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Development of high purity large forgings for nuclear power plants
Journal of Nuclear Materials 417 (2011) 854–859
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Development of high purity large forgings for nuclear power plants Yasuhiko Tanaka ⇑, Ikuo Sato The Japan Steel Works, Ltd., 1-11-1 Osaki, Shinagawa, Tokyo 141-0032, Japan
a r t i c l e
i n f o
Article history: Available online 1 January 2011
a b s t r a c t The recent increase in the size of energy plants has been supported by the development of manufacturing technology for high purity large forgings for the key components of the plant. To assure the reliability and performance of the large forgings, refining technology to make high purity steels, casting technology for gigantic ingots, forging technology to homogenize the material and consolidate porosity are essential, together with the required heat treatment and machining technologies. To meet these needs, the double degassing method to reduce impurities, multi-pouring methods to cast the gigantic ingots, vacuum carbon deoxidization, the warm forging process and related technologies have been developed and further improved. Furthermore, melting facilities including vacuum induction melting and electro slag re-melting furnaces have been installed. By using these technologies and equipment, large forgings have been manufactured and shipped to customers. These technologies have also been applied to the manufacture of austenitic steel vessel components of the fast breeder reactors and components for fusion experiments. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Growing world energy demand has led to the construction of higher performance and larger scale energy plants. Dramatic increases in plant performance have been achieved by increasing the size of the major system components. Fig. 1 shows the trend of maximum unit power generating capacity for nuclear and fossil power plants. The power generating capacity of nuclear power plants dramatically increased in the late 1960s and recently reached 1600 MW with the construction of the first European Pressurized Water Reactor (EPR). Large forgings allow the reactor pressure vessels to use integrated advanced designs with reduced numbers of components. The size of forgings for low-pressure steam turbines and for reactor pressure vessels (RPV) components is sure to increase further in the future. The serviceability of these components must be secured even through the forging size increased, because the operating conditions will be more severe. The properties required of component materials are homogeneity, free from harmful defects, high fracture toughness, resistance to ageing embrittlement and resistance to environmental damage, good inspectability and so on. These properties are achieved through state-of-the-art production technologies that include refining technology for high-purity materials, casting technology for very large ingots, forging technology for large components, heat treatment technology, and machining technology for larger and heavier components. This paper addresses the history and recent developments in manufacturing
⇑ Corresponding author. Tel.: +81 03 5745 2046; fax: +81 03 5745 2049. E-mail address: [email protected] (Y. Tanaka). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.305
technology for large forgings. In addition, the production methods for austenitic material is introduced.
2. Experience with large forgings for nuclear power plants in the Japan Steel Works The main reason for the evolution in the size of forging components, is a great demand for increasing in plant capacity. Another critical reason is the increase of safety and reliability through the integration of components. Fig. 2 shows examples of integration in low pressure (LP) steam turbines and in nuclear reactor pressure vessels. Stress corrosion cracking (SCC) sometimes occurs in the LP turbines of the nuclear power plants. Early large LP rotor components were made by shrink fitting of the shaft and disks [1]. The causes of SCC are considered to be high strength of the material used in shrink-on designs and crevice corrosion. Therefore, these shrunk fit type rotor forgins were replaced by a monoblock type using lower strength material. In the case of reactor pressure vessels, the integration of components minimizes the number of weld seams of pressure vessels by adopting the ring forgings and heads made from large ingots. The dimensions of rotor shaft forgings have increased significantly. The first monoblock LP rotor was manufactured in 1977, and since then monoblock forgings have been used in many fossil and nuclear power plants. In Japan, many shrunk-on type LP rotor forgings in nuclear power plants were replaced by the monoblock type in order to prevent a stress corrosion cracking failure. Recently, in USA, the LP turbines of aged nuclear power plant have been replaced by new ones. Fig. 3 shows a monoblock LP rotor forging. The Japan Steel Works (JSW) has shipped 223 monoblock
Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859
855
Fig. 3. Monoblock LP rotor forging made from a 600 ton ingot.
Fig. 1. History of unit power generating capacity for nuclear and fossil power plants.
