BACKGROUND OF THE INVENTION
[0001] This invention relates to novel steam turbines, and more particularly it deals with
a superhigh temperature and pressure steam turbine operating at a steam temperature
of 600° to 650°C and under a steam pressure of 4000 to 5003psi which turbine is superior
in thermal fatigue resistant property.
[0002] Owing to petroleum shortage and a consequent rise in the price of petroleum, it has
been taken to consideration to raise the temperature and pressure of steam used in
generating plants so as to increase efficiency. In steam generating plants, it is
now usual practice to operate the turbine under a steam condition of 538°C, and the
rotor shaft is formed of low alloy steel such as a Cr-Mo-V steel. However, in a high
temperature and pressure generating plant in which the turbine is operated at a steam
temperature of over 600°C and under a pressure of over 4000 psi, it has been revealed
that the material now used is not fit for use from the point of view of strength because
it is markedly low in creep rupture strength, thermal fatigue strength and strength
at high temperatures. Meanwhile steam turbines operating at a main steam temperature
of over 600°C are disclosed in "Transaction of the ASME", October 1960, for example.
However, this type of steam turbines are low in thermal fatigue resistant property,
and not suitable for use as steam turbines that only operate at peak load. More specifically,
when a steam turbine is repeatedly started up and shut down, the starting-up and ceasing
impose severe thermal fatigue conditions. Thus it becomes necessary to provide a steam
turbine having a rotor and rotor blades having a fatigue life that can withstand this
transitory condition, high strength at elevated temperature and high ductility at
elevated temperature. Austenitic alloys are considered suitable from the point of
view of strength as materials for the rotor and rotor blades operating under conditions
of a steam pressure exceeding 4000 psi and a steam temperature in the range over 600°C.
However, these alloys have the risks that, since various deposit phases are precipitated
at high temperature, embrittlement thereof would be accelerated and their strength
would be markedly lowered at elevated temperature.
SUMMARY OF THE INVENTION
[0003] An object of this invention is to provide a steam turbine including a turbine shaft
having superior thermal fatigue resistant property at a main steam temperature of
600° to 650°C.
[0004] Another object is to provide a superhigh temperature and pressure steam turbine of
high reliability, particularly a steam turbine of the type described including a rotor
shaft formed of austenitic-forged steel having high strength at elevated temperature,
high ductility at elevated temperature and low thermal embrittlement under the steam
condition of a temperature range between 600°C and 650°
C.
[0005] Still another object is to provide a steam turbine having a rotor shaft and rotor
blades of high thermal fatigue resistant property under the steam condition of a temperature
range between 600° and 650°C.
[0006] According to the invention, there is provided a superhigh temperature and pressure
steam turbine comprising a casing, rotor blades in the casing receiving steam jet
streams for rotation and a rotor shaft in the casing supporting the rotor blades for
rotation, said casing including an inner casing member supporting static blades for
guiding the steam jet and an outer casing member having a substantially spherical
external shape enclosing the inner casing member, wherein the rotor shaft and the
rotor blades are formed of austenitic steel having 1000 hour creep rupture strength
of over 25 kg/mm
2 at 650°C and have high thermal fatigue resistant strength.
[0007] The blade sections of the rotor shaft are preferably equidistantly located axially
of the rotor shaft in the casing and are preferably unsymmetrical at the center position
with respect to the axial direction.
[0008] The outer casing member is preferably formed of Cr-Ni austenitic cast steel or Cr-Mo-V
cast steel having a bainite structure.
[0009] The inner casing member is preferably formed of Cr-Ni austenitic cast steel.
[0010] The rotor shaft is made of austenitic forged steel consisting essentially by weight
of less than 0.04% C , not more than 2% Mn, not more than 1.5% Si, 10-20% Cr, 20-30%
Ni, 0.5-3% Mo, 0.5-3% Ti, 0.5-1% A1, 0.002-0.01% B and the balance Fe and having a
y' phase precipitated in the austenite matrix. Such rotor shaft has 1000 hour creep
rupture strength of not less than 25 kg/mm
2 and excellent thermal fatigue resistant property at 650°C.
