Comprehensive nuclear materials 4 03 ferritic steels and advanced ferritic–martensitic steels

Comprehensive nuclear materials 4 03 ferritic steels and advanced ferritic–martensitic steels Comprehensive nuclear materials 4 03 ferritic steels and advanced ferritic–martensitic steels Comprehensive nuclear materials 4 03 ferritic steels and advanced ferritic–martensitic steels Comprehensive nuclear materials 4 03 ferritic steels and advanced ferritic–martensitic steels Comprehensive nuclear materials 4 03 ferritic steels and advanced ferritic–martensitic steels

4.03 Ferritic Steels and Advanced Ferritic–Martensitic Steels B Raj and M Vijayalakshmi Indira Gandhi Centre for Atomic Research, Kalpakkam, India ß 2012 Elsevier Ltd All rights reserved 4.03.1 4.03.2 4.03.3 4.03.4 4.03.4.1 4.03.4.2 4.03.4.3 4.03.4.4 4.03.4.5 4.03.4.5.1 4.03.5 4.03.6 4.03.7 References Introduction Basic Metallurgy of Ferritic–Martensitic Steels Radiation Damage of Core Components in Fast Reactors Development of Ferritic Steels for Fast Reactor Core Influence of Composition and Microstructure on Properties of Ferritic Steels Void Swelling Resistance Irradiation Hardening in Ferritic Steels Irradiation Creep Resistance of Ferritic Steels Irradiation Embrittlement in Ferritic Steels GBE to reduce embrittlement in ferritic steels Development of Advanced ODS Ferritic Steels Ferritic Steels for Out-of-Core Applications: Improvements in Joining Summary Abbreviations bcc CSL DBTT DICTRA dpa EBR EBSD fcc FFTF GBCD GBE HAADF HAZ HFIR ITER ODS steel PAGS PFR PWHT RIS SIPA SIPN TEM ▽DBTT Body-centered cubic Coincident site lattice Ductile to brittle transition temperature Diffusion-controlled transformations Displacements per atom Experimental breeder reactor Electron back scattered diffraction Face-centered cubic Fast flux test facility Grain boundary character distribution Grain boundary engineering High angle annular dark field Heat-affected zone High flux isotope reactor International Thermonuclear Experimental Reactor Oxide dispersion strengthened steel Prior austenite grain size Power fast reactor Postweld heat treatment Radiation-induced segregation Stress-induced preferential absorption Stress-induced preferential nucleation Transmission electron microscopy Change in DBTT 97 98 101 102 103 105 106 108 110 112 114 116 119 119 4.03.1 Introduction The widespread acceptance of nuclear energy depends1 on the improved economics, better safety, sustainability, proliferation resistance, and waste management Innovative technological solutions are being arrived at, in order to achieve the above goals The anticipated sustainability, rapid growth rate, and economic viability can be ensured by the judicious choice of fast reactor technology with a closed fuel cycle option The fast reactor technology has attained (http://www.world-nuclear.org/info/inf98.html) a high level of maturity in the last three decades, with 390 years of successful operation The emerging international collaborative projects (http://www.iaea.org/ INPRO/; http://www.gen4.org/) have, therefore, chosen fast reactors as one of the important constituents of the nuclear energy in the twenty-first century The nuclear community has been constantly striving for improving the economic prospects of the technology The short-term strategies include the development of radiation-resistant materials and extension of the lifetime of the components The achievement of materials scientists in this field is remarkable Three generations of materials have been developed,2 increasing the burn-up of the fuel from 45 dpa for 316 austenitic stainless steel to above 180 dpa for ferritic steels Presently, efforts are in 97 98 Ferritic Steels and Advanced Ferritic–Martensitic Steels progress to achieve a target burn-up of 250 dpa, using advanced ferritic steels The attempts by nuclear technologists to enhance the thermal efficiency have posed the challenge of improving the high temperature capability of ferritic steels Additionally, there is an inherent disadvantage in ferritic steels, that is, their susceptibility to undergo embrittlement, which is more severe under irradiation It is necessary to arrive at innovative solutions to overcome these problems in ferritic steels In the long time horizon, advanced metallic fuels and coolants for fast reactors are being considered for increasing the sustainability and thermal efficiency respectively Fusion technology, which is ushering (http://www.iter.org/proj) in a new era of optimism with construction of the International Thermonuclear Experimental Reactor (ITER) in France, envisages