U.S. patent application number 12/811450 was filed with the patent office on 2010-12-09 for welding structure of tube stubs and tube header.
This patent application is currently assigned to MITSUBISHI HEAVY INDUSTRIES, LTD.. Invention is credited to Masahiko Hokano, Toshiyuki Imazato, Nobuyoshi Komai, Takeshi Miyazawa, Fumio Nishi, Nobuhiko Saito, Takumi Tokiyoshi.
Application Number | 20100307429 12/811450 |
Document ID | / |
Family ID | 42100529 |
Filed Date | 2010-12-09 |
United States Patent
Application |
20100307429 |
Kind Code |
A1 |
Komai; Nobuyoshi ; et
al. |
December 9, 2010 |
WELDING STRUCTURE OF TUBE STUBS AND TUBE HEADER
Abstract
In order to provide a welding structure of a tube header and
tube stubs which successfully improves the durability thereof
against creep and fatigue damage of the tube stubs without
interposing a component of a different material between the tube
header and the tube stubs, i.e. requiring additional components, a
welding structure of the present invention comprises: a tube header
(2) being made of ferritic heat resisting steel; a plurality of
tube stubs (4) which are welded onto an outer surface of the tube
header, each of the tube stubs having a bended section, in which
the plurality of tube stubs (4) are made of austenite stainless
steel, and welded to the tube header (2) by using nickel base alloy
as a welding material. Moreover annular chases (10) are
respectively formed on the outer surface of the tube header (2) to
surround welding portions of the tube header (2); joining portions
(14) are formed on an inner side of the annular chases (10); butt
grooves are formed by joining surfaces of the tube stubs (4) and
the joining portions (14) in a continuous manner and welded by
using the nickel base alloy as a welding material, the tube stubs
having a diameter substantially same as those of the joining
portions.
Inventors: |
Komai; Nobuyoshi; (
Nagasaki, JP) ; Miyazawa; Takeshi; (Nagasaki, JP)
; Nishi; Fumio; ( Nagasaki, JP) ; Tokiyoshi;
Takumi; (Nagasaki, JP) ; Saito; Nobuhiko; (
Nagasaki, JP) ; Imazato; Toshiyuki; ( Nagasaki,
JP) ; Hokano; Masahiko; ( Nagasaki, JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW, SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
MITSUBISHI HEAVY INDUSTRIES,
LTD.
Tokyo
JP
|
Family ID: |
42100529 |
Appl. No.: |
12/811450 |
Filed: |
September 30, 2009 |
PCT Filed: |
September 30, 2009 |
PCT NO: |
PCT/JP2009/067016 |
371 Date: |
August 12, 2010 |
Current U.S.
Class: |
122/235.15 |
Current CPC
Class: |
F28F 21/082 20130101;
F22B 37/22 20130101; F28F 2265/26 20130101; B23K 2101/14 20180801;
F28F 9/182 20130101; F28F 9/18 20130101; F22B 37/104 20130101 |
Class at
Publication: |
122/235.15 |
International
Class: |
F22B 37/10 20060101
F22B037/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 7, 2008 |
JP |
2008-260701 |
Claims
1. A welding structure of a tube header and tube stubs, comprising:
a tube header being made of ferritic heat resisting steel; and a
plurality of tube stubs which are welded onto an outer surface of
the tube header, each of the tube stubs having a bended section,
wherein the plurality of tube stubs are made of austenite stainless
steel, and welded to the tube header by using nickel base alloy as
a welding material.
2. The welding structure of the tube heater and the tube stubs
according to claim 1, wherein: annular chases are respectively
formed on the outer surface of the tube header to surround welding
portions of the tube header onto which the tube stubs are to be
welded; joining portions are formed on an inner side of the annular
chases; butt grooves are formed between the tube stubs and the
joining portions by joining surfaces of the tube stubs and the
joining portions in a continuous manner, the tube stubs having a
diameter that is substantially same as those of the joining
portions; and the butt grooves formed between the tube stubs and
the joining portions are welded by using the nickel base alloy as a
welding material.
3. The welding structure of the tube heater and the tube stubs
according to claim 1, wherein: through-holes are formed on the
outer surface of the tube header; ends of the tube stubs are
respectively inserted in the through-holes; and the ends of the
tube stubs inserted in the through-holes are welded to an inner
surface of the tube header by using the nickel base alloy as a
welding material.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a welding structure of a
tube header and tube stubs, especially those suitable for use in a
final superheater and a reheater of a boiler installed in a thermal
power plant.