[0011] The rotor shaft is preferably formed of austenitic forged steel consisting essentially
by weight of 0.015-0.03% C, 0.5-1.5% Mn, 0.3-1% Si, 14-17% Cr, 24-28% Ni, 1-2% Mo,
1.5-2.5% Ti, 0.15-0.4% A1, 0.004 - 0.008% B and the balance Fe and having a y' phase
precipitated in the austenite matrix. Further, it is preferred to add vanadium of
0.05-0.5 wt% in the steel alloy of the rotor, and more preferably 0.2-0.3% vanadium
is added.
[0012] The rotor blades of the superhigh temperature and pressure turbine according to the
invention consist essentially by weight of 0.01-0.1% C, not more than 1.5% Si, not
more than 2% Mn, 10-20% Cr; 20 30% Ni, 0.5-3% Mo, 1.5-3% Ti, 0.1-0.5% A1, 0.002-0.01%
B and the balance Fe. Preferably the rotor blades consist essentially by weight of
0.04-0.7% C, 0.5-1.5% Mn, 0.3-1% Si, 14-17% Cr, 24-28% Ni, 1-2% Mo, 1.5-2.5% Ti, 0.15-0.4%
Al, 0.004-0.008% B and the balance Fe and inevitable impurities. Further, it is preferred
to add vanadium of 0.05-0.5 wt% in the steel alloy of the rotor blade, and more preferably
0.2-0.3% vanadium is added.
[0013] The rotor shaft and the rotor blades of the superhigh temperature and pressure steam
turbine according to the invention preferably have, with respect to the aforesaid
chemical composition, the following Ni equivalent and Cr equivalent:
Ni equivalent [Ni (%) + ((30 x C (%))) + ((0.5 x Mn (%)))] = 23 - 29
Cr equivalent [Cr (%) + Mo (%) + ((1.5 x Si (%)))] = 12 - [(

x Ni equivalent) + 9]
[0014] The reasons why the elements are limited to the respective ranges of values in the
aforesaid composition are as follows:
Carbon
[0015] Strength at elevated temperature increases with an increase in C but a reduction
in toughness and thermal embrittlement are accelerated with an increase in C. The
carbon content in the material of the rotor shaft that increases strength at elevated
temperature without reducing toughness nor thermal embrittlement is less than 0.04%,
more preferably between 0.015 and 0.03%, and the carbon content of material of the
rotor blades is 0.01-0.1%, preferably between 0.04 and 0.07%.
Manganese
[0016] This element is the most important deoxidizing component in production. However,
when the elment is too large in amount, toughness and oxidation resistance are adversely
affected. Thus the amount is limitted to be not more than 2%, preferably between 0.5
and 1.5%.
Nickel
[0017] This element is an important component for improving high temperature mechanical
strength of the steel according to the invention, particularly creep rupture strength
and thermal fatigue life thereof. More specifically, this element is conducive to
formation of a stable austenite structure and increased high temperature strength.
However, in view of its expensiveness and the high temperature ductility and yield
strength of the steel being reduced when its amount is too high, the amount is limitted
between 20 and 30%, preferably between 24 and 28%.
Silicon
[0018] Similarly to manganese, this element is a deoxidizing component necessary for production.
When the amount is over 1.5%, however, the forgibility of the steel diminishes and
its high temperature toughness is reduced. Thus the upper limit is 1.5 wt%, preferably
between 0.3 and 1%.
Molybdenum
[0019] This element improves creep rupture strength by strengthening the austenite matrix
and forming a carbide. However, the steel has its high temperature ductility reduced
and its workability deteriorates when the amount of the element is too large. Thus
the amount is limitted between 0.5 and 3 wt%, preferably between 1 and 2%.
