BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
[0001] The present invention relates to a novel steam turbine of high efficiency and high
temperature, and more particularly to a steam turbine in which a main steam temperature
and/or a reheat steam temperature are/is 620 (°C) or above. It also relates to a steam-turbine
power plant which employs such steam turbines.
2. DESCRIPTION OF THE RELATED ART:
[0002] Conventional steam turbines have had a steam temperature of 566 (°C) at maximum and
a steam pressure of 246 (atg).
[0003] It is desired, however, to heighten the efficiencies of thermal power plants from
the viewpoints of the exhaustion of fossil fuel such as petroleum and coal, the saving
of energy, and the prevention of environmental pollution. For enhancing the power
generation efficiencies, it is the most effective expedient to raise the steam temperatures
of the steam turbines. Regarding materials for such high-efficiency turbines, 1Cr-1Mo-1/4V
ferritic low-alloy forged steel and 11Cr-1Mo-V-Nb-N forged steel are known as rotor
materials, while 1Cr-1Mo-1/4V ferritic low-alloy cast steel and 11Cr-1Mo-V-Nb-N cast
steel are known as casing materials. Among these materials, austenitic alloys disclosed
in the official gazette of Japanese Patent Applications Laid-open No. 180044/1987
and No. 23749/1986, and martensitic steel disclosed in the official gazette of Japanese
Patent Applications, Laid-open No. 147948/1992, No. 290950/1990 and No. 371551/1992
are especially known as materials whose high-temperature strengths are superior.
[0004] Although, in the laid-open applications mentioned above, the rotor materials, the
casing materials, etc. are disclosed, almost no consideration is given to the steam
turbines and the thermal power plants which are accompanied by the higher steam temperatures
as stated above.
[0005] Further, a supercritical steam turbine is known from the official gazette of Japanese
Patent Applications, Laid-open No. 248806/1987, but a plant system as a whole is not
considered at all.
SUMMARY OF THE INVENTION
[0006] An object of the present invention is to provide a steam turbine which permits a
heightened steam temperature of 610 - 660 (°C) by heat-resisting ferritic steel and
which exhibits a high thermal efficiency, and a steam-turbine power plant which employs
the steam turbine.
[0007] Another object of the present invention is to provide steam turbines whose running
temperatures are 610 - 660 (°C) and whose basic designs are substantially the same,
and a steam-turbine power plant which employs the steam turbines.
[0008] The present invention consists in improvement to a steam-turbine power plant having
a high-pressure turbine and an intermediate-pressure turbine which are joined to each
other, and low-pressure turbines which are connected in tandem. The improvement comprises
that steam inlet of each of the high-pressure and intermediate-pressure turbines which
leads to moving blades of a first-stage included in each of the high-pressure and
intermediate-pressure turbines being at a temperature of 610 - 660 (°C) (preferably
615 - 640 (°C), and more preferably 620 - 630 (°C)). Further, that steam inlet of
each of the low-pressure turbines which leads to moving blades of a first stage included
in each of the low-pressure turbines is at a temperature of 380 - 475 (°C) (preferably
400 - 430 (°C)), and a rotor shaft, at least first stage ones of each of the moving
blades and the fixed blades, and a casing, which are included in each of the high-pressure
and intermediate-pressure turbines and which are exposed to the temperature of the
steam inlet of each of the high-pressure and intermediate-pressure turbines, are made
of high-strength martensitic steel which contains 8 - 13 (weight-%) of Cr.
[0009] Further, the present invention consists in a steam turbine comprising a rotor shaft,
moving blades which are assembled on the rotor shaft, fixed blades which guide inflow
of steam to the moving blades, and an inner casing which holds the fixed blades. The
steam flows into a first stage of the moving blades at a temperature of 610 - 660
(°C) and under a pressure of at least 250 (kg/cm
2) (preferably 246 - 316 (kg/cm
2)) or 170 - 200 (kg/cm
2). The rotor shaft and, at least, first-stage ones of the moving blades and the fixed
blades are made of high-strength martensitic steel of fully-tempered martensitic structure
which exhibits a 10
5-hour creep rupture strength of at least 15 (kg/mm
2) (preferably 17 (kg/mm
2)) at a temperature corresponding to the respective steam temperatures (preferably
610 (°C), 625 (°C), 640 (°C), 650 (°C) and 660 (°C)), and which contains 9.5 - 13
(weight-%) (preferably 10.5 - 11.5 (weight-%)) of Cr, and the inner casing is made
of martensitic cast steel which exhibits a 10
5-hour creep rupture strength of at least 10 (kg/mm
2) (preferably 10.5 (kg/mm
2)) at the temperature corresponding to the respective steam temperatures, and which
contains 8 - 9.5 (weight-%) of Cr. Here at least first stage ones of the moving blades
may be made of Ni-based alloy which exhibits tensile strength of at least 90 (kg/mm
2) at a room temperature.
[0010] Further, the present invention consists in an improvement to a steam turbine having
a rotor shaft, moving blades which are assembled on the rotor shaft, fixed blades
which guide inflow of steam to the moving blades, and an inner casing which holds
the fixed blades. In the improvement, the rotor shaft and, at least, first-stage ones
of the moving blades and the fixed blades are made of high-strength martensitic steel
which contains 0.05 - 0.20 (%) of C, at most 0.15 (%) of Si, 0.03 - 1.5 (%) of Mn,
9.5 - 13 (%) of Cr, 0.05 - 1.0 (%) of Ni, 0.05 - 0.35 (%) of V, 0.01 - 0.20 (%) of
Nb, 0.01 - 0.06 (%) of N, 0.05 - 0.5 (%) of Mo, 1.0 - 4.0 (%) of W, 2 - 10 (%) of
Co and 0.0005 - 0.03 (%) of B, and which has at least 78 (%) of Fe, the percentages
being given in terms of weight, and the inner casing is made of high-strength martensitic
steel which contains 0.06 - 0.16 (%) of C, at most 0.5 (%) of Si, at most 1 (%) of
Mn, 0.2 - 1.0 (%) of Ni, 8 - 12 (%) of Cr, 0.05 - 0.35 (%) of V, 0.01 - 0.15 (%) of
Nb, 0.01 - 0.8 (%) of N, at most 1 (%) of Mo, 1 - 4 (%) of W and 0.0005 - 0.03 (%)
of B, and which has at least 85 (%) of Fe, the percentages being given in terms of
weight. The moving blades, at least first stage one thereof, may be made of Ni-based
alloy which contains 0.03 - 0.20(%) of C, at most 0.3 (%) of Si, at most 0.2 (%) of
Mn, 12 - 20 (%) of Cr, 9 - 20 (%) of Mo, 0.5 - 1.5 (%) of Al, 2 - 3 (%) of Ti, at
most 5 (%) of Fe, 0.003 - 0.015 (%) of B.
[0011] Further, the present invention consists in the improvement to a high-pressure steam
turbine having a rotor shaft, moving blades which are assembled on the rotor shaft,
fixed blades which guide inflow of steam to the moving blades, and an inner casing
which holds the fixed blades, wherein the moving blades are arranged including at
least 10 stages on each side in a lengthwise direction of the rotor shaft, except
a first stage which is of double flow, and the rotor shaft has a distance (L) of at
least 5000 (mm) (preferably 5200 - 5500 (mm)) between centers of bearings in which
it is journaled, and a minimum diameter (D) of at least 600 (mm) (preferably 620 -
700 (mm)) at its parts which correspond to the fixed blades, a ratio (L/D) between
the distance (L) and the diameter (D) being 8.0 - 9.0 (preferably 8.3 - 8.7), and
it is made of high-strength martensitic steel which contains 9 - 13 (weight-%) of
Cr.
[0012] Further, the present invention consists in the improvement in an intermediate-pressure
steam turbine having a rotor shaft, moving blades which are assembled on the rotor
shaft, fixed blades which guide inflow of steam to the moving blades, and an inner
casing which holds the fixed blades, wherein the moving blades have a double-flow
construction in which at least 6 stages are included on each side in a lengthwise
direction of the rotor shaft, in a bilaterally symmetric arrangement on both sides,
and in which the first stages of the arrangement are assembled on a central part of
the rotor shaft in the lengthwise direction, and the rotor shaft has a distance (L)
of at least 5200 (mm) (preferably 5300 - 5800 (mm)) between centers of bearings in
which it is journaled, and a minimum diameter (D) of at least 620 (mm) (preferably
620 - 680 (mm)) at its parts which correspond to the fixed blades, a ratio (L/D) between
the distance (L) and the diameter (D) being 8.2 - 9.2 (preferably 8.5 - 9.0), and
it is made of high-strength martensitic steel which contains 9 - 13 (weight-%) of
Cr.
[0013] Further, the present invention consists in the improvement to a low-pressure steam
turbine having a rotor shaft, moving blades which are assembled on the rotor shaft,
fixed blades which guide inflow of steam to the moving blades, and an inner casing
which holds the fixed blades, wherein the moving blades have a double-flow construction
in which at least 8 stages are included on each side in a lengthwise direction of
the rotor shaft, in a bilaterally symmetric arrangement on both sides, and in which
the first stages of the arrangement are assembled on a central part of the rotor shaft
in the lengthwise direction, the rotor shaft has a distance (L) of at least 7200 (mm)
(preferably 7400 - 7600 (mm)) between centers of bearings in which it is journaled,
and a minimum diameter (D) of at least 1150 (mm) (preferably 1200 - 1350 (mm)) at
its parts which correspond to the fixed blades, a ratio (L/D) between the distance
(L) and the diameter (D) being 5.4 - 6.3 (preferably 5.7 - 6.1), and it is made of
Ni-Cr-Mo-V low-alloy steel which contains 3.25 - 4.25 (weight-%) of Ni, and each of
the final-stage moving blades of the arrangement has a length of at least 40 (inches)
and is made of a Ti-based alloy.
[0014] Further, the present invention consists in the improvement to a steam-turbine power
plant having a high-pressure turbine and an intermediate-pressure turbine which are
joined to each other, and two low-pressure turbines which are connected in tandem,
wherein that steam inlet of each of the high-pressure and intermediate-pressure turbines
which leads to moving blades of a first stage included in each of the high-pressure
and intermediate-pressure turbines is at a temperature of 610 - 660 (°C), that steam
inlet of the low-pressure turbine which leads to moving blades of a first stage included
in the low-pressure turbine is at a temperature of 380 - 475 (°C), the first-stage
moving blade of the high-pressure turbine, and that part of a rotor shaft of the high-pressure
turbine on which the first-stage moving blade is assembled are held at metal temperatures
which are not, at least, 40 (°C) lower than the temperature of the steam inlet of
the high-pressure turbine leading to the first-stage moving blade (preferably, the
metal temperatures are 20 - 35 (°C) lower than the steam temperature), the first-stage
moving blade of the intermediate-pressure turbine, and that part of a rotor shaft
of the intermediate-pressure turbine on which the first-stage moving blades are assembled
are held at metal temperatures which are not, at least, 75 (°C) lower than the temperature
of the steam inlet of the intermediate-pressure turbine leading to the first-stage
moving blade (preferably, the metal temperatures are 50 - 70 (°C) lower than the steam
temperature), and the rotor shaft of each of the high-pressure and intermediate-pressure
turbines and, at least, the first-stage one of the moving blades of each of the high-pressure
and intermediate-pressure turbines are made of martensitic steel which contains 9.5
- 13 (weight-%) of Cr.
[0015] Further, the present invention consists in the improvement to a coal-fired power
plant having a coal-fired boiler, steam turbines which are driven by steam developed
by the boiler, and one or more, preferably two, generators which are driven by the
steam turbines and which can generate an output of at least 1000 (MW), wherein the
steam turbines include a high-pressure turbine, an intermediate-pressure turbine which
is joined to the high-pressure turbine, and two low-pressure turbines, that steam
inlet of each of the high-pressure and intermediate-pressure turbines which leads
to moving blades of a first stage included in each of the high-pressure and intermediate-pressure
turbines is at a temperature of 610 - 660 (°C), that steam inlet of the low-pressure
turbine which leads to moving blades of a first stage included in the low-pressure
turbine is at a temperature of 380 - 475 (°C), the steam heated by a superheater of
the boiler to a temperature which is at least 3 (°C) (preferably 3 - 10 (°C), more
preferably 3 - 7 (°C)) higher than the temperature of the steam inlet of the high-pressure
turbine leading to the first-stage moving blade thereof is caused to flow into the
first-stage moving blade of the high-pressure turbine, the steam having come out of
the high-pressure turbine is heated by a reheater of the boiler to a temperature which
is at least 2 (°C) (preferably 2 - 10 (°C), more preferably 2 - 5 (°C)) higher than
the temperature of the steam inlet of the intermediate-pressure turbine leading to
the first-stage moving blade thereof, whereupon the heated steam is caused to flow
into the first-stage moving blade of the intermediate-pressure turbine, and the steam
having come out of the intermediate-pressure turbine is heated by an economizer of
the boiler to a temperature which is at least 3 (°C) (preferably 3 - 10 (°C), more
preferably 3 - 6 (°C)) higher than the temperature of the steam inlet of the low-pressure
turbine leading to the first-stage moving blade thereof, whereupon the heated steam
is caused to flow into the first-stage moving blade of the low-pressure turbine.
[0016] Further, the present invention consists in the improvement to the low-pressure steam
turbine stated before; wherein that steam inlet of the low-pressure turbine which
leads to a first-stage one of the moving blades is at a temperature of 380 - 475 (°C)
(preferably 400 - 450 (°C)), and the rotor shaft is made of low-alloy steel which
contains 0.2 - 0.3 (%) of C, at most 0.05 (%) of Si, at most 0.1 (%) of Mn, 3.25 -
4.25 (%) of Ni, 1.25 - 2.25 (%) of Cr, 0.07 - 0.20 (%) of Mo, 0.07 - 0.2 (%) of V
and at least 92.5 (%) of Fe, the percentages being given in terms of weight.
[0017] The present invention consists in the improvement to the high-pressure steam turbine
stated before; wherein the moving blades are arranged including at least 7 stages
(preferably 9 - 12 stages), and they have lengths of 35 - 210 (mm) in a region from
an upstream side of the steam flow to a downstream side thereof, diameters of those
parts of the rotor shaft on which the moving blades are assembled are larger than
diameters of those parts of the rotor shaft which correspond to the fixed blades;
and widths of the moving-blade assembling parts of the rotor shaft in an axial direction
of the rotor shaft being stepwise larger on the downstream side than on the upstream
side at, at least, 3 stages (preferably 4 - 7 stages), and the ratios of these widths
to the lengths of the moving blades decrease from the upstream side toward the downstream
side within a range of 0.6 - 1.0 (preferably 0.65 - 0.95).
[0018] Further, in the high-pressure steam turbine stated before, the present invention
consists in the improvement wherein the moving blades are arranged including at least
7 stages, and they have lengths of 35 - 210 (mm) in a region from an upstream side
of the steam flow to a downstream side thereof, ratios between the lengths of the
moving blades of the respectively adjacent stages are at most 1.2 (preferably 1.10
- 1.15), and they increase gradually toward the downstream side, and the lengths of
the moving blades are larger on the downstream side than on the upstream side.
[0019] Further, in the high-pressure steam turbine stated before, the present invention
consists in the improvement wherein the moving blades are arranged including at least
7 stages, and they have lengths of 35 - 210 (mm) in a region from an upstream side
of the steam flow to a downstream side thereof, and widths of those parts of the rotor
shaft which correspond to the fixed blades, the widths being taken in an axial direction
of the rotor shaft, are stepwise smaller on the downstream side than on the upstream
side at, at least, 2 stages (preferably 2 - 4 stages), and the ratios of these widths
to the lengths of the downstream-side moving blades decrease stepwise toward the downstream
side within a range of 0.65 - 1.8 (preferably 0.7 - 1.7).
