Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invetltion relates to a method of welding cast
-- steel materials containing a vanadium component, and more par-
ticularly to a method of welding such cast steel materials
while preventing the occurrence oE cracks in the region of the
weld.
The recent development oE high temperature, high
; pressure steam turbines has required the rapid development of
suitable high temperature and high pressure-withstanding cas~
steel products to be used therein, for example as turbine
casings, valve cases for steam valves, etc. In order to improve
the high temperature withstanding characteristics of such
materials, an attempt has been made to increase the creep strength
at elevated temperatures by using Cr-Mo-V cast steels which, as
can be seen, contain a vanadium component.
Although Cr-Mo-V cast steel products have the necessary
creep strength at elevated temperatures, they also have a con-
siderably higher susceptibility to cracking than other cast
steel products owing to the presence of the vanadium component.
Consequently, cracking is very liable to occur during welding.
That is to say, when welding Cr-Mo-V cast steel products, a
secondary hardening occurs after a stress relief annealing step,
owing to the influence of the welding heat, and cracking of the
welded parts is disadvantageously liable to take place because
of the influence of the thermal stress during the stress relief
annealing step.
Therefore, when welding Cr-Mo-V cast steel products, a
complicated welding technique has been required, such as the
application of peening as a means for relieving the residual
stress from the welded parts. For example, such welding has
30 been carried out by skilled welding engineers in well adjusted, -
good welding atmospheres, such as in a welding shop. However,
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when s~ch Cr-~o-V cast steel products are actually used in
steam turbine casings, the welding of the products must be
carried out outdoors at the erection site of a power plant,
and thus it is almost impossible to maintain a good welding
- atmosphere. Even if the welding is carried out when taking the
best possible precautions it is very difficult to prevent the
occurrence of cracks due to the inclusion of vanadium component.
One of the methods previously employed for overcoming
this difficulty comprises preparing forged parts free from the
vanadium component and having cross-sectional shapes identical
to those of the parts to be welded and consisting of high tem-
perature, high pressure materials having good high temperature- -~
withstanding characteristics. These forged parts are then
individually welded to the necessary parts made of the Cr-Mo-V
cast steel products within a shop where it is possible to weld
the parts under suitable conditions. The forged parts are then
welded to each other at the erection site, thereby preventing
the occurrence of cracks in the welded parts due to the presence
of the vanadium component.
However, in this welding method, the forged parts to ` ~ `
be shop welded to the Cr-Mo-V cast steel products must be pre-
pared separately, and furthermore it is necessary to make the
cross-sectional shapes of the forged parts identical to those
of the vanadium-containing parts. Thus, much additional labor
is required.
An object of the present invention is to thus provide
a method for welding cast steel materials containing a vanadium ;-~
component in a simple manner while preventing ~the occurrence of
cracks in the welded parts.
According to the present invention there is provided
a method of welding a cast steel material containing a vanadium
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component, comprising a first step of build-up welding a first
vanadium-Eree metallic material, having as high a creep strength
; at elevated temperatures as said cast steel material, to a part
of said cast steel material, and a second step of coalescing
the built-up weldecl part to a second vanadium-free metallic
material by welding therebetween a third vanadium-free metallic
material, having as high a creep strength at elevated temperatures
as said cast steel material.
An advantage of the present invention, at least in its
preferred forms, is that it can provide a method for welding
cast steel materials containing a vanadium component to each
other in a simple manner even under unfavourable welding con-
ditions such as at an erection site of a power plant, while
preventing the occurrence of cracks in the welded parts.
A preferred embodiment of the present invention will
be described in detail in the following with reference to the ~`
accompanying drawings, in which: `
Figure 1 is a vertical cross-sectional view of one
embodiment of the present invention showing a steam turbine
casing consisting of a Cr-Mo-V cast steel material;
Figure 2 is a view along line II-II of Figure l; and
Figure 3 is a partial cross-sectional view along line
III-III of Figure 2.
