INFLUENCE OF LEAD AND Pb Bi EUTECTIC MELTS ON THE MECHANICAL PROPERTIES OF 20Kh13 FERRITIC MARTENSITIC STEEL

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1 Materials Science, Vol. 48, No. 3, November, 2012 (Ukrainian Original Vol. 48, No. 3, May June, 2012) INFLUENCE OF LEAD AND Pb Bi EUTECTIC MELTS ON THE MECHANICAL PROPERTIES OF 20Kh13 FERRITIC MARTENSITIC STEEL O. І. Yas kiv 1,2 and V. M. Fedirko 1 UDC : We have investigated the influence of a liquid-metal medium on the mechanical properties of 20Kh13 ferritic martensitic steel in the temperature range С. It is shown that lead and lead bismuth eutectic melts facilitate a decrease in the ultimate strength of 20Kh13 steel by 10 20% against that in vacuum, and with increase in temperature, this effect is enhanced. We have found the influence of the liquid-metal medium on the plastic deformability of 20Kh13 steel at elevated temperatures. It has been established that 20Kh13 steel is susceptible to liquid-metal embrittlement in the temperature range С, which manifests itself weaker in lead and more substantially in the lead bismuth eutectic. The decrease in the percentage elongation against that in vacuum depends on the medium: it is equal to 11% in Pb at a temperature of 450 С, and ranges from 15 to 30% in the Pb Bi eutectic melt in the temperature range С. Keywords: ferritic martensitic structural steel, lead melts and lead bismuth eutectics, short-term strength and plasticity, liquid-metal embrittlement. The materials-science and technological problems of the development and production of reactor materials are among main problem on the way to realization of the projects of next generation nuclear power plants [1 4]. It is anticipated that austenitic and ferritic martensitic steels with a fast decrease in the induced activity will be used as main structural materials of nuclear power plants. A substantial disadvantage of austenitic steels (based on the Fe Cr Ni system) is the development of vacancy porosity under high-temperature neutron irradiation. Ferritic martensitic steels (based on the Fe Cr system) are promising materials for the first walls and blankets of synthesis reactors. and fuel elements of fast reactors. They are characterized by high thermophysical and mechanical characteristics and demonstrate an acceptable compatibility with main coolants and low susceptibility to swelling and helium embrittlement in comparison with austenitic steels [5, 6]. Melts of heavy metals (Рb, Ві, Pb Bi, and their eutectic mixture), due to their nuclear and thermophysical properties, are candidate coolants for fast (BREST-type) reactors and accelerator-driven subcritical hybrid systems (ADS) [7 9]. A serious threat that arises in the operation of nuclear plants is the negative influence of melts of hard metals on the mechanical properties of structural materials. One of the causes of degradation of the mechanical characteristics is liquid-metal embrittlement (LME). Investigations of the influence of liquid-metal media on the degradation of the mechanical properties of steels are predominantly collecting and unsystematic, which complicates the generalization of information on the mechanisms of initiation and propagation of cracks and on the character of fracture depending on media and temperature and does not enable one to predict diverse manifestations of interactions between liquid metal and solid metals and, hence, predict reliably the behavior of structural materials. It should be noted that investigations of the adsorption influence of melts on the mechanical properties of solid metals under static and cyclic loads are aimed at clarifying the nature of LME and predicting the 1 Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. 2 Corresponding author; oleh.yaskiv@ipm.lviv.ua. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 48, No. 3, pp , May June, Original article submitted February 22, X/12/ Springer Science+Business Media New York