[0020] The present invention consists in the improvement in the intermediate-pressure steam
turbine stated before, wherein the moving blades have a double-flow construction in
which at least 6 stages (preferably 6 - 9 stages) are included on each side in a lengthwise
direction of the rotor shaft, in a bilaterally symmetric arrangement on both sides,
and they have lengths of 100 - 300 (mm) in a region from an upstream side of the steam
flow to a downstream side thereof, diameters of those parts of the rotor shaft on
which the moving blades being assembled are larger than diameters of those parts of
the rotor shaft which correspond to the fixed blades, and widths of the moving-blade
assembling parts of the rotor shaft in an axial direction of the rotor shaft being
stepwise larger on the downstream side than on the upstream side at, at least, 2 stages
(preferably 3 - 6 stages), and the ratios of these widths to the lengths of the moving
blades decrease from the upstream side toward the downstream side within a range of
0.45 0.75 (preferably 0.5 - 0.7).
[0021] Further, in the intermediate-pressure steam turbine stated before, the present invention
consists in the improvement wherein the moving blades have a double-flow construction
in which at least 6 stages are included on each side in a lengthwise direction of
the rotor shaft, in a bilaterally symmetric arrangement on both sides, and they have
lengths of 100 - 300 (mm) in a region from an upstream side of the steam flow to a
downstream side thereof, and the lengths of the respectively adjacent moving blades
are larger on the downstream side than on the upstream side, and their ratios are
at most 1.3 (preferably 1.1 - 1.2) and increase gradually toward the downstream side.
[0022] Further, in the intermediate-pressure steam turbine stated before, the present invention
consists in the improvement wherein the moving blades have a double-flow construction
in which at least 6 stages are included on each side in a lengthwise direction of
the rotor shaft, in a bilaterally symmetric arrangement on both the sides, and they
have lengths of 100 - 300 (mm) in a region from an upstream side of the steam flow
to a downstream side thereof, and widths of those parts of the rotor shaft which correspond
to the fixed blades, the widths being taken in an axial direction of the rotor shaft,
are stepwise smaller on the downstream side than on the upstream side at, at least,
2 stages (preferably 3 - 6 stages), and the ratios of these widths to the lengths
of the downstream-side moving blades decrease stepwise toward the downstream side
within a range of 0.45 - 1.60 (preferably 0.5 - 1.5).
[0023] The present invention consists in the improvement in the low-pressure steam turbine
stated before; wherein the moving blades have a double-flow construction in which
at least 8 stages (preferably 8 - 10 stages) are included on each side in a lengthwise
direction of the rotor shaft, in a bilaterally symmetric arrangement on both sides,
and they have lengths of 90 -1300 (mm) in a region from an upstream side of the steam
flow to a downstream side thereof; diameters of those parts of the rotor shaft on
which the moving blades are assembled are larger than diameters of those parts of
the rotor shaft which correspond to the fixed blades; and widths of the moving-blade
assembling parts of the rotor shaft in an axial direction of the rotor shaft are stepwise
larger on the downstream side than on the upstream side at, at least, 3 stages (preferably
4 - 7 stages), and the ratios of these widths to the lengths of the moving blades
decrease from the upstream side toward the downstream side within a range of 0.15
- 1.0 (preferably 0.15 - 0.91).
[0024] Further, in the low-pressure steam turbine stated before, the present invention consists
in the improvement wherein the moving blades have a double-flow construction in which
at least 8 stages are included on each side in a lengthwise direction of the rotor
shaft, in a bilaterally symmetric arrangement on both sides, and they have lengths
of 90 - 1300 (mm) in a region from an upstream side of the steam flow to a downstream
side thereof; and the lengths of the moving blades of the respectively adjacent stages
are larger on the downstream side than on the upstream side, and their ratios increase
gradually toward the downstream side within a range of 1.2 - 1.7 (preferably 1.3 -
1.6).
[0025] Further, in the low-pressure steam turbine stated before, the present invention consists
in the improvement wherein the moving blades have a double-flow construction in which
at least 8 stages are included on each side in a lengthwise direction of the rotor
shaft, in a bilaterally symmetric arrangement on both sides, and they have lengths
of 90 - 1300 (mm) in a region from an upstream side of the steam flow to a downstream
side thereof; and widths of those parts of the rotor shaft which correspond to the
fixed blades, the widths being taken in an axial direction of the rotor shaft, are
stepwise larger on the downstream side than on the upstream side at, at least, 3 stages
(preferably 4 - 7 stages), and the ratios of these widths to the lengths of the respectively
adjacent moving blades on the downstream side decrease stepwise toward the downstream
side within a range of 0.2 - 1.4 (preferably 0.25 - 1.25).
[0026] The present invention consists in the improvement in a high-pressure steam turbine
having a rotor shaft, moving blades which are assembled on the rotor shaft, fixed
blades which guide inflow of steam to the moving blades, and an inner casing which
holds the fixed blades; wherein the moving blades are arranged including at least
7 stages; diameters of those parts of the rotor shaft which correspond to the fixed
blades are smaller than diameters of those parts of the rotor shaft which correspond
to the assembled moving blades; widths of the rotor shaft parts corresponding to the
fixed blades, in an axial direction of the rotor shaft are stepwise larger on an upstream
side of the steam flow than on a downstream side thereof at, at least, 2 of the stages
(preferably 2 - 4 stages), and the width between the final stage of the moving blades
and the stage thereof directly preceding the final stage is 0.75 - 0.95 (preferably
0.8 - 0.9, more preferably 0.84 - 0.88) times as large as the width between the second
stage and the third stage of the moving blades; and widths of the rotor shaft parts
corresponding to the assembled moving blades, in the axial direction of the rotor
shaft are stepwise larger on the downstream side of the steam flow than on the upstream
side thereof at, at least, 3 of the stages (preferably 4 - 7 stages), and the axial
width of the final stage of the moving blades is 1 - 2 (preferably 1.4 - 1.7) times
as large as the axial width of the second stage of the moving blades.
[0027] The present invention consists in the improvement in an intermediate-pressure steam
turbine having a rotor shaft, moving blades which are assembled on the rotor shaft,
fixed blades which guide inflow of steam to the moving blades, and an inner casing
which holds the fixed blades; wherein the moving blades are arranged including at
least 6 stages; diameters of those parts of the rotor shaft which correspond to the
fixed blades are smaller than diameters of those parts of the rotor shaft which correspond
to the assembled moving blades; widths of the rotor shaft parts corresponding to the
fixed blades, in an axial direction of the rotor shaft are stepwise larger on an upstream
side of the steam flow than on a downstream side thereof at, at least, 2 of the stages
(preferably 3 - 6 stages), and the width between the final stage of the moving blades
and the stage thereof directly preceding the final stage is 0.55 - 0.8 (preferably
0.6 - 0.7) times as large as the width between the first stage and the second stage
of the moving blades; and widths of the rotor shaft parts corresponding to the assembled
moving blades, in the axial direction of the rotor shaft are stepwise larger on the
downstream side of the steam flow than on the upstream side thereof at, at least,
2 of the stages (preferably 3 - 6 stages), and the axial width of the final stage
of the moving blades is 0.8 - 2 (preferably 1 - 1.5) times as large as the axial width
of the first stage of the moving blades.
[0028] The present invention consists in the improvement in a low-pressure steam turbine
having a rotor shaft, moving blades which are assembled on the rotor shaft, fixed
blades which guide inflow of steam to the moving blades, and an inner casing which
holds the fixed blades; wherein the moving blades have a double-flow construction
in which at least 8 stages are included on each side in an axial direction of the
rotor shaft, in a bilaterally symmetric arrangement on both sides; diameters of those
parts of the rotor shaft which correspond to the fixed blades are smaller than diameters
of those parts of the rotor shaft which correspond to the assembled moving blades;
widths of the rotor shaft parts corresponding to the fixed blades, in the axial direction
of the rotor shaft are stepwise larger on an upstream side of the steam flow than
on a downstream side thereof at, at least, 3 of the stages (preferably 4 - 7 stages),
and the width between the final stage of the moving blades and the stage thereof directly
preceding the final stage is 1.5 - 2.5 (preferably 1.7 - 2.2) times as large as the
width between the first stage and the second stage of the moving blades; and widths
of the rotor shaft parts corresponding to the assembled moving blades, in the axial
direction of the rotor shaft, are stepwise larger on the downstream side of the steam
flow than on the upstream side thereof at, at least, 3 of the stages (preferably 4
- 7 stages), and the axial width of the final stage of the moving blades is 2 - 3
(preferably 2.2 - 2.7) times as large as the axial width of the first stage of the
moving blades.
[0029] The designs of the high-pressure, intermediate-pressure and low-pressure turbines
described above can be rendered similar for any of the service steam temperatures,
610 - 660 (°C) of the respective turbines.
[0030] In the rotor material of the present invention, alloy contents should preferably
be controlled so as to become 4 - 8 in terms of a Cr equivalent which is computed
by a formula given below, in order that a superior high-temperature strength, a low-temperature
toughness and a high fatigue strength may be attained from the fully-tempered martensitic
structure.
[0031] Besides, in the heat-resisting cast steel of the present invention, which is used
as the casing material, alloy contents should preferably be controlled so as to become
4 - 10 in terms of the Cr equivalent which is computed by the formula given below,
in order that a superior high-temperature strength, a low-temperature toughness and
a high fatigue strength may be attained by controlling the alloying constituents so
as to establish a martensitic structure tempered to at least 95 (%), in other words,
containing at most 5 (%) of δ (delta) ferrite.
- Cr equivalent =
- Cr + 6Si + 4Mo + 1.5W + 11V + 5Nb
- 40C - 30N - 30B - 2Mn - 4Ni - 2Co
[0032] Regarding the 12Cr heat-resisting steel of the present invention, especially in a
case where the steel is used with steam at or above 621 (°C), it should preferably
be endowed with a 625-°C 10
5-h creep rupture strength of at least 10 (kgf/mm
2) and a room-temperature absorbed impact energy of at least 1 (kgf-m).
[0033] Now, the materials specified in the present invention will be itemized as (1) - (3)
below.
(1) There will be elucidated the reasons for restricting the constituents of the heat-resisting
ferritic steel which is used in the present invention for making the rotors, blades,
nozzles and inner-casing tightening bolts of the high-pressure and intermediate-pressure
steam turbines, and the first-stage diaphragm of the intermediate-pressure portion:
[0034] The constituent C (carbon) is an element which is indispensable to ensuring hardenability
upon quenching, and precipitating carbides in a tempering heat-treatment process so
as to enhance a high-temperature strength. Besides, the element C is required at a
level of at least 0.05 (%) in order to attain a high tensile strength. However, in
a case where the C content exceeds 0.20 (%), the ferritic steel comes to have an unstable
metallographic structure and spoils the long-time creep rupture strength thereof when
exposed to high temperatures for a prolonged period of time. Therefore, the C content
is restricted to within 0.05 - 0.20 (%). It should desirably be within 0.08 - 0.14
(%), and particularly preferably be within 0.09 - 0.14 (%).
[0035] The constituent Mn (manganese) is added as a deoxidizer etc., and the deoxidizing
effect thereof is achieved by a small amount of addition. A large amount of addition
exceeding 1.5 (%) is unfavorable because it lowers the creep rupture strength. Especially,
a range of 0.03 - 0.20 (%) or a range of 0.3 - 0.7 (%) is preferable, and a range
of 0.35 - 0.65 (%) is more preferable for the latter. As the Mn content is made lower,
a higher strength is attained. On the other hand, as the Mn content is made higher,
the workability of the ferritic steel improves.
[0036] The constituent Si (silicon) is also added as a deoxidizer, but the Si deoxidation
is dispensed if a steelmaking technique such as the vacuum C deoxidation or the like
is made. A lower Si content is effective to prevent the production of the deleterious
δ ferrite structure, and to prevent the degradation of the toughness of the ferritic
steel attributed to grain-boundary segregation, etc. Accordingly, the addition of
the constituent Si needs to be suppressed to 0.15 (%) or below. The Si content of
the ferritic steel should desirably be at most 0.07 (%), and should particularly preferably
be at most 0.05 (%).
[0037] The constituent Ni (nickel) is an element which is very effective to heighten the
toughness and to prevent the production of the δ ferrite. The addition of the element
Ni at a level of less than 0.05 (%) is unfavorable because it has an insufficient
effect, and the addition thereof at more than 1.0 (%) is also unfavorable because
of degradation in the creep rupture strength. Especially, a range of 0.3 - 0.7 (%)
is preferable, and a range of 0.4 - 0.65 (%) is more preferable.
[0038] The constituent Cr (chromium) is an element which is indispensable to enhancing the
high-temperature strength and high-temperature oxidation resistance of the ferritic
steel. The element Cr is required at least 9 (%). However, when the Cr content exceeds
13 (%), the deleterious δ ferrite structure is produced, which lowers the high-temperature
strength and the toughness. Therefore, the Cr content is restricted to within 9 -
12 (%). Especially, a range of 10 - 12 (%) is preferable, and a range of 10.8 - 11.8
(%) is more preferable.
[0039] The addition of the constituent Mo (molybdenum) is intended to enhance the high-temperature
strength. However, in a case where the constituent W (tungsten) is contained at a
level of more than 1 (%), as in the steel of the present invention, Mo addition at
a level of exceeding 0.5 (%) lowers the toughness and fatigue strength of the ferritic
steel. Therefore, the Mo content is limited to, at most, 0.5 (%). Especially, a range
of 0.05 - 0.45 (%) is preferable, and a range of 0.1 - 0.3 (%) is more preferable.
[0040] The constituent W (tungsten) suppresses the coarsening of carbides due to the agglomerations
thereof at high temperatures, and it turns the matrix of the ferritic steel into a
solid solution and strengthens this matrix. It is therefore effective to remarkably
enhance the long-term strength of the ferritic steel at the high temperatures of at
least 620 (°C). The W content of the ferritic steel should preferably be 1 - 1.5 (%)
at 620 (°C), 1.6 - 2.0 (%) at 630 (°C), 2.1 - 2.5 (%) at 640 (°C), 2.5 - 3.0 (%) at
650 (°C) and 3.1 - 3.5 (%) at 660 (°C). Besides, when the W content exceeds 3.5 (%),
the δ ferrite is produced, which lowers the toughness. Therefore, the W content is
restricted to within 1 - 3.5 (%). Especially, a range of 2.4 - 3.0 (%) is preferable,
and a range of 2.5 - 2.8 (%) is more preferable.
[0041] The constituent V (vanadium) is effective to heighten the creep rupture strength
by precipitating the carbonitrides of this constituent V. When the V content of the
ferritic steel is less than 0.05 (%), the effect is insufficient. On the other hand,
when the V content exceeds 0.3 (%), the δ ferrite is produced, which lowers the fatigue
strength. Especially, a range of 0.10 - 0.25 (%) is preferable, and a range of 0.15
- 0.25 (%) is more preferable.
[0042] The constituent Nb (niobium) is an element which is very effective to precipitate
NbC (niobium carbide) and enhance the high-temperature strength. However, when the
element Nb is added in an excessively large amount, the coarse grains of eutectic
NbC appear, especially in a large-sized steel ingot, which causes significant lowering
of the strength and precipitation of the δ ferrite, which lowers the fatigue strength.
It is therefore necessary to suppress the amount of the element Nb to 0.20 (%) or
below. On the other hand, when the Nb amount is less than 0.01 (%), the effect is
insufficient. Especially, a range of 0.02 - 0.15 (%) is preferable, and a range of
0.04 - 0.10 (%) is more preferable.
[0043] The constituent Co (cobalt) is an important element, and is a feature which distinguishes
the present invention from the prior-art techniques. In the present invention, owing
to the addition of the element Co, the high-temperature strength is remarkably improved,
and the toughness is also heightened. These effects are considered to be based on
the interaction between the elements Co and W, and they are the characterizing phenomena
of the alloy of the present invention containing the element W in the amount of at
least 1 (%). In order to realize such effects of the element Co, the lower limit of
the Co amount in the alloy of the present invention is set at 2.0 (%). On the other
hand, even when the element Co is added in excess, greater effects are not attained,
and moreover, the ductility of the ferritic steel is lowered. Therefore, the upper
limit of the Co amount is set at 10 (%). The Co amount should desirably be selected
from 2 - 3 (%) for 620 (°C), 3.5 - 4.5 (%) for 630 (°C), 5 - 6 (%) for 640 (°C), 6.5
- 7.5 (%) for 650 (°C), and 8 - 9 (%) for 660 (°C). But an efficient strength can
be obtained by addition of at least 2 (%) of Co for any degree of temperature at most
650 (°C).