LP rotor forgings as of 2008. Half speed four pole generators are another typical large forging for nuclear power plants. The first four pole generator, shipped in 1971 for the Biblis plant in Germany, was one of the first successful experiences in application of a 400 ton ingot, the largest in the world at that time. Since then, 162 four poles generators have been shipped from JSW as of 2008. As for nuclear pressure vessels, JSW has supplied the plates for the RPV in the Japan Power Demonstration Reactor (JPDR) of JAERI and RPV of Tokai #1 which was build in 1961 and the first commercial nuclear power plant in Japan, a Calder Hall type reactor. The first forged component was a flange ring manufactured and shipped in 1968 for the Tsuruga #1. The first shell ring forging was shipped in 1971 for the Biblis-B nuclear power plant in Germany. Ring forgings for core region components were first manufactured in 1974. Since then, a total 559 RPV forged components including shells, flanges and heads had been supplied as of 2008. The dimensions of these components have been further increased for the construction of advanced BWR (ABWR) plants. The first ABWR nuclear power plant in Japan is Kashiwazaki-Kariha
#6, #7 [2,3]. The first 600 ton ingot was used for an RPV component, the bottom petal. Recently, the nozzle shell for European Pressurized Water Reactor (EPR) was also made from a 600 ton ingot as is shown in Fig. 4. At the same time the integration of the components has also proceeded. The weld seams can be placed outside the core region by use of tall core region shells. The closure heads were also developed and have been used in PWRPV. Steam generator (SG) components including shells, primary heads and secondary heads have been developed and used for new and replacement plants [4]. High strength steel, SA508 Gr.3 Cl.2 was successfully used for components of SG. Another important development in forging technology is the use of austenitic stainless steel components for the fast breeder reactors (FBR). Components of the reactor vessel for ‘‘Monju’’, the prototype FBR in Japan, were manufactured using type 304 stainless steel. Significant development of production technologies were made to fabricate the austenitic steel components [5]. Using austenitic materials presented considerable problems that were solved in the manufacturing technologies. Furthermore, the forging of austenitic steels for fusion reactor (FR) components has been developed. The material and production technologies were established and the components were shipped for FR experiments.
Fig. 2. Monoblock rotor forging and integration of RPV components.
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Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859
Fig. 4. Nozzle shell for EPR made from a 600 ton ingot.
3. Development of manufacturing technology for forgings for large power generation plants The evolution of the large forging is supported by the development of many technologies in steelmaking and ingot making, forging, heat treatment, non-destructive examination, etc. Fig. 5 shows the history of the major manufacturing technology in JSW focusing on the steelmaking, forging and heat treatment [6–8]. 3.1. Refining and casting technologies Though not shown in the figure, in the early 1950s when the first rotor shaft forgings were manufactured by JSW, melting and refining were performed by basic and acid open hearth furnaces (OHF). The ability of the OHF to reduce the impurity content was quite limited, and the purity of steels produced at that time was not very good. Moreover, an inherent problem of the OHF was that the molten steel tended to dissolve a significant amount of hydrogen which could cause flaking of the steel. Vacuum casting using mechanical pumps was applied in the late 1950s as a solution to the hydrogen problem. The introduction of steam ejectors
in the late 1960s permitted reduction of the hydrogen content to less than 1 ppm. By the late 1960s the OHF was replaced by basic electric arc furnaces (EAF) and impurity contents were very effectively reduced through desulphurization and dephosphorization. By reducing the Si content, vacuum carbon deoxidizaton (VCD) proceeds was become applicable through the evolution of CO during the vacuum casting process [9]. The VCD process is also effective in reducing macro segregations. In the early 1980s, ladle refining furnaces (LRF) were installed. Fig. 6 shows the advanced refining process called double degassing used for large forgings. In this refining process, phosphorus is removed in the EAF using an oxidizing slag. After careful deslagging to avoid rephosphorization during the reladle process, the molten steel is desulphurized in LRF using a basic reducing slag. By this process sulfur and phosphorus can be reduced to less than 10 ppm and 40 ppm, respectively. Double degassing by the vacuum treatment in the ladle furnace aided by intensive argon stirring followed by vacuum stream degassing results in hydrogen contents of less than 0.5 ppm and low oxygen levels. Fig. 7 shows the historical