the use of radiation-resistant advanced ferritic steels Thus, the newly emerging scenario in nuclear energy imposes the necessity to reevaluate the materials technology of today for future applications The genesis of the development of ferritic steels is, indeed, in the thermal power industry The development of creep-resistant, low alloy steels for boilers and steam generators has been one of the major activities in the last century Today, the attempt to develop ultra super critical steels is at an advanced stage Extensive research of the last century is responsible for identifying certain guidelines to address the concerns in the ferritic steels The merit of ferritic steels for the fast reactor industry was established3 in the 1970s and since then, extensive R&D has been carried out4 on the application of ferritic steels for nuclear core component A series of commercial ferritic alloys have been developed, which show excellent void swelling resistance The basic understanding of the superior resistance of the ferrite lattice to void swelling, the nature of dislocations and their interaction with point defects generated during irradiation have been well understood The strengthening and deformation mechanisms of ferrite, influence of various alloying elements, microstructural stability, and response of the ferrite lattice to irradiation temperature and stress have been extensively investigated The mechanism of irradiation hardening, embrittlement and methods to overcome the same are studied in detail Of the different steels evaluated, 9–12% Cr ferritic–martensitic steels are the immediate future solution for fast reactor core material, with best void swelling resistance and minimum propensity for embrittlement The high temperature capability of the ferritic steels has been improved from 773 to 973 K, by launching the next generation ferritic steels, which are currently under evaluation for nuclear applications, namely the oxide dispersion strengthened (ODS) ferritic steels (see Chapter 4.08, Oxide Dispersion Strengthened Steels) Conceptually, this series of steels combines the merits of swelling resistance of the ferrite matrix and the creep resistance offered by inert, nanometer sized, yttria dispersions to enhance the high temperature limit of the ODS steels to temperature beyond 823 K The concerns of this family of materials include optimization of the chemistry of the host lattice, cost effective fabrication procedure, and stability of the dispersions under irradiation, which will be discussed in this article The present review begins with a brief introduction to the basic metallurgy of ferritic steels, summarizing the influence of chemistry on stability of phases, decomposition modes of austenite, different types of steels and structure–property correlations The main thrust is on the development of commercial ferritic steels for core components of fast reactors, based on their chemistry and microstructure Hence, the next part of the review introduces the operating conditions and radiation damage mechanisms of core components in fast reactors The irradiation response of ferritic steels with respect to swelling resistance, irradiation hardening, and irradiation creep are highlighted The in-depth understanding of the damage mechanisms is explained The main concerns of ferritic steels such as the inferior high temperature irradiation creep and severe embrittlement are addressed The current attempts to overcome the problems are discussed Finally, the development of advanced creep-resistant ferritic steels like the ODS steels, for fission and fusion applications are presented The application of ferritic steels for steam generator circuits and the main concerns in the weldments of ferritic steels are discussed briefly The future trends in the application of ferritic steels in fast reactor technology are finally summarized 4.03.2 Basic Metallurgy of Ferritic–Martensitic Steels The advanced ferritic and ferritic–martensitic steels of current interest have evolved5 from their predecessors, the creep-resistant ferritic steels, over nearly a century The first of the series was the carbon and C–Mn steels with a limited application to about Ferritic Steels and Advanced Ferritic–Martensitic Steels Liquid 1500 Liq Liquid + a + g uid a 1400 9Cr steel 1300 Temperature (ЊC) 1200 a a+g g 1100 1000 900 a + g + (CrFe)7C3 800 a + g + (FeCr)3C 700 600 a + (CrFe)7C3 + (CrFe)4C a + (FeCr)3C + (CrFe)7C3 a + (CrFe)7C3 a + (FeCr)3C 10 a + (CrFe)4C 15 20 25 Chromium (%) (a) Ae3 Ferrite Temperature (ЊC) 523 K Subsequent developments through different levels