[0044] The constituent N (nitrogen) is also an important element and is a feature which
distinguishes the present invention from the prior-art techniques. The element N is
effective to improve the creep rupture strength and to prevent the production of the
δ ferrite structure. However, when the N content of the ferritic steel is less than
0.01 (%), the effects are not sufficient. On the other hand, when the N content exceeds
0.05 (%), the toughness is lowered, and the creep rupture strength is also lowered.
Especially, a range of 0.01 - 0.03 (%) is preferable, and a range of 0.01 - 0.025
(%) is more preferable.
[0045] The constituent B (boron) is effective to enhance the high-temperature strength by
the action of intensifying grain boundaries, and the action of turning into solid
solutions in carbides M
23C
6 to hinder the M
23C
6 type carbides from coarsening due to the agglomerations thereof. It is effective
to add the constituent B to a level in excess of 0.001 (%). However, when the B content
exceeds 0.03 (%), the weldability and forgeability of the ferritic steel are degraded.
Therefore, the B content is limited to within 0.001 - 0.03 (%). It should desirably
be 0.001 - 0.01 (%) or 0.01 - 0.02 (%).
[0046] The addition of the constituent/constituents Ta (tantalum), Ti (titanium) or/and
Zr (zirconium) is effective to heighten the toughness. A sufficient effect is attained
by adding at most 0.15 (%) of Ta, at most 0.1 (%) of Ti or/and at most 0.1 (%) of
Zr singly or in combination. In a case where the constituent Ta is added at a level
of 0.1 (%) or above, the addition of the constituent Nb (niobium) can be omitted.
[0047] The rotor shaft and, at least, the first-stage ones of the moving blades and fixed
blades in the present invention should preferably be made for a steam temperature
of 620 - 630 (°C) out of steel of fully-tempered martensitic structure which contains
0.09 - 0.20 (%) of C, at most 0.15 (%) of Si, 0.05 - 1.0 (%) of Mn, 9.5 - 12.5 (%)
of Cr, 0.1 - 1.0 (%) of Ni, 0.05 - 0.30 (%) of V, 0.01 - 0.06 (%) of N, 0.05 - 0.5
(%) of Mo, 2 - 3.5 (%) of W, 2 - 4.5 (%) of Co, 0.001 - 0.030 (%) of B, and at least
77 (%) of Fe (iron). Besides, they should preferably be made for a steam temperature
of 635 - 660 (°C) out of steel of fully-tempered martensitic structure in which the
aforementioned Co content is replaced with 5 - 8 (%), and which contains at least
78 (%) of Fe. Especially, a high strength is attained by decreasing the Mn content
to 0.03 - 0.2 (%) and the B content to 0.001 - 0.01 (%) for both the aforementioned
temperatures. The martensitic steel should particularly preferably contain 0.09 -
0.20 (%) of C, 0.1 - 0.7 (%) of Mn, 0.1 - 1.0 (%) of Ni, 0.10 - 0.30 (%) of V, 0.02
- 0.05 (%) of N, 0.05 - 0.5 (%) of Mo, and 2 - 3.5 (%) of W, along with 2 - 4 (%)
of Co and 0.001 - 0.01 (%) of B for a temperature of or below 630 (°C) or 5.5 - 9.0
(%) of Co and 0.01 - 0.03 (%) of B for a temperature of 630 - 660 (°C). The martensitic
steel including the former percentage of Co can be used at the temperature between
620 - 650 (%).
[0048] The Cr equivalent which is obtained by the formula mentioned before is set at 4 -
10.5 for the rotor shafts of the high-pressure and intermediate-pressure steam turbines,
and a range of 6.5 - 9.5 is particularly preferable therefor. The same applies to
the other components of these steam turbines stated before.
[0049] Regarding the rotor material of the high-pressure and intermediate-pressure steam
turbines of the present invention, the fatigue strength and the toughness lower due
to the coexistence of the δ ferrite structure. Therefore, the tempered martensitic
structure which is homogeneous, is favorable for the heat-resisting ferritic steel.
In order to obtain the tempered martensitic structure, the Cr equivalent which is
computed by the formula mentioned before must be set at, at most, 10 by controlling
the alloy contents. On the other hand, when the Cr equivalent is too small, it lowers
the creep rupture strength, and hence, it must be set at, at least, 4. Especially,
a range of 5 - 8 is preferable as the Cr equivalent.
[0050] At least one of the rotor shaft, each moving blade and each fixed blade in the present
invention should preferably be made of steel which satisfies at least one of conditions;
a (B + N) content of 0.050 (%) or below, an (N/B) ratio of 1.5 or above (preferably,
1.5 - 2.0), a (B/Co) ratio of 0.0035 or above (preferably, 0.0035 - 0.008, and more
preferably, 0.004 - 0.006), a (Co/Mo) ratio of 18 or below (preferably, 8 - 18, and
more preferably, 11 - 16), and a (Co/Nb) ratio of 30 or above (preferably, 30 - 70).
Steel which satisfies all the conditions, is more preferable. These elements correlate
organically.
[0051] Further, there will be elucidated the reasons for restricting the constituents of
each of the Ni-based precipitation-strengthened alloys which can be applied to, at
least, the first stages of the moving blades of the high-pressure and intermediate-pressure
turbines in the present invention.
[0052] When added at least 0.03 (%), the element C (carbon) precipitates carbides during
use in a solid-solution state or at high temperatures, to thereby enhance the yield
strength and creep strength of the alloy at high temperatures. However, in a case
where the C content exceeds 0.2 (%), the carbides are drastically precipitated during
the use at the high temperatures, to thereby lower the high-temperature pulling contraction
percentage of the alloy. Therefore, the C content should preferably be 0.03 - 0.15
(%).
[0053] The element Cr (chromium) needs to be contained at least 12 (%) in order to enhance
the yield strength and creep strength of the alloy at high temperatures and to further
enhance the high-temperature oxidation resistance and sulfurization-corrosion resistance
of the alloy, in the solid-solution state of this element Cr in the alloy. However,
when the Cr content exceeds 20 (%), the σ phase of steel is precipitated to lower
the contraction percentage of the alloy in the high-temperature tensile test thereof.
Therefore, the preferable range of the Cr content is 12 - 20 (%).
[0054] When added in excess of 9 (%), the element Mo (molybdenum) remarkably enhances the
yield strength of the alloy at high temperatures and also the creep rupture strength
thereof, in the solid-solution state of this element Mo in the alloy. However, when
contained in excess of 20 (%), the element Mo abruptly degrades the yield strength
at the high temperatures contrariwise. Further, it spoils the cold working property
of the alloy, and it precipitates the σ phase to lower the contraction percentage
of the alloy in the high-temperature tension thereof. Therefore, the preferable range
of the Mo content is 12 - 20 (%).
[0055] When added at most 12 (%), the element Co (cobalt) remarkably enhances the creep
rupture strengths of the alloy at the room temperature and at high temperatures, in
the solid-solution state of this element Co in the alloy. However, when contained
in excess of 12 (%), the element Co abruptly degrades the high-temperature ductility
of the alloy, and it precipitates the σ phase to lower the contraction percentage
of the alloy in the high-temperature tension thereof. Therefore, the preferable range
of the Co content is 5 - 12 (%).
[0056] When added 0.5 - 1.5 (%), the element Al (aluminum) turns into a solid solution in
the alloy and precipitates the γ prime phase of steel during use at high temperatures
for a long time period, to thereby enhance the yield strength and creep rupture strength
of the alloy in the high-temperature tension thereof. However, in a case where the
Al content exceeds 1.5 (%), the contraction percentage of the alloy degrades in the
high-temperature tension thereof. Therefore, the preferable range of the Al content
is 0.5 - 1.2 (%).
[0057] When added 2 - 3 (%), the element Ti (titanium) turns into a solid solution in the
alloy and precipitates the γ prime phase during use at high temperatures for a long
time period, to thereby enhance the yield strength and creep rupture strength of the
alloy in the high-temperature tension thereof. However, in a case where the Ti content
exceeds 3 (%), the contraction percentage of the alloy degrades in the high-temperature
tension thereof.
[0058] Since the element Fe (iron) lowers the creep rupture strength of the alloy, the containment
thereof ought to be avoided as far as possible. Even in a case where the element Fe
is contained as an impurity, it ought to be limited to, at most, 5 (%).
[0059] The elements Si (silicon) and Mn (manganese) are respectively added at most 0.3 (%)
and at most 0.2 (%) as deoxidizers or in order to enhance the hot working property
of the alloy. It is the most preferable, however, to add neither of these elements
Si and Mn.
[0060] The element B (boron) segregates at the austenitic grain boundary of steel in a very
small amount, and enhances the creep rupture strength and high-temperature ductility
of the alloy. This element B is effective when contained at least 0.003 (%). However,
it degrades both the hot plastic working property and high-temperature ductility of
the alloy when contained in excess of 0.015 (%). Therefore, the B content ought to
be set at 0.003 - 0.015 (%).
[0061] The element Mg (magnesium) and the rare-earth elements segregate at the austenitic
grain boundaries of the alloy, and enhance the creep rupture strength thereof. Besides,
the element Zr (zirconium) is an intense element for forming a carbide. When added
in a very small amount, this element Zr enhances the creep rupture strength of the
alloy owing to its action synergic with the formation of other carbides by the elements
Ti etc. However, when these elements are added in excess, the ductility of the alloy
at high temperatures degrades for such reasons that the binding powers of grain boundaries
lower and that coarse carbide grains are formed. Therefore, it is preferable to add
the element Mg at most 0.1 (%), the rare-earth elements at most 0.5 (%) and the element
Zr at most 0.5 (%), and it is particularly preferable to add the element Mg 0.005
- 0.05 (%), the rare-earth elements 0.005 - 0.1 (%) and the element Zr 0.01 - 0.2
(%).
Structure and Annealing:
[0062] The alloy according to the present invention is subjected to a treatment for turning
an ingot into a solid solution, and thereafter to a treatment for ageing.
[0063] The solid-solution treatment is carried out by holding the ingot at 1,050 - 1,200
(°C) for 30 (minutes) - 10 (hours), and subsequently cooling the heated ingot with
water, air or the like. The water cooling is performed by throwing the alloy at a
predetermined temperature into the body of water.
Alternatively, when the alloy is flat, water is sprayed onto the surfaces of
the alloy at a predetermined temperature.
[0064] The ageing treatment is carried out in such a way that the material subjected to
the above solid-solution treatment is heated and held at 700 - 870 (°C) for 4 - 24
(hours).
Melting:
[0065] The alloy according to the present invention should preferably be molten in a non-oxidizing
atmosphere. Raw materials to be used for the alloy according to the present invention
are pure metals. From the standpoints of heightening the available percentages of
the alloy elements and preventing the dispersion of the alloy composition, therefore,
the raw materials should preferably be heated in vacuum until they are about to melt
and drop, and they are thereafter molten by introducing a non-oxidizing gas.
[0066] Further, the raw materials thus molten are subjected to vacuum arc remelting or electroslag
remelting. Then, the desired alloy can be obtained.
[0067] Each of the Ni-based precipitation-strengthened alloys in the present invention should
exhibit a preferable tensile strength of 90 (kg/mm
2) or above, or a more preferable one of 100 (kg/mm
2) or above, at the room temperature, and a preferable tensile strength of 80 (kg/mm
2) or above, at 732 (°C). It should also exhibit a preferable elongation percentage
of 10 (%) or
above.
[0068] Regarding each of the rotors in the present invention, alloying raw materials to
be brought into the desired composition are melted in an electric furnace, the molten
materials are deoxidized by carbon vacuum deoxidation, the deoxidized materials are
cast into a metal mold, and the molded article is forged into an electrode. The electrode
thus fabricated is subjected to electroslag remelting, and the resulting slag is forged
and formed into the shape of the rotor. The forging must be carried out at a temperature
of 1150 (°C) or below in order to prevent forging cracks. After the forged steel has
been annealed, it is heated to 1000 - 1100 (°C) and then quenched, and it is tempered
twice in the sequence of a temperature range of 550 - 650 (°C) and a temperature range
of 670 - 770 (°C). Thus, the steam turbine rotor which is usable in steam at or above
620 (°C) can be manufactured.
[0069] Regarding each of the components in the present invention, which includes the blades,
nozzles and inner-casing tightening bolts of the high-pressure and intermediate-pressure
steam turbines, and the first-stage diaphragm of the intermediate-pressure portion,
an ingot is prepared in such a way that the alloying raw materials to be brought into
the desired composition are melted by vacuum melting, and that the molten materials
are cast in a metal mold in vacuum. The ingot is hot-forged into a predetermined shape
at the same temperature as stated before. After the forged ingot has been heated to
1050 - 1150 (°C), it is subjected to water cooling or oil quenching. Subsequently,
the resulting ingot is tempered in a temperature range of 700 - 800 (°C), and it is
machined into the component of desired shape. The vacuum melting is carried out under
a vacuum condition of 10
-1 - 10
-4 (mmHg). In particular, although the heat-resisting steel in the present invention
can be applied to all the stages of the blades and nozzles of the high-pressure portion
and intermediate-pressure portion, they are especially necessary for the first stages
of both the sorts of component.
[0070] The steam-turbine rotor shaft made of the 12 weight-% Cr type martensitic steel in
the present invention should preferably be so constructed that buildup welding layers
of good bearing characteristics are formed on the surface of the parent metal forming
each journal portion of the rotor shaft. More specifically, the buildup welding layers
are formed in the number of, at least 3, preferably 5 - 10 by the use of a weld metal
being steel. The Cr content of the steel as the weld metal is lowered successively
from the first layer to any of the second - fourth layers, whereas the layers of and
behind the fourth layer are formed of the steel having an identical Cr content. Herein,
the Cr content of the weld metal for the deposition of the first layer is rendered
about 2 - 6 (weight-%) less than that of the parent metal, and the Cr contents of
the welding layers of and behind the fourth layer are set at 0.5 - 3 (weight-%), preferably
at 1 - 2.5 (weight-%).
[0071] In the present invention, the buildup welding is favorable for the improvement of
the bearing characteristics of the journal portion in view of the highest safety,
but it becomes very difficult due to increase in the B content of the steel. Therefore,
in the case where the B content is set at 0.02 (%) or above in order to attain a higher
strength, it is recommended to adopt a construction in which the journal portion is
inserted into a sleeve made of low-alloy steel having a Cr content of 1 - 3 (%), through
shrinkage fit. The material composition of the sleeve is the same as that of the buildup
welding layers to be explained later.
[0072] The buildup welding layers according to the method of the present invention are preferably
in the number of 5 - 10. Abrupt decrease in the amount of Cr in the first welding
layer causes the development of high residual tensile stress or welding cracks, so
that the Cr content of the weld metal of the first welding layer cannot be sharply
lowered. As stated before, therefore, the Cr contents need to be gradually lowered
with the enlarged number of welding layers. Further, since the desired Cr content
and a desired thickness need to be held as the surface layer of the journal portion,
the welding layers need to be in the number of 5 or more. By the way, even when the
number of welding layers is larger than 10, no greater effect is achieved. Regarding
a large-sized structural member such as the steam-turbine rotor shaft, the buildup
welding layers must not have their composition influenced by the parent metal and
need to be endowed with the desired composition as well as the desired thickness.
Herein, three layers are required as a thickness for preventing the influence of the
parent metal. Besides, layers of desired characteristics need to be stacked on the
three layers to a desired thickness, and at least two layers are required as the desired
thickness. By way of example, a thickness of about 18 (mm) is required as the desired
thickness of the finally finished buildup welding layers. In order to form such a
thickness, at least five buildup welding layers are necessitated even when a final
finish margin to be machined is excluded. The third layer et seq. should preferably
be mainly made of the tempered martensitic structure from which the carbides have
been precipitated. Especially, the composition of the fourth welding layer et seq.
should preferably contain in terms of weight, 0.01 - 0.1 (%) of C, 0.3 - 1 (%) of
Si, 0.3 - 1.5 (%) of Mn, 0.5 - 3 (%) of Cr and 0.1 - 1.5 (%) of Mo, the balance being
Fe.
