Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)10Development of 700°C Class Steam Turbine Technology

EIJI SAITO*1

SHIN NISHIMOTO*2

HIROYUKI ENDO*3

RYUICHI YAMAMOTO*4

KENJI KAWASAKI*5

JUN SATO*6

Mitsubishi Hitachi Power Systems, Ltd. (MHPS) has carried out the development of the700°C class A-USC steam turbine through the auspices of a Japanese national project since 2008.
We have developed a mainly Ni-based material with a high-temperature creep strength exceeding
100 MPa at 700°C for 100,000 hours with the successful production of a large-scale forged rotor
weighing more than 10 tons, as well as fabrication technology for a dissimilar-material welded
rotor using the Ni-based material and high Cr steel. We then conducted a full-scale, long-term
rotational turbine test to verify these development technologies under test conditions where thesteam turbine with a dissimilar-material welded rotor was utilized at a temperature higher than700°C at 3600 rpm. This paper describes these development technologies.

|1. Introduction
Since the production of Japan's first land steam turbine (500 kW) in 1908, MHPS's steamturbines have accumulated over 360 GW of output to date. The main reason why steam turbines
continue to be widely used even today is that steam turbines have a proven track record ofsupplying stable electric power successfully for many years. Other important factors include thefact that the steam conditions have improved and that steam turbines have continued to evolve andrespond to electric power energy demand by rapidly adopting the latest analysis technologies intheir design and applying up-to-date technologies for higher efficiency and reliability to actual
products. In the face of environmental concerns, there has been further increasing demand for theexpansion of the output capacity of single turbines and improvement in efficiency in recent years.
Figure 1 lists the historical changes in the steam conditions of coal-fired thermal powerplants. The steam conditions of coal-fired thermal power plants have changed from subcriticalpressure, through super critical pressure (SC), and to 600°C class ultra-super critical pressure(USC) power generation. These steam conditions, which were best suited for the steam turbines ofeach era, contributed to the improvement of the efficiency of the entire plant together with theadoption of technologies for the improvement of efficiency in steam turbines that have evolved
according to the times. As a continuation of this improvement of steam conditions, the need for thepractical realization of 700°C class A-USC (Advanced Ultra Super Critical), the temperature ofwhich was further increased by 100°C in comparison with 600°C class USC, has increased
globally(1). If A-USC is realized, the turbine efficiency will exceed 50% and a plant efficiency of46%HHV (higher heating value) or more can be expected. This efficiency improvement, whenconverted into fuel cost, can save about 1.6 to 2 billion yen annually in comparison with USC(estimated from the coal price as of January 2017: 60 to 80 dollars per ton(2)), and also contributes
to a CO2 reduction of 25% to 27% in comparison with the global average. In addition, A-USC has
the same system configuration as existing coal-fired thermal power plants, so it can be easily used
as a replacement for aging thermal power plants. In Japan, a national project for the practicalrealization of A-USC has been carried out as a subsidized effort of the Ministry of Economy, Tradeand Industry since 2008. The development of A-USC is described below.
*1 Chief Staff Manager, Steam Turbine Engineering Department, Steam Turbine Technology & Production Integration Division, Turbomachinery
Headquarters, Mitsubishi Hitachi Power Systems, Ltd.
*2 Chief Staff Manager, Steam Turbine Engineering Department, Steam Turbine Technology & Production Integration Division, Turbomachinery
Headquarters, Mitsubishi Hitachi Power Systems, Ltd.
*3 Steam Turbine Engineering Department, Steam Turbine Technology & Production Integration Division, Turbomachinery Headquarters,
Mitsubishi Hitachi Power Systems, Ltd.
*4 Manager, Materials Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.
*5 Manufacturing Technology Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.
*6 Chief Staff Manager, Thermal Power Systems Research Department, Research & Development Center, Mitsubishi Hitachi Power Systems, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)
11Figure 1History of changes in steam conditions|2. Issues and schedule of development
The allowable application limit of advanced 12 Cr steel that is currently under consideration
for commercialization is up to the 630°C level, and the attainment of even higher steam
temperatures requires the use of Ni-based alloy. However, there is an issue with Ni-based alloy
characteristics derived from manufacturing in that the structure is highly sensitive to temperaturechange. Therefore, when a larger material is manufactured, a temperature gradient tends to occur
between the surface and the inside, generating the problem of segregation, which is the unevenconcentration and distribution of component elements. Even if a high strength characteristic isgained with a test piece or small amount of material, it is difficult to attain the strength in a large
amount of material that satisfies the objective set by the designer. For this reason, the developmentof a several-ton class Ni-based alloy material that can be used for steam turbine rotors and theverification of its long-term reliability are the most important issues for the development ofA-USC. In addition, it is necessary to design components so that the use of expensive Ni-based
alloy is limited. In particular, it is difficult to manufacture a large forged rotor, and therefore themanufacturing of a Ni-based alloy welded rotor is one of the important development issues.
Figure 2 provides the master schedule of the development(3). Between 2008 and 2012,
materials and element technologies related to boilers, turbines and valves were developed. In orderto verify the reliability of the development of these element technologies, actual boiler tests andturbine rotation tests were conducted between 2013 and 2016 for evaluation.