2 INFLUENCE OF LEAD AND Pb Bi EUTECTIC MELTS ON THE MECHANICAL PROPERTIES OF 20Kh13 STEEL 309 behavior of various metallic materials under load in melts were pioneered by scientists of the Karpenko Physicomechanical Institute in the 1960s [10, 11]. A substantial contribution of these investigations is the phenomenological model of LME [12, 13], which agrees fairly well with experimental data and remains one of the most convincing models in the description of the phenomenon of LME. However, the study of the scientific and technological aspects of the influence of lead melts (Рb and Рb Bi) on the mechanical properties of ferritic martensitic steels remains urgent for the prospective development of nuclear technologies in Ukraine. The aim of the work is to establish regularities of the influence of lead and lead bismuth eutectic melts on the mechanical properties of 20Kh13 structural ferritic martensitic steel. Methods We investigated specimens of 20Kh13 ferritic martensitic steel with a working section of 2 mm cut off from a sheet material 1 mm thick. As a result of rolling and etching, the steel sheets in the as-delivered state have a surface defective layer, which, before investigations, was removed by mechanical polishing with the use of abrasive paper and diamond paste, and the surface was finished to a surface roughness R a = 0.4 μm. Immediately prior to treatment, specimens were washed in benzene, acetone, and alcohol and then dried. Before testing, specimens were annealed in vacuum at a temperature of 1050 С for 30 min to relieve stresses that arouse in operation and polishing. We investigated the microstructure of the near-surface layers and matrix of the steel before and after tests on straight microsections using a Neophot-2 metallographic microscope equipped with a camera and a computer attachment with recording images in a digital form. The microsections were etched by a 1 vol. % НF + 1 vol. % H 2 NO vol. % glycerol reagent. Overetched microsections were lightened with a 5 ml НF + 95 ml H 2 NO 3 reagent The distribution of elements, microstructure, and fractography of fractures were investigated using a Carl Zeiss EVO 40 scanning electron microscope with a detector for electron-probe microanalysis by energy-dispersive X-ray spectrometry (EDX). The qualitative character of distribution of interstitial elements in saturation was assessed from secondary electron and backscattered images of the surfaces of microsections by scanning with an electron beam with an area of μm. The phase composition of the surfaces of the steel was determined by X-ray phase analysis on a DRON-3.0 diffractometer in CuK α radiation with focusing by the Bragg Brentano scheme. The voltage on the anode of the X-ray tube was 30 kv, and the current was 20 mа. We obtained X-ray diffraction spectra of the specimens in the form of a set of values of the interplanar spacing d i and the relative intensity I i under automatic control by computer programs (Powder Cell-2.3 [14], Sietronix, and Full Prof software packages) and identified them by comparing with the JCPDS-ASTM file [15]. Mechanical tests under active loading in uniaxial tension were performed on an R-0.5 tensile testing machine in vacuum ( Pa) and in lead and lead bismuth eutectic melts. The strain rate of the specimens was sec 1. A tensile stress strain diagram was obtained in elongation load coordinates. To determine the mechanical properties in metallic melts, we used special equipment for operation with liquid metals (a pressure chamber, equipment for melting and pouring of liquid-metal media). Investigations were performed at room temperature (20 С) and in the temperature range С. In testing in melts, the space above specimens was filled by spectroscopically pure argon, in which the oxygen content did not exceed mm Hg. Prior to testing, the specimens were plated by lead and the eutectic to improve the wetting of the metals.

3 310 O. І. YAS KIV AND V. M. FEDIRKO Fig. 1. X-ray diffraction patterns of 20Kh13 steel in the initial state (1) at 20 C and after holding in vacuum (2) and lead melt (3) at 450 C. Table 1. Ultimate Strength σ u and Yield Strength σ 0.2 of 20Kh13 Steel in Vacuum, Lead Melt, and Eutectic Melt T, С Medium σ u /σ 0.2, MPa Vacuum 495/ / / / /105 93/75 Pb 214/ / / / /87 82/70 Pb Bi 160/ / / / / /65 83/63 Results and Their Discussion The X-ray diffraction analysis of the surface of steel specimens after fracture showed only reflexes of α - Fe in the phase composition, and their shape does not change with increase in the test temperature and after changing the medium, whereas reflexes of a new phase are absent (Fig. 1). Thus, the tensile tests of specimens in the liquid-metal media, including tests at elevated temperatures, do not influence the phase-structural state of 20Kh13 steel. Let us analyze the change in the ultimate strength of specimens depending on temperature in different media. The ultimate strength of 20Kh13 steel at room temperature (20 С) is 495 MPа. Heating in vacuum up to a temperature of 350 С leads to a substantial decrease in the strength of the metal (210 МPа). Further increase in temperature (up to 600 С) is accompanied by a decrease in the ultimate strength: its value at 600 С is almost three times smaller than at 350 С (Table 1). Temperature dependences of the softening coefficient of the material in the investigated media testify to the character of the influence of the melts on the ultimate strength (Fig. 2а). The coefficient K was determined as