[0073] Moreover, in the buildup welding layers, the Cr content is lowered successively from
the first layer to any of the second - fourth layers. In performing the buildup welding,
welding rods whose Cr contents are gradually lowered are used for the respective layers.
Then, the buildup welding layers of the desired composition can be formed without
incurring the problem of lowered ductility or welding cracks of the first-layer welding
zone attributed to the sharp decrease of the chromium content in the first-layer welding
zone. In this way, the present invention can form the buildup welding layers in which
the chromium contents in the vicinities of the parent metal and the first-layer zone
do not exhibit a very large difference, and in which the final layer has the good
bearing characteristics as stated above.
[0074] The weld metal which is applied to the first-layer welding has its chromium content
rendered about 2 - 6 (weight-%) lower than the chromium content of the parent metal.
When the Cr content of the weld metal is less than 2 (%) that of the parent metal,
the pertinent Cr content of the buildup welding layer cannot be lowered sufficiently,
and the effect is slight. To the contrary, when the value exceeds 6 (%), the Cr content
of the buildup welding layer lowers suddenly from that of the parent metal, and the
difference between the Cr contents gives rise to a large difference between the coefficients
of thermal expansion of both the metals, to thereby cause development of high residual
tensile stress or welding cracks. Incidentally, since a higher Cr content results
in a smaller coefficient of thermal expansion, the buildup welding layer of lower
Cr content has larger coefficient of thermal expansion than the parent metal and is
formed with the high residual tensile stress by the welding. Therefore, the welding
with steel of still lower Cr content produces a hard layer due to the high residual
stress and causes development of welding cracks. Accordingly, the Cr content of the
weld metal needs to be set at, at most, 6 (%) smaller than that of the parent metal.
Owing to the use of such a weld metal, the chromium content of the first-layer welding
layer becomes lower than that of the parent metal by as little as about 1 - 3 (%)
because the weld metal mixes with the parent metal. Thus, favorable welding is attained.
[0075] In the method of the present invention, the layers of and behind the fourth layer
need to be formed using weld metal which is made of steel having an identical Cr content.
In the buildup welding, the buildup welding layers up to the third layer are influenced
by the composition of the parent metal. Since, however, the fourth buildup welding
layer et seq. are composed only of the employed weld metal without this influence,
they can be formed to satisfy the characteristics required for the journal portion
of the steam-turbine rotor shaft. Besides, as stated before, the thickness of the
buildup welding layers required for the large-sized structural member operating as
the steam-turbine rotor shaft is about 18 (mm). Accordingly, in order to ensure the
alloying constituents required for the final layer and the sufficient thickness required
in the case of the constituents, two or more layers are deposited as the fourth layer
et seq. by the use of the weld metal having the same Cr content. Thus, the final layer
which satisfies the characteristics required for the journal portion as stated before
can be formed having the sufficient thickness.
(2) There will be elucidated the reasons for restricting the constituents of the heat-resisting
ferritic steel which is used in the present invention for making the inner casings,
control-valve valve casings, combinational-reheater-valve valve casings, main-steam
leading pipes, main-steam inlet pipes and reheat-steam inlet pipes of the high-pressure
and intermediate-pressure steam turbines, the nozzle box of the high-pressure turbine,
the first-stage diaphragm of the intermediate-pressure turbine, and the main-steam
inlet flange and elbow and the main-steam stop valve of the high-pressure turbine:
[0076] In the casing material of the heat-resisting ferritic cast steel, especially, the
Ni/W ratio is controlled to 0.25 - 0.75, thereby obtaining the casing material of
the heat-resisting cast steel which meets a 625-°C 10
5-h creep rupture strength of at least 9 (kgf/mm
2) and a room-temperature absorbed impact energy of at least 1 (kgf-m) that are required
of the high-pressure and intermediate-pressure inner casings, main-steam stop valve
and control valve casing of the turbine under the ultra-supercritical pressure of
at least 250 (kgf/cm
2) at 621 (°C).
[0077] In the heat-resisting cast steel of the present invention used as the casing material,
the Cr equivalent which is computed in terms of the alloy contents (weight-%) of the
following formula should preferably be controlled so as to become 4 - 10, in order
to attain a superior high-temperature strength, a superior low-temperature toughness
and a high fatigue strength:
- Cr equivalent =
- Cr + 6Si + 4Mo + 1.5W + 11V + 5Nb
- 40C - 30N - 30B - 2Mn - 4Ni - 2Co
[0078] Since the 12Cr heat-resisting steel of the present invention is used in the steam
at or above 621 (°C), it must be endowed with the 625-°C 10
5-h creep rupture strength of at least 9 (kgf/mm
2) and the room-temperature absorbed impact energy of at least 1 (kgf-m). Further,
in order to ensure a still higher reliability, this steel should preferably be endowed
with a 625-°C 10
5-h creep rupture strength of at least 10 (kgf/mm
2) and a room-temperature absorbed impact energy of at least 2 (kgf-m).
[0079] The constituent C (carbon) is an element which is required at a level of least 0.06
(%) in order to attain a high tensile strength. However, in a case where the C content
exceeds 0.16 (%), the steel comes to have an unstable metallographic structure and
degraded the long-time creep rupture strength thereof when exposed to high temperatures
for a long time period. Therefore, the C content is restricted to within 0.06 - 0.16
(%). It should preferably be within 0.09 - 0.14 (%).
[0080] The constituent N (nitrogen) is effective to improve the creep rupture strength and
to prevent the production of the δ ferrite structure. However, when the N content
of the steel is less than 0.01 (%), the effects are not sufficient. On the other hand,
even when the N content exceeds 0.1 (%), no remarkable effects are attained. Moreover,
the toughness is lowered, and the creep rupture strength is also lowered. Especially,
a range of 0.02 - 0.4 (%) is preferable.
[0081] The constituent Mn (manganese) is added for a deoxidizer, and the effect thereof
is achieved by a small amount of addition. A large amount of addition exceeding 1
(%) is unfavorable because it lowers the creep rupture strength. Especially, a range
of 0.4 - 0.7 (%) is preferable.
[0082] The constituent Si (silicon) is also added as a deoxidizer, but the Si deoxidation
is dispensed with when the steelmaking technique employed is vacuum C deoxidation
or the like. A lower Si content is effective to prevent the production of the deleterious
δ ferrite structure. Accordingly, the addition of the constituent Si needs to be suppressed
to 0.5 (%) or below. The Si content of the steel should preferably be 0.1 - 0.4 (%).
[0083] The constituent V (vanadium) is effective to heighten the creep rupture strength.
When the V content of the steel is less than 0.05 (%), the effect is insufficient.
On the other hand, when the V content exceeds 0.35 (%), the δ ferrite is produced
which lowers the fatigue strength. Especially, a range of 0.15 - 0.25 (%) is preferable.
[0084] The constituent Nb (niobium) is an element which is very effective to enhance the
high-temperature strength. However, when the element Nb is added in an excessively
large amount, the coarse grains of eutectic NbC (niobium carbide) appear especially
in a large-sized steel ingot, to thereby cause substantial lowering of the strength
and precipitation of the δ ferrite which lowers the fatigue strength. It is therefore
necessary to suppress the amount of the element Nb to 0.15 (%) or below. On the other
hand, when the Nb amount is less than 0.01 (%), the effect is insufficient. Especially
in the case of the large-sized steel ingot, a range of 0.02 - 0.1 (%) is preferable,
and a range of 0.04 - 0.08 (%) is more preferable.
[0085] The constituent Ni (nickel) is an element which is very effective to heighten the
toughness and to prevent the production of the δ ferrite. The addition of the element
Ni at a level of less than 0.2 (%) is unfavorable because of insufficient effects,
and the addition thereof at the level of more than 1.0 (%) is also unfavorable because
of degradation in the creep rupture strength. Especially, a range of 0.4 - 0.8 (%)
is preferable.
[0086] The constituent Cr (chromium) is effective to improve the high-temperature strength
and high-temperature oxidation resistance of the 12Cr steel. Herein, a Cr content
exceeding 12 (%) causes production of the deleterious δ ferrite structure, and a Cr
content below 8 (%) results in an insufficient oxidation resistance to the high-temperature
high-pressure steam. Besides, the addition of the element Cr is effective to enhance
the creep rupture strength. However, the Cr addition in an excessive amount causes
production of the deleterious δ ferrite structure and for lowering of the toughness.
Especially, a range of 8.0 - 10 (%) is preferable, and a range of 8.5 - 9.5 (%) is
more preferable.
[0087] The constituent W (tungsten) is effective to remarkably enhance the high-temperature
long-term strength of the 12Cr steel. When the amount of the element W is smaller
than 1 (%), the effect is insufficient as the heat-resisting steel which is used at
620 - 660 (°C). On the other hand, when the amount of the element W exceeds 4 (%),
the toughness is lowered. The W content of the steel should preferably be selected
according to the temperature, such as 1.0 - 1.5 (%) at 620 (°C), 1.6 - 2.0 (%) at
630 (°C), 2.1 - 2.5 (%) at 640 (°C), 2.6 - 3.0 (%) at 650 (°C) and 3.1 - 3.5 (%) at
660 (°C). Particularly, the steel contains 1.5 - 1.9 (%) of W can be used at the temperature
at most 650 (°C).
[0088] The constituents W and Ni correlate with each other. The 12Cr steel whose strength
and toughness are both superior, can be obtained by setting the Ni/W ratio at 0.25
- 0.75.
[0089] The addition of the constituent Mo (molybdenum) is intended to enhance the high-temperature
strength. However, in a case where the constituent W (tungsten) is contained at a
level of more than 1 (%) as in the cast steel of the present invention, the Mo addition
exceeding 1.5 (%) lowers the toughness and fatigue strength of the steel. Therefore,
the Mo content is recommended to be at most 1.5 (%). Especially, a range of 0.4 -
0.8 (%) is preferable, and a range of 0.55 - 0.70 (%) is more preferable.
[0090] The addition of the constituent/constituents Ta (tantalum), Ti (titanium) or/and
Zr (zirconium) is effective to heighten the toughness. A sufficient effect is attained
by adding at most 0.15 (%) of Ta, at most 0.1 (%) of Ti or/and at most 0.1 (%) of
Zr singly or in combination. In a case where the constituent Ta is added at a level
of 0.1 (%) or above, the addition of the constituent Nb (niobium) can be omitted.
[0091] Regarding the heat-resisting cast steel of the present invention which is used as
the casing material, the fatigue strength and the toughness are lowered due to the
coexistence of the δ ferrite structure. Therefore, the tempered martensitic structure
which is homogeneous is favorable. In order to obtain the tempered martensitic structure,
the Cr equivalent which is computed by the formula mentioned before must be set at,
at most, 10 by controlling the alloy contents. On the other hand, when the Cr equivalent
is too small, it lowers the creep rupture strength, and hence, it must be set at 4
or above. Especially, a range of 6 - 9 is preferable as the Cr equivalent.
[0092] The addition of the constituent B (boron) remarkably enhances the high-temperature
(620 (°C) or above) creep rupture strength of the steel. Herein, when the B content
of the steel exceeds 0.003 (%), the weldability thereof worsens. Therefore, the upper
limit of the B content is set at 0.003 (%). Especially, the upper limit of the B content
of the large-sized casing should preferably be set at 0.0028 (%). Further, a range
of 0.0005 - 0.0025 (%) is preferable, and a range of 0.001 - 0.002 (%) is particularly
preferable.
[0093] Since the casing covers the high-pressure steam at temperatures of at least 620 (°C),
it undergoes a high stress ascribable to the internal pressure thereof. From the viewpoint
of preventing the creep rupture of the casing, therefore, the 10
5-h creep rupture strength of at least 10 (kgf/mm
2) is required of the steel. Moreover, during the starting operation of the turbine,
the casing undergoes a thermal stress at the time of a low metal temperature. From
the viewpoint of preventing the brittle fracture of the casing, therefore, the room-temperature
absorbed impact energy of at least 1 (kgf-m) is required of the steel. For the higher
temperature side of the steam, the steel can be strengthened by containing at most
10 (%) of Co (cobalt). Especially, the Co content should more preferably be selected
from 1 - 2 (%) for 620 (°C), 2.5 - 3.5 (%) for 630 (°C), 4 - 5 (%) for 640 (°C), 5.5
- 6.5 (%) for 650 (°C), and 7 - 8 (%) for 660 (°C). But even the steel which does
not contain Co can be used at each temperature.
[0094] The casing in the present invention should preferably be made of steel which satisfies
at least either of conditions; a (W/Mo) ratio of 2.85 or above (preferably, 2.85 -
4.50, and more preferably, 3 - 4), and an (Mo/Cr) ratio of 0.04 - 0.08 (preferably,
0.05 - 0.06). Steel which satisfies both the conditions, is more preferable.
[0095] In fabricating the casing having few defects, a high degree of manufacturing technology
is required because the casing is a large-sized structural member whose ingot has
a weight of about 50 (tons). As the casing material of the heat-resisting ferritic
cast steel in the present invention, a satisfactory one can be prepared in such a
way that alloying raw materials to be brought into the desired composition are melted
by an electric furnace and then refined by a ladle, and that the resulting raw materials
are thereafter cast into a sand mold. The cast steel in which casting defects such
as shrinkage holes are involved in small numbers, can be obtained by sufficiently
refining and deoxidizing the raw materials before the casting.
[0096] After the cast steel has been annealed at 1000 - 1150 (°C), it is normalized by heating
it to 1000 - 1100 (°C) and then quenching it. Subsequently, the resulting steel is
tempered twice in the sequence of a temperature range of 550 - 750 (°C) and a temperature
range of 670 - 770 (°C). Thus, the steam turbine casing which is usable in the steam
at or above 620 (°C) can be manufactured. When the annealing and normalizing temperatures
are below 1000 (°C), carbonitrides cannot be sufficiently turned into a solid solution,
and when they are excessively high, grain coarsening is caused. Besides, the two tempering
operations decompose retained austenite entirely, so that the steel can be rendered
the tempered martensitic structure which is homogeneous. Owing to the above method
of preparation, the 625-°C 10
5-h creep rupture strength of at least 10 (kgf/mm
2) and the room-temperature absorbed impact energy of at least 1 (kgf-m) are attained,
and the prepared steel can be fabricated into the steam turbine casing which is usable
in steam at or above 620 (°C).
[0097] The casing material in the present invention is set at the Cr equivalent stated before,
and the δ ferrite content thereof should preferably be 5 (%) or less, and more preferably
0 (%).
[0098] Except for the inner casing for the intermediate-pressure steam turbine, which is
made of cast steel, the components mentioned before should preferably be made of forged
steel.
(3) Others:
[0099]
(a) The rotor shaft of the low-pressure steam turbine should preferably be made of
low-alloy steel having a fully-tempered bainitic structure which contains in terms
of weight, 0.2 - 0.3 (%) of C, at most 0.1 (%) of Si, at most 0.2 (%) of Mn, 3.2 -
4.0 (%) of Ni, 1.25 - 2.25 (%) of Cr, 0.1 - 0.6 (%) of Mo and 0.05 - 0.25 (%) of V.
The low-alloy steel should preferably be manufactured by the same manufacturing method
as the ferritic steel of the high-pressure and intermediate-pressure rotor shafts,
as explained before. Especially, the manufacture should preferably be a superclean
(highly pure) one employing raw materials which contain at most 0.05 (%) of Si and
at most 0.1 (%) of Mn, and in which impurities such as P, S, As, Sb and Sn are decreased
to the utmost so as to amount to 0.025 (%) or less in total. It is favorable that
each of the P and S contents of the raw materials is at most 0.010 (%), that each
of the Sn and As contents is at most 0.005 (%), and that the Sb content is at most
0.001 (%).
(b) Blades for the low-pressure turbine except a final-stage moving one, and nozzles
therefor should preferably be made of fully-tempered martensitic steel which contains
0.05 - 0.2 (%) of C, 0.1 - 0.5 (%) of Si, 0.2 - 1.0 (%) of Mn, 10 - 13 (%) of Cr and
0.04 - 0.2 (%) of Mo.