[0003] 2. Description of the Related Art
[0004] Currently, there are mainly three methods to generate power,
that is nuclear power, thermal power and hydraulic power. Among
those, thermal power generation is a very potential method for
generating electricity, which is comparatively safe and responsive
to load change and is expected to remain valuable in the field of
power generation technology.
[0005] As a main component of a boiler for generating power in a
thermal power plant, there is a tube header which is installed in
each of various devices such as an evaporator, a superheater and a
reheater and which distributes or collects inner fluids. The tube
header has a plurality of mount holes for mounting heat exchange
tubes and pipes.
[0006] FIG. 1 is a schematic view showing an overall structure of
tube stubs having bended sections respectively and the tube header
to which the tube stubs are welded. The plurality of tube stubs 4
each having a bended section are welded to the tube header 2 of a
straight pipe type so as to form a tube header part. For instance,
in a boiler for generating electricity, there is a plurality of
tube stubs 4 mounted on one tube header 2 by welded connection.
[0007] As the tube header 2 is used at a high temperature in a
final superheater and a repeater of a boiler, ferritic
heat-resisting steel is often used, such as 1Cr steel, 2Cr steel,
9Cr steel and 12Cr steel. The ferritic heat resisting steels
(hereinafter referred to as ferritic steel) have high-temperature
strength, low thermal expansion coefficiency in the perspective of
preventing thermal deformation and thermal stress, and high
heat-transfer coefficient. Further, it is common to use ferritic
steels such as 1Cr steel, 2Cr steel, 9Cr steel and 12Cr steel for
the tube stubs same as the tube header so as not to generate
thermal stress. Herein, the tube stubs are made of ferritic steel
such as JIS STBA28, JIS STBA23, JIS STBA24 as well as STBA23J1,
STBA28, STBA29 and SUS410J3TB of Japanese standards for Thermal
Power Technology and the tube header is made of materials
equivalent of the above standard. The materials of ASME equivalent
of the above can be used as well.
[0008] Recently, in thermal power plants, frequent start-stop
operation is needed to conform to demand fluctuation of
electricity. Under such operation conditions of the thermal power
plant, some damages caused by the thermal stress from the
temperature difference between the tube stubs and ceiling wall of a
furnace or self-weight of the heat exchanger tubes are
acknowledged, such as fatigue damages or creep damages at the
welding portion on the tube header. Especially, as for a tube
header with tube stubs welded thereon of a final superheater and
repeater in a USC boiler (USC: Ultra Super Critical) whose maximum
steam condition is improved to about 600.degree. C. in order to
enhance power generation efficiency, when the tube header with tube
stubs are made of ferritic steel, a welded heat-affected zone of
the materials has lower creep strength in comparison with the base
material, which makes it prone to creep damage. As a result,
regular inspections or changing of tubes are required more
frequently than before.
[0009] FIG. 5 is a side view of a bended section (section B of FIG.
1) of the tube stub 4. FIG. 6 illustrates sectional views of the
tube stub 4 taken along the lines A-A, B-B, C-C, D-D, and E-E of
FIG. 5. As shown in FIG. 6, a cross-sectional shape of the tube
stub 4 is not uniform but flattened at the bended section. Thus,
when internal pressure acts thereon, bending stress takes
place.
[0010] Further, another factor of occurrence of the creep damages
may be decline in the creep strength at a dorsal side of the bended
section due to decreased thickness and plastic deformation thereof
in comparison to a straight section thereof.
[0011] This has already been verified by creep tests in which
internal pressure was loaded to a test piece of a tube type having
a bended section and made of 12Cr steel. There was substantial
creep damage to the bended section compared to the straight section
and the test piece exploded from the bended section and it was
affirmed that the bending stress at the bended section was a factor
to the creep damage.
[0012] A yet another factor of occurrence of the creep damages is
that the steam temperature varies from one tube stub to another.
Thus, the temperature of the tube stub subjected to the steam
temperature becomes higher than a set temperature, and thus
experiences a shorter creep life than other tube stubs.
[0013] In order to solve the problems caused from the creep damage,
it is possible to make the tube stubs thicker but this is not
economical and still fails to solve the problem within realistic
dimensions of the tube stub. Therefore, it is difficult to suppress
the occurrence of the creep damage simply by making the tube stub
walls thicker.