4 INFLUENCE OF LEAD AND Pb Bi EUTECTIC MELTS ON THE MECHANICAL PROPERTIES OF 20Kh13 STEEL 311 Fig. 2. Temperature dependence of the influence coefficient of the medium K on the ultimate strength (a) and plasticity (b) of 20Kh13 steel in the Pb melt (1) and Pb Bi eutectic melt (2). Table 2. Percentage Elongation δ of 20Kh13 Steel in Vacuum, Lead Melt and Eutectic Melt T, С Medium δ, % Vacuum Pb Pb Bi the ratio of the strength of the material in the melt to the strength in vacuum at the corresponding temperatures K =σ Me /σ vac. We performed tests in the lead melt in the temperature range С. The lead medium does not change qualitatively the temperature dependences of the ultimate strength and yield strength. At temperatures of 350 and 450 С, the ultimate strength in the melt is almost equal to that in vacuum (Fig. 2а, curve 1). At a temperature of 400 С, the strength decreases by 8% against that in vacuum. Its maximum decrease was observed in the lead melt (up to 25%) in the temperature range С. In the Pb Bi eutectic melt, tests were carried out in the temperature range С. The regularities of the temperature dependences of the ultimate strength and yield strength of the material in the eutectic melt are almost analogous to those in lead (Table 1). The minimum decrease in the ultimate strength was observed at a temperature of 450 С. At other investigated temperatures, we observed a more substantial decrease in the ultimate strength, namely, by 11 12% at 350 and 400 С and by 11 20% in the temperature range С (Fig. 2а, curve 2). These results indicate that, in comparison with vacuum, the investigated liquid-metal media soften the metal. Let us analyze the change in the percentage elongation of the specimens with temperature in different media (Table 2). At room temperature, the percentage elongation of the material is equal to 13.4%. With increase in temperature, the plasticity of the steel after testing in vacuum decreases significantly and is equal to 9.8% at 350 C. A further increase in temperature up to 400 С does not influence the deformability of the material.

5 312 O. І. YAS KIV AND V. M. FEDIRKO Fig. 3. SEM images of fracture surfaces of 20Kh13 steel in the central zone (a) and near the surface (b) after fracture at a temperature of 350 C in vacuum. An insignificant decrease in the percentage elongation (down to 9.1%) is observed at a temperature of 450 С. Further increase in temperature causes the gradual restoration and an increase in the deformability of the steel: at 600 С, the plasticity is equal to 11.3%. However, the values characteristic of the material at room temperature are not attained in the investigated temperature range. The fractographic analysis of fracture surfaces of 20Kh13 steel in vacuum indicates that fracture occurs by the ductile mechanism (Fig. 3). In the central zone, the formation and coagulation of voids are observed, and this leads to the formation of a dimple microstructure with deep cups and a network of high combs on the fracture surface (Fig. 3а). In regions adjacent to the surface of the specimens, we can see numerous ductile combs and the absence of brittle cleavage facets (Fig. 3b). Temperature dependences of the coefficient K, which characterizes the change in the plasticity under the influence of the melts against the plasticity obtained in vacuum are presented in Fig. 2b. In the temperature range С, the percentage elongation in the lead melt is smaller than that after testing in vacuum (Fig. 2b, curve 1). In particular, at a temperature of 450 С, a decrease in the percentage elongation (from 9.1% to 8%) is observed, which is equal to 11% of the decrease in plasticity in comparison with that obtained in vacuum. This may be a manifestation of liquid-metal embrittlement. With further increase in temperature, the plasticity is restored faster than in a vacuum medium (the maximum percentage elongation at 550 С is 12.1%). Only at a temperature of 600 С, the plasticity of the specimens in vacuum and plasticity in lead become practically equal. Tests of specimens of 20Kh13 steel in the lead bismuth eutectic melt gave somewhat different results. The regularity of the decrease in the percentage elongation with temperature against the percentage elongation at room temperature is retained (Table 2). However, in contrast to tests in lead, here, in the temperature range С, we observe a more substantial decrease in the percentage elongation (Fig. 2b, curve 2): at 400 С, it attains a minimum value (6.5%), which constitutes 30% of the decrease against the percentage elongation in vacuum; then (from a temperature of 450 С), the deformability of the metal increases, and, in the range С, becomes almost identical to that in the lead melt. This indicates that, in the temperature range С, in the Pb melt and Pb Bi melt, the liquid-metal embrittlement of 20Kh13 steel shows up, and its manifestation in the eutectic is more noticeable (at temperatures of 350 and 400 С), than in lead (at temperatures of 350 and 450 С). An analysis of tensile stress strain diagrams of 20Kh13 steel in vacuum and the melts confirms the regularities described above (Fig. 4). In particular, tension in vacuum at temperatures of 350; 400, and 500 С reveals the ductile character and substantial expenditure of energy to fracture of the material. In the lead melt, at these temperatures, the ductile character of fracture is retained, and in the eutectic melt, excluding 350 and 400 С,

6 INFLUENCE OF LEAD AND Pb Bi EUTECTIC MELTS ON THE MECHANICAL PROPERTIES OF 