Journal of Nuclear Materials 417 (2011) 854–859

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Journal of Nuclear Materials

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Development of high purity large forgings for nuclear power plants

Yasuhiko Tanaka ⇑, Ikuo SatoThe 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

0022-3115/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.jnucmat.2010.12.305

⇑ Corresponding author. Tel.: +81 03 5745 2046; faE-mail address: yasuhiko_tanaka@jsw.co.jp (Y. Tan

a b s t r a c t

The recent increase in the size of energy plants has been supported by the development of manufacturingtechnology for high purity large forgings for the key components of the plant. To assure the reliability andperformance of the large forgings, refining technology to make high purity steels, casting technology forgigantic 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 doubledegassing method to reduce impurities, multi-pouring methods to cast the gigantic ingots, vacuum car-bon deoxidization, the warm forging process and related technologies have been developed and furtherimproved. Furthermore, melting facilities including vacuum induction melting and electro slag re-melt-ing furnaces have been installed. By using these technologies and equipment, large forgings have beenmanufactured and shipped to customers. These technologies have also been applied to the manufactureof austenitic steel vessel components of the fast breeder reactors and components for fusion experiments.

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1. Introduction

Growing world energy demand has led to the construction ofhigher performance and larger scale energy plants. Dramaticincreases in plant performance have been achieved by increasingthe size of the major system components. Fig. 1 shows the trendof maximum unit power generating capacity for nuclear and fossilpower plants. The power generating capacity of nuclear powerplants dramatically increased in the late 1960s and recentlyreached 1600 MW with the construction of the first EuropeanPressurized Water Reactor (EPR). Large forgings allow the reactorpressure vessels to use integrated advanced designs with reducednumbers of components. The size of forgings for low-pressuresteam turbines and for reactor pressure vessels (RPV) componentsis sure to increase further in the future.

The serviceability of these components must be secured eventhrough the forging size increased, because the operating condi-tions will be more severe. The properties required of componentmaterials are homogeneity, free from harmful defects, high frac-ture toughness, resistance to ageing embrittlement and resistanceto environmental damage, good inspectability and so on. Theseproperties are achieved through state-of-the-art production tech-nologies that include refining technology for high-purity materials,casting technology for very large ingots, forging technologyfor large components, heat treatment technology, and machiningtechnology for larger and heavier components. This paperaddresses the history and recent developments in manufacturing

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technology for large forgings. In addition, the production methodsfor austenitic material is introduced.

2. Experience with large forgings for nuclear power plants inthe Japan Steel Works

The main reason for the evolution in the size of forging compo-nents, is a great demand for increasing in plant capacity. Anothercritical reason is the increase of safety and reliability through theintegration of components. Fig. 2 shows examples of integrationin low pressure (LP) steam turbines and in nuclear reactor pressurevessels. Stress corrosion cracking (SCC) sometimes occurs in the LPturbines of the nuclear power plants. Early large LP rotor compo-nents were made by shrink fitting of the shaft and disks [1]. Thecauses of SCC are considered to be high strength of the materialused in shrink-on designs and crevice corrosion. Therefore, theseshrunk fit type rotor forgins were replaced by a monoblock typeusing lower strength material. In the case of reactor pressure ves-sels, the integration of components minimizes the number of weldseams of pressure vessels by adopting the ring forgings and headsmade from large ingots.

The dimensions of rotor shaft forgings have increased signifi-cantly. The first monoblock LP rotor was manufactured in 1977,and since then monoblock forgings have been used in many fossiland nuclear power plants. In Japan, many shrunk-on type LP rotorforgings in nuclear power plants were replaced by the monoblocktype in order to prevent a stress corrosion cracking failure.Recently, in USA, the LP turbines of aged nuclear power plant havebeen replaced by new ones. Fig. 3 shows a monoblock LP rotorforging. The Japan Steel Works (JSW) has shipped 223 monoblock

Fig. 1. History of unit power generating capacity for nuclear and fossil powerplants.