Chromium
[0020] This element is an important component for improving the high temperature oxidization
property of the material according to the invention. As shown in Fig. 1 (b), no satisfactory
effect is achieved if the amount is under 10%. When the amount is over 20%, embrittlement
increases after prolonged holding at ele-- vated temperature. The amount is thus preferably
between 14 and 17%.
Titanium
[0021] Besides being used as a deoxidizing agent, this element has the effect of hardening
alloys by the precipitation thereof pounds. However, when the amount exceeds 3%, the
element reduces the ductility and toughness of the steel and accelerates notch deterioration.
To increase the high temperature strength of the steel, it is necessary that the amount
of this element be not less than 0.5%. Preferably the amount is between 1.5 and 2.5%.
Aluminium
[0022] Aluminium is added as a deoxidizing agent in the amount of 0.1-0.5%. Preferably in
0.15-0.4%. This element is combined with titanium to cause precipitation of an intermetallic
compound, to thereby increase high temperature strength. However, when the amount
is too great, it tends to reduce strength. Thus the amount thereof is limitted between
0.1-0.5%.
Boron
[0023] This element has the effects of markedly strengthening grain boundary and providing
high temperature ductility. However, workability deteriorates when the amount is too
large. Thus the amount is limitted between 0.002 and 0.01%, preferably between 0.004
and 0.008%.
Vanadium
[0024] This element is added to improve creep strength. When the amount thereof is below
0.05%, no satisfactory effect is achieved. When the amount is over 0.5%, however,
ductility and toughness are both adversely affected. The amount is preferably between
0.2 and 0.3%.
Nickel Equivalent
[0025] Fig. 7a shows the relation between the nickel equivalent (% Ni + 30 x % C + 0.5 x
% Mn) regarding an austenite heat resisting steel now in use and the 1000 hour creep
rupture strength obtained at 650°C. As shown, an increase in the nickel equivalent
is accompanied by an improvement in creep rupture strength. This relation tends to
be saturated with the nickel equivalent being about 35%. If the 1000 hour creep rupture
strength at 650°C used as a target is made to agree with the scatter band value of
26-34 kg/mm
2 (1000 hour creep rupture strength at 550°C) now used for the currently used material,
then the optimum value of the nickel equivalent is between 23 and 29%.
Chromium Equivalent
[0026] Chromium equivalent is 12 < (% Cr equivalent) ≦

x (% nickel equivalent) + 9. Fig. 7b shows the relation between the chromium content
and the increment for high temperature oxidation. Fig. 7b shows that the chromium
content should be not less than 12% if its addition is to have any effect in oxidization
resistance at elevated temperature. Fig. 7c shows the result obtained with the relation
shown in Fig. 7b and the optimum range of the aforesaid nickel equivalent as inserted
in Shefla's diagram. In Fig. 7c, if the upper limit of the chromium equivalent is
set corresponding to the nickel equivalent on condition that a stable austenite structure
is obtained, then the value is 7/10 x (% nickel equivalent) + 9 (see Fig. 7c) according
to Shefla's diagram. The hatching in Fig. 7c indicates the nickel equivalent and chromium
equivalent as limited by the invention.
[0027] Additional and other objects, features and advantages of the invention will become
apparent from the description set forth hereinafter when considered in conjunction
with the accompanying drawings.