[0014] Patent Document 1 discloses a structure of welding tube
stubs of ferritic steel and a header of ferritic steel, in which a
tube of austenite steel is interposed between the tube stubs and
the tube header and the tube is set to the length which is extended
to the position separated by 1/2 or more of the outer diameter of
the tube from the welding end of the tube to the header. Further,
in this welding structure, a protrusion part is formed on the
header at the welding position by cutting around the welding
position while a tip of the tube is cut to form a tapered end so as
to form a butted groove with the protrusion part of the header. The
butted groove is a root and the root of the protrusion part is made
longer than the root of the header.
[0015] However, according to Patent Document 1, not only that this
structure requires more components by interposing the tube of
austenite steel between the tube header and tube stubs but it takes
twice more hours for welding the parts as the tube stubs of
ferritic steel and the tube of austenite steel, and the header of
ferritic steel and the tube of austenite steel needed to be welded
respectively. Therefore, when the structure disclosed in Patent
Document 1 is applied to a boiler for power generation having
hundreds of tube stubs to be welded to a header, it takes enormous
time and money.
[0016] Furthermore, in the case of the tube stubs having bended
sections respectively, the welding structure disclosed in Patent
Document 1 hardly suppresses the creep damage at the bended
sections of the tube stubs.
[0017] Moreover, the protrusion part of the header and the cut end
of the tube are joined so as to form a trapezoidal groove and thus
the structure around the welding position becomes complex and the
workability at the welding position is compromised. In addition, at
the welding end of the tube stub at which the bending stress works
heavily, it may be difficult to ensure sufficient diameter of the
tube stub and the welding material together.
[Prior Art]
[Patent Document 1] JP8-152991A
SUMMARY OF THE INVENTION
[0018] In view of the problems above, an object of the present
invention is to provide a welding structure of a tube header and
tube stubs which successfully improves the durability thereof
against creep and fatigue damage of the tube stubs without
interposing a different material between the tube header and the
tube stubs or additional components.
[Means to Solve the Problems]
[0019] To achieve the object, the present invention proposes a
welding structure of a tube header and tube stubs, comprising: a
tube header being made of ferritic heat resisting steel; a
plurality of tube stubs which are welded onto an outer surface of
the tube header, each of the tube stubs having a bended section,
wherein the plurality of tube stubs are made of austenite stainless
steel, and welded to the tube header by using nickel base alloy as
a welding material.
[0020] The plurality of tube stubs are made of austenite stainless
steel which has excellent creep strength so as to improve the
durability thereof against the bending stress thereof and
temperature fluctuation. Further, the austenite stainless steel has
superior heat resistance and thus it can be used in a boiler under
higher steam temperature condition than a conventional one. And
simply changing the material of the tube stubs of the structure
same as the conventional case from ferritic steel of the
conventional case to austenite stainless steel of the present
embodiment, the steam temperature condition can be improved from
600.degree. C. to 630.degree. C.
[0021] As for the coefficiency of thermal expansion (1/.degree. C.)
when the temperature is raised from the ambient temperature to
600.degree. C., the coefficiency of 9Cr steel which is one of
ferritic steels is approximately 12.6.times.10.sup.-6 and that of
austenite steel is approximately 18.4.times.10.sup.-6. Therefore,
by using as a welding material nickel base alloy whose thermal
expansion coefficiency is 16.0.times.10.sup.-6 between that of 9Cr
steel and that of austenite stainless steel, the thermal stress
occurred between the weld metal and 9Cr steel and between the weld
metal and austenite stainless steel can be minimized. Examples of
nickel base alloy as a weld material are inconel-type weld
materials specified in JISZ3334 YniCr-3, AWS A5.14 ER NiCr-3 and
the like.
[0022] By using nickel base alloy as a weld material in the above
manner, the thermal stress between the welding material and the
tube stubs of austenite stainless steel and also between the
welding material and the tube header of ferritic steel is
suppressed but cannot be completely diminished.
[0023] However, according to the analysis of the thermal stress we
conducted, it was found that austenite stainless steel used for the
tube stubs had such high creep strength that the creep and the
fatigue damage due to the thermal stress was too small to cause an
practical issue.
[0024] Further, it is preferable that annular chases are
respectively formed on the outer surface of the tube header to
surround welding portions of the tube header onto which the tube
stubs are to be welded; joining portions are formed on an inner
side of the annular chases; butt grooves are formed between the
tube stubs and the joining portions by joining surfaces of the tube
stubs and the joining portions in a continuous manner, the tube
stubs having a diameter that is substantially same as those of the
joining portions; and the butt grooves formed between the tube
stubs and the joining portions are welded by using the nickel base
alloy as a welding material.