of chromium, molybdenum have increased the high temperature limit to 873, leading to the current ferritic and ferritic–martensitic steels, that is, the 9–12% Cr–Mo steels In addition to being economically attractive, easy control of microstructure using simple heat treatments is possible in this family of steels, resulting in desired mechanical properties The propensity to retain different forms of bcc ferrite, that is, ferrite or martensite or a mixture at room temperature in Cr–Mo steels, depends crucially on the alloying elements Extent of the phase field traversed by an alloy on heating also depends on the amount of chromium, silicon, molybdenum, vanadium, and carbon in the steel The combined effect of all the elements can be represented by the net chromium equivalent, based on the effect of the austenite and ferrite stabilizing elements A typical pseudobinary phase diagram6 is shown in Figure 1(a) Increase in chromium equivalent by addition of ferrite stabilizers or V or Nb would shift the Fe–9Cr alloy into the duplex phase field at the normalizing temperature The phase field at the normalizing temperature and the decomposition mode7–9 of high temperature austenite (Figure 1(b)) dictate the resulting microstructure at room temperature and hence, the type of steel Accordingly, the 9CrMo family of steels can either be martensitic (9Cr–1Mo (EM10) or stabilized 9Cr–1MoVNb (T91)), ferritic (12Cr–1MoVW (HT9)) or ferritic–martensitic (9Cr–2Mo–V–Nb (EM12)) steel The stabilized variety of 9–12 CrMo steels could result10 in improved strength and delayed grain coarsening due to the uniform distribution of fine niobium or vanadium carbides or carbonitrides The transformation temperatures and the kinetics of phase transformations depend strongly on the composition of the steels Sixteen different 9Cr steels have been studied11,12 and the results, which provide the required thermodynamic database are shown in Figure 2, with respect to the dependence of melting point, Ms temperature and the continuous heating transformation diagrams The constitution and the kinetics of transformations dictate microstructure and the properties In the early stages, the oxidation resistance and creep strength were of prime importance, since the Cr–Mo steels were developed4 for thermal power stations In addition to the major constituent phases discussed above, the minor carbides which form at temperatures less than 1100 K, dictate the long term industrial performance of the steels Evaluation of tensile and creep properties of Cr–Mo steels exposed 99 Pearlite Ws Bs Widmanstatten ferrite Upper bainite Bainite Lower bainite Ms Martensite (b) Log {time} Figure (a) Pseudobinary phase diagram for a Fe–Cr–C steel with 0.01% C Reprinted, with permission, from High chromium ferritic and martensitic steels for nuclear applications, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428 (b) Decomposition modes of high-temperature austenite during cooling to elevated temperature for prolonged durations have been extensively studied.5,13,14 The following trends were established: The optimized initial alloy composition considered was 9Cr, W–2Mo ¼ 3, Si ¼ 0.5, with C, B, V, Nb, and Ta in small amounts Higher chromium content has two effects: it increases the hardenability leading to the formation of martensite and also promotes the formation of d-ferrite thereby reducing the toughness A reduction in the chromium 100 Ferritic Steels and Advanced Ferritic–Martensitic Steels (Mn+Ni)/135 Mod 9Cr 1Mo Mod 9Cr 1Mo: base model (Mn+Ni)/1.85 Mod 9Cr 1Mo (Mn+Ni)/2.32 Mod 9Cr 1Mo (Mn+Ni)/1.7 Mod 9Cr 1Mo 1800 Mod 9Cr 1Mo 1805 10 1795 625 (a) 1810 0.6 Si added 9Cr 1Mo 650 0.24 Si added 9Cr 1Mo 675 Plain 9Cr 1Mo Melting point (K) Ms, experimental (K) 1815 700 600 600 Experimental Estimated 1820 Ms/K = 904 - 474 (C + 0.46(N - 0.15Nb ) - 0.046Ta) -{17Cr + 33Mn + 21Mo + 20Ni + 39V + 5W) -45Mn2 - 25Ni2 - 100V2 + 10Co } - 44.5Ta 0.42 Si added 9Cr 1Mo 9Cr–ferritic martensitic steels 725 1W-0.23V-0.05Ta 9Cr 1Mo 750 1790 625 650 675 700 725 750 1785 Ms, empirical estimate (K) (b) 11 Steel designation 1000 9Cr–ferritic steel 99 Continuous heating transformation (CHT) diagram 1248 Austenite 60 Ac3 40 1198 20 900 10 50% transformed Ac1 850 15 Ferrite+ austenite+ carbide 1098 Ferrite + carbide 800 100 101 (c) 102 1148 Temperature (K) Temperature (ЊC) 950 103 Time (s) Figure Influence of chemistry on transformation temperatures (Ms and melting point) and kinetics of transformation of g ! a ỵ carbide, in various ferritic steels content lowers the oxidation resistance If W ỵ Mo concentration is kept