Development of High- and Intermediate-Pressure Steam Turbine Rotors for Efficient Fossil Power Generation Technology

Development of High- and Intermediate-Pressure
Steam Turbine Rotors
for Efficient Fossil Power Generation Technology
Koichi Takasawa, Ph.D* Kazuhiro Miki**-SynopsisThis article summarizes the history of the development of large turbine rotors for fossil power generation in the world,
including recent topics of research and development at JSW. The first half of the article describes the engineering currentof thermal condition of the fossil power plant and the progress of high chromium ferritic heat resistant steels for ultra super
critical (USC) power generation. The second half introduces the present status of the development of turbine rotors foradvanced-ultra super critical (A-USC) power generation in Japan, followed by our recent manufacturing experience, inwhich we successfully manufactured 10-ton class A-USC turbine rotors of a Ni-Fe base superalloy with a fine grain structureand sufficient permeability of ultrasonic waves. These superalloy rotors are expected to contribute to further developmentsof fossil power generation as the high chromium steel rotors have greatly helped fossil power plants become more and more
efficient and reduce the emission of greenhouse effect gases.

1. Introduction
The World Energy Agency predicts global energy
demand will increase by about 30%(1) and global
power demand by about 70%(2) between 2011 and2035. In addition, it predicts global CO2 emissions
will increase by about 20% from 30 billion tons in2011 to 35.7 billion tons in 2035, with remarkableincreases in China and India(1). In Japan, theshutting down of all domestic nuclear power plants
after the March 2011 accident at the Fukushima
Daiichi Nuclear Power Plant resulted in
dependence on fossil power generation, causing an
increase of 112 million tons in CO2 emissions frompower generation in fiscal 2012. Although there isan urgent need to reduce greenhouse gas emissions
significantly, renewable energy sources are notsufficient to meet global power demand.
Accordingly, fossil fuels remain the mainstream
fuels for power generation worldwide. Amongthem, consumption of coal is expected to continue
for a long time into the foreseeable future due tolarge reserves and low costs. However, the CO2
emissions intensity of coal-fired thermal power
generation is the highest among fossil fuels (LNG,
oil, and coal). Therefore, there are demands toraise the efficiency of coal-fired thermal power
generation plants. Because generating efficiency
can be enhanced by increasing the temperatureand pressure of steam when it enters a steam
turbine, development of heat-resistant materials
to achieve that has been promoted for a number ofyears. At present, the maximum steam temperaturereached to 620 ˚C, and ferritic heat-resistant steel
with a 9-12% Cr content is used as the material
exposed to such high-temperature steam. To attaingreater efficiency, development of a Ni-based
superalloy as a material that is resistant to 700 ˚Cor higher temperatures is currently underway
worldwide.
JSW has manufactured various kinds ofcomponents for power generation facilities; e.g.
high- and intermediate-pressure turbine rotor(3-5),
low-pressure turbine rotor(6-7), monoblock-typehigh- and low-pressure turbine rotor(8-11), generator
rotor, retaining rings(12-13), turbine casing(14-16),
boiler pipes(17-18), and gas turbine disks(19-20). This
article focuses on materials for high- andintermediate-pressure turbine rotors, and outlines
technological trends as steam temperatures rise atfossil power plants.

*Muroran Plant, Forgings, Castings and Pressure Vessels Engineering Department**Muroran Plant, Muroran Research Laboratory

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Development of High- and Intermediate-Pressure Steam Turbine Rotors for Efficient Fossil Power Generation Technology

2. Changes in Thermal Conditions of a
Fossil Power Plant
Figure 1 shows changes in thermal steam
conditions at the inlets of steam turbines for fossil
power plants in Japan. Steam conditions havechanged from sub-critical, which is below thecritical point of water (374 ˚C, 22.1 MPa) throughsuper critical (Pressure ≧ 22.1 MPa, Temperature≦ 566 ˚C), to ultra super critical (“USC”: Pressure≧ 22.1 MPa, Temperature ≧ 593°C). Pressure andtemperature of 4.1 MPa and 450 ˚C in 1950 rose to
16.6 MPa and 566 ˚C in 1959. In 1967, Super
critical steam, of which pressure was 24.1 MPaand the temperatures of main and reheat steam
were 538 and 566 ˚C, respectively, was adopted forthe first time at Anegasaki power station, unit 1(output: 600 MW). Subsequent efforts were madeto increase power plant capacity, reaching 1000
MW capacity for a single unit at Kashima thermal
power station unit 5 in 1974. However, hightemperatures were not addressed for more than 30
years. Although oil-fired power generation plants
were constructed one after another in the 1960s,
after the oil crises in the 1970s, coal-fired power
plants were introduced from the perspectives ofenergy saving, energy security, and economy, andhigher efficiency has been pursued. Consequently,
the first plant employing USC power generation
with a 593 °C reheat steam temperature wasrealized at Hekinan No. 3 unit (700 MW) in 1993,
followed by further improvements in temperature
resistance capabilities: thermal conditions wereachieved of 600 ˚C/600 ˚C at Misumi unit 1 andHaramachi unit 2 (both 1000 MW) in 1998, and in2000 of 600/610 ˚C at Tachibanawan thermal