[0025] The joining portions are formed on the tube header so that
the tube stubs of austenite stainless steel with high thermal heat
expansion rate and the tub header can be connected in a continuous
manner, and the tube stubs and the joining portions of the tube
header are butted so that thermal stress can be reduced by the
annular chases.
[0026] The above structure is similar to that of butt-welding of
small diameter tubes and the durability of butt welding of the
small diameter tubes are already well proven.
[0027] Further, the shape of the annular chases such as the depth
thereof can be set depending on the temperature or bending stress.
However, the tube header must have thickness enough to accommodate
the depth of the chases.
[0028] In this, the absolute figure of the thermal stress occurred
between the weld material and the tube stubs of austenite stainless
steel and between the weld material and the tube header of ferritic
steel is so small that there is no practical issue of the creep
damage or fatigue damage.
[0029] It is also preferable that through-holes are formed on the
outer surface of the tube header; ends of the tube stubs are
respectively inserted in the through-holes; and the ends of the
tube stubs inserted in the through-holes are welded to an inner
surface of the tube header by using the nickel base alloy as a
welding material.
[0030] In this, the bending stress of the tube stub due to the
thermal stress is largest at contact positions where the tube stubs
and the outer surface of the tube header are in contact. However,
there is no welding at the contact positions and thus the creep
damage due to the bending stress is very little even when the
bending stress acts at the contact positions.
[0031] Further, in order to insert the tube stubs in the
through-holes, the tube stubs may need to be cooled beforehand or
the tube header may need to be warmed beforehand.
[0032] Furthermore, the tube header needs to be seal-welded from
inside and the tube header used in a final superheater or repeater
of a USC boiler usually has an inner diameter of 300 mm. If it is
difficult to manually perform the welding, a robot for automatic
welding or the like may be used.
[0033] At the same time, the tube stubs of austenite stainless
steel are subjected to compression stress from the tube header of
ferritic steel. However, the creep life of the tube stubs of
austenite stainless steel is higher than that of the tube header
made of ferritic steel and thus there is enough creep strength at
the contact positions.
[Effects of the Invention]
[0034] According to the present invention described above, a
welding structure of a tube header and tube stubs which
successfully improves the durability thereof against creep and
fatigue damages of the tube stubs without interposing a different
material between the tube header and the tube stubs or additional
components can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a schematic view of an overall structure of tube
stubs each having a bended section and a tube header having the
stubs welded thereon.
[0036] FIG. 2 is a sectional view of the tube header with the tube
stubs welded thereon in relation to a first preferred
embodiment.
[0037] FIG. 3 is a sectional view of the tube header with the tube
stubs welded thereon in relation to a second preferred
embodiment.
[0038] FIG. 4 is a sectional view of the tube header with the tube
stubs welded thereon in relation to a third preferred
embodiment.
[0039] FIG. 5 is a side view illustrating a bended section of the
tube stub.
[0040] FIG. 6 shows sectional views of the tube stub taken along
lines A-A, B-B, C-C, D-D, and E-E of FIG. 5
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] A preferred embodiment of the present invention will now be
described in detail with reference to the accompanying drawings. It
is intended, however, that unless particularly specified,
dimensions, materials, shape, its relative positions and the like
shall be interpreted as illustrative only and not limitative of the
scope of the present.
First Preferred Embodiment
[0042] In a first preferred embodiment, an example structure of a
tube header and a plurality of tube stubs, each of which has a
bended section, to be used in a final superheater and a repeater of
USC boiler of 600.degree. C. grade. The overall structure of the
tube header and the tube stubs welded thereto is the same as the
known structure shown in FIG. 1 and thus it will not be explained
further.
[0043] FIG. 2 is a sectional view of the tube header with the tube
stubs welded thereon in relation to the first preferred embodiment
and illustrates an enlarged view of a section A of FIG. 1.
[0044] In FIG. 2, a tube header 2 made of 9Cr steel of ferritic
steel has through-holes 3 formed on the outer surface thereof.
