Fundamental Materials Technologies for Supporting Highly-Reliable Power-Generation Plants

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1 Hitachi Review Vol. 47 (1998), No Fundamental Materials Technologies for Supporting Highly-Reliable Power-Generation Plants Masateru Suwa Hideyo Kodama Takao Iwayanagi Abstract: Finding a best-mix energy policy is a fundamental issue in meeting the increasing demand for electrical power in Japan. New fossil-fuel thermal power plants are needed that have higher efficiency, are smaller, and emit less pollutants. One approach is to use combined-cycle power-generation systems. Several promising processes for these types of systems are being investigated, including integrated coal gasification combined cycle (IGCC) and pressurized fluidized bed combustion (PFBC). The most important issue for light-water nuclear reactors is increasing plant safety, in addition to improving plant availability, developing lifetime estimation techniques, and improving the reliability of the structural materials. Developing fusionenergy reactors is also underway. Hitachi has been researching and developing technologies that are aimed at achieving higher efficiency, higher reliability, lower cost, and lower emissions. Engineers and researchers are working on developing new materials for power plants that provide better insulation and have better anti-corrosion and joining properties, achieved by molecular or atomic-level structural control and high-quality analytical instrument utilization. INTRODUCTION THE rapidly increasing demand for electrical power world-wide, particularly in Asia, necessitates the construction of new power plants with high reliability and acceptable environmental effects. Development of new materials is necessary to support such highly reliable power plants. To answer these needs, Hitachi is promoting the development of heat-resistant and radiation-resistant materials, along with improving fundamental materials technologies. In this paper we discuss the current state of our development work related to fundamental materials technologies. MATERIALS FOR THERMAL POWER PLANTS Steam Turbine Materials Ultra super critical (USC) steam turbine rotor materials Higher efficiencies can be achieved by increasing the steam temperature, which can also result in lower emissions. By optimizing its chemical composition and Fig. 1 Intermediate-Pressure Turbine Rotor for 1,000-MWe Class Steam Turbine Operated at 600 C (diameter: 1,230 mm, length: 7,680 mm). The rotor was machined from 80-ton-class electro-slag remelting ingot.

2 226 Fundamental Materials Technologies for Supporting Highly-Reliable Power-Generation Plants CrMoWVNb steel Creep rupture stress (MPa) CrMoV steel Fig. 2 Estimated Creep Rupture Strength of 12Cr Steels After 100,000 Hours of Exposure. The creep rupture strength was increased by finely dispersing NbC precipitates Applied temperature ( C) by stabilizing the carbide phases in its microstructure, 12CrMoWVNb steel has been made applicable to turbine rotors (Fig. 1), under steam conditions of 24.5 OM (600 C/600 C). This ferritic steel was used to construct the turbine rotor installed in the No. 2 unit of Tohoku Electric Power Co., Inc. s Haramachi thermal power station commissioned in Another promising material for operation under high steam temperatures (+650 C) is 12CrWCoMoVNbB steel. Increasing the creep rupture strength of this steel has been the main focus in making it practical for turbine rotors. One approach has been to add Co and B to 12CrMoWVNb steel while increasing the W content, resulting in a lower Mo/W ratio 1). High-pressure/low-pressure monolithic rotor material A smaller turbine size can reduce plant cost by reducing the amount of materials used for the parts and the construction area necessary for the plant. Unification of the high-pressure (HP) and low-pressure (LP) turbine rotors results in a single casing. Furthermore, a longer blade at the final stage of an LP turbine rotor can increase its efficiency. Hitachi has developed an HP/LP monolithic rotor with the hightemperature strength required for the HP side and the low temperature toughness necessary for the LP side. Compared to a conventional CrMoV steel rotor, a large monolithic cylindrical rotor (1,700 mm in diameter and 8,800 mm long) has excellent mechanical properties, including a 1.2-times higher creep rupture strength for 100,000 hours and 10-times higher impact absorbed energy. 43-inch-long blade in final stage of LP rotor Adding Nb to conventional 12Cr steel increases its tensile strength from 1,128 to 1,275 MPa, enabling the blade to be lengthened to 43 inches (1,092 mm). Gas Turbine Materials Temperatures at gas-turbine combustion outlets have now reached the 1,400 C-level, resulting in a combined efficiency of 48%. It is expected that they will soon reach the 1,500 C-level, producing a total efficiency as high as 54%. The primary development issues for gas turbines are related to the fast-rotating parts, i.e., the blades and the supporting disc, particularly the thermal barrier coating (TBC) applied to the blades. Single-crystal growth technology Strength at high-tem peratures can be improved by using single-crystal materials because the lack of grain boundaries reduces the degradation in creep strength. Hitachi has developed a large-scale investment casting technology for manufacturing single-crystal turbine blades longer than 200 mm with fine and complex cooling passages. Disc materials Adding Nb to a super-clean steel with lower Si and Mn contents has resulted in higher strength and toughness (Fig. 2). Niobium promotes a finer grain size, which suppresses the embrittlement of superclean steel that occurs with aging. Thermal barrier coating Hitachi has developed a four-layer TBC for the