Fig. 3. Monoblock LP rotor forging made from a 600 ton ingot.

Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859 855

LP rotor forgings as of 2008. Half speed four pole generators are an-other typical large forging for nuclear power plants. The first fourpole generator, shipped in 1971 for the Biblis plant in Germany,was one of the first successful experiences in application of a400 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 forthe RPV in the Japan Power Demonstration Reactor (JPDR) of JAERIand RPV of Tokai #1 which was build in 1961 and the firstcommercial nuclear power plant in Japan, a Calder Hall type reac-tor. The first forged component was a flange ring manufacturedand shipped in 1968 for the Tsuruga #1. The first shell ring forgingwas shipped in 1971 for the Biblis-B nuclear power plant inGermany. Ring forgings for core region components were firstmanufactured in 1974. Since then, a total 559 RPV forged compo-nents including shells, flanges and heads had been supplied asof 2008. The dimensions of these components have been furtherincreased for the construction of advanced BWR (ABWR) plants.The first ABWR nuclear power plant in Japan is Kashiwazaki-Kariha

Fig. 2. Monoblock rotor forging and

#6, #7 [2,3]. The first 600 ton ingot was used for an RPV compo-nent, the bottom petal. Recently, the nozzle shell for EuropeanPressurized Water Reactor (EPR) was also made from a 600 toningot as is shown in Fig. 4. At the same time the integration ofthe components has also proceeded. The weld seams can be placedoutside the core region by use of tall core region shells. The closureheads were also developed and have been used in PWRPV. Steamgenerator (SG) components including shells, primary heads andsecondary heads have been developed and used for new andreplacement plants [4]. High strength steel, SA508 Gr.3 Cl.2 wassuccessfully used for components of SG.

Another important development in forging technology is theuse of austenitic stainless steel components for the fast breederreactors (FBR). Components of the reactor vessel for ‘‘Monju’’, theprototype FBR in Japan, were manufactured using type 304 stain-less steel. Significant development of production technologieswere made to fabricate the austenitic steel components [5]. Usingaustenitic materials presented considerable problems that weresolved in the manufacturing technologies. Furthermore, the forgingof austenitic steels for fusion reactor (FR) components has beendeveloped. The material and production technologies were estab-lished and the components were shipped for FR experiments.

integration of RPV components.

Fig. 4. Nozzle shell for EPR made from a 600 ton ingot.

856 Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859

3. Development of manufacturing technology for forgings forlarge power generation plants

The evolution of the large forging is supported by the develop-ment of many technologies in steelmaking and ingot making, forg-ing, heat treatment, non-destructive examination, etc. Fig. 5 showsthe history of the major manufacturing technology in JSW focusingon 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 thefirst rotor shaft forgings were manufactured by JSW, melting andrefining were performed by basic and acid open hearth furnaces(OHF). The ability of the OHF to reduce the impurity content wasquite limited, and the purity of steels produced at that time wasnot very good. Moreover, an inherent problem of the OHF wasthat the molten steel tended to dissolve a significant amount ofhydrogen which could cause flaking of the steel. Vacuum castingusing mechanical pumps was applied in the late 1950s as a solu-tion to the hydrogen problem. The introduction of steam ejectors

Fig. 5. History of production technology fo

in the late 1960s permitted reduction of the hydrogen content toless than 1 ppm. By the late 1960s the OHF was replaced by basicelectric arc furnaces (EAF) and impurity contents were very effec-tively reduced through desulphurization and dephosphorization.By reducing the Si content, vacuum carbon deoxidizaton (VCD)proceeds was become applicable through the evolution of CO dur-ing the vacuum casting process [9]. The VCD process is also effec-tive in reducing macro segregations. In the early 1980s, ladlerefining furnaces (LRF) were installed. Fig. 6 shows the advancedrefining process called double degassing used for large forgings.In this refining process, phosphorus is removed in the EAF usingan oxidizing slag. After careful deslagging to avoid rephosphoriza-tion during the reladle process, the molten steel is desulphurized inLRF using a basic reducing slag. By this process sulfur and phospho-rus can be reduced to less than 10 ppm and 40 ppm, respectively.Double degassing by the vacuum treatment in the ladle furnaceaided by intensive argon stirring followed by vacuum streamdegassing results in hydrogen contents of less than 0.5 ppm andlow oxygen levels. Fig. 7 shows the historical decrease of P and Scontents in castings. It should be mentioned that the residual ele-ments such as As, Sn, Sb and Cu must be controlled by the selectionof raw material, since these elements are difficult to remove fromthe molten steel in the refining process.