Around each of the through-holes, a concave part 7 is formed by
chipping off the edge of the through-hole of the tube header 2, an
edge of a tube stub 4 is butted against and connected to the
concave part 7 such that a passage 5 inside the tube stub 4 having
a bended section faces the through-hole 3. Herein, the tube stubs
are made of austenite stainless steel such as JIS SUS304, JIS SUS
321HTB, and JIS SUS347HTB as well as SUSTP347HTB, SUS321J1HTB,
SUS321J4HTB, SUS310J1TB, SUS310J2TB, SUS310J3TB and the like of
Japanese standards for Thermal Power Technology and materials
equivalent of the above according to ASME.
[0045] The above connection of the tube stub to the tube header is
performed by fillet welding using Ni-based alloy such as inconel
alloy as a welding material thereby forming a welding spot 6.
[0046] As described above, the tube header 2 and the tube stubs 4
are welded together and when bending stress acts on the tube stubs,
the part most susceptible to the creep damage is the welding end of
the tube stub 4 (section C of FIG. 2) at which there is a
discontinuity in the welding shape and is adjacent to a weld
heat-affected zone. However, in the preferred embodiment, as the
tube stubs are made of austenite stainless steel which has an
excellent creep strength, the durability of the tube stubs 4
against the bending stress is significantly improved and thus the
creep damage to the welding ends of the tube stubs (section C of
FIG. 2) due to the bending stress on the tube stubs is suppressed
to the degree that practically causes no issues.
[0047] The bending stress takes place when there is an internal
pressure at the bending section where the cross-sectional shape
thereof is flattened regardless of materials thereof. However, when
tube stubs are made of austenite stainless steel, which has better
creep strength than ferritic steel, it is possible to set a tube
thickness in consideration of the creep durability at the bended
section so as to solve this issue.
[0048] As for the coefficiency of thermal expansion (1/.degree. C.)
when the temperature is raised from the ambient temperature to
600.degree. C., the coefficiency of 9Cr steel of which the tube
header 2 is made is approximately 12.6.times.10.sup.-6 and that of
austenite steel of which the tube stubs are made is approximately
18.4.times.10.sup.-6. Further, the thermal expansion coefficiency
of inconel which is nickel base alloy and used as the welding
material herein, is 16.0.times.10.sup.-6 between that of 9Cr steel
and that of austenite stainless steel. Therefore, nickel base alloy
is used as a welding material so as to suppress the thermal stress
occurred between the weld metal and 9Cr steel and between the weld
metal and austenite stainless steel.
[0049] However, there is a large difference of the thermal
expansion coefficiency between austenite stainless steel and 9Cr
steel as explained above and thus even with nickel base alloy used
as the welding material, the thermal stresses between the welding
material and the tube stubs of austenite stainless steel and
between the welding material and the tube header of ferritic steel
are suppressed but cannot be completely diminished.
[0050] Therefore, analysis of the thermal stress at the welding
portions were conducted and it was found that austenite stainless
steel used for the tube stubs had such high creep strength that the
creep and the fatigue damage due to the thermal stress was too
small to cause any practical issue.
[0051] Furthermore, austenite stainless steel has excellent
corrosion resistance and thus it can be used in a boiler under
higher steam temperature condition than a conventional one. And
simply changing the material of the tube stubs with the structure
same as the conventional case from ferritic steel of the
conventional case to austenite stainless steel of the present
embodiment, the steam temperature condition can be improved from
600.degree. C. to 630.degree. C.
[0052] As explained above, by using austenite stainless steel for
the tube stubs, the bending stress caused by the bended section
where a cross-sectional shape is flatted and the damages at the
welding portions of the tube stubs can be minimized and further,
tolerance for temperature fluctuation is significantly improved. As
a result, the creep damage can be suppressed to the degree that
practically causes no issue.
[0053] Moreover, by using austenite stainless steel for the tube
stubs, the creep damage due to the thermal stress becomes so
minimal that it practically causes no issue.
Second Preferred Embodiment
[0054] In a second preferred embodiment, another example structure
of a tube header and a plurality of tube stubs to be used in a
final superheater and a reheater of USC boiler of 600.degree. C.
grade is explained.
[0055] FIG. 3 is a sectional view of the tube header with the tube
stubs welded thereon in relation to the second preferred
embodiment.
[0056] In FIG. 3, the tube header 2 made of 9Cr steel of ferritic
steel has through-holes 6 formed on the outer surface thereof and
the tube stubs are arranged such that the through holes 3 face the
passages 5 of the tube stubs 4 with bended sections respectively.