3 Hitachi Review Vol. 47 (1998), No Wear loss (mg/km) Water temperature: 288 C Water pressure: 8.3 MPa Applied load: 98 N Sliding speed: *Normal speed (m/s) **High speed Roller Pin * 1.9** 0.01* 1.9** Co base alloy Ni base developed alloy Fig. 3 Wear Loss of Developed Alloy (Co Free) in Hot Water. High wear resistance was achieved by structurally controlling the carbide dispersion and matrix configuration. combustor, nozzles, and blades. The nozzles and blades are exposed to a corrosive environment, and this coating relaxes the thermal stresses while functioning as a corrosion shield and a thermal barrier. It has twice the durability of a conventional two-layer TBC, plus an about 90 C-higher temperature difference across its thickness. MATERIALS FOR NUCLEAR REACTORS Light-Water Reactor Materials Using materials with better corrosion and wear resistance will lead to higher reliability and longer life for the core structure. Hitachi has developed new technologies to meet these requirements. Corrosion-resistant materials for internal components Austenitic stainless steels are highly sensitive to stress corrosion, exhibiting cracking along their grain boundaries due to neutron irradiation during long-time operation of the light-water reactors. This phenomenon is called irradiation-assisted stress corrosion cracking (IASCC). One possible cause of this cracking is the existence of impurities, like P and N, along the grain boundaries. Hitachi has determined that reducing the P and N contents improves the cracking resistance. Irradiation experiments conducted at a commercial reactor in the United States showed that the cracking resistance of 316L stainless steel with low P and N contents is remarkably better than that of conventional 304 stainless steel 2). Potential applications and the basic engineering properties of these low P and N content steels are currently being investigated as a Joint Study of Japan BWR Owners Group. Hitachi is also working on developing IASCC-free materials for future applications. The basic idea is to prevent the depletion of Cr from the grain boundaries, which is caused by neutron irradiation. One approach is to add Zr for trapping the vacancies, which couple with the Cr atoms and stimulate their depletion from the grain boundaries. The other approach is to add Mn, which has a larger atomic size than Cr. An appropriate arrangement of the Mn and Cr atoms can suppress the depletion of Cr from the grain boundaries. Electron irradiation experiments 3 5) conducted at 400 C in a high-voltage electron microscope (1 MeV) have confirmed the positive effects of adding both Zr and Mn on Cr depletion. Corrosion-resistant materials for fuel assemblies Due to the high burn-up of fuel in light-water reactors, the Zr alloy Zircalloy is used extensively as a cladding material for the fuel tubing because it does not suffer from the local oxidation process called nodular corrosion. Hitachi has developed a new alloy with added Fe and Ni that suppresses this corrosion by decreasing the anion-defect concentration 6). Wear-resistant materials for control-rod guiding parts The guiding parts of the control rods are usually made of Co base alloys because of their good wearresistant properties. Hitachi is developing Co-free Ni and Fe base alloys with better wear-resistance, which is achieved by dispersing carbide to harden the alloy (Fig. 3).

4 228 Fundamental Materials Technologies for Supporting Highly-Reliable Power-Generation Plants Optical fiber Probe Motor Fig. 4 Examination of Thermal Degradation. The expected life-time of the elevator motor was diagnosed by assembling a compact instrument (2 kg) with a large-diameter (1.5 mm) optical fiber. Fusion Reactor Materials The most important objective for fusion reactors is developing heat-resistant materials for the divertor, which experiences significant thermal loads. The Japan Atomic Energy Research Institute has been investigating a water-cooling system fabricated by joining carbon-carbon fibers (C-C material) to a Cu heat-sink. Hitachi has delivered a 1.5-m-long curvedsurface divertor module for testing. It is designed to withstand a heat load as high as 20 MW/m 2. This structure is characterized by its highly reliable metallic bonding of the long heat-sink material, which has a complex structure, with the C-C material. Further research is continuing on developing a new heat-sink material as a substitute for the oxygen-free highly conductive Cu material. FUNDAMENTAL MATERIALS TECHNOLOGIES Insulation Technologies Insulating materials are important for achieving reliable and safe electrical devices. Hitachi has been working on developing insulating materials with higher quality and lower manufacturing cost. When a highly reliable insulating layer is exposed to an elevated temperature environment, it may undergo thermal Object Corrosion suppresion effect by prefilming (compared with an untreated specimen) Nuclear power plant tube (stainless steel) 5-times suppresion (i.e., amount of corrosion was 1/5.) Generator coil (copper) 20-times suppression Fig. 5 Proof of High Corrosion Resistance by Prefilming Technology. High corrosion resistance compared to crude materials was achieved by using prefilming technology. Magnetic head (permalloy) Absorption chiller (carbon steel) 10-times suppression 100-times suppression