decrease of P and S contents in castings. It should be mentioned that the residual elements such as As, Sn, Sb and Cu must be controlled by the selection of raw material, since these elements are difficult to remove from the molten steel in the refining process. The casting technology for large ingots is the key to producing the large components and was developed using the ladle refining furnace [1–3]. Fig. 8 shows the multi pouring process. After the melting and oxidizing refining in an electric furnace, the molten steel was poured into a ladle. Further refining is done in the electric furnace and the molten steel is kept at optimum conditions until the time of casting. Multiple ladles are prepared to keep all the needed molten steel at optimum conditions and then all are poured successively into an large ingot. Chemistry of the each ladle is carefully controlled and poured to minimize the segregation. Thus the capacity to cast ingots up to 600 tons by using the fully ladle refined melt method [6–8] has been available since 1987, Installation of facilities for casting 650 ton ingot is under way at JSW. Utilization of other secondary refining processes such as electro-slag-re-melting (ESR) contribute to the production of high-purity materials. Presently, steels denoted ‘‘superclean’’, in which impurity and gas elements are reduced as low as possible while Si and Mn are also held to low levels, can be manufactured
Fig. 5. History of production technology for forgings in the JSW Muroran plant.
Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859
857
Fig. 6. Double degassing process for high purity steels.
through these processes [10]. The material is confirmed to have no sensitivity to temper embrittlement in NiCrMoV steels for rotor shaft forgings [11].
3.2. Forging technologies Consolidation of porosity formed in the ingot during solidification and homogenization are the major aim of initial stage of forging. After that the material is forged to form the shape of desired products. Specific forging processes have been evolved to lead to the soundness of the forging. The early stage of the forging pro-
cess upset the ingot to reduce the height and increase the diameter. This improves the homogeneity and increases the forging ratio. To consolidate porosity in the large ingot, the forging effect must reach into the center of the ingot, and this required developing and applying processes which optimize the forging temperature, shape and dimension of the dies, and pressing sequence. The forging ratio generally required to develop a homogeneous microstructure is around 3 for conventional ingots. In case of ESR ingots, a lower forging ratio is acceptable due to the inherently good solidification structure. Dies and hot working steps are carefully designed to exert the largest forging effects. Increasing the size of the forging makes it difficult to forge the material in a free forging press. In order to manufacture the large diameter rings and heads, facilities to perform the forging outside of the press were
Fig. 7. Historical changes in sulfur and phosphorus contents.
Fig. 8. Multiple pouring process for gigantic ingots.
Fig. 9. Outside pressing for large diameter shell rings.
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Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859
developed. The vertical motion of the forging press is converted to a horizontal motion, and Fig. 9 shows the operation of outside pressing to forge a shell ring. Shell rings of 10 m diameter and 4.35 m height can be manufactured by this process.
ties and microstructure. A stress relief heat treatment follows the quality heat treatment. After the forging process, preliminary heat treatment is performed aiming at the relaxation of strain introduced by hot working and refining of coarse grain formed during the forging process. Since large forgings it is generally difficult to develop small grain structure through the dynamic recrystallization during hot working, a preliminary heat treatment is important to build the fine grained microstructure needed for toughness and inspectability by ultrasonic testing (UT). Target properties required in the forgings are controlled by the quenching heat treatment followed by the tempering heat treatment for ferritic steel. Quenching is the heat treatment accompanied by rapid cooling from an austenitizing temperature, which is commonly selected at the temperature to dissolve the carbides in steels and to obtain desired material properties such as creep strength. Attention should be paid, however, to avoiding the excessive grain coarsening at this high temperature. In order to attain the maximum cooling effects during quenching, a quenching bath with agitation of the quenching media is used for shell rings. For rotor forgings, quenching is performed under rotation in a vertical furnace to attain the homogeneous microstructure needed for thermal stability. Between and after these processes, machining and inspection are repeated. The machining of large forgings has also required advances in several technologies.