(c) Both inner and outer casings for the low-pressure turbine should preferably be
made of cast carbon steel which contains 0.2 - 0.3 (%) of C, 0.3 - 0.7 (%) of Si and
at most 1 (%) of Mn.
(d) The casings of a main-steam stop valve and a steam control valve for the low-pressure
turbine should preferably be made of fully-tempered martensitic steel which contains
0.1 - 0.2 (%) of C, 0.1 - 0.4 (%) of Si, 0.2 - 1.0 (%) of Mn, 8.5 - 10.5 (%) of Cr,
0.3 - 1.0 (%) of Mo, 1.0 - 3.0 (%) of W, 0.1 - 0.3 (%) of V, 0.03 - 0.1 (%) of Nb,
0.03 - 0.08 (%) of N and 0.0005 - 0.003 (%) of B.
(e) A Ti alloy is employed for the final-stage moving blade of the low-pressure turbine.
More specifically, the Ti alloy contains 5 - 8 (weight-%) of Al and 3 - 6 (weight-%)
of V for the length of the final-stage moving blade exceeding 40 (inches), and these
contents can be increased with the length. Especially, a high-strength material containing
5.5 - 6.5 (%) of Al and 3.5 - 4.5 (%) of V is preferable for the length of 43 (inches),
and a high-strength material containing 4 - 7 (%) of Al, 4 - 7 (%) of V and 1 - 3
(%) of Sn for the length of 46 (inches).
(f) Outer casings for the high-pressure and intermediate-pressure steam turbines should
preferably be fabricated of cast steel of fully-tempered bainitic structure which
contains 0.05 - 0.20 (%) of C, 0.05 - 0.5 (%) of Si, 0.1 - 1.0 (%) of Mn, 0.1 - 0.5
(%) of Ni, 1 - 2.5 (%) of Cr, 0.5 - 1.5 (%) of Mo and 0.1 - 0.3 (%) of V, and which
favorably contains at least either of 0.001 - 0.01 (%) of B and at most 0.2 (%) of
Ti.
BRIEF DESCRIPTION OF THE DRAWINGS
[0100] Fig. 1 is a sectional design view of a high-pressure steam turbine made of ferritic
steel according to the present invention.
[0101] Fig. 2 is a sectional design view of an intermediate-pressure steam turbine made
of ferritic steel according to the present invention.
[0102] Fig. 3 is a sectional design view of a low-pressure steam turbine according to the
present invention.
[0103] Fig. 4 is an arrangement diagram of a coal-fired power plant according to the present
invention.
[0104] Fig. 5 is a sectional view of a rotor shaft for the high-pressure steam turbine according
to the present invention.
[0105] Fig. 6 is a sectional view of a rotor shaft for the intermediate-pressure steam turbine
according to the present invention.
[0106] Fig. 7 is a graph showing the creep rupture strengths of rotor shaft and blade materials.
[0107] Fig. 8 is a graph showing the relationships between the creep rupture time periods
and Co contents of alloys.
[0108] Fig. 9 is a graph showing the relationships between the creep rupture time periods
and B contents of the alloys.
[0109] Fig. 10 is a graph showing the relationship between the creep rupture strengths and
W contents of the alloys.
[0110] Fig. 11 is a graph showing the creep rupture strengths of casing materials.
[0111] Fig. 12 is a table (Table 1) exemplifying the specifications of a boiler which is
operated under specified steam conditions.
[0112] Fig. 13 is a table (Table 2) indicating the specifications of a steam turbine which
is operated under specified conditions.
[0113] Fig. 14 is a table (Table 3) for explaining the alloy contents of steel materials
which are employed in the present invention.
[0114] Fig. 15 is a table (Table 4) for explaining the mechanical properties and heat treatment
conditions of the steel materials which are listed in Table 3.
[0115] Fig. 16 is a table (Table 5) indicating the chemical constituents of welding rods
which were used in buildup welding.
[0116] Fig. 17 is a table (Table 6) indicating those welding rods in Table 5 which were
used in the respective layers of the buildup welding.
[0117] Fig. 18 is a table (Table 7) indicating the chemical constituents of moving blades
materials according to the present invention.
[0118] Fig. 19 is a table (Table 8) for explaining the strengths of the rotor shaft and
blade materials (in Fig. 7) according to the present invention.
[0119] Fig. 20 is a table (Table 9) indicating the ratios of percentages of the constituents.
[0120] Fig. 21 is a table (Table 10) indicating the chemical constituents of inner casing
materials according to the present invention.
[0121] Fig. 22 is a table (Table 11) indicating the test results of the inner casing materials
(in Fig. 21) according to the present invention.
PREFERRED EMBODIMENTS OF THE INVENTION
(Embodiment 1)
[0122] Due to sudden rises in the prices of fuel after the Oil crisis, a pulverized-coal
direct-fired boiler and a steam turbine at steam temperatures of 600 - 649 (°C) have
been required in order to increase thermal efficiencies on the basis of enhanced steam
conditions. One example of the boiler which is operated under such steam conditions,
is indicated in Table 1 of Fig. 12.
[0123] Since steam oxidation is attendant upon such a higher-temperature operation, 8 -
10-% Cr steel is employed instead of conventional 2.25-% Cr steel. Besides, since
the maximum sulfur content and the maximum chlorine content become 1 (%) and 0.1 (%),
respectively, regarding high-temperature corrosion ascribable to pulverized-coal direct-firing
gas, an austenitic stainless steel pipe functioning as a superheater tube is made
of a material which contains 20 - 25 (%) of Cr and 20 - 35 (%) of Ni, which contains
Al and Ti in very small amounts of at most 0.5 (%), and 0.5 - 3 (%) of Mo, and which
more preferably contains at most 0.5 (%) of Nb. Pulverized-coal direct firing becomes
high-temperature burning. Accordingly, it is desirable, from the view point of decreasing
nitrogen oxides NO
x, to employ a burner which makes flames of higher temperatures by feeding inner peripheral
air and secondary outer peripheral air that form burning flames based on the primary
air and pulverized coal, and also reducing flames around the burning flames.
[0124] The pulverized-coal fired boiler becomes larger in size as its capacity enlarges.
The boiler has a width of 31 (m) and a depth of 16 (m) in the class of 1050 (MW),
and a width of 34 (m) and a depth of 18 (m) in the class of 1400 (MW).
[0125] Table 2 in Fig. 13 indicates the main specifications of a steam turbine plant which
has an output of 1050 (MW) and a steam temperature of 625 (°C). In this embodiment,
a cross-compound type 4-flow exhaust system is adopted, and a final-stage blade in
each of the low-pressure turbines (LP's) is 43 (inches) long. An HP (high-pressure
turbine) - IP (intermediate-pressure turbine) connection has a rotational speed of
3600 (r/min), while the two LP's have a rotational speed of 1800 (r/min). In a high-temperature
portion, components are made of principal materials which are listed in the table.
The high-pressure portion (HP) undergoes the steam temperature of 625 (°C) and a pressure
of 250 (kg/cm
2). The intermediate-pressure portion (IP) has its steam heated to 625 (°C) by a reheater
(R/H), and is operated under a pressure of 170 - 180 (kg/cm
2). Steam enters the low-pressure portions (LP's) at a temperature of 450 (°C), and
it is sent to a condenser at a temperature of at most 100 (°C) and in a vacuum of
722 (mmHg).
[0126] Fig. 1 is a sectional design view of the high-pressure steam turbine. This high-pressure
steam turbine is provided with a high-pressure rotor shaft 23 on which high-pressure
moving blades 16 are assembled in a high-pressure inner casing 18 and a high-pressure
outer casing 19 surrounding the inner casing 18. The steam at the high temperature
and under the high pressure as stated before is generated by the boiler explained
before. The generated steam is passed through a main steam pipe and then through a
main steam inlet 28 defined by a flange and elbow 25, whereupon it is guided to the
moving blades 16 of the double-flow first stage from a nozzle box 38. The first stage
has the double-flow construction, and eight other stages are disposed on each of the
two sides of the high-pressure steam turbine along the rotor shaft 23. Fixed blades
are respectively provided in correspondence with the moving blades 16. The moving
blades 16 are double-tenon type tangential entry dovetail blades, and the first-stage
blade is about 35 (mm) long. The length of the rotor shaft 23 between the centers
of bearings 1 and 2 is about 5.25 (m), and the smallest diameter part of this rotor
shaft corresponding to the fixed blades has a diameter of about 620 (mm), so that
the ratio of the length to the diameter is about 8.5.
[0127] The widths of those parts of the rotor shaft 23 on which the moving blades 16 are
assembled are substantially equal at the first stage and final stage, and they become
smaller toward the downstream side of the steam stepwise at the five types stages,
namely the first stage, second stage, third - fifth stages, sixth stage and seventh
- eighth stages. The axial width of the assembled part of the second stage is 0.64
times as wide as that of the assembled part of the final stage.
[0128] Those parts of the rotor shaft 23 which correspond to the fixed blades are smaller
in diameter than those parts thereof on which the moving blades 16 are assembled.
The axial widths of the parts corresponding to the fixed blades become smaller stepwise
from the width between the second-stage moving blade and the third-stage moving blade,
to the width between the final-stage moving blade and the penultimate-stage moving
blade, the latter width being 0.86 times as wide as the former width. Concretely,
the axial widths of the parts corresponding to the fixed blades become smaller at
the second - sixth stages and the sixth - ninth stages.
[0129] In this embodiment, the blades and nozzles of the first stage are made of materials
indicated in Table 3 of Fig. 14 to be explained later, whereas those of all the other
stages are made of 12-% Cr steel which contains no W, Co or B. The blade parts of
the moving blades 16 in this embodiment are 35 - 50 (mm) long at the first stage,
and become longer at the respective stages from the second stage toward the final
stage. Especially, the lengths of the blade parts of the second - final stages are
set at 65 - 210 (mm), depending upon the output of the steam turbine. The number of
the stages is 9 - 12. Herein, the lengths of the blade parts at the respective stages
increase at ratios of 1.10 - 1.15 in terms of the lengths of the downstream-side blade
parts adjoining the upstream-side ones, and the ratios gradually enlarge on the downstream
side.
[0130] As stated above, those parts of the rotor shaft 23 on which the moving blades 16
are assembled are larger in diameter compared with those parts thereof which correspond
to the fixed blades. In this regard, the axial widths of the moving-blade assembled
parts become larger with the lengths of the blade parts of the moving blades 16. The
ratios of the aforementioned axial widths to the lengths of the blade parts of the
moving blades 16 are 0.65 - 0.95 at the second - final stages, and become smaller
stepwise from the second stage toward the final stage.
[0131] As also stated above, the axial widths of those parts of the rotor shaft 23 which
correspond to the fixed blades become smaller stepwise from the width between the
second stage and the third stage, to the width between the final stage and the penultimate
stage. The ratios of the aforementioned axial widths to the lengths of the blade parts
of the moving blades 16 are 0.7 - 1.7 at the second - final stages, and become smaller
stepwise from the upstream-side blade part toward the downstream-side blade part.
[0132] The high-pressure steam turbine shown in Fig. 1 further includes a thrust bearing
5, a first shaft packing 10, a second shaft packing 11, a high-pressure spacer 14,
a front bearing box 26, a journal portion 27, a high-pressure steam exhaust port 30,
a reheat steam inlet 32, and a thrust-bearing wear interrupter 39.
[0133] Fig. 2 is a sectional view of the intermediate-pressure steam turbine. This intermediate-pressure
steam turbine rotates a generator (G in Fig. 13) conjointly with the high-pressure
steam turbine, by the use of steam which is obtained in such a way that steam exhausted
from the high-pressure steam turbine is heated again to 625 (°C) by a reheater (R/H
in Fig. 13). Herein, the intermediate-pressure turbine has a rotational speed of 3600
(revolutions/min). Likewise to the high-pressure turbine, the intermediate-pressure
turbine includes an intermediate-pressure inner casing 21 and an outer casing 22.
It is provided with fixed blades in opposition to intermediate-pressure moving blades
17. The moving blades 17 have a double-flow construction of six stages, and they are
disposed in a substantially symmetrical arrangement on both sides of an intermediate-pressure
rotor shaft 24 in the lengthwise direction thereof. The distance between the centers
of bearings 3 and 4 in which the rotor shaft 24 is journaled, is about 5.5 (m). The
moving blade of the first stage has a length of about 92 (mm), and that of the final
stage has a length of about 235 (mm). The dovetail of the double-flow construction
is in an inverted-chestnut shape. That part of the rotor shaft 24 which corresponds
to the fixed blade preceding the final-stage moving blade 17 has a diameter of about
630 (mm), and the ratio of the inter-bearing distance of this rotor shaft to the aforementioned
diameter is about 8.7.
[0134] The axial widths of those parts of the rotor shaft 24 of the intermediate-pressure
steam turbine of this embodiment on which the moving blades 17 are assembled become
larger toward the downstream side of the steam stepwise at the three sorts of stages
of the first stage, the fourth and fifth stages and the final stage. The axial width
of the assembled part of the final stage is about 1.4 times as large as that of the
assembled part of the first stage.
[0135] Besides, those parts of the rotor shaft 24 of the intermediate-pressure steam turbine
which correspond to the fixed blades are smaller in diameter than those parts thereof
on which the moving blades 17 are assembled. The axial widths of the parts corresponding
to the fixed blades become smaller toward the downstream side of the steam stepwise
at the four moving-blade stages of the first stage, second stage, third stage and
final stage, and the axial width at the final stage is about 0.7 time as large as
the axial width at the first stage.
[0136] In this embodiment, the blades and nozzles of the first stage are made of the materials
indicated in Table 3 of Fig. 14 to be explained later, whereas those of all the other
stages are made of the 12-% Cr steel which contains no W, Co or B. The blade parts
of the moving blades 17 in this embodiment become longer at the respective stages
from the first stage toward the final stage. The lengths of the blade parts of the
first - final stages are set at 90 - 350 (mm), depending upon the output of the steam
turbine. The number of the stages is 6 - 9. Herein, the lengths of the blade parts
at the respective stages increase at ratios of 1.1 - 1.2 in terms of the lengths of
the downstream-side blade parts adjoining the upstream-side ones.
[0137] As stated above, those parts of the rotor shaft 24 on which the moving blades 17
are assembled are larger in diameter compared with those parts thereof which correspond
to the fixed blades. In this regard, the axial widths of the moving-blade assembled
parts become larger with the lengths of the blade parts of the moving blades 17. The
ratios of the aforementioned axial widths to the lengths of the blade parts of the
moving blades 17 are 0.5 - 0.7 at the first - final stages, and become smaller stepwise
from the first stage toward the final stage.
[0138] As also stated above, the axial widths of those parts of the rotor shaft 24 which
correspond to the fixed blades become smaller stepwise from the width between the
first stage and the second stage, to the width between the final stage and the penultimate
stage. The ratios of the aforementioned axial widths to the lengths of the blade parts
of the moving blades 17 are 0.5 - 1.5, and become smaller stepwise from the upstream-side
blade part toward the downstream-side blade part.
[0139] The intermediate-pressure steam turbine shown in Fig. 2 further includes shaft packings
12 and 13, an intermediate-pressure spacer 15, a first inner casing 20 (associated
with the second inner casings 21), reheat steam inlets 29, steam exhaust ports 30,
crossover pipes 31, and a warming steam inlet 40.