Figure 1 Changes in thermal conditions at the inlets of steam turbines forfossil power generation in Japan.

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JSW TECHNICAL REVIEW No.20

power station units 1 and 2 (1050 MW) in 2000.
Currently, the highest steam temperature is
600/620 ˚C at Isogo thermal power station unit 2(600 MW), of which commercial operation started
in 2009.
Meanwhile, in the United States, the first USC
plant was established in 1957, adopting thermal
conditions of 31 MPa, 621 ˚C at Philo 6 (125 MW).
In 1960, the world’s highest thermal conditions of
34.5 MPa, 649 ˚C were adopted at Eddystone
station unit 1 (325 MW). However, because
Eddystone unit 1 experienced high costs foraustenitic steel and Ni-base alloy which were used
to address elevated temperatures, and otherproblems such as damage to boilers, thermal
conditions of 27 MPa, 538 ˚C became themainstream for newer plants(21).

3. Development of Rotor Materials for
USC Power Generation
To improve thermal conditions, heat-resistantmaterials that can be used at high temperaturesand under high pressures are indispensable. To
this end, many heat-resistance materials havebeen developed for turbine rotors, turbine blades,
casings and boiler pipes/tubes, and valves. This
chapter reviews the development history ofmaterials focusing mainly on high chromium (Cr)ferritic heat resistant steels, which are used forhigh- and intermediate-pressure steam turbine
rotors—important components of USC power
generation plants.
Concerning metal materials used at hightemperatures, creep phenomena, which cause
deformation over time, are needed to be taken into
account even the loaded stress is below their yieldstrength. Because steels present remarkable creep
deformation at approximately 500 ˚C or higher, it
is crucial to develop materials with high creep
strength to address increasingly high temperaturesat plants. For high- and intermediate-pressure
turbine rotors, a 100,000-hour creep rupture
strength is required to be 100 MPa or greater. The
100,000-hour creep rupture strength of eachmaterial is shown in Figure 2(22). The creep
strengths of Advanced and new 12Cr steel andtheir derivatives are shown by the band in the
figure. 1Cr-1Mo-0.25V steel (referred to “1CrMoV”
below) has been widely used at 566 ˚C or below.
Both 10CrMoVNbN steel developed in the UnitedStates and 10CrMoVTaN steel developed inDevelopment of High- and Intermediate-Pressure Steam Turbine Rotors for Efficient Fossil Power Generation Technology

Japan(23), which have higher creep strengths than
1CrMoV steel, were used mainly for 566 ˚Cintermediate-pressure turbine rotors. However,
because a 100,000-hour creep rupture strength of
100 MPa can be sustained at approximately 570
˚C, a new high-strength ferritic heat-resistant steel
for USC that can address 593 ˚C had to be
developed.

Figure 3 USC/A-USC technological development projects.

Figure 2 100,000-hour creep rupture strength of materials for turbine
rotors (22).