3.3. Heat treatment
4. Production of austenitic stainless steel components
The role of heat treatment is not only the development of target mechanical properties, such as strength and toughness, but also to built adequate microstructures with sufficient inspectability and thermal stability. These features in a forging are achieved by developing a fine and uniform microstructure. The overall heat treating process involves several steps in the heating pattern and it largely depends on the end use of the component. The difference in heating rate, cooling rate and hold time from surface to center of the large diameter forging need to be considered to attain the target properties. Heat treatment of forgings generally consists of a preliminary heat treatment which is first performed after forging and a subsequent quality heat treatment to develop the desired proper-
JSW has experience in the production of many austenitic steel forgings. In 1986, JSW manufactured components for the prototype FBR in Japan, ‘‘Monju’’. In manufacturing of the large components made from austenitic steel, several issues were raised. In the steelmaking process, reducing impurities including gas elements was needed and a decrease of the C content is important to avoid sensitization. Casting must use specific technologies to minimize any segregation in large steel ingots. Since there are no phase transformation in the austenitic steels, control of grain size in the forging processes is important. The initial stage of the forging processes must effect the homogenization of the solidified structure and consolidation of porosity. But, in the latter processes, it becomes very
Fig. 10. Concept of the forging process for austenitic steels.
Fig. 11. History of austenitic non-magnetic steels produced by JSW.
Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859
859
for international thermonuclear experimental reactor (ITER). The material was developed to have high strength and high fracture toughness at 4 K. Compared to type 316L steel, the Mo and N content are increased for higher strength. Mn is also increased to allow higher nitrogen solubility, and Cr content is reduced to avoid the formation of delta ferrite by the increase of Cr equivalent when Mo is added. Fig. 12 shows the strength vs. toughness balance of JJ1 steel and weld metal, and the data demonstrates that JJ1 satisfactorily meets the ITER coil case requirements. 5. Conclusion
Fig. 12. Toughness vs. strength balance at 4 K of JJ1 steel and weldments.
The quality and performance properties of recent large forgings are satisfactorily controlled through the evolution of the technology to produce these large components. However, the trend for growth of plant capacity and thus size will continue and plants with new and advanced designs will demand even larger size components. The established technology will be further improved and new technologies will be developed in JSW to realize the advanced energy plants. References
important to control the amount of forging strain and the corresponding temperature in order to attain the homogeneity in the microstructures by recrystallization. Fig. 10 shows schematically the heating temperature and anticipated grain size produced in the forging of austenitic stainless steel. Rapid cooling rate is desired in the solution heat treatment, so the shape of the components should be designed to attain these rapid cooling rates, since the thermal conductivity of the austenitic steel is smaller than that of ferritic steels. The thin material, as shells for example, tends to deform easily and careful attention must be taken to avoid it. In machining these steels, several problems arose in cutting, carrying and handling, so these had to be solved. The typical material for major components of fusion reactors is the non-magnetic austenitic stainless steels. These materials also need to have excellent fracture toughness vs. strength balance for use at cryogenic temperature. Fig. 11 shows development of some of these steels for fusion reactors. Among them the JJ1 steel is highlighted for the application in the toroidal field (TF) coil cases