[0140] Fig. 3 is a sectional view of the low-pressure turbine. Two low-pressure turbines
are connected in tandem, and they have the same design. Moving blades 41 are provided
as eight stages on both sides of a rotor shaft 44 in the lengthwise direction thereof,
and these moving blades on both sides are substantially in a bilaterally symmetric
arrangement. Besides, fixed blades 42 are disposed in correspondence with the moving
blades 41. The moving blade 41 of the final stage is 43 (inches) long, and is made
of a Ti-based alloy. The moving blades 41 of all the stages are double-tenon type
tangential entry dovetail blades. A nozzle box 45 is of double-flow type. The Ti-based
alloy is subjected to age hardening, and it contains 6 (%) of Al and 4 (%) of V in
terms of weight. The rotor shafts 44 are made of forged steel of fully-tempered bainitic
structure prepared from superclean materials (high purity materials) which consist
of 3.75 (%) of Ni, 1.75 (%) of Cr, 0.4 (%) of Mo, 0.15 (%) of V, 0.25 (%) of C, 0.05
(%) of Si, 0.10 (%) of Mn, and the balance of Fe. All the moving blades and the fixed
blades except the final-stage ones are made of 12-% Cr steel containing 0.1 (%) of
Mo. Cast steel containing 0.25 (%) of C is employed as the material of the inner and
outer casings. The distance between the centers of bearings 43 in this embodiment
is 7500 (mm). Those parts of the rotor shaft 44 which correspond to the fixed blades
42 have a diameter of about 1280 (mm), while those parts thereof on which the moving
blades 41 are assembled have a diameter of about 2275 (mm). The ratio of the inter-bearing
distance to the smaller diameter of the rotor shaft 44 is about 5.9.
[0141] In the low-pressure turbine of this embodiment, the axial widths of the moving-blade
assembled parts of the rotor shaft 44 gradually enlarge at the five sorts of stages
of the first - third stages, the fourth stage, the fifth stage, the sixth - seventh
stages and the eighth stage. The width of the final stage is about 2.5 times as large
as that of the first stage.
[0142] Besides, those parts of the rotor shaft 44 which correspond to the fixed blades 42
are smaller in diameter than those parts thereof on which the moving blades 41 are
assembled. The axial widths of the parts corresponding to the fixed blades 42 become
larger toward the downstream side of the steam gradually at the three sorts of moving-blade
stages of the first stage, the fifth - sixth stages and the seventh stage, and the
width at the final stage is about 1.9 times as large as the width at the first stage.
[0143] The blade parts of the moving blades 41 in this embodiment become longer at the respective
stages from the first stage toward the final stage. The lengths of the blade parts
of the first - final stages are set at 90 - 1270 (mm), depending upon the output of
the steam turbine. The number of the stages is 8 or 9. Herein, the lengths of the
blade parts at the respective stages enlarge at ratios of 1.3 - 1.6 in terms of the
lengths of the downstream-side blade parts adjoining the upstream-side ones.
[0144] As stated above, those parts of the rotor shaft 44 on which the moving blades 41
are assembled are larger in diameter compared with those parts thereof which correspond
to the fixed blades 42. In this regard, the axial widths of the moving-blade assembled
parts become larger with the lengths of the blade parts of the moving blades 41. The
ratios of the aforementioned axial widths to the lengths of the blade parts of the
moving blades 41 are 0.15 - 0.91 at the first - final stages, and become smaller stepwise
from the first stage toward the final stage.
[0145] Also, the axial widths of those parts of the rotor shaft 44 which correspond to the
fixed blades 42 become smaller stepwise from the width between the first stage and
the second stage, to the width between the final stage and the penultimate stage.
The ratios of the aforementioned axial widths to the lengths of the blade parts of
the moving blades 41 are 0.25 - 1.25, and become smaller stepwise from the upstream-side
blade part toward the downstream-side blade part.
[0146] Apart from this embodiment, it is also possible to similarly construct a large-capacity
power plant of 1000 (MW) class in which steam inlets to a high-pressure steam turbine
and an intermediate-pressure steam turbine are at a temperature of 610 (°C), while
steam inlets to two low-pressure steam turbines are at a temperature of 385 (°C).
[0147] Fig. 4 is a diagram showing the typical plant layout of a coal-fired high-temperature
high-pressure steam turbine plant.
[0148] The high-temperature high-pressure steam turbine plant in this embodiment is chiefly
configured of a coal-fired boiler 51, a high-pressure turbine 52, an intermediate-pressure
turbine 53, low-pressure turbines 54 and 55, steam condensers 56, condensate pumps
57, a low-pressure feed water heater system 58, a deaerator 59, a pressuring pump
60, a boiler feed pump 61 and a high-pressure feed water heater system 63. Herein,
ultra-supercritical steam generated by the boiler 51 enters the high-pressure turbine
52 to generate power. Thereafter, exhaust steam from the high-pressure turbine 52
is reheated by the boiler 51, and the resulting steam enters the intermediate-pressure
turbine 53 to generate power again. Exhaust steam from the intermediate-pressure turbine
53 enters the low-pressure turbines 54 and 55 to generate power, and it is thereafter
condensed by the condensers 56. The resulting condensate is sent to the low-pressure
feed water heater system 58 and deaerator 59 by the condensate pumps 57. Feed water
deaerated by the deaerator 59 is sent by the pressurizing pump 60 and boiler feed
pump 61 to the high-pressure feed water heater system 63, in which the water is heated
and from which it is returned to the boiler 51.
[0149] Here in the boiler 51, the feed water is turned into high temperature and high pressure
steam by passing through an economizer 64, a vaporizer 65 and a superheater 66. Meantime,
the combustion gas of the boiler 51 having heated the steam comes out of the economizer
64, and it thereafter enters an air heater 67 to heat air. In the illustrated plant,
the boiler feed pump 61 is driven by a boiler feed pump driving turbine which is operated
by steam extracted from the intermediate-pressure turbine 53.
[0150] In the high-temperature high-pressure steam turbine plant thus constructed, the temperature
of the feed water having emerged from the high-pressure feed water heater system 63
is much higher than a feed water temperature in the prior-art thermal power plant,
and hence, the temperature of the combustion gas having emerged from the economizer
64 disposed in the boiler 51 becomes much higher than in the prior-art boiler as an
inevitable consequence. Therefore, heat is recovered from the exhaust gas of the boiler
51 so as to prevent the gas temperature from lowering.
[0151] Numerals 68 in Fig. 4 indicate generators which are respectively joined to the HP
- IP connection and the tandem LP connection.
[0152] By the way, apart from this embodiment, it is possible to similarly construct a tandem
compound type power plant in which the same high-pressure turbine, intermediate-pressure
turbine and one or two low-pressure turbines as described above are joined in tandem
so as to rotate a single generator for power generation. In the generator whose output
power is in the 1050 (MW) class as in this embodiment, a shaft of higher strength
is employed for the generator. Especially, the generator shaft should preferably be
made of steel of fully-tempered bainitic structure which contains 0.15 - 0.30 (%)
of C, 0.1 - 0.3 (%) of Si, at most 0.5 (%) of Mn, 3.25 - 4.5 (%) of Ni, 2.05 - 3.0
(%) of Cr, 0.25 - 0.60 (%) of Mo and 0.05 - 0.20 (%) of V, and which has a room-temperature
tensile strength of 93 (kg/mm
2) or above, particularly 100 (kg/mm
2) or above, and a 50-% FATT (Fracture Appearance Transition Temperature) of 0 (°C)
or below, particularly -20 (°C) or below. The steel should preferably be such that
a magnetizing force at 21.2 (kG) is at most 985 (AT/cm), that impurities P, S, Sn,
Sb and As contained is at the total amount of most 0.025 (%), and that a ratio Ni/Cr
is at most 2.0.
[0153] Figs. 5 and 6 are front views showing examples of the high-pressure and intermediate-pressure
turbine rotor shafts, respectively. The exemplified high-pressure turbine shaft has
a construction which consists of a multistage side and a single-stage side, and in
which blades totaling eight stages are assembled on both sides so as to laterally
center on the first-stage blade of the multistage side. The exemplified intermediate-pressure
turbine shaft has a construction in which multistage blades are assembled in a bilaterally
symmetric arrangement so as to total six stages on each side and to be substantially
bounded by the laterally central part of this shaft. Although the rotor shaft for
each low-pressure turbine is not specifically exemplified, the rotor shaft of any
of the high-pressure, intermediate-pressure and low-pressure turbines is formed with
a center hole, through which the presence of defects is examined by a ultrasonic test,
a visual test and a fluorescent penetrant inspection. Incidentally, numerals 27 in
each of Figs. 5 and 6 denote the journal parts of the corresponding rotor shaft.
[0154] Table 3 in Fig. 14 indicates the chemical constituents (weight-%) which were used
for the principal components of the high-pressure turbine, intermediate-pressure turbine
and low-pressure turbines in an example of this embodiment. In this example, all the
high-temperature parts of the high-pressure and intermediate-pressure turbines were
made of the steel materials of ferritic crystalline structure which had a coefficient
of thermal expansion of 12 × 10
-6 (/°C), so that no problems ascribable to discrepancy in the coefficients of thermal
expansion occurred.
[0155] Regarding each of the rotors of the high-pressure and intermediate-pressure portions,
an electrode was prepared in such a way that the heat-resisting cast steel mentioned
in Table 3 was melted in an amount of 30 (tons) by an electric furnace, that the molten
steel was subjected to carbon vacuum deoxidation and then poured into a metal mold,
and that the molded steel was forged. Further, the electrode was subjected to electroslag
remelting so as to melt from the upper part of the cast steel to the lower part thereof,
and the resulting steel was forged into a rotor shape having a maximum diameter of
1050 (mm) and a length of 5700 (mm). The forging was carried out at temperatures of
at most 1150 (°C) in order to prevent any forging cracks. Besides, after the forged
steel was annealed, it was heated to 1050 (°C) and was subjected to water spray quenching.
Subsequently, the resulting steel was tempered twice at temperatures of 570 (°C) and
690 (°C), and it was machined into the shape shown in Fig. 5 or Fig. 6. In this example,
the upper part side of the electroslag ingot was used as the first-stage blade side
of the rotor shaft, and the lower part side as used as the final-stage blade side.
[0156] Regarding the blades and nozzles of the high-pressure portion and intermediate-pressure
portion, the heat-resisting steel materials also mentioned in Table 3 of Fig. 14 were
melted by a vacuum arc furnace, and they were forged and molded into the shapes of
blade and nozzle blanks each having a width of 150 (mm), a height of 50 (mm) and a
length of 1000 (mm). The forging was carried out at temperatures of at most 1150 (°C)
in order to prevent any forging cracks. Besides, the forged steel was heated to 1050
(°C), subjected to oil quenching and tempered at 690 (°C). Subsequently, the resulting
steel was machined into the predetermined shapes.
[0157] Regarding the inner casings of the high-pressure portion and intermediate-pressure
portion, the casing of each main-steam stop valve and the casing of each steam control
valve, the heat-resisting cast steel materials mentioned in Table 3 were melted by
an electric furnace and then refined by a ladle. The resulting materials were thereafter
poured into sand molds. The cast steel, which did not suffer any casting defects such
as shrinkage holes, could be obtained by sufficiently refining and deoxidizing the
materials before the pouring. The weldability of each of the casing materials was
evaluated in conformity with "JIS Z3158". A preheating temperature, an inter-pass
temperature and a post-heating starting temperature were set at 200 (°C), and a post-heating
treatment was conducted at 400 (°C) for 30 (minutes). No welding cracks were noted
in either of the materials of the present invention, and the weldability was good.
[0158] Table 4 shown in Fig. 15 indicates the heat treatment conditions of the ferritic
steel materials listed in Table 3 of Fig. 14, and the mechanical properties of the
principal members of the high-temperature steam turbines made of the materials as
tested by cutting these members.
[0159] From results of the tests of the central parts of the rotor shafts, it has been verified
that the special qualities (625-°C 10
5-h strength ≧ 13 kgf/mm
2, and 20-°C absorbed impact energy ≧ 1.5 kg-m) required of the high-pressure and intermediate-pressure
turbine rotors can be satisfactorily met. Thus, it has been proved that the steam
turbine rotors usable in steam of 620 (°C) or above can be manufactured.
[0160] Besides, as the results of the property tests of the blades, it has been verified
that the special quality (625-°C 10
5-h strength ≧ 15-kgf/mm
2) required of the first-stage blades of the high-pressure and intermediate-pressure
turbines can be satisfactorily met. Thus, it has been proved that the steam turbine
blades usable in steam of 620 (°C) or above can be manufactured.
[0161] Further, as the results of the property tests of the casings, it has been verified
that the special qualities (625-°C 10
5-h strength ≧ 10 kgf/mm
2, and 20-°C absorbed impact energy ≧ 1 kg-m) required of the high-pressure and intermediate-pressure
turbine casings can be satisfactorily met, and that weld metal materials can be deposited
to the casings. Thus, it has been proved that the steam turbine casings usable in
steam of 620 (°C) or above can be manufactured.
[0162] Fig. 7 is a graph showing the relationships between the 10
5-h creep rupture strength and temperature for different rotor shaft materials. It
has been found that the materials according to the present invention satisfy the requirements,
which the 10
5-h creep rupture strength is equal to or stronger than 13kg/mm
2, at 610 - 640 (°C). Incidentally, the 12Cr rotor material is the prior-art material
which contains no B, W or Co.
[0163] In an example of this embodiment, bearing characteristics were improved in such a
way that Cr - Mo low-alloy steel was deposited on each journal portion of the rotor
shaft by buildup welding. The buildup welding was carried out as stated below.
[0164] Welding rods employed in the buildup welding were shielded-arc ones having diameters
of 4.0 (mm). Table 5 shown in Fig. 16 indicates the chemical constituents (weight-%)
of deposited metals which were formed by the welding operations with the shielded-arc
welding rods. The constituents of the deposited metals were substantially the same
as those of the corresponding weld metals (which shall be called the "welding rods
A - D" below).
[0165] The conditions of the buildup welding were a welding current of 170 (A), a voltage
of 24 (V) and a rate of 26 (cm/min).
[0166] Eight layers were welded onto the surface of the parent metal described before, as
the buildup welding by combining the used welding rods A - D for the respective layers
as listed in Table 6 of Fig. 17. The thickness of each layer was 3 - 4 (mm), the total
thickness of the eight layers forming each of samples Nos. 1 - 3 was about 28 (mm),
and the surface of each sample was ground to about 5 (mm).
[0167] As the conditions of execution of the welding operations, a preheating temperature,
an inter-pass temperature and a stress-relief-annealing (SR) starting temperature
were 250 - 350 (°C), and the SR was conducted by holding the deposited layers at 630
(°C) for 36 (hours).
[0168] All the samples Nos. 1 - 3 conformed to the present invention, and the chemical constituents
of the fifth layer et seq. in each sample were C or D mentioned in Table 5 of Fig.
16.
[0169] In order to confirm the quality of such a welding zone, buildup welding was similarly
conducted on a flat member, and the resulting flat member was subjected to a side-bend
test of 160°. All this time, no cracks were noted in the welding zone.
[0170] Further, when the bearings were subjected to a slide test on the basis of the revolutions
of the rotor shafts in the present invention, and none of them were not adversely
affected. The oxidation resistances of the bearings were also excellent.
[0171] Apart from this embodiment, it is possible to similarly construct a tandem type power
plant in which the high-pressure turbine, intermediate-pressure turbine and one or
two low-pressure turbines are joined in tandem so as to rotate a single generator
at 3600 (r/min).
[0172] Table 7 in Fig. 18 lists the chemical constituents of the Ni-based precipitation-strengthened
alloys each of which was applied to the first - third stages of the moving blades
in the high-pressure turbine and the first stage of the moving blades in the intermediate-pressure
turbine. Each of these alloys was produced in such a way that an ingot was prepared
by vacuum arc remelting, followed by hot forging, that the hot-forged ingot was subsequently
heated at 1,070 - 1,200 (°C) for 1 - 8 (hours) in accordance with the alloy composition
of the pertinent alloy as a treatment for turning the ingot into a solid solution,
followed by air cooling, and that the air-cooled material was heated at 700 - 870
(°C) for 4 - 24 (hours) as an ageing treatment.
(Embodiment 2)
[0173] Each of a number of alloys, having chemical constituents listed in Table 8 of Fig.
19, was cast into an ingot of 10 (kg) by vacuum induction melting, and the ingot was
forged into a rod of 30 (mm-square). Table 9 of Fig. 20 indicates the ratios of percentages
of the constituents. Regarding the rotor shaft of a large-sized steam turbine, each
of the alloys was quenched under the conditions of 1050 (°C) × 5 (hours) and 100 (°C/h)
cooling and was subjected to primary tempering of 570 (°C) × 20 (hours) and the secondary
tempering of 690 (°C) × 20 (hours), by simulating the central part of the rotor shaft.