Figures 1 to 3 show a steam turbine casing 10 made
from a Cr-Mo-V cast steel material. The casing consists of a
semi-cylindrical upper half casing 11 and a lower half casing
12, and has an upper half 13 of a steam chest and a lower half
14 of a steam chest, each of which is in the form of a steam
chamber for guiding steam into the casing. Furthermore, the
upper half casing 11 has a coupling flange 15 for connection to
a crossover pipe for guiding steam to the low pressure side of
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tlle turblnc casin~, allcl tlle lower llalf casing 12 has a number
of connection terminals, i.e. a maln steam inlet terminal 18,
wh~ch is connected to a main steam inlet pipe 28 for guiding
high temperature, high pressure steam from a boiler into the
casing, a low temperature reheatlng pipe terminal 16, which is
whicll is connected to a -Low temperature relleatlng p;pe 26
for guiding steam from the inside of the casing to a reheater,
a high temperature reheating pipe terminal 17, which is
connected to a high temperature reheating pipe 27 for guiding
high temperature steam into the casing from a reheater, and a
steam extraction pipe terminal 19, which is connected to a steam
extraction pipe 29 for extracting steam from the casing. All
of these pipe terminals are forged integrally with the casing.
Each of these pipe terminals 16, 17, 18 and 19 require not only
high pressure and high temperature withstanding properties,
but also high creep strength at elevated temperatures, because
the pipe terminals come into contact with high temperature steam.
Consequently, Cr-Mo-V cast steel is used as the material of con-
struction for these pipe terminals.
When a combined reheating valve, comprising a reheating
stop valve and an interceptor, is used as a steam valve in a
portion of the high temperature reheating pipe 27 near the casing
- 10, that portion must also be in an integrated structure with
the valve casing, and thus must also be made from a Cr-Mo-V cast
steel material.
The low temperature reheating pipe 26 and the steam
extraction pipe 29, are made from Cr-Mo cast steel containlng no
vanadium component 5 because the temperature of the steam passing
through these pipes is not very higll. Ultimately these steam
pipes 26, 27, 28 and 29 are welded to the respective pipe terminals
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16, 17, 18 and 19 on the turbine casing 10. Since the casing
material is Cr-Mo-V cast steel, direct welding will bring
about cracking of the welded parts. Thus, as shown ln Figures
2 and 3, when the reheating pipe terminal 17 is to be welded
to the reheating pipe 27, built-up welding parts 31 and 32 are
respectively welded to welding-requiring part 17a of the high
temperature reheating pipe terminal 17 and welding-requiring
part 27a of the high temperature reheating pipe 27, both of
which are made of Cr-Mo-V material, with a welding rod of Cr-Mo
steel containing no vanadium component and having as high a
creep strength at elevated temperatures as that of the casing
10 made of Cr-Mo-V material. This step is carried out in a
welding shop in a good welding atmosphere. Then, the outer
surfaces of the built-up welded parts 31 and 32 of Cr-Mo steel
material are processed to provide welding surfaces 31a and 32a,
respectively. Furthermore9 a peening treatment is applied to
the built-up welded parts 31 and 32 to release the residual
stress. Then, the turbine casing 10, steam pipe 27, etc. are
transported to the erection site of a power plant, and a welding
20 part 33 is welded between the built-up welded part 31a of the
casing side and the built-up welded part 32a of the high tem- ~ -
perature reheating pipe 27 in the unfavourable welding conditions,
using a welding rod of Cr-Mo steel having a high creep strength -~
at elevated temperatures, thereby coalescing the built-up
welded parts 31 and 32 to each other. Then, a peening treatment
is applied to the welded part 33 to release the residual stress,
` whereby the welding operation is completed.
By carrying out the welding operation in the above
manner, the welding can be attained effectively with a good
30 operability without the occurrence of any cracks in the welded
parts, even although Cr-Mo-V cast steel materials are welded in
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un~avourable welding atmospheres.
The Eor~golng embodiment only illustrates the welding
together of two Cr-Mo-V cast steel products, but when the lower
half casing 12 of Cr-Mo-V cast steel material is to be welded to
a pipe made of Cr-Mo cast steel material, for example, such as
the low temperature reheating pipe 26 or the steam extraction
pipe 29, the build-up welding technique using a welding rod of
Cr-Mo steel material containing no vanadium component and having
a high creep strength at elevated temperatures need only be
applied to the welding-requiring part of the lower half casing
12 in the favourable welding atmosphere~ and then the built-up
welded part of the casing may be welded and coalesced to the
low temperature reheating pipe 26 or steam extraction pipe 29
with a welding rod of Cr-Mo steel material containing no vanadium
and having a high creep strength at elevated temperatures under
the unfavourable welding conditions at the erection site. That ~ ;
is, the parts containing no vanadium components are welded to
each other in the unfavourable welding atmosphere, whereby the
occurrence of cracks in the welded parts due to the inclusion of
2~ the vanadium component can be prevented.
These welding methods can thus facilitate the welding
operation and prevent the occurrence of welding cracks when
cast steel materials containing a vanadium component are welded
together, or to a cast steel material containing no vanadium.
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