Figure 3 shows the trends of USC technologicaldevelopment. In Japan, technological developmentof USC fossil power plants was conducted byElectric Power Development Co., Ltd (EPDC). andheavy electrical equipment manufacturers during
the period 1980-2001. Compared to conventional
thermal conditions of 24.1 MPa, 538/566 ˚C, thegoals of Phase 1 in 1980-1994, were 31.4 MPa,593/593/593 ˚C (Step 1), which were possible byextending conventional technologies (ferritic steel),and 34.3 MPa, 649/593/593 ˚C (Step 2), which
requires new materials and technologies (austenitic
steel). The goal of Phase II in 1994-2001 was set to30.0 MPa, 630/630 ˚C using ferritic steel. The
power generation efficiency of the 1000 MW model
was estimated to be 44.2%, 44.9%, 44.16%, each,against the conventional 42.1%(24).
Based on intensive research by Fujita and otherson 9-12Cr steel, some kinds of 600 ˚C-class
Advanced 12Cr steels including TOS107(25),
HR1100(26), TMK1(27), and TMK2(28) have beendeveloped in the 1980s. As shown in Table 1, thefeature of these steels is the compositions withdifferent Mo/W balances and similar Mo equivalent(defined as [mass%Mo] + 0.5[mass%W]) of 1.5(29).
Compared to conventional 12Cr steel, toughness
and microstructural stability have been improvedby reducing C and increasing Ni. Through the trial
production of an Advanced 12Cr steel rotor
supposed to be used as an intermediate-pressure
rotor of a 1000 MW model, its manufacturability,
soundness of internal properties, and physical andTable 1 Chemical compositions of high Cr ferritic steel for USC turbine rotor.

17Development of High- and Intermediate-Pressure Steam Turbine Rotors for Efficient Fossil Power Generation Technology

mechanicalproperties
wereverified(30).
Furthermore, at the aforementioned Step 1, arotating test was conducted using an Advanced
12Cr steel rotor at 593 ˚C and a 50 MW demo test
was conducted at EPDC Wakamatsu work(31),followed by a series of onsite applications at 600
˚C-class plants.
Because ferritic steel is not strong enough at 649
˚C, A286 alloy (Fe-15Cr-26Ni-1.5MoVAlTiB),
austenitic iron-base superalloy, became a candidate
material. To make the alloy suitable for large
forged components, chemical components weremodified; for example, C content was lowered tosuppress the precipitation of coarse carbides andTi content was also reduced to improve segregation
without degrading creep strength, notch sensitivity
and ductility(32). An approximately 1.6 tons turbine
rotor of Discaloy (Fe-13Cr-25Ni-3MoAlTi) wasused at Eddystone in the United States on thepremise of base load operation. In Step 2, a
5000-hours rotating test with 151 times of startand-stop operation, which simulates daily startand stop (DSS) at power stations, was conductedusing an A286 rotor of approximately 12.5 tons at
649 ˚C(24). Some issues concerning austenitic steel
such as an operational limitations owing to thermal
fatigue caused by low thermal conductivity andlarge thermal expansion coefficient were clarified.
There also found an economic issues due to highmaterial costs. Figure 4 shows an A286 turbine
rotor manufactured by JSW using a 20-ton steel
ingot prepared by electroslag remelting process(ESR). The maximum diameter of the turbine rotor
was 892 mm, and its weight was 7.54 tons.

TOS110(33), HR1200(34), and MTR10A(35) are kinds
of New 12Cr steels for 630 ˚C. Performances of
TOS110 and HR1200 toward practical application
were evaluated in Phase II through anapproximately 500-hours rotating test at 650 ˚C,as well as JSW’s in-company test about
manufacturability. Containing Co and B forsuppressing the generation of δ-ferrite, andimproving creep strength, respectively, is thenoteworthy feature of these steels. They also
contain more W compared to the Advanced 12Cr
steel and reduced amounts of Ni and Mn. Figure 5
is an exterior view of the HR1200 trial turbine
rotor made from a ESR ingot of approximately 80
tons. Figure 6 is an exterior view of a TOS110
turbine rotor with an overlay welding on journals.
Overlay welding on journals of a high Cr steel
turbine rotor is necessary to prevent the adhesive
wearing with bearings, even the B containing
steels with poor weldability(33).

Figure 5 HR1200 trial turbine rotor.
(Body diameter: 1,215 mm, Body length: 3,366 mm, Total length: 8,064 mm)

Figure 6 TOS110 trial turbine rotor.
(Body diameter: 1,296 mm, Body length: 3,040 mm, Total length: 7,734 mm,
47.1t)

Figure 4 A286 turbine rotor.
(Body diameter: 892 mm, Total length: 6,044 mm, 7.54t)

In Phase II, with emphasis on economic efficiency
and operability compared to Phase I, Step 2, whichused austenitic steel, research was conductedaiming for the early realization of a 30 MPa, 630 ˚C
USC plant that uses high-strength ferritic steel.