On the other hand, regarding the blade of the turbine, each alloy was quenched under
the condition of 1100 (°C) × 1 (hour) and was tempered under the condition of 750
(°C) × 1 (hour). Thereafter, the creep rupture tests of such alloys were executed
under the conditions of 625 (°C) and 30 (kgf/mm
2). The results obtained are also listed in Table 8 of Fig. 19.
[0174] It is seen from Table 8 that the alloys No. 1 - No. 9 according to the present invention
have a much longer creep rupture lifetime than the comparative alloy No. 10.
[0175] Incidentally, the comparative alloy No. 10 does not contain Co unlike the alloys
of the present invention.
[0176] Figs. 8 and 9 are graphs showing those influences of the Co content and B content
of the alloys (listed in Table 8 of Fig. 19) which are respectively exerted on the
creep rupture strength.
[0177] As shown in Fig. 8, the creep rupture time period of the alloy becomes longer with
increase in the Co content. However, the increase of the Co content by a large amount
is unfavorable for the reason that the alloy is liable to become brittle when heated
at 600 - 660 (°C). In order to enhance both the strength and toughness of the alloy,
therefore, the Co content should preferably be 2 - 5 (%) for 620 - 630 (°C) and 5.5
- 8 (%) for 630 - 660 (°C).
[0178] As shown in Fig. 9, the strength of the alloy is prone to lower with increase in
the B content. It is understood that the alloy exhibits a superior strength when the
B content is 0.03 (%) or below. The strength is increased by setting the B content
to 0.001 - 0.01 (%) and the Co content to 2 - 4 (%) in a temperature range of 620
- 630 (°C), and by increasing the B content to 0.01 - 0.03 (%) and the Co content
to 5 - 7.5 (%) on a higher temperature side of 630 - 660 (°C).
[0179] It has been revealed that the alloy is strengthened more by a lower N content at
the temperatures exceeding 600 (°C) in this embodiment. This is also apparent from
the fact that sample No. 2 in Table 8 of Fig. 19 exhibits a higher strength than sample
No. 8 having a higher N content. The N content of the alloy should preferably be 0.01
- 0.04 (%). Since the constituent N was hardly contained by the vacuum melting, the
parent alloy was doped with the element N.
[0180] It is seen from Fig. 7 concerning Embodiment 1 that all the alloys according to the
present invention as listed in Table 8 exhibit high strengths. The rotor material
indicated in Embodiment 1 corresponds to the alloy of the sample No. 2 in this embodiment.
[0181] As shown in Fig. 9, the sample No. 8 having a low Mn content of 0.09 (%) exhibits
a higher strength, subject to equal Co contents. As is also apparent from this fact,
the Mn content of the alloy should preferably be set at 0.03 - 0.20 (%) in order to
attain a higher strength.
(Embodiment 3)
[0182] Table 10 shown in Fig. 21 indicates chemical constituents (weight-%) which concern
the inner casing materials of the present invention. With the thick part of a large-sized
casing assumed, the ingot of each of the listed samples was prepared in such a way
that each alloy was melted in an amount of 200 (kg) by a high-frequency induction
furnace, and that the molten alloy was poured into a sand mold having a maximum thickness
of 200 (mm), a width of 380 (mm) and a height of 440 (mm). The samples Nos. 3 - 7
are materials according to the present invention, whereas the samples Nos. 1 and 2
are materials of the prior art. The materials of the samples Nos. 1 and 2 are Cr-Mo-V
cast steel and 11Cr-1Mo-V-Nb-N cast steel respectively which are currently used in
turbines. After having been annealed by furnace cooling of 1050 (°C) × 8 (h), the
samples were heat-treated (normalized and tempered) under the following conditions,
assuming the thick part of the casing of the large-sized steam turbine:
Sample No. 1:
[0183]
Air cooling of 1050 (°C) × 8 (h)
Air cooling of 710 (°C) × 7 (h)
Air cooling of 710 (°C) × 7 (h)
Samples No. 2 - No. 7:
[0184]
Air cooling of 1050 (°C) × 8 (h)
Air cooling of 710 (°C) × 7 (h)
Air cooling of 710 (°C) × 7 (h)
[0185] The weldability of each of the samples was evaluated in conformity with "JIS Z3158".
A preheating temperature, an inter-pass temperature and a post-heating starting temperature
were set at 150 (°C), and a post-heating treatment was conducted at 400 (°C) for 30
(minutes).
[0186] Table 11 shown in Fig. 21 indicates the test results of the samples Nos. 1 - 7 listed
in Table 10 of Fig. 21, concerning tensile characteristics at room temperature, V-notch
Charpy impact absorption energy at 20 (°C), a 650-°C 10
5-h creep rupture strength, and a welding crack.
[0187] The creep rupture strength and the absorbed impact energy of each of the materials
of the present invention (samples Nos. 3, 4, 6 and 7) doped with appropriate amounts
of B, Mo and W, fully satisfy the special qualities (625-°C 10
5-h strength ≧ 8 kgf/mm
2, and 20-°C absorbed impact energy ≧ 1 kg-m) required of the high-temperature high-pressure
turbine casing. Especially, the samples Nos. 3, 6 and 7 exhibit high strengths exceeding
9 (kgf/mm
2). Moreover, none of the materials of the present invention (except the sample No.
3) suffer from welding cracks and all have good weldability. As the result of a test
concerning the relationship between the B content and the welding crack of the alloy,
when the B content exceeded 0.0035 (%), the welding crack appeared. The alloy of the
sample No. 3 was considered to be somewhat cracked. Regarding the influences of the
constituent Mo on the mechanical properties, the alloy whose Mo content was as high
as 1.18 (%) had a high creep rupture strength, but it exhibited a small impact energy
value and could not meet the required toughness. On the other hand, the alloy whose
Mo content was 0.11 (%) had a high toughness, but it exhibited a low creep rupture
strength and could not meet the required strength.
[0188] As the result of the investigation of the influences of the constituent W on the
mechanical properties, when the W content exceeds 1.1 (%), the creep, rupture strength
becomes remarkably high, but when it exceeds 2 (%), the room-temperature absorbed
impact energy becomes low. Especially, the Ni/W ratio of the alloy is controlled to
0.25 - 0.75, thereby obtaining the casing material of the heat-resisting cast steel
which meets a 625-°C 10
5-h creep rupture strength of at least 9 (kgf/mm
2) and a room-temperature absorbed impact energy of at least 1 (kgf-m) that are required
of the high-pressure and intermediate-pressure inner casings, and main-steam stop
valve and control valve casings of the high-temperature high-pressure turbine under
a pressure of at least 250 (kgf/cm
2) at a temperature of 621 (°C). Especially, the W content and the Ni/W ratio are respectively
controlled to 1.2 - 2 (%) and 0.25 - 0.75, thereby obtaining the excellent casing
material of the heat-resisting cast steel which meets a 625-°C 10
5-h creep rupture strength of at least 10 (kgf/mm
2) and a room-temperature absorbed impact energy of at least 2 (kgf-m).
[0189] Fig. 10 is a graph showing the relationship between the W content and the creep rupture
strength for the alloys explained above. As indicated in the figure, when the W content
is at least 1.0 (%), the strength is remarkably increased, and a value of at least
8.0 (kg/mm
2) is attained, especially for a W content of at least 1.5 (%).
[0190] Fig. 11 is a graph showing the relationship between the 10
5-hour creep rupture strength and the rupture temperature for the alloys explained
above. The alloy of the sample No. 7 in the present invention satisfactorily meets
the required strength at temperatures of, at most, 640 (°C).
[0191] In an example, alloying raw materials to be brought into the desired composition
of the heat-resisting cast steel in the present invention were melted in an amount
of 1 (ton) by an electric furnace and then refined by a ladle, and the resulting raw
materials were thereafter poured into a sand mold. Thus, the inner casing for the
high-pressure portion or intermediate-pressure portion as described in Embodiment
1 was obtained.
[0192] After having been annealed by furnace cooling of 1050 (°C) × 8 (h), the cast steel
stated above was normalized by air-blast quenching of 1050 (°C) × 8 (h) and was tempered
twice by furnace cooling of 730 (°C) × 8 (h). The casing which was manufactured by
way of trial and which had a fully-tempered martensitic structure, was cut and investigated.
As a result, it was verified that the manufactured casing fully satisfies the special
qualities (625-°C 10
5-h strength ≧ 9 kgf/mm
2, and 20-°C absorbed impact energy ≧ 1 kg-m) required of the casing of the high-temperature
high-pressure turbine of 250 (atm.) and 625 (°C), and that it can be subjected to
welding.
(Embodiment 4)
[0193] This embodiment sets the steam temperature of a high-pressure steam turbine and an
intermediate-pressure steam turbine at 649 (°C) in place of 625 (°C) in Embodiment
1, and the construction and size thereof are obtained by substantially the same design
as in Embodiment 1. Different here from Embodiment 1 are the rotor shafts, first-stage
moving blades, first-stage fixed blades and inner casings of the high-pressure and
intermediate-pressure steam turbines that come into direct contact with the steam
of the above temperature. In the materials of these components except the inner casings,
the B content and the Co content are respectively increased to 0.01 - 0.03 (%) and
5 - 7 (%) in the foregoing materials listed in Table 8 of Fig. 19. Further, as the
material of the inner casings, the W content of the material in Embodiment 1 as indicated
in Table 3 of Fig. 14 is increased to 2 - 3 (%), and Co is added 3 (%). In this way,
required strengths are fulfilled, and the design in the prior art can be used very
meritoriously. More specifically, in this embodiment, the design concept in the prior
art as it is can be used in the point that all the structural materials to be exposed
to the high temperature are formed of ferritic steel. By the way, since the second-stage
moving blades and fixed blades of the high-pressure and intermediate-pressure steam
turbines are subject to steam inlet temperatures of about 610 (°C), the material used
for the first stage in Embodiment 1 should preferably be employed for these components.
[0194] Further, the steam temperature of low-pressure steam turbines in this embodiment
becomes about 405 (°C) which is somewhat higher than about 380 (°C) in Embodiment
1. However, since the material in Embodiment 1 has a satisfactorily high strength
for the rotor shafts themselves of the low-pressure steam turbines, the same superclean
material (high purity material) is employed.
[0195] Still further, although a turbine configuration in this embodiment is of the cross-compound
type, the tandem type in which all the turbines are directly connected can also be
realized at a rotational speed of 3600 (r.p.m.).
[0196] The present invention thus far described brings forth effects as stated below.
[0197] According to the present invention, it is possible to obtain heat-resisting martensitic
cast steel the creep rupture strength and room-temperature toughness of which are
high at 610 - 660 (°C). Therefore, all principal members for ultra-supercritical pressure
turbines at individual temperatures can be made of heat-resisting ferritic steel,
the basic designs of prior-art steam turbines can be used as they are, and a thermal
power plant of high reliability can be built.
[0198] Heretofore, an austenitic alloy has been inevitably used at such temperatures. It
has therefore been impossible to manufacture a nondefective large-sized rotor, from
the viewpoint of manufactural properties. In contrast, according to heat-resisting
ferritic forged steel of the present invention, the nondefective large-sized rotor
can be manufactured.
[0199] Moreover, the high-temperature steam turbine made entirely of the ferritic steel
according to the present invention does not use the austenitic alloy which has a large
coefficient of thermal expansion. Therefore, the steam turbine has such advantages
as being rapidly started with ease and being less susceptible to thermal fatigue damage.
1. A steam turbine for use with steam of increased temperature, specifically in the range
of 610 to 660 °C, comprising a rotor shaft with moving blades and an inner casing
with fixed blades, wherein the rotor shaft and the inner casing are made of martensitic
steel containing 8 to 13 weight-% of Cr.
2. A steam turbine as defined in claim 1, wherein said rotor shaft is made of high-strength
martensitic steel of fully-tempered martensitic structure which exhibits a 105-hour creep rupture strength of at least 15 (kg/mm2), and which contains 9 - 13 (weight-%) of Cr; and said inner casing is made of martensitic
steel which exhibits a 105-hour creep rupture strength of at least 10 (kg/mm2) at the temperature corresponding to said inflowing steam temperature, and which
contains 8 - 13 (weight-%) of Cr.
3. A steam turbine as defined in claim 1 or 2, wherein at least one of first-stage blades
of at least one of said moving blades and said fixed blades is made of martensitic
steel which exhibits a 105-hour creep rupture strength of at least 15 (kg/mm2) at a temperature corresponding to the inflowing steam temperature of the first-stage
moving blade, or which exhibits an anti-tension of at least 90 (kg/mm2) at room temperature, and which contains 8 - 13 (weight-%) of Cr.
4. A steam turbine as defined in claim 1 or 2, wherein at least a first-stage blade of
said moving blades is made of Ni-based deposited strengthen alloy which exhibits an
anti-tension of at least 100 (kg/mm2) at room temperature.
5. A steam turbine as defined in claim 1, 2, 3 or 4, wherein said rotor shaft is made
of high-strength martensitic steel which contains 0.05 - 0.20 (%) of C, at most 0.15
(%) of Si, 0.03 - 1.5 (%) of Mn, 9.5 - 13 (%) of Cr, 0.05 - 1.0 (%) of Ni, 0.05 -
0.35 (%) of V, 0.01 - 0.20 (%) of Nb, 0.01 - 0.06 (%) of N, 0.05 - 0.5 (%) of Mo,
1.0 - 3.5 (%) of W, 2 - 10 (%) of Co and 0.0005 - 0.03 (%) of B, and which has at
least 78 (%) of Fe, the percentages being given in terms of weight; and said inner
casing is made of high-strength martensitic steel which contains 0.06 - 0.16 (%) of
C, at most 0.5 (%) of Si, at most 1 (%) of Mn, 0.2 - 1.0 (%) of Ni, 8 - 12 (%) of
Cr, 0.05 - 0.35 (%) of V, 0.01 - 0.15 (%) of Nb, 0.01 - 0.1 (%) of N, at most 1.5
(%) of Mo, 1 - 4 (%) of W and 0.0005 - 0.03 (%) of B, and which has at least 85 (%)
of Fe, the percentages being given in terms of weight.
6. A high-pressure steam turbine as defined in claim 1, 2, 3, 4, or 5, wherein said moving
blades are arranged including at least 7 stages on each side in a lengthwise direction
of said rotor shaft, except a first stage which is of double flow; and said rotor
shaft has a distance (L) of at least 5000 (mm) between centers of bearings in which
it is journaled, and a minimum diameter (D) of at least 600 (mm) at its parts which
correspond to said fixed blades, a ratio (L/D) between the distance (L) and the diameter
(D) being 8.0 - 9.0, and it is made of high-strength martensitic steel which contains
9 - 13 (weight-%) of Cr.
7. An intermediate-pressure steam turbine as defined in claim 1, 2, 3, 4 or 5, wherein
said moving blades have a double-flow construction in which at least 6 stages are
included on each side in a lengthwise direction of said rotor shaft, in a bilaterally
symmetrical arrangement on both sides, and in which the first stages of the arrangement
are assembled on a central part of said rotor shaft in the lengthwise direction; and
said rotor shaft has a distance (L) of at least 5000 (mm) between centers of bearings
in which it is journaled, and a minimum diameter (D) of at least 600 (mm) at its parts
which correspond to said fixed blades, a ratio (L/D) between the distance (L) and
the diameter (D) being 8.2 - 9.2, and it is made of high-strength martensitic steel
which contains 9 - 13 (weight-%) of Cr.
8. A high-pressure steam turbine as defined in claim 1, 2, 3, 4, 5 or 6, wherein said
moving blades are arranged including at least 7 stages, and they have lengths of 35
- 210 (mm) in a region from an upstream side of the steam flow to a downstream side
thereof; diameters of those parts of said rotor shaft on which said moving blades
are assembled are larger than diameters of those parts of said rotor shaft which correspond
to said fixed blades; and widths of the moving-blade assembling parts of said rotor
shaft in an axial direction of said rotor shaft are stepwise larger on the downstream
side than on the upstream side, and the ratios of these widths to the lengths of said
moving blades decrease from said upstream side toward said downstream side within
a range of 0.6 - 1.0.