[1] R. Yanagimoto, T. Jin, Y. Ikeda, T. Ohhashi, N. Kanno, The Japan Steel Works, Ltd. Technical Review, 42, 1986, pp. 111–118. [2] H. Tsukada, K. Suzuki, I. Sato, The Thermal and Nuclear Power 44 (1993) 499– 507. [3] K. Suzuki, I. Sato, The Japan Steel Works, Ltd. Technical Review, 47, 1992, pp. 93–98. [4] T. Sasaki, E. Murai, I. Sato, K. Suzuki, M. Kusuhashi, H. Tsukada, The Thermal and Nuclear Power 52 (2001) 675–682. [5] H. Tsukada, K. Suzuki, I. Sato, R. Miura, The Thermal and Nuclear Power 39 (1988) 21–29. [6] Y. Tanaka, T. Ishiguro, Physica Status Solidi (A) 160 (1996) 305–320. [7] Y. Tanaka, T. Iwadate, T. Ishiguro, The Japan Steel Works, Ltd. Technical Review, 54, 1998, pp. 1–19. [8] T. Takenouchi, The Japan Steel Works, Ltd. Technical Review, 46, 1992, pp. 108–124. [9] G.E. Danner, Paper Presented at the Vaccum Deoxidation of steels for Large Ingots, New York University, 1960 [10] Y. Ikeda, H. Yoshida, Y. Tanaka, T. Fukuda, in: J. Nutting, R. Viswanathan (Eds.), Clean Steel:Superclean Steel, The Institute of Materials, 1996, pp. 71–87. [11] Y. Tanaka, T. Azuma, N. Yaegashi, Y. Ikeda, in: J. Nutting, R. Viswanathan (Eds.), Clean Steel:Superclean Steel, The Institute of Materials, 1996, pp. 181–193.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Development of high purity large forgings for nuclear power plants Yasuhiko Tanaka ⇑, Ikuo Sato The Japan Steel Works, Ltd., 1-11-1 Osaki, Shinagawa, Tokyo 141-0032, Japan
a r t i c l e
i n f o
Article history: Available online 1 January 2011
a b s t r a c t The recent increase in the size of energy plants has been supported by the development of manufacturing technology for high purity large forgings for the key components of the plant. To assure the reliability and performance of the large forgings, refining technology to make high purity steels, casting technology for gigantic ingots, forging technology to homogenize the material and consolidate porosity are essential, together with the required heat treatment and machining technologies. To meet these needs, the double degassing method to reduce impurities, multi-pouring methods to cast the gigantic ingots, vacuum carbon deoxidization, the warm forging process and related technologies have been developed and further improved. Furthermore, melting facilities including vacuum induction melting and electro slag re-melting furnaces have been installed. By using these technologies and equipment, large forgings have been manufactured and shipped to customers. These technologies have also been applied to the manufacture of austenitic steel vessel components of the fast breeder reactors and components for fusion experiments. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Growing world energy demand has led to the construction of higher performance and larger scale energy plants. Dramatic increases in plant performance have been achieved by increasing the size of the major system components. Fig. 1 shows the trend of maximum unit power generating capacity for nuclear and fossil power plants. The power generating capacity of nuclear power plants dramatically increased in the late 1960s and recently reached 1600 MW with the construction of the first European Pressurized Water Reactor (EPR). Large forgings allow the reactor pressure vessels to use integrated advanced designs with reduced numbers of components. The size of forgings for low-pressure steam turbines and for reactor pressure vessels (RPV) components is sure to increase further in the future. The serviceability of these components must be secured even through the forging size increased, because the operating conditions will be more severe. The properties required of component materials are homogeneity, free from harmful defects, high fracture toughness, resistance to ageing embrittlement and resistance to environmental damage, good inspectability and so on. These properties are achieved through state-of-the-art production technologies that include refining technology for high-purity materials, casting technology for very large ingots, forging technology for large components, heat treatment technology, and machining technology for larger and heavier components. This paper addresses the history and recent developments in manufacturing
⇑ Corresponding author. Tel.: +81 03 5745 2046; fax: +81 03 5745 2049. E-mail address: [email protected] (Y. Tanaka). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.305
technology for large forgings. In addition, the production methods for austenitic material is introduced.
