out, and film thickness (.mu.m) of
the alumina barrier layer formed in the welded part inside the
specimen piece and area percentage (%) of Al.sub.2O.sub.3 were
measured. The measurement methods will be described below, and the
measurement results are described in Table 5 as "film thickness"
and "area percentage".
<Measurement of Film Thickness>
[0241] Thickness of an alumina barrier layer for the welded part
surface of a specimen piece was measured by a SEM (scanning
electron microscope). The specimen in which an alumina barrier
layer was not generated, and the specimen in which the site having
a thickness of less than 0.05 .mu.m (including the thickness of
zero) appeared intermittently in a part of the alumina barrier
layer, are marked with the character N (No) in Table 5.
<Measurement of Area Percentage of Film>
[0242] Area percentage of Al.sub.2O.sub.3 to the surface of the
welded part of the specimen piece was determined by using a SEM/EDX
(scanning analytical electron microscope) measurement tester.
Measurement was conducted for a region of 1.35 mm.times.1 mm on the
surface of the welded part of the specimen piece, and distribution
condition of Al was surface analyzed, and the distribution quantity
was converted to area percentage.
<Ductility Test>
[0243] A tensile test piece was prepared from a specimen tube in
conformance with JIS 22201, and a ductility test was conducted.
[0244] Concretely, the test piece was processed to have a parallel
part diameter of 10 mm and a parallel part length of 50 mm
including the welded part, and ductility test was conducted
according to the metal material tensile test method of JIS 22241.
The test was conducted at room temperature because difference
arises more clearly, than as it is conducted at high
temperature.
[0245] The results of the aforementioned tests are shown in Table
5. In Table 5, the indication of "-" means that the measurement or
the test was not conducted.
TABLE-US-00005 TABLE 5 FILM AREA TENSILE SPECIMEN THICKNESS
PERCENTAGE DUCTILITY NO. (.mu.m) (%) (%) 501 0.07 94.6 9.5 502 0.8
86.6 25.4 503 1.8 82.5 12.5 504 0.6 99.5 10.5 505 1 88.3 12.2 506
0.9 96.2 18.2 507 0.8 76.3 -- 508 1.1 71.8 -- 611 1.7 98.0 0.4 612
N 62.4 11.4 613 N 53.1 --
<Discussion of Test Results>
[0246] Table 5 reveals that specimen tubes No. 501 to No. 508 which
are inventive examples show better film thickness of the alumina
barrier layer and area percentage in comparison with specimen tubes
No. 611 to No. 613 which are comparative examples.
[0247] In the discussion of inventive examples, it can be found
that any film thickness falls within the preferred range of 0.05
.mu.m or more and 3 .mu.m or less. It is also revealed that the
tensile ductility is sufficient.
[0248] Comparison of inventive examples reveals that specimen tubes
No. 507 and No. 508 are inferior in film thickness and area
percentage to other inventive examples, and this is attributed to
the fact that surface roughness by a surface treatment of specimen
tube No. 507 is large, and surface roughness by a surface treatment
of specimen tube No. 508 is too fine. Therefore, it can be found
that the surface treatment conducted on the welded part preferably
makes the surface roughness (Ra) of 0.05 .mu.m to 2.5 .mu.m for
making the alumina barrier layer of the welded part 80 area % or
more.
[0249] On the other hand, as to the comparative example, specimen
tube No. 611 is inferior in tensile ductility although a preferred
alumina barrier layer is formed. This is because the content of Al
in the alloy chemical composition exceeds 4%. Therefore, it is
revealed that the content of Al is preferably 4% or less.
[0250] Further, in specimen tubes No. 612 and No. 613, a sufficient
alumina barrier layer is not formed. In specimen tube No. 612, the
content of Ni in the alloy chemical composition is smaller than
18%, and as a result the content of Fe is relatively large, and
Cr--Fe--Mn oxide is more likely to be generated on the surface of
the cast body, so that generation of an alumina barrier layer is
inhibited.
In specimen No. 613, while the alloy chemical composition falls
within the range of the present invention, the surface roughness is
large, and generation of an alumina barrier layer is inhibited as a
result of not conducting a surface treatment.
[0251] These reveal that a preferred alumina barrier layer is
formed in the specimen tube which is an inventive example, in
comparison with the specimen tube which is a comparative
example.
<Section Analysis>
[0252] For specimen pieces obtained from specimen tube No. 504
which is an example of the present invention, and specimen tube No.
613 which is a comparative example, photographs of the section
perpendicular to the welded part were taken, and section SEM
analysis was conducted. For section SEM analysis, the specimen
piece was plated with Ni, covered with a stainless steel sheet, and
further coated with resin thereon.
[0253] The obtained section photographs of the inventive example
and the comparative example are respectively shown in FIG. 33 and
FIG. 34, and enlarged photographs by section SEM analysis in the
inventive example and the comparative example are respectively
shown in FIG. 35 and FIG. 36.
[0254] From these drawings, it can be seen that in the inventive
example, an alumina barrier layer having a film thickness of 0.5
.mu.m is uniformly formed on the surface of the base material. On
the other hand, it can be seen that in the comparative example, the
asperity of the surface is significant, and an alumina barrier
layer is not successfully formed.
[0255] Also from these section photographs, the advantage of the
present invention would be recognized.
[0256] As shown in the above examples, in the cast body of the
present invention, a uniform alumina barrier layer can be formed on
the entire surface of the cast body including a welded part by
conducting a heat treatment after subjecting the welded part to a
surface treatment, so that entry of oxygen, carbon, nitrogen and
the like from the external atmosphere is effectively inhibited, and
the cast body as a whole including the welded part is able to keep
the excellent repeated oxidation resistance at high temperature,
cementation resistance, nitriding resistance, corrosion resistance
and the like for a long term.
[0257] The present invention is useful as a cast product having an
alumina barrier layer and a method for producing the same.
* * * * *
References