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JSW TECHNICAL REVIEW No.20

In Europe, with the goal of 29.4 MPa, 600 ˚C/620
˚C, “COST501”—a COST program (COST: Cooperation in the field of Science and Technology)
was implemented in 1986-1997, resulting in thedevelopment of 600 ˚C-class steel rotor E(36)containing 1%Mo and 1%W, and rotor F(36)containing 1.5%Mo. These compositions areequivalent to those of TOS107 and TMK1.
Moreover, FB2(37) containing Co and B wasDevelopment of High- and Intermediate-Pressure Steam Turbine Rotors for Efficient Fossil Power Generation Technology

developed to address 620 ˚C. In the project of
COST522 which was held in 1998-2003, the target
was 29.4 MPa, 620 ˚C/650 ˚C. In COST536 which
was held in 2004-2009, materials development andlong-term creep tests were conducted aiming forthe goal of 29.4 MPa, 630-650 ˚C. As a result of
these developments, it became possible to raise thesteam temperature range at European plants from
530-565 ˚C to 580-620 ˚C(38). Pursuing further
improvements in heat resistance capability,
development of high Cr steels is continuing inJapan and overseas(39). Figure 7 is an exterior viewof a COST E intermediate-pressure rotor (1,210
mm in diameter) made from vacuum carbon
deoxidation process (VCD) and Figure 8 shows a
COST FB2 intermediate-pressure rotor (1,365 mm
in diameter) made from ESR. Both products were
the first models we manufactured. The COST E
rotor was made from a 113-ton VCD steel ingot
and the COST FB2 rotor was made from a 102-ton
ESR steel ingot. Although the heating time of theserotors during heat treatments was inevitably long
due to the large body diameters, and then, grains

Figure 7 COST E turbine rotor (first model).
(Body diameter: 1,210 mm, Body length: 3,895 mm, Total length:6,242 mm, 37.9t)

Figure 8 COST FB2 turbine rotor.
(Body diameter: 1,365 mm, Body length: 3,981 mm, Total length: 9,301 mm,
45.95t)Table 2 Number of 12Cr rotors shipped.

are easy to coarsen by high quenching temperaturesof 1090-1100 ˚C, controlling the grain growth at
the time of manufacturing made the permeabilityof ultrasonic waves sufficiently good. Table 2 shows
the number of 12Cr rotors we have shipped. The
first shipments of a conventional 12Cr steel rotor,
an Advanced 12Cr steel rotor, and a New 12Cr
steel rotor were in 1971, 1989, and 1998,
respectively.
The total number of such rotors shipped by theend of January 2018 was 309 excluding trial
turbine rotors. All of them were for actual plants.

4. Development of turbine rotor
forgings for A-USC in JSW
4.1 Trend of A-USC in Japan
A-USC power generation has been paid muchattention as a promising technology of efficient
power generation. In Japan, the national project of
the study of element technologies for thedevelopment of A-USC power plant has launchedin 2008 at the initiative of Agency for National
Resources and Energy(40). JSW have joined the
project and developed the nickel-base superalloy
turbine rotor, collaborating with heavy electricmachinery companies in Japan(41-42). One of our
example of the manufacturing A-USC turbine
rotor is shown as below.
4.2 Manufacturing the A-USC turbine rotor of
FENIX-700
FENIX-700 (Ni-36Fe-16Cr-1.3Al-2Nb, in mass%)
is a modified alloy of Alloy706 that is widely used
as a material for high temperature components
such as gas turbine disks. Compared to Alloy706,
FENIX-700 contains less niobium and a large
amount of aluminum, and the stability of the
microstructure at high temperatures andmanufacturabilityoflarge
forgingsare(43).
ameliorated
So far, we have succeeded in
producing a FENIX-700 large ESR ingot of 1,050
mm in diameter without macrosegregation for the
first time in the world, and examined the properties
concerning the manufacturability of large forgings
in detail(44). For example, the lower limit of theforging temperature of FENIX-700 is lower thanthat of Alloy617, which is the candidate alloy
mainly studied in Europe, indicating that
FENIX-700 is advantageous to forge. Moreover,the behavior of grain growth and the effects of themass effect, which is the large difference in thecooling rate between the surface and center of the19Development of High- and Intermediate-Pressure Steam Turbine Rotors for Efficient Fossil Power Generation Technology