9. A high-pressure steam turbine as defined in claim 1, 2, 3, 4, 5, 6, or 8, wherein
said moving blades are arranged including at least 7 stages, and they have lengths
of 35 - 210 (mm) in a region from an upstream side of the steam flow to a downstream
side thereof; ratios between the lengths of said moving blades of the respectively
adjacent stages are at most 1.2, and they increase gradually toward the downstream
side; and said lengths of said moving blades are larger on said downstream side than
on the upstream side.
10. A high-pressure steam turbine as defined in claim 1, 2, 3, 4, 5, 6, 8 or 9, wherein
said moving blades are arranged including at least 7 stages, and they have lengths
of 35 - 210 (mm) in a region from an upstream side of the steam flow to a downstream
side thereof; and widths of those parts of said rotor shaft which correspond to said
fixed blades, the widths being taken in an axial direction of said rotor shaft, are
stepwise smaller on the downstream side than on the upstream side, and the ratios
of these widths to the lengths of the downstream-side moving blades decrease stepwise
toward said downstream side within a range of 0.65 - 1.8.
11. An intermediate-pressure steam turbine as defined in claim 1, 2, 3, 4, 5, or 7, wherein
said moving blades have a double-flow construction in which at least 6 stages are
included on each of two sides in a lengthwise direction of said rotor shaft, in a
bilaterally symmetrical arrangement on both sides, and they have lengths of 100 -
300 (mm) in a region from an upstream side of the steam flow to a downstream side
thereof; diameters of those parts of said rotor shaft on which said moving blades
are assembled are larger than diameters of those parts of said rotor shaft which correspond
to said fixed blades; and widths of the moving-blade assembling parts of said rotor
shaft in an axial direction of said rotor shaft are larger on the downstream side
than on the upstream side, and the ratios of these widths to the lengths of said moving
blades decrease from said upstream side toward said downstream side within a range
of 0.45 - 0.75.
12. An intermediate-pressure steam turbine as defined in claim 1, 2, 3, 4, 5, 7 or 11
wherein said moving blades have a double-flow construction in which at least 6 stages
are included on each of two sides in a lengthwise direction of said rotor shaft, in
a bilaterally symmetrical arrangement on both sides, and they have lengths of 100
- 300 (mm) in a region from an upstream side of the steam flow to a downstream side
thereof; and the lengths of the respectively adjacent moving blades are larger on
the downstream side than on the upstream side, and the ratios of the lengths of adjacent
blades are at most 1.3 and increase gradually toward said downstream side.
13. An intermediate-pressure steam turbine as defined in claim 1, 2, 3, 4, 5, 7, 11 or
12, wherein said moving blades have a double-flow construction in which at least 6
stages are included on each of two sides in a lengthwise direction of said rotor shaft,
in a bilaterally symmetrical arrangement on both sides, and they have lengths of 100
- 300 (mm) in a region from an upstream side of the steam flow to a downstream side
thereof; and widths of those parts of said rotor shaft which correspond to said fixed
blades, the widths being taken in an axial direction of said rotor shaft, are stepwise
smaller on the downstream side than on the upstream side, and the ratios of these
widths to the lengths of the downstream-side moving blades decrease stepwise toward
said downstream side within a range of 0.45 - 1.60.
14. A high-pressure steam turbine as defined in claim 1, 2, 3, 4, 5, 6, 8, 9 or 10, wherein
said moving blades are arranged including at least 7 stages; diameters of those parts
of said rotor shaft which correspond to said fixed blades are smaller than diameters
of those parts of said rotor shaft which correspond to the assembled moving blades;
widths of the rotor shaft parts corresponding to said fixed blades, in an axial direction
of said rotor shaft are stepwise larger on an upstream side of the steam flow than
on a downstream side thereof at, at least, 2 of said stages, and the width between
the final stage of said moving blades and the stage thereof directly preceding said
final stage is 0.75 - 0.95 times as large as the width between the second stage and
the third stage of said moving blades; and widths of the rotor shaft parts corresponding
to said assembled moving blades, in the axial direction of said rotor shaft, are stepwise
larger on the downstream side of said steam flow than on the upstream side thereof
at, at least, 3 of said stages, and the axial width of said final stage of said moving
blades is 1 - 2 times as large as the axial width of said second stage of said moving
blades.
15. An intermediate-pressure steam turbine as defined in claim 1, 2, 3, 4, 5, 7, 11, 12,
or 13, wherein said moving blades are arranged including at least 6 stages; diameters
of those parts of said rotor shaft which correspond to said fixed blades are smaller
than diameters of those parts of said rotor shaft which correspond to the assembled
moving blades; widths of the rotor shaft parts corresponding to said fixed blades,
in an axial direction of said rotor shaft are stepwise larger on an upstream side
of the steam flow than on a downstream side thereof at, at least, 2 of said stages,
and the width between the final stage of said moving blades and the stage thereof
directly preceding said final stage is 0.6 - 0.8 times as large as the width between
the first stage and the second stage of said moving blades; and widths of the rotor
shaft parts corresponding to said assembled moving blades, in the axial direction
of said rotor shaft, are stepwise larger on the downstream side of said steam flow
than on the upstream side thereof at, at least, 2 of said stages, and the axial width
of said final stage of said moving blades is 0.8 - 2 times as large as the axial width
of said first stage of said moving blades.
16. In a steam-turbine power plant having a high-pressure turbine, an intermediate-pressure
turbine and a low-pressure turbine; the improvement therein wherein that steam inlet
of each of the high-pressure and intermediate-pressure turbines which leads to moving
blades of a first stage included in each of said high-pressure and intermediate-pressure
turbines is at a temperature of 610 - 660 (°C); that steam inlet of said low-pressure
turbine which leads to moving blades of a first stage included in said low-pressure
turbine is at a temperature of 380 - 475 (°C); said high-pressure being as defined
in claim 1, 2, 3, 4, 5, 6, 8, 10 or 14, and said intermediate-pressure turbines being
as defined in claim 1, 2, 3, 4, 5, 7, 11, 12, 13 or 15.
17. A steam-turbine power plant as defined in claim 16, wherein the first-stage moving
blade of said high-pressure turbine, and that part of a rotor shaft of said high-pressure
turbine on which said first-stage moving blade is assembled are held at metal temperatures
which are not more than 40 (°C) lower than a temperature of said steam inlet of said
high-pressure turbine leading to said first-stage moving blade; and the first-stage
moving blade of said intermediate-pressure turbine, and that part of a rotor shaft
of said intermediate-pressure turbine on which said first-stage moving blades is assembled
are held at metal temperature which is not more than 75 (°C) lower than a temperature
of said steam inlet of said intermediate-pressure turbine leading to said first-stage
moving blade.
18. A coal-fired power plant having a coal-fired boiler, steam turbines which are driven
by steam developed by the boiler, and one or two generators which are driven by the
steam turbines and which can generate an output of at least 1000 (MW); the improvement
therein wherein said steam turbines include a high-pressure turbine, an intermediate-pressure
turbine which is joined to said high-pressure turbine, and two low-pressure turbines;
that steam inlet of each of the high-pressure and intermediate-pressure turbines which
leads to moving blades of a first stage included in each of said high-pressure and
intermediate-pressure turbines is at a temperature of 610 - 660 (°C); that steam inlet
of said low-pressure turbine which leads to moving blades of a first stage included
in said low-pressure turbine is at a temperature of 380 - 450 (°C); said high-pressure
turbine being as defined in claim 1, 2, 3, 4, 5, 6, 8, 10 or 14, and said intermediate-pressure
turbine being as defined in claim 1, 2, 3, 4, 5, 7, 11, 13 or 15.
19. In a coal-fired power plant having a coal-fired boiler, steam turbines which are driven
by steam developed by the boiler, and one or two generators which are driven by the
steam turbines and which can generate an output of at least 1000 (MW); the improvement
therein wherein said steam turbines include a high-pressure turbine, an
intermediate-pressure turbine which is joined to said high-pressure turbine, and two
low-pressure turbines; that steam inlet of each of the high-pressure and intermediate-pressure
turbines which leads to moving blades of a first stage included in each of said high-pressure
and intermediate-pressure turbines is at a temperature of 610 - 660 (°C); that steam
inlet of said low-pressure turbine which leads to moving blades of a first stage included
in said low-pressure turbine is at a temperature of 380 - 450 (°C); the steam heated
by a superheater of said boiler to a temperature which is at least 3 (°C) higher than
the temperature of said steam inlet of said high-pressure turbine leading to the first-stage
moving blade thereof is caused to flow into said first-stage moving blade of said
high-pressure turbine; the steam having come out of said high-pressure turbine is
heated by a reheater of said boiler to a temperature which is at least 2 (°C) higher
than the temperature of said steam inlet of said intermediate-pressure turbine leading
to the first-stage moving blade thereof, whereupon the heated steam is caused to flow
into said first-stage moving blade of said intermediate-pressure turbine; and the
steam having come out of said intermediate-pressure turbine is heated by an economizer
of said boiler to a temperature which is at least 3 (°C) higher than the temperature
of said steam inlet of said low-pressure turbine leading to the first-stage moving
blade thereof, whereupon the heated steam is caused to flow into said first-stage
moving blade of said low-pressure turbine.
20. A power plant as defined in claim 16, 17, 18 or 19, wherein said low-pressure steam
turbine has a rotor shaft, moving blades which are assembled on the rotor shaft, fixed
blades which guide inflow of steam to the moving blades, and an inner casing which
holds the fixed blades; said moving blades have a double-flow construction in which
at least 8 stages are included on each side in a lengthwise direction of said rotor
shaft, in a bilaterally symmetrical arrangement on both sides, and in which the first
stages of the arrangement are assembled on a central part of said rotor shaft in the
lengthwise direction; said rotor shaft has a distance (L) of at least 7000 (mm) between
centers of bearings in which it is journaled, and a minimum diameter (D) of at least
1150 (mm) at its parts which correspond to said fixed blades, a ratio (L/D) between
the distance (L) and the diameter (D) being 5.4 - 6.3, and it is made of Ni-Cr-Mo-V
low-alloy steel which contains 1 - 2.5 (weight-%) of Cr and 3.0 - 4.5 (weight-%) of
Ni; and each of the final-stage moving blades of said arrangement has a length of
at least 40 (inches) and is made of a Ti-based alloy.
21. A power plant as defined in claim 16, 17, 18, 19 or 20, wherein said low-pressure
steam turbine has a rotor shaft, moving blades which are assembled on the rotor shaft,
fixed blades which guide inflow of steam to the moving blades, and an inner casing
which holds the fixed blades; steam inlet of said low-pressure turbine which leads
to a first-stage one of said moving blades is at a temperature of 380 - 450 (°C);
and said rotor shaft is made of low-alloy steel which contains 0.2 - 0.3 (%) of C,
at most 0.05 (%) of Si, at most 0.1 (%) of Mn, 3.0 - 4.5 (%) of Ni, 1.25 - 2.25 (%)
of Cr, 0.07 - 0.20 (%) of Mo, 0.07 - 0.2 (%) of V and at least 92.5 (%) of Fe, the
percentages being given in terms of weight.
22. A power plant as defined in claim 16, 17, 18 or 19, wherein said low-pressure steam
turbine has a rotor shaft, moving blades which are assembled on the rotor shaft, fixed
blades which guide inflow of steam to the moving blades, and an inner casing which
holds the fixed blades; said moving blades have a double-flow construction in which
at least 8 stages are included on each of two sides in a lengthwise direction of said
rotor shaft, in a bilaterally symmetric arrangement on both sides, and they have lengths
of 90 - 1300 (mm) in a region from an upstream side of the steam flow to a downstream
side thereof; diameters of those parts of said rotor shaft on which said moving blades
are assembled are larger than diameters of those parts of said rotor shaft which correspond
to said fixed blades; and widths of the moving-blade assembling parts of said rotor
shaft in an axial direction of said rotor shaft are larger on the downstream side
than on the upstream side, and the ratios of these widths to the lengths of said moving
blades decrease from said upstream side toward said downstream side within a range
of 0.15 - 1.0.
23. A power plant as defined in claim 16, 17, 18, 19, 20, 21 or 22, wherein said low-pressure
steam turbine has a rotor shaft, moving blades which are assembled on the rotor shaft,
fixed blades which guide inflow of steam to the moving blades, and an inner casing
which holds the fixed blades; said moving blades have a double-flow construction in
which at least 8 stages are included on each of two sides in a lengthwise direction
of said rotor shaft, in a bilaterally symmetrical arrangement on both sides, and they
have lengths of 90 - 1300 (mm) in a region from an upstream side of the steam flow
to a downstream side thereof; and the lengths of said moving blades of the respectively
adjacent stages are larger on the downstream side than on the upstream side, and their
ratios increase gradually toward said downstream side within a range of 1.2 - 1.7.
24. A power plant as defined in claim 16, 17, 18, 19, 20, 21, 22 or 23, wherein said low-pressure
steam turbine has a rotor shaft, moving blades which are assembled on the rotor shaft,
fixed blades which guide inflow of steam to the moving blades, and an inner casing
which holds the fixed blades; said moving blades have a double-flow construction in
which at least 8 stages are included on each of two sides in a lengthwise direction
of said rotor shaft, in a bilaterally symmetrical arrangement on both sides, and they
have lengths of 90 - 1300 (mm) in a region from an upstream side of the steam flow
to a downstream side thereof; and widths of those parts of said rotor shaft which
correspond to said fixed blades, the widths being taken in an axial direction of said
rotor shaft, are larger on the downstream side than on the upstream side, and the
ratios of these widths to the lengths of the respectively adjacent moving blades on
said downstream side decrease stepwise toward said downstream side within a range
of 0.2 - 1.4.
25. A power plant as defined in claim 16, 17, 18, 19, 20, 21, 22, 23 or 24, wherein said
low-pressure steam turbine has a rotor shaft, moving blades which are assembled on
the rotor shaft, fixed blades which guide inflow of steam to the moving blades, and
an inner casing which holds the fixed blades; said moving blades have a double-flow
construction in which at least 8 stages are included on each of two sides in an axial
direction of said rotor shaft, in a bilaterally symmetrical arrangement on both sides;
diameters of those parts of said rotor shaft which correspond to said fixed blades
are smaller than diameters of those parts of said rotor shaft which correspond to
the assembled moving blades; widths of the rotor shaft parts corresponding to said
fixed blades, in the axial direction of said rotor shaft, are stepwise larger on an
upstream side of the steam flow than on a downstream side thereof at, at least, 3
of said stages, and the width between the final stage of said moving blades and the
stage thereof directly preceding said final stage is 1.5 - 2.5 times as large as the
width between the first stage and the second stage of said moving blades; and widths
of the rotor shaft parts corresponding to said assembled moving blades, in said axial
direction of said rotor shaft are stepwise larger on the downstream side of said steam
flow than on the upstream side thereof at, at least, 3 of said stages, and the axial
width of said final stage of said moving blades is 2 - 3 times as large as the axial
width of said first stage of said moving blades.
26. A steam-turbine as defined in one of claims of 1 to 17, further comprising buildup
welding layers made of metal which has better bearing characteristics than these of
a parent metal of said rotor shaft, formed on the surface of each journal portion
of said rotor shaft.
27. A steam-turbine as defined in claim 26, wherein said buildup welding layers are formed
in the number of at least three.
28. A steam-turbine as defined in claim 27, wherein said buildup welding layers are formed
in the number of five to ten.
29. A steam-turbine as defined in claim 26, 27 or 28, wherein the most peripheral layer
of said buildup welding layers is made of low-alloy steel which contains 0.5 - 3 (weight-%)
of Cr.
30. A steam-turbine as defined in claim 29, wherein the most peripheral layer of said
buildup welding layers is made of low-alloy steel which contains 0.01 - 0.15 (weight-%)
of C, 0.3 - 1 (weight-%) of Si, 0.3 - 1.5 (weight-%) of Mn, and 0.5 - 3 (weight-%)
of Cr, 0.1 - 1.5 (weight-%) of Mo.