The casting technology for large ingots is the key to producingthe large components and was developed using the ladle refiningfurnace [1–3]. Fig. 8 shows the multi pouring process. After themelting and oxidizing refining in an electric furnace, the moltensteel was poured into a ladle. Further refining is done in the electricfurnace and the molten steel is kept at optimum conditions untilthe time of casting. Multiple ladles are prepared to keep all theneeded molten steel at optimum conditions and then all arepoured successively into an large ingot. Chemistry of the each ladleis carefully controlled and poured to minimize the segregation.Thus the capacity to cast ingots up to 600 tons by using the fullyladle refined melt method [6–8] has been available since 1987,Installation of facilities for casting 650 ton ingot is under way atJSW.

Utilization of other secondary refining processes such aselectro-slag-re-melting (ESR) contribute to the production ofhigh-purity materials. Presently, steels denoted ‘‘superclean’’, inwhich impurity and gas elements are reduced as low as possiblewhile Si and Mn are also held to low levels, can be manufactured

r forgings in the JSW Muroran plant.

Fig. 6. Double degassing process for high purity steels.

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through these processes [10]. The material is confirmed to have nosensitivity to temper embrittlement in NiCrMoV steels for rotorshaft forgings [11].

3.2. Forging technologies

Consolidation of porosity formed in the ingot during solidifica-tion and homogenization are the major aim of initial stage offorging. After that the material is forged to form the shape of de-sired products. Specific forging processes have been evolved to leadto the soundness of the forging. The early stage of the forging pro-

Fig. 7. Historical changes in sulfur and phosphorus contents.

Fig. 8. Multiple pouring process for gigantic ingots.

cess upset the ingot to reduce the height and increase the diame-ter. This improves the homogeneity and increases the forgingratio. To consolidate porosity in the large ingot, the forging effectmust reach into the center of the ingot, and this required develop-ing and applying processes which optimize the forging tempera-ture, shape and dimension of the dies, and pressing sequence.The forging ratio generally required to develop a homogeneousmicrostructure is around 3 for conventional ingots. In case of ESRingots, a lower forging ratio is acceptable due to the inherentlygood solidification structure. Dies and hot working steps are care-fully designed to exert the largest forging effects. Increasing thesize of the forging makes it difficult to forge the material in a freeforging press. In order to manufacture the large diameter rings andheads, facilities to perform the forging outside of the press were

Fig. 9. Outside pressing for large diameter shell rings.

Fig. 10. Concept of the forging process for austenitic steels.

858 Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859

developed. The vertical motion of the forging press is converted toa horizontal motion, and Fig. 9 shows the operation of outsidepressing to forge a shell ring. Shell rings of 10 m diameter and4.35 m height can be manufactured by this process.

3.3. Heat treatment

The role of heat treatment is not only the development of targetmechanical properties, such as strength and toughness, but also tobuilt adequate microstructures with sufficient inspectability andthermal stability. These features in a forging are achieved by devel-oping a fine and uniform microstructure. The overall heat treatingprocess involves several steps in the heating pattern and it largelydepends on the end use of the component. The difference in heat-ing rate, cooling rate and hold time from surface to center of thelarge diameter forging need to be considered to attain the targetproperties. Heat treatment of forgings generally consists of a preli-minary heat treatment which is first performed after forging and asubsequent quality heat treatment to develop the desired proper-

Fig. 11. History of austenitic non-m

ties and microstructure. A stress relief heat treatment follows thequality heat treatment.