turbine rotor, are clarified(45).
The ingots were prepared using the doublemelting process consisting of vacuum induction
melting (VIM) and ESR. The ingots were forgedusing a 14,000-ton free hydraulic press to destroy
the solidification structure and homogenize themicrostructure of the forgings. Then, the forgings
were annealed at 980 ˚C, followed by aging at 840
˚C and 730 ˚C. The forgings after heat treatment
were underwent machining and ultrasonicinspection. Two trial rotors were produced in thepresent work. The ingots for the first and second
trial rotors were 1,050 mm in diameter, 2,750 mm
in length, and 19 tons in weight, and 1,050 mm indiameter, 2,380 mm in length, and 17 tons inweight, respectively. The melting process wascompleted normally. To assess the effects of theforging conditions on the microstructure andmechanical properties, we set the temperature of
the final forging process for the second trial rotor
50 ˚C lower than that of the first one, and increased
the forging ratio by approximately 60%. In addition,
we increased the forging ratio by approximately
50% at the center of the second trial rotor comparedto the first one. As shown in Figure 9, the 10-tonclass turbine rotor forgings of FENIX-700 was
successfully manufactured.

of the first one, especially, the grains in center at
the position of Xn = 0.5 were remarkably refined.
As mentioned above, the temperature of the final
forging process was lowered and the forging ratio
was increased, especially at the center, in thesecond trial. In the manufacturing process of thenickel or nickel-iron base superalloy, the strainenergy accumulated during the final process drives
the grain growth in the following solution heat
treatment. Therefore, the downstream grain sizeand strain of the forgings play very important roles
in controlling the final grain size of the rotor. The
grain refinement of the second trial rotor is
attributed to the large strain and suppression of
the grain growth during the final process. Because
the surface is more prone to strain than the center,
the grain at the surface is considered to be morerefined.

Figure 10 Distribution of grain size number of FENIX-700 turbine rotors.

Figure 9 FENIX-700 1st trial forging.

Figure 10 shows the distribution of grain size
number of two trial rotors. Because the lengths ofthe trial rotors are not equal, the specimen
positions are described by Xn, which is the distancefrom the bottom side normalized by the rotor
length. The values of Xn of the bottom, middle, andtop parts of the first and second trial rotors arealmost equal, meaning that the relative position of
the specimen can be regarded as the same. The
grains of the second trial rotor are finer than that

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JSW TECHNICAL REVIEW No.20

The effect of grain refinement was also
recognized in the difference of permeability ofultrasonic wave at the ultrasonic examination.
Minimum detectable flaw size (MDFS) of thesecond trial forgings was 1.6 to 1.9 mm, which was
obviously smaller than those of the first one 3.7 to
4.8 mm, indicating that the permeability of theultrasonic wave in the second trial rotor was better
than that of the first one. In general, the ultrasonicwave is strongly attenuated by the large particles.
Thus, the excellent permeability of the second trial
rotor is realized by the grain refinement, improving
the reliability of the ultrasonic inspection.
Furthermore, toughness and ductility are also
confirmed to be improved by grain refinement, andestimated creep strength at 700 ˚C, 105 hours
exceeds 100 MPa regardless of fine grain(41). This
result is highly appreciated by domestic andoverseas customers who are interested in theDevelopment of High- and Intermediate-Pressure Steam Turbine Rotors for Efficient Fossil Power Generation Technology

efficient power generation technologies, and theknowledges obtained in this trial production has
stimulated us to develop the nickel-base superalloy
large turbine rotors with higher quality for thecommercialization of A-USC in the future.

5. Conclusions
JSW has been engaged in the practicalapplication of new materials mainly through thedevelopment of materials and manufacturing
technologies and the trials of prototype turbine
rotor productions. Consequently, since our first
shipment of a high Cr steel rotor in 1971, we havedelivered more than 300 turbine rotors to actual
power plants. JSW also successfully developed a
Ni-Fe base superalloy FENIX-700 turbine rotor,
which is compatible with 700 ˚C-class A-USC. As
high Cr ferritic heat resistant steel rotors havecontributed significantly to the higher efficiency offossil power plants and reduction of emissions ofgreenhouse effect gases, superalloy rotors currently
under development are also expected to contributeto the further progress of high-efficiency fossil
power plants.
Acknowledgement
A part of the present work was sponsored by
Ministry of Economy, Trade and Industry of Japan.

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