[0028] For producing the alloys used in the present invention, it is preferred to effect
melting by use of argon-oxygen blowing decarburization process or vacuum decarburization
process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029]
Fig. 1 is a schematic view of one example of the superhigh temperature and pressure
steam turbine according to the invention;
Fig. 2 is a diagrammatic representation showing a relation between absorbed energy
and the amount of C in the material according to the invention;
Fig. 3 is a diagrammatic representation of the working stress caused in the rotor
shaft according to the operation pattern of the steam turbine now in use;
Fig. 4a is a notional view of the turbine rotor surface temperature-strain (stress)
pattern of the steam turbine in actual use;
Fig. 4b is a view showing a model of a high temperature low cycle fatigue pattern
simulating the thermal fatigue operation pattern of Fig. 4a which model is a strain-holding
type;
Fig. 4c is another model of the fatigue pattern similar to Fig. 4b in which model
the strain-holding time is removed;
Fig. 5 is a diagrammatic representation of the results of high temperature low cycle
fatigue tests conducted on the materials according to the invention and material of
the prior art;
Fig. 6 is a diagrammatic representation of the influence of born exerted on the high
temperature low cycle fatigue life;
Fig. 7a is a diagrammatic representation of the relation between creep rupture strength
and nickel equivalent;
Fig. 7b is a diagrammatic representation of the relation between high temperature
oxidization increment and the amount of chromium;
Fig. 7c is a Shefla's diagram;
Fig. 8 is a diagram showing the 103 hour creep rupture strength of the material according to the invention and the material
of the prior art as extrapolated by the Rallson-Mirror process;
Fig. 9 is a diagrammatic representation of the high temperature low cycle fatigue
of the material according to the invention and the material of the prior art; and
Fig. 10 is a microscopic photograph of the specimen structure.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0030] Fig. 1 is a sectional view showing the essential portions of one embodiment of the
superhigh temperature and pressure steam turbine in conformity with the invention,
in which steam is introduced through a main steam line 1 into the turbine and is jetted
in a predetermined direction by static blades 3 attached to an inner casing member
2 to thereby rotate rotor blades 5 mounted on a rotor shaft 4. After doing work, the
steam flows through a gap between an outer casing member 6 and the inner casing member
2 and then is exhausted through a cooled steam outlet port 7, an exhaust outlet port
8 and an auxiliary exhaust outlet port 9. The exhausted steam is forwarded to a next
steam turbine operating at a lower temperature. 10 is the center of each bearing of
the rotor shaft 4. 11 and 12 are a gland and a intermediate gland leak outlet port
respectively. 13 is a nozzle box. Arrows indicate the direction of flow of the steam.
[0031] The embodiment of the invention of the aforesaid construction will be described in
detail with reference to the inner casing member 2 and the outer casing member 6 formed
of Cr-Ni austenite cast steel and Cr-Ni austenite forged steel, respectively.
Example 1
[0032] Table 1 shows the chemical composition of alloys used in experiments. Each of the
alloys used in the present invention is prepared by the steps of vacuum are melting,
forging, solid-solution treatment of holding it at 980°C for one hour with water-cooling
thereafter, and aging treatment of holding at 720°C for 16 hours with air-cooling
being effected thereafter, while each of the conventional alloys for comparison is
prepared by the steps of vacuum are melting, forging and succeeding necessary treatments
shown hereinbelow. The alloys used in the present invention has a microstructure in
which a y' phase is precipitated in the austenite matrix.
[0033] Conventional Cr-Mo-V steel serving as a comparative material was cooled by air-blowing
after heating at 970°C for 15 hours, and then reheated at 670°C for 48 hours before
being cooled in the furnace.
[0034] 12 Cr steel also serving as another comparative material was cooled by spraying of
water in atomized particles after heating at 1050°C for 24 hours, and then subjected
to tempering at 650°C for 20 hours. These steel alloys were subjected to V-notch Charpy
impact tests and creep rupture tests.
[0035] Fig. 2 shows the results of impact tests conducted on the influences of the amount
of C on thermal embrittlement by heating at 650°C for 1500 hours. The results show
that, whereas absorbed energy of non-heat treated blanks is substantially constant
irrespective of the amount of C, the absorbed energy reduces as the amount of C increases
in the material heated at 650°C for 1500 hours, showing a marked thermal embrittlement
in material with high C content. The values of absorbed energy at 20° C for 12 Cr
steel and Cr-Mo-V steel which are now in use for producing rotors are specified as
not less than 1.1 kg - m and 0.69 kg - m respectively. Particularly, in a case where
the amount of C is not more than 0.03% by weight, the absorbed energy becomes not
less than 1.5 kg - m, that is, superior thermal embrittlement resistant property can
be obtained. The thermal embrittlement resistant property of each alloy used as a
material of the turbine shaft in the present invention is larger than those of the
conventional 12% Cr steel and Cr-Mo-V steel.