2. Experience with large forgings for nuclear power plants in the Japan Steel Works The main reason for the evolution in the size of forging components, is a great demand for increasing in plant capacity. Another critical reason is the increase of safety and reliability through the integration of components. Fig. 2 shows examples of integration in low pressure (LP) steam turbines and in nuclear reactor pressure vessels. Stress corrosion cracking (SCC) sometimes occurs in the LP turbines of the nuclear power plants. Early large LP rotor components were made by shrink fitting of the shaft and disks [1]. The causes of SCC are considered to be high strength of the material used in shrink-on designs and crevice corrosion. Therefore, these shrunk fit type rotor forgins were replaced by a monoblock type using lower strength material. In the case of reactor pressure vessels, the integration of components minimizes the number of weld seams of pressure vessels by adopting the ring forgings and heads made from large ingots. The dimensions of rotor shaft forgings have increased significantly. The first monoblock LP rotor was manufactured in 1977, and since then monoblock forgings have been used in many fossil and nuclear power plants. In Japan, many shrunk-on type LP rotor forgings in nuclear power plants were replaced by the monoblock type in order to prevent a stress corrosion cracking failure. Recently, in USA, the LP turbines of aged nuclear power plant have been replaced by new ones. Fig. 3 shows a monoblock LP rotor forging. The Japan Steel Works (JSW) has shipped 223 monoblock
Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859
855
Fig. 3. Monoblock LP rotor forging made from a 600 ton ingot.
Fig. 1. History of unit power generating capacity for nuclear and fossil power plants.
LP rotor forgings as of 2008. Half speed four pole generators are another typical large forging for nuclear power plants. The first four pole generator, shipped in 1971 for the Biblis plant in Germany, was one of the first successful experiences in application of a 400 ton ingot, the largest in the world at that time. Since then, 162 four poles generators have been shipped from JSW as of 2008. As for nuclear pressure vessels, JSW has supplied the plates for the RPV in the Japan Power Demonstration Reactor (JPDR) of JAERI and RPV of Tokai #1 which was build in 1961 and the first commercial nuclear power plant in Japan, a Calder Hall type reactor. The first forged component was a flange ring manufactured and shipped in 1968 for the Tsuruga #1. The first shell ring forging was shipped in 1971 for the Biblis-B nuclear power plant in Germany. Ring forgings for core region components were first manufactured in 1974. Since then, a total 559 RPV forged components including shells, flanges and heads had been supplied as of 2008. The dimensions of these components have been further increased for the construction of advanced BWR (ABWR) plants. The first ABWR nuclear power plant in Japan is Kashiwazaki-Kariha
#6, #7 [2,3]. The first 600 ton ingot was used for an RPV component, the bottom petal. Recently, the nozzle shell for European Pressurized Water Reactor (EPR) was also made from a 600 ton ingot as is shown in Fig. 4. At the same time the integration of the components has also proceeded. The weld seams can be placed outside the core region by use of tall core region shells. The closure heads were also developed and have been used in PWRPV. Steam generator (SG) components including shells, primary heads and secondary heads have been developed and used for new and replacement plants [4]. High strength steel, SA508 Gr.3 Cl.2 was successfully used for components of SG. Another important development in forging technology is the use of austenitic stainless steel components for the fast breeder reactors (FBR). Components of the reactor vessel for ‘‘Monju’’, the prototype FBR in Japan, were manufactured using type 304 stainless steel. Significant development of production technologies were made to fabricate the austenitic steel components [5]. Using austenitic materials presented considerable problems that were solved in the manufacturing technologies. Furthermore, the forging of austenitic steels for fusion reactor (FR) components has been developed. The material and production technologies were established and the components were shipped for FR experiments.
Fig. 2. Monoblock rotor forging and integration of RPV components.
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Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859
Fig. 4. Nozzle shell for EPR made from a 600 ton ingot.