Figure 2 Master Schedule of the A-USC National Project in Japan

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)12|3. Development of new Ni-based alloy
Table 1 shows the Ni-based alloys that we developed. It is estimated that these materials willattain a high-temperature creep strength of more than 100 MPa for 100,000 hours, which wastargeted in the national project development. FENIX700(4) and LTES700R(5) were used toprototype a 10-ton class large forged rotor successfully. In addition, these materials used for theprototype of a roughly φ1,000 large forged rotor resulted in the nondestructive inspection detection
dimension of about 2 mm or less. In addition, USC141(6) and USC800(7) have excellent
high-temperature strength and superior workability, and they are expected to be applied in not only
blades and bolts, but also boiler piping, etc. The long-term creep strength of each of these materials
will actually be tested for more than 100,000 hours continuously, so the long-term reliability isunder verification. In addition, the material and mechanical characteristics required for turbine
design were obtained and reflected in the turbine rotor design for rotation tests.
Table 1 New Ni-based SuperalloysMaterial name
FENIX700

LTES700R

USC141

USC800C
NiCrMoCo
WNbAlTi
Fe0.014216―――21.3
1.7Bal.
Iron-nickel-based alloy with a material price that is about two-thirds of that of typical Ni-based alloys.
Excellent manufacturability for large forged products. Candidate material for the rotor.
0.03
Bal.
126.2
―
7―
1.65
0.65
―
Ni-based alloy with a linear expansion coefficient that is suppressed to approximately the same levelas high Cr steel. Excellent weldability. Candidate material for the rotor.
0.03
Bal.
2010―
―
―
1.2
1.6
―
High-temperature creep strength of about 180 MPa for 100,000 hours at 700°C class. Candidate
material for the turbine blade, bolt, and boiler heat transfer tube.
0.04
Bal.
176232―
4――
High-strength Ni-based alloy with excellent hot forgeability. High-temperature creep strength of about
270 MPa for 100,000 hours at 700°C. Candidate material for the turbine blade, bolt, boiler heat transfer
tube, and large piping.

|4. Development of manufacturing technology
4.1 Prototype of welded rotor
Because of the manufacturing limitations of Ni-based alloy forged rotors and to reduce thecost, technology for manufacturing dissimilar-material welded rotors using Ni-based alloy and Crsteel is one the key technologies for realizing A-USC. Figure 3 is an example of a welded rotor.
MHPS has already manufactured welded rotors made of high Cr steel or similar materials andextended the manufacturing technologies to Ni-based alloy. The welding method used was proven
TIG welding. We welded an actual-sized mockup, verified the performance of coupling (structure,mechanical characteristics, etc.), and thereafter made a welded rotor for a rotation test.
On the other hand, nondestructive inspection is critical technology to perform reliability
inspection of the manufactured welded rotor. The supersonic wave permeability of the Ni-based
alloy is inferior to steel. However, the grain size of LTES700R is fine, and its MDDS (Minimum
Detectable Defect Size) is 2 or smaller. In addition, we evaluated the supersonic wave permeation
property of the weld and developed a sensor which reduced the scattered wave noise of the weldingborder. Figure 3 indicates the nondestructive inspection results of the weld. No problems werefound in the nondestructive inspection of the materials and welds and their soundness was verified.

4.2 Manufacturing
Turbine blade and rotor blade groove structures are complicated and their working tolerancesin assembly and manufacturing are severe. To keep working tolerances similar to the conventional
levels in cutting difficult-to-cut materials such as Ni-based alloy, it is necessary to improve themanufacturing technology. Table 2 shows the cutting of a forged blade as an example of theturbine manufacturing process. The Ni-based alloy is a difficult-to-cut material with hard andviscous properties, and since the heat conductivity is low and the cutting heat is concentrated in one
place while cutting, there is a problem that local tool damage occurs remarkably early in theprocess. At the early phase of manufacturing, we had difficulty in selecting tool tips and managing
cutting oil. Currently, these problems have been overcome and processing accuracy similar to thatof conventional manufacturing can be obtained.
We also planned a control stage blade that can realize partial load operation on the rotation
test rotor. In recent years, since renewable energy is often adopted, despite achieving performanceMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)13

improvements in A-USC, we thought that we should consider partial load operation as well. Figure 4
shows an example of the manufacture of a control stage blade. MHPS's control stage blade consistsof three integrated blades and the blade root has a fork structure, which is the most difficult andcomplicated structure in manufacturing. Therefore, the prototyping of this speed-adjustable bladestructure has greatly contributed to the improvement of manufacturing, processing, and assembling
technologies for difficult-to-cut materials.