[0036] In the rotor shaft material for the steam turbine according to the invention, intracrystalline
rupture (white triangle) prevailed when the material had a C ccntent of below 0.04
wt%. However, when the C content had a value not less than 0.04 wt%, the rupture form
has transferred to a grain boundary rupture type (black triangle). The results of
analysis have shown that this change in rupture type is accounted as the deterioration
of the grain boundary due to precipitation of the Mx type and M
23C
6 type carbides in the grain boundary. Thus, to diminish thermal embrittlement in interrelation
with thermal fatigue, the amount of C is limitted to be less than 0.04%, preferably
between 0.015 and 0.03% in the material for rotor shafts according to the invention.
[0037] Fig. 3 shows an operation pattern of the severest condition for conventional steam
turbines that is operated at a steam temperature of 566°C, that is, the starting-up
and shutting-down of the turbine is repeated every 12 hours. When this operation pattern
is applied to the superhigh temperature and pressure steam turbine according to the
invention, the rotor shaft would be subjected to harsh low cycle fatigue due to high
stress at the time of startup and shutdown as shown. Particularly the rotor shaft,
unlike the rotor blades, would be subjected to harsh low cycle fatigue due to the
combined actions of thermal stress and centrifugal stress because arise in temperature
is gradual in the rotor. Also, it would be subjected to creep due to centrifugal forces
in steadystate operation. In Fig. 3, working stress represents thermal stress combined
with centrifugal stress.
[0038] Table 2 shows the results of tension tests, creep rupture tests and low cycle fatigue
tests conducted on the steel according to the invention at 650°C and on the steel
of the prior art at 550°C. It will be seen in the table that the material according
to the invention is.equal to or higher than the Cr-Mo-V steel in tensile strength
and 0.2% yield strength, and that the rate of elongation thereof is 1.5 to 1.75 times
as high as in the Cr-Mo-V steel. Creep rupture strength is 1.1 to 1.20 times as high
in the material according to the invention as in the Cr-Mo-V steel and low cycle fatigue
is equal to or slightly higher in the former than in the latter in spite of the difference
of test temperature.
[0039] The results of the low cycle fatigue tests represent the number of repetition continued
until rupture occurs at a strain rate of 0.1%/second and strain amounts of 1.0% and
0.65% without holding strain (, that is, in the same manner as shown in Fig. 4c).

[0040] From the foregcing, it will be appreciated that the material according tc the invention
which is used at a temperature of 650°C meets requirements regarding mechanical strength
required in the case of Cr-Mo-V steel used at present at 550°C. Thus it has been made
clear that the material described above can be used for forming the rotor shaft of
a superhigh temperature and pressure steam turbine operating at a steam temperature
of 600°-650°C and under a steam pressure of 4000-5000 psi.
Example 2
[0041] Table 3 shows the chemical composition (wt%) of materials for forming the rotor blades
used in the superhigh temperature and pressure steam turbine according to the invention.
Each material has been obtained by performing vacuum arc melting, forging, and grain
size regulation into a range cf ASTM G.S. 2.5 - 4 was effected by holding it at 1050°C
for 3 hours. Then each material was water-cooled to room temperature in the same manner
as in Example 1 after having been subjected to solid solution treatment of holding
it at-899°C for 2 hours. Then, each material was subjected aging treatments of two
steps, i.e., first step was the holding it at 760°C for 16 hours with air-cooling
thereafter and the second step was the holding at 718°C for 6 hours with air-cooling
thereafter, The blanks obtained by processing the materials through the aforesaid
treatments had a microstructure having a y' phase precipitated in the austenite matrix.
The blanks were machined to provide test pieces of predetermined dimensions.