On the outer surface of the tube header 2, annular chases 10 are
respectively formed to surround joining portions 14 of the tube
header. Further, butt grooves are formed between the tube stubs 4
and the joining portions 14 by joining surfaces of the tube stubs
and the joining portions in a continuous manner and the tube stubs
4 having a diameter that is substantially same as those of the
joining portions 14.
[0057] In order to weld the tube header 2 and the tube stubs having
the above structures, the butt grooves formed between the tube
stubs and the joining portions 14 are welded by using the nickel
base alloy as a welding material and welding portions 8 are formed
there.
[0058] According to the second preferred embodiment, by forming the
annular chases 10, in addition to the effects similar to those
obtained in the first preferred embodiment, the thermal stress due
to the difference in material of the tube header 2 and the tube
stubs 4 can be reduced.
[0059] According to the thermal stress analysis, with the above
structure, the absolute figure of the thermal stress occurred
between the weld material and the tube stubs of austenite stainless
steel and between the weld material and the tube header of ferritic
steel is so small that there is no practical issue of the creep
damage.
[0060] As explained above, by using austenite stainless steel for
the tube stubs, the bending stress caused by the bended section
where a cross-sectional shape is flatted and the damages at the
welding portions of the tube stubs can be minimized and further,
tolerance for temperature fluctuation is significantly improved. As
a result, the creep damage can be suppressed to the degree that
practically causes no issue.
[0061] Moreover, by using austenite stainless steel for the tube
stubs, the creep damage due to the thermal stress becomes so
minimal that it practically causes no issue.
Third Preferred Embodiment
[0062] In a third preferred embodiment, different from those of the
first and second embodiments, another example structure of a tube
header tube stubs used in a final superheater and a reheater of USC
boiler of 600.degree. C. grade is explained.
[0063] FIG. 4 is a sectional view of the tube header with the tube
stubs welded thereon in relation to the third preferred
embodiment.
[0064] In FIG. 4, the tube header 2 made of 9Cr steel of ferritic
steel has through-holes 3 formed on the outer surface thereof and
the through holes 3 have a diameter that is approximately same as
the outer diameter of the tube stubs 4 each of which has a bended
section. Ends of the tube stubs 4 are respectively inserted in the
through-holes 3 approximately to the point where the ends of the
tube stubs 4 reach an inner surface of the tube header 2. Further,
to facilitate the insertion of the tube stubs in the tube header,
it is efficient to cool the tube stubs 4 or warm the tube header 2
beforehand. For instance, when only few of the tube stubs 4 need to
be inserted for repair, the tube stubs 4 are cooled beforehand and
in contrast, when a number of the tube stubs 4 need to be inserted
in the tube header on occasions such as installing a new facility,
the tube header is heated beforehand.
[0065] Then, the ends of the inserted tube stubs 4 are welded to
the inner surface of the tube header 2 by using nickel base alloy
as a welding material, and thus forming welding portions 12. It
requires seal weld from inside of the tube header 2. The tube
header used in a final superheater or repeater of a USC boiler
usually has an inner diameter of 300 mm and thus, if it is
difficult to manually perform the welding, a robot for automatic
welding or the like may be used.
[0066] According to the third preferred embodiment, the bending
stress of the tube stub due to the thermal stress is largest at
contact positions (section D of FIG. 4) where the tube stubs and
the outer surface of the tube header are in contact. However, there
is no welding at the contact positions and thus the creep damage
due to the bending stress is very little even when the bending
stress acts at the contact positions.
[0067] At the same time, the tube stubs of austenite stainless
steel are subjected to compression stress from the tube header of
ferritic steel. However, according to the stress analysis we
performed, it was found that the creep life of the tube stubs of
austenite stainless steel was higher than that of the tube header
made of ferritic steel and thus there was enough creep strength at
the contact positions.
[0068] As explained above, by using austenite stainless steel for
the tube stubs, the durability of the tube stubs against the
bending stress caused from the bended section there of or the
temperature fluctuation is significantly improved, and the creep
damage is suppressed to the degree that practically causes no
issue.
[0069] Moreover, by using austenite stainless steel for the tube
stubs, the creep damage due to the thermal stress is so minimal
that it practically causes no issue.
INDUSTRIAL APPLICABILITY
[0070] The present invention is applicable as a welding structure
of a tube header and tube stubs which successfully improves the
durability thereof against creep and fatigue damage of the tube
stubs without a different material between the tube header and the
tube stubs or additional components.
* * * * *