Figure 3 Example of welded rotor application andnondestructive inspection

Figure 4
Manufacturing example
of control stage blade

Table 2 Example of forged blade cutting testNo.

#1

#2

#3

①A dent on the profile②Bite into contact partof groove and cover①Profile surface
becomes wavy shape①Profile end thickness
is inappropriate②Out of tolerance ofgrove dimension#4

Prototyping

Three-dimensional
measurementresult

Point at issue✓Acceptable level|5. High-temperature field turbine rotation test
We performed a high-temperature field rotation test to verify the reliability of the turbine
component. Figure 5 provides the details of the manufacture and verification of the rotor for thehigh-temperature field rotation test. This test was intended mainly to confirm the manufacturabilityof an actual-sized Ni-based alloy turbine and evaluate the remaining life of the high-temperaturefield rotation-tested components, in particular to verify the reliability of the welded joint part.
Based on the trial design of the double reheat steam turbine structure(3), MHPS adopted, for thestructure of the rotor used in the rotation test, a rotor that includes same-material welded joints of
LTES700R and the dissimilar-material welds of LTES700R and MTR10A and simulates thecontrol stage, IP sixth stage, and IP seventh stage where the shape of the blade has relative
difficulty in manufacturing (Figure 6).
Regarding control stage blades, as described above, it was confirmed that themanufacturability and assembly properties of the complicated blade structure were established withNi-based alloy. With regard to IP blades, the manufacturability of a forged blade was confirmed.
Dissimilar-material welded joints are provided between IP stages assuming adoption on actualequipment.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)
14

Figure 5 Rotation test verification items

Figure 6 Rotor design concept for rotation test

Figure 7 shows the turbine rotation test facility, the rotor structure and the design plan
structure. This facility features the capability of simulating a 700°C or higher thermal field using
heat radiated from a heater and a driving motor capable of long-term testing at the rated rotation of
3,600 rpm in a vacuum. During this test, the highest stress occurs at the turbine blade groove, but itwould not be exposed to a temperature as high as 700°C in actual operation. Therefore, this life
evaluation is an acceleration test against actual operation.
Initially, we planned a creep acceleration test of 100,000 hours by conducting a test for 1,500
hours maintaining the ambient temperature of the control stage blade at approximately 730°C.
However, because the joint temperature between the IP stage blades increased due to windage, theequipment and rotor were modified and finally the rotation test was carried out for 1,051 hours at arated speed of 3,600 rpm.
During the rotational test, we monitored shaft vibration, shaft expansion, heater panel
ambient temperature, and rotor temperature 24 hours a day. Figure 8 shows the photographs of thecontrol stage blades during the test and the test rotor. During the rotation test, the control stage anddissimilar-metal welded joint are kept under a 700°C or higher and a roughly 600°C thermal field,
respectively. Thus, the dissimilar-metal welded joint and the control stage blade achieved creep
damage corresponding to 160,000 hours and 10,000 hours, respectively, in this 1,051-hourMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)15

continuous rotation test. As a result of having performed visual inspection, nondestructiveinspection, and rotor deflection measurement, etc., after the rotation test for a comparison between
before and after the test, there were no particular problems and the soundness of the test turbine
rotor was confirmed. We disassembled the rotor to extract test specimens and are evaluating theremaining life.

Figure 7 Rotation test rotor and design plan structure

Figure 8

Test turbine rotor and control stage blade|6. Conclusion
A-USC turbine development has been going on for nine years since 2008 as a subsidized
project of the Ministry of Economy, Trade and Industry and New Energy and Industrial
Technology Development Organization (NEDO) and was completed as originally planned. Due tothe technology development, prospects for the manufacture of actual 700°C class steam turbines
have become clear. At the same time, in addition to the turbine, actual equipment testing of theboiler was also completed. In this actual equipment test, the components experienced 700°C for
13,000 hours. After extubation, test samples were extracted and reliability verification is being
carried out. Some creep tests of the materials and the welded joints of both the boiler and theturbine have exceeded 100,000 hours, and the long-term reliability verification is about to becompleted. We established the practical application of USC in Japan in the past, and in the samemanner, we would like to contribute widely to the world through the practical application ofA-USC.

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