[0042] Generally, the blades of a steam turbine are directly subjected to the jetted steam,
so that their temperatures rise relatively quickly after the steam turbine is actuated
and becomes equal to that of steam. Meanwhile a rise in the temperature of the rotor
shaft is relatively slow after the commencement of actuation of the steam turbine
because it has a high thermal capacity and austenite steel has a low thermal conductivity.
Thus the relatively large difference in temperature between the blades and rotor shaft
exists for a
' substantial period of time. Because of this, the working stress (thermal stress plus
centrifugal stress) to which the rotor shaft is subjected is very high at starting-up
or ceasing (or when transferring to idling) as shown in Fig. 3, with the result that
the thermal fatigue suffered'by the blades is relatively lower than that suffered
by the rotor shaft. Thus the amount of C for the materials of the blades is limitted
between 0.01 and 0.1%, preferably between 0.04 and 0.07%.
[0043] Fig. 4a shows a conceptual pattern of a relation between temperature and strain (stress)
to which the surface of the rotor is subjected in the steam turbine of superhigh temperature
and pressure according to the invention having the blades referred to hereinabove
and the rotor shaft described by referring to Example 1. To assess the thermal fatigue
life experimentarily, the operation conditions shown in Fig. 4a were converted to
a trapezoidal strain cycle (constant temperature) with strain-holding (Fig. 4b), and
a tension-compression triangular wave form (constant temperature) without strain-holding
(Fig. 4c). The aforesaid conversion has been made on the basis of the hypothesis that
there is a correlation between a thermal fatigue phenomenon (varying temperature)
and a low cycle fatigue phenomenon (constant temperature).
[0044] Fig. 5 is a diagram showing the results of high temperature low cycle fatigue tests
conducted by controlling strain of gauge length at a strain rate of 0.1%/sec and at
a temperature of 650°C. As shown, it will be seen that the materials of the invention
containing 0.002-0.008% B has about twice as long service life as a conventional material
containing no B in a low cycle region having a strain range of 0.5-1.2%, indicating
that the addition of B has the effect of improving thermal fatigue resistant property.
[0045] Fig. 6 shows the influences exerted on fatigue life by the amount of B at a temperature
of 650°C and a strain speed of 0.1%/sec in a strain region of 1.0%. As shown, fatigue
life has a peak in the vicinity of 0.006% B and is about twice as long as that of
material containing B in an amount outside the range of the invention.
[0046] From the foregoing, it will be appreciated that the blades for steam turbines according
to the invention has superior thermal fatigue resistant property and a prolonged service
life.
Example 3
[0047] Materials were subjected to vacuum arc melting, forging and regulating of grain size
under the same conditions as described in Example 1. Then the materials were subjected
to solid solution treatment and aging under the heat treating conditions shown in
Table 7, and machines to produce materials for blades and rotor shafts. The materials
Nos. 10 and 11 are those used for producing blades, and the materials Nos. 12 and
13 are those used for producing rotor shafts. Tables-5 and 6 show the results of creep
rupture tests and fatigue tests conducted on these materials, respectively.
[0049] Fig. 8 shows the results of extrapolation cf the creep rupture strength of 105 hours
conducted by the Rollson-Mirror process. According to this relation, the materials
Nos. 10, 11 , 12 and 13 covered by the claims of the invention have strength of about
133 MPa at 650°C which is similar to mean creep rupture strength of 127 MPa of the
conventional Cr-Mo-V steel tested at 550°C for comparison.
[0050] Fig. 9 shows the results of high temperature low cycle fatigue tests effected by
controlling strain of gauge length, at a strain rate of 0.1%Isec. The results show
that in the entire strain range the fatigue life of the materials according to the
present invention at 650°C is equal to or longer than that of the Cr-Mo-V steel at
550°C.
[0051] Fig. 10 is a microscopic photograph at a magnification 1000x of the No. 1 alloy having
a microstructure in which a y' phase is precipitated in the austenite matrix.