After the forging process, preliminary heat treatment is per-formed aiming at the relaxation of strain introduced by hot work-ing and refining of coarse grain formed during the forging process.Since large forgings it is generally difficult to develop small grainstructure through the dynamic recrystallization during hot work-ing, a preliminary heat treatment is important to build the finegrained microstructure needed for toughness and inspectabilityby ultrasonic testing (UT).

Target properties required in the forgings are controlled by thequenching heat treatment followed by the tempering heat treat-ment for ferritic steel. Quenching is the heat treatment accompa-nied by rapid cooling from an austenitizing temperature, whichis commonly selected at the temperature to dissolve the carbidesin steels and to obtain desired material properties such as creepstrength. Attention should be paid, however, to avoiding the exces-sive grain coarsening at this high temperature. In order to attainthe maximum cooling effects during quenching, a quenching bathwith agitation of the quenching media is used for shell rings. Forrotor forgings, quenching is performed under rotation in a verticalfurnace to attain the homogeneous microstructure needed forthermal stability.

Between and after these processes, machining and inspectionare repeated. The machining of large forgings has also required ad-vances in several technologies.

4. Production of austenitic stainless steel components

JSW has experience in the production of many austenitic steelforgings. In 1986, JSW manufactured components for the prototypeFBR in Japan, ‘‘Monju’’. In manufacturing of the large componentsmade from austenitic steel, several issues were raised. In the steel-making process, reducing impurities including gas elements wasneeded and a decrease of the C content is important to avoid sen-sitization. Casting must use specific technologies to minimize anysegregation in large steel ingots. Since there are no phase transfor-mation in the austenitic steels, control of grain size in the forgingprocesses is important. The initial stage of the forging processesmust effect the homogenization of the solidified structure and con-solidation of porosity. But, in the latter processes, it becomes very

agnetic steels produced by JSW.

Fig. 12. Toughness vs. strength balance at 4 K of JJ1 steel and weldments.

Y. Tanaka, I. Sato / Journal of Nuclear Materials 417 (2011) 854–859 859

important to control the amount of forging strain and the corre-sponding temperature in order to attain the homogeneity in themicrostructures by recrystallization. Fig. 10 shows schematicallythe heating temperature and anticipated grain size produced inthe forging of austenitic stainless steel. Rapid cooling rate isdesired in the solution heat treatment, so the shape of the compo-nents should be designed to attain these rapid cooling rates, sincethe thermal conductivity of the austenitic steel is smaller than thatof ferritic steels. The thin material, as shells for example, tends todeform easily and careful attention must be taken to avoid it. Inmachining these steels, several problems arose in cutting, carryingand handling, so these had to be solved.

The typical material for major components of fusion reactors isthe non-magnetic austenitic stainless steels. These materials alsoneed to have excellent fracture toughness vs. strength balancefor use at cryogenic temperature. Fig. 11 shows development ofsome of these steels for fusion reactors. Among them the JJ1 steelis highlighted for the application in the toroidal field (TF) coil cases

for international thermonuclear experimental reactor (ITER). Thematerial was developed to have high strength and high fracturetoughness at 4 K. Compared to type 316L steel, the Mo and N con-tent are increased for higher strength. Mn is also increased to allowhigher nitrogen solubility, and Cr content is reduced to avoid theformation of delta ferrite by the increase of Cr equivalent whenMo is added. Fig. 12 shows the strength vs. toughness balance ofJJ1 steel and weld metal, and the data demonstrates that JJ1 satis-factorily meets the ITER coil case requirements.

5. Conclusion

The quality and performance properties of recent large forgingsare satisfactorily controlled through the evolution of the technol-ogy to produce these large components. However, the trend forgrowth of plant capacity and thus size will continue and plantswith new and advanced designs will demand even larger size com-ponents. The established technology will be further improved andnew technologies will be developed in JSW to realize the advancedenergy plants.

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