5 Hitachi Review Vol. 47 (1998), No degradation. Hitachi has developed a simple nondestructive technique for estimating the lifetime of insulating materials (Fig. 4). The thermal degradation of an insulating material is related to the absorbance ratio measured for two different radiations with nearinfrared wavelengths. Comparison of the measured absorbance ratio with the diagnostic master curve yields the remaining lifetime of the material. Wider application of this technology is currently under consideration. Corrosion-Resistance Technologies Achieving lower costs and maintenance-free performance for manufactured products requires highly reliable materials with better corrosion resistance. For this purpose, Hitachi has developed a surface-finishing inhibitor treatment and cathodic protection technologies. One of these technologies is called prefilming, in which thin oxide layers are formed on the surface of materials. This method has been applied to the structural materials used in large nuclear plants, to the pipes of absorption-type refrigeration equipment, and to small magnetic thin-film heads (Fig. 5). Promising anti-corrosion behavior has been obtained for these commercial products. Based on these concepts, Hitachi has developed an artificial passive-film treatment technique for manufacturing self-healing anti-corrosion films. It is based on controlling the ionic movement. Joining Technologies Hitachi has developed highly reliable technologies for joining electrical devices. Laser-beam welding uses a thermal source with a high energy density that enables deep penetration into the butt-welded zone with only a small deflection. This welding can be applied to very difficult and dissimilar welding processes, such as to the butt-welding of austenitic stainless steel and carbon steel for gas-insulated switchgear and to aluminum alloys for large structures. CONCLUSIONS By utilizing high-technology-based techniques and processes, such as the control of microstructures on the nano-mesoscopic level, Hitachi is continuing its efforts to develop materials with exceptional properties that can support a best-mix energy policy. We acknowledge the valuable assistance of the domestic electrical power companies and other materials users in the preparation of this report. REFERENCES (1) R. Kaneko et al., Development of Steam Turbine Materials at Targeted Temperature 650 C, The Thermal and Nuclear Power 46, No. 9, (1995-9), pp (2) S. Kasahara et al., Proceedings of the Sixth International Symposium on Environmental Degradation of the Materials in Nuclear Power Systems Water Reactors, p. 615, August 1-5 (1993), San Diego, CA, USA. (3) T. Kato et al., J. Nucl. Mater., 189 (1992) 167. (4) T. Kato et al., Mater. Trans. JIM, 32 (1991) 921 (5) T. Kato et al., J. Atom. Enr. Soc. of Japan, 39, No. 9 (1992) 889. (6) M. Inagaki et al., Effect of Alloying Elements in Zircaloy on Photo-Electrochemical Characteristics of Zirconium Oxide Films, ASTM STP 1132, (1991), pp ABOUT THE AUTHORS Masateru Suwa, D.Eng. Joined Hitachi, Ltd. in Currently working in Hitachi s Research Laboratory, researching materials. Member of the Japan Institute of Metals, Japan Foundry Engineering Society, the Surface Finishing Society of Japan, Japanese Society of Tribologists, and the Japan Society for Heat Treatment. msuwa@hrl.hitachi.co.jp Hideyo Kodama, D.Eng. Joined Hitachi, Ltd. in Currently working in Hitachi s Research Laboratory, researching materials. Member of the Japan Institute of Metals, the Iron and Steel Institute of Japan, and Japan Foundry Engineering Society. hkodama@hrl.hitachi.co.jp Takao Iwayanagi, D.Eng. Joined Hitachi, Ltd. in Currently working in Hitachi s Advanced Research Laboratory, researching materials. Member of American Chemical Society, Chemical Society of Japan, Polymer Society of Japan, and Japanese Society of Applied Physics. takao@harl.hitachi.co.jp