3. Development of manufacturing technology for forgings for large power generation plants The evolution of the large forging is supported by the development of many technologies in steelmaking and ingot making, forging, heat treatment, non-destructive examination, etc. Fig. 5 shows the history of the major manufacturing technology in JSW focusing on the steelmaking, forging and heat treatment [6–8]. 3.1. Refining and casting technologies Though not shown in the figure, in the early 1950s when the first rotor shaft forgings were manufactured by JSW, melting and refining were performed by basic and acid open hearth furnaces (OHF). The ability of the OHF to reduce the impurity content was quite limited, and the purity of steels produced at that time was not very good. Moreover, an inherent problem of the OHF was that the molten steel tended to dissolve a significant amount of hydrogen which could cause flaking of the steel. Vacuum casting using mechanical pumps was applied in the late 1950s as a solution to the hydrogen problem. The introduction of steam ejectors
in the late 1960s permitted reduction of the hydrogen content to less than 1 ppm. By the late 1960s the OHF was replaced by basic electric arc furnaces (EAF) and impurity contents were very effectively reduced through desulphurization and dephosphorization. By reducing the Si content, vacuum carbon deoxidizaton (VCD) proceeds was become applicable through the evolution of CO during the vacuum casting process [9]. The VCD process is also effective in reducing macro segregations. In the early 1980s, ladle refining furnaces (LRF) were installed. Fig. 6 shows the advanced refining process called double degassing used for large forgings. In this refining process, phosphorus is removed in the EAF using an oxidizing slag. After careful deslagging to avoid rephosphorization during the reladle process, the molten steel is desulphurized in LRF using a basic reducing slag. By this process sulfur and phosphorus can be reduced to less than 10 ppm and 40 ppm, respectively. Double degassing by the vacuum treatment in the ladle furnace aided by intensive argon stirring followed by vacuum stream degassing results in hydrogen contents of less than 0.5 ppm and low oxygen levels. Fig. 7 shows the historical decrease of P and S contents in castings. It should be mentioned that the residual elements such as As, Sn, Sb and Cu must be controlled by the selection of raw material, since these elements are difficult to remove from the molten steel in the refining process. The casting technology for large ingots is the key to producing the large components and was developed using the ladle refining furnace [1–3]. Fig. 8 shows the multi pouring process. After the melting and oxidizing refining in an electric furnace, the molten steel was poured into a ladle. Further refining is done in the electric furnace and the molten steel is kept at optimum conditions until the time of casting. Multiple ladles are prepared to keep all the needed molten steel at optimum conditions and then all are poured successively into an large ingot. Chemistry of the each ladle is carefully controlled and poured to minimize the segregation. Thus the capacity to cast ingots up to 600 tons by using the fully ladle refined melt method [6–8] has been available since 1987, Installation of facilities for casting 650 ton ingot is under way at JSW. Utilization of other secondary refining processes such as electro-slag-re-melting (ESR) contribute to the production of high-purity materials. Presently, steels denoted ‘‘superclean’’, in which impurity and gas elements are reduced as low as possible while Si and Mn are also held to low levels, can be manufactured
Fig. 5. History of production technology for forgings in the JSW Muroran plant.
Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859
857
Fig. 6. Double degassing process for high purity steels.
through these processes [10]. The material is confirmed to have no sensitivity to temper embrittlement in NiCrMoV steels for rotor shaft forgings [11].
3.2. Forging technologies Consolidation of porosity formed in the ingot during solidification and homogenization are the major aim of initial stage of forging. After that the material is forged to form the shape of desired products. Specific forging processes have been evolved to lead to the soundness of the forging. The early stage of the forging pro-
cess upset the ingot to reduce the height and increase the diameter. This improves the homogeneity and increases the forging ratio. To consolidate porosity in the large ingot, the forging effect must reach into the center of the ingot, and this required developing and applying processes which optimize the forging temperature, shape and dimension of the dies, and pressing sequence. The forging ratio generally required to develop a homogeneous microstructure is around 3 for conventional ingots. In case of ESR ingots, a lower forging ratio is acceptable due to the inherently good solidification structure. Dies and hot working steps are carefully designed to exert the largest forging effects. Increasing the size of the forging makes it difficult to forge the material in a free forging press. In order to manufacture the large diameter rings and heads, facilities to perform the forging outside of the press were
Fig. 7. Historical changes in sulfur and phosphorus contents.
Fig. 8. Multiple pouring process for gigantic ingots.
Fig. 9. Outside pressing for large diameter shell rings.