[0052] The alloy according to the invention has high temperature strength required of the
materials for the blades and the rotor operating at a steam temperature of 600°-650°C
and is suitable for use as materials for the rotor blades and the rotor.
Example 4
[0053] Alloys having chemical compositions shown in Table 8 were produced in the same manner
as Example 1.
[0054] Figs. 11 and 12 show a relation between aging temperature and tensile strength and
another relation between aging temperature and creep rupture time at 650°C regarding
the above-described alloys of the present invention, respectively. As apparent from
Figs. 11 and 12, low aging temperature not more than 740°C is preferred for obtaining
improved creep rupture strength and tensile strength. Further, the amount of carbon
does not cause much influence regarding the enhancement of mechanical strength, however,
the lower the amounts carbon and titanium in the alloys, the higher the elongation
and reduction of area thereof become.
Example 5
[0055] Table 9 shows the chemical composition of the rotor shaft 4 of the superhigh temperature
and pressure steam turbine according to the invention.
[0056] Raw materials for constituting the aforesaid composition were subjected to vacuum
induction melting under a vacuum of 10
-3 to produce electrodes of about 1000 mm in diameter. The electrodes were remelted
by an electro-slag-remelting process (ESR) by use of flux consisting of CaF
2 of 55%, A1
20
3 of 35% and Ti0
2 of 10% and cast into columnar ingots. The ingots were diffusion- annealed at a temperature
of 1100°-1500°C and forged at a temperature below 1050°C to produce a columnar blank
of 850 mm in diameter and 6000 mm in length. The blank was held for 3 hours at a temperature
of 1050°C to control the grain size into a range of ASTM G.S. 2.5-4 while rotating
the blank at a rate of 3 times per one minute.

[0057] After subjecting the blank to solution treatment by holding same at a temperature
in the range between 900 and 1000°C for 1 hour, they were water-cooled by jetting
water thereagainst while vertically holding and rotating it at a rate of three revolutions
per minute to room temperature. Thereafter, the blank was held at a temperature between
700° and 730°C for 16 hours to effect aging while rotating it in the same manner,
to provide a microstructure in which a y' phase is precipitated in austenite matrix.
The blank was then machined to obtain a rotor shaft having predetermined dimensions
shown in Fig. 13. Specimens for experiments were taken from the left hand end of the
rotor shaft as shown in Fig. 13 and were subjected to the tests of tensile strength
and creep rupture strength, with the result that there were obtained strength and
elongation both substantially similar to those of the specimen No. 17 described above.
Example 6
[0058] Table 10 shows the chemical composition (wt%) of a blade used in the superhigh temperature
and pressure steam turbine of the present invention.

[0059] Raw materials for constituting this chemical composition were subjected to vacuum
induction melting to produce electrode (600 mm in diameter) for ESR. The electrodes
were remelted by ESR process by use of flux consisting of CaFe of 50%, Cao of 25%,
Ti0
2 of 15% and
A1203 of
10%, and the molten metal was cast into blanks. Then, each of the blanks was heat-treated
to control the grain size thereof. After that, there was effected the solution heat
treatment of holding the blank at 899°C for 2 hours and of water-cooling thereafter.
Then, the blank was subjected to aging treatment of two steps, i.e., in the first
step the blank was held at 760°C for 16 hours with air-cooling being effected thereafter
and in the second step the blank was held at 718°C for 6 hours with air-cooling thereafter,
so that there was obtained a blank having microstructure in which y' phase is precipitated
in austenite matrix. Such blank was subjected to mechanical working to obtain the
blade having predetermined dimensions. Specimens were picked from the blade, which
specimens were subjected to the test of evaluating tensile strength, creep rupture
strength and high temperature lower cycle fatigue resistant property, with the result
that there were obtained values in a degree approximately similar to those of the
specimen No. 1 shown in Table 2.