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Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859
developed. The vertical motion of the forging press is converted to a horizontal motion, and Fig. 9 shows the operation of outside pressing to forge a shell ring. Shell rings of 10 m diameter and 4.35 m height can be manufactured by this process.
ties and microstructure. A stress relief heat treatment follows the quality heat treatment. After the forging process, preliminary heat treatment is performed aiming at the relaxation of strain introduced by hot working and refining of coarse grain formed during the forging process. Since large forgings it is generally difficult to develop small grain structure through the dynamic recrystallization during hot working, a preliminary heat treatment is important to build the fine grained microstructure needed for toughness and inspectability by ultrasonic testing (UT). Target properties required in the forgings are controlled by the quenching heat treatment followed by the tempering heat treatment for ferritic steel. Quenching is the heat treatment accompanied by rapid cooling from an austenitizing temperature, which is commonly selected at the temperature to dissolve the carbides in steels and to obtain desired material properties such as creep strength. Attention should be paid, however, to avoiding the excessive grain coarsening at this high temperature. In order to attain the maximum cooling effects during quenching, a quenching bath with agitation of the quenching media is used for shell rings. For rotor forgings, quenching is performed under rotation in a vertical furnace to attain the homogeneous microstructure needed for thermal stability. Between and after these processes, machining and inspection are repeated. The machining of large forgings has also required advances in several technologies.
3.3. Heat treatment
4. Production of austenitic stainless steel components
The role of heat treatment is not only the development of target mechanical properties, such as strength and toughness, but also to built adequate microstructures with sufficient inspectability and thermal stability. These features in a forging are achieved by developing a fine and uniform microstructure. The overall heat treating process involves several steps in the heating pattern and it largely depends on the end use of the component. The difference in heating rate, cooling rate and hold time from surface to center of the large diameter forging need to be considered to attain the target properties. Heat treatment of forgings generally consists of a preliminary heat treatment which is first performed after forging and a subsequent quality heat treatment to develop the desired proper-
JSW has experience in the production of many austenitic steel forgings. In 1986, JSW manufactured components for the prototype FBR in Japan, ‘‘Monju’’. In manufacturing of the large components made from austenitic steel, several issues were raised. In the steelmaking process, reducing impurities including gas elements was needed and a decrease of the C content is important to avoid sensitization. Casting must use specific technologies to minimize any segregation in large steel ingots. Since there are no phase transformation in the austenitic steels, control of grain size in the forging processes is important. The initial stage of the forging processes must effect the homogenization of the solidified structure and consolidation of porosity. But, in the latter processes, it becomes very
Fig. 10. Concept of the forging process for austenitic steels.
Fig. 11. History of austenitic non-magnetic steels produced by JSW.
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for international thermonuclear experimental reactor (ITER). The material was developed to have high strength and high fracture toughness at 4 K. Compared to type 316L steel, the Mo and N content are increased for higher strength. Mn is also increased to allow higher nitrogen solubility, and Cr content is reduced to avoid the formation of delta ferrite by the increase of Cr equivalent when Mo is added. Fig. 12 shows the strength vs. toughness balance of JJ1 steel and weld metal, and the data demonstrates that JJ1 satisfactorily meets the ITER coil case requirements. 5. Conclusion
Fig. 12. Toughness vs. strength balance at 4 K of JJ1 steel and weldments.
The quality and performance properties of recent large forgings are satisfactorily controlled through the evolution of the technology to produce these large components. However, the trend for growth of plant capacity and thus size will continue and plants with new and advanced designs will demand even larger size components. The established technology will be further improved and new technologies will be developed in JSW to realize the advanced energy plants. References
important to control the amount of forging strain and the corresponding temperature in order to attain the homogeneity in the microstructures by recrystallization. Fig. 10 shows schematically the heating temperature and anticipated grain size produced in the forging of austenitic stainless steel. Rapid cooling rate is desired in the solution heat treatment, so the shape of the components should be designed to attain these rapid cooling rates, since the thermal conductivity of the austenitic steel is smaller than that of ferritic steels. The thin material, as shells for example, tends to deform easily and careful attention must be taken to avoid it. In machining these steels, several problems arose in cutting, carrying and handling, so these had to be solved. The typical material for major components of fusion reactors is the non-magnetic austenitic stainless steels. These materials also need to have excellent fracture toughness vs. strength balance for use at cryogenic temperature. Fig. 11 shows development of some of these steels for fusion reactors. Among them the JJ1 steel is highlighted for the application in the toroidal field (TF) coil cases
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