1. A superhigh temperature and pressure steam turbine comprising a casing, blades
in said casing receiving streams of steam jets for rotation, and a rotor shaft supporting
said blades for rotation, said casing including an inner casing member having static
blades secured thereto for guiding the streams of steam jets, and an outer casing
member enclosing said inner casing member; said rotor shaft being made of an alloy
consisting essentially, by weight, of less than 0.04% C, not more than 2% Mn, not
more than 1.5% Si, 10-20% Cr, 20-30% Ni, 0.5-3% Mo, 0.5-3% Ti, 0.1-0.5% Al, 0.002-0.01%
B and the balance Fe and inevitable impurities and having a microstructure in which
a y' phase is precipitated in austenite matrix,
2. A superhigh temperature and pressure steam turbine comprising a casing, blades
in said casing receiving streams of steam jets for rotation, and a rotor shaft supporting
said blades for rotation, said casing including an inner casing member having static
blades secured thereto for guiding the streams of steam jets, and an outer casing
member enclosing said.inner casing member; said rotor shaft being made of an alloy
consisting essentially, by weight, of less than 0.04% C, not more than 2% Mn, not
more than 1.5% Si, 10-20% Cr, 20-30% Ni, 0.5-3% Mo, 0.5-3% Ti, 0.1-0.5% A1, 0.002-0.01%
B and the balance Fe and inevitable impurities and having a microstructure in which
a y' phase is precipitated in austenite matrix, and said blades being made of an alloy
consisting essentially, by weight, of 0.01-0.1% C, not more than 1.5% Si, not more
than2% Mn, 10-20% Cr, 20-30% Ni, 0.5-3% Mo, 1.5-3% Ti, 0.1-0.5% A1, 0.002-0.01% B
and the balance Fe and inevitable umpurities and having a microstructure in which
a y' phase is precipitated in austenite matrix,
3. A superhigh temperature and pressure steam turbine comprising a casing, blades
in said casing receiving streams of steam jets for rotation, and a rotor shaft supporting
said blades for rotation, said casing including an inner casing member having static
blades secured thereto for guiding the streams of steam jets, and an outer casing
member enclosing said inner casing member; said rotor shaft being made of an alloy
consisting essentially, by weight, of less than 0.04% C, not more than 2% Mn, not
more than 1.5% Si, 10-20% Cr, 20-30% Ni, 0.5-3% Mo, 0.5-3% Ti, 0.1-0.5% Al, 0.002-0.01%
B and the - balance Fe and inevitable impurities, and the Ni equivalent of said alloy
is:
Ni equivalent [Ni(%) + {(30 x C(%) + (0.5 x Mn(%)}] = 23 - 29; and the Cr equivalent
thereof is:

and the alloy being austenitic forged steel having a microstructure with a γ' phase
precipitated in austenite matrix,
4. A superhigh temperature and pressure steam turbine as claimed in any one of claims
1-3, wherein said rotor shaft is formed of an alloy consisting essentially, by weight,
of 0.015-0.03% C, 0.5-1.5% Mn, 0.3-1% Si, 14-17% Cr, 24-28% Ni, 1-2% Mo, 1.5-2.5%
Ti, 0.15-0.4% Al, 0.004-0.008% B, 0.05-0.5% V and the balance Fe and inevitable impurities.
5. A superhigh temperature and pressure steam turbine as claimed in claim 2, wherein
said blades are formed of an alloy consisting essentially, by weight, of 0.04-0.07%
C, 0.5-1.5% Mn, 0.3-1% Si, 14-17% Cr, 24-28% Ni, 1-2% Mo, 1.5-2.5% Ti, 0.15-0.4% A1,
0.004-0.008% B, 0.05-0.5% V and the balance Fe and inevitable impurities.
6. A superhigh-temperature and pressure steam turbine as claimed in any one of claims
1-3, wherein said rotor shaft has 1000 hour creep rupture strength of 26-34 kglmm2 at 650°C.
7. A superhigh temperature and pressure steam turbine as claimed in any one of claims
1-3, wherein said stream of steam jets have a temperature in a range between 600°
and 650°C and the steam pressure is in a range between 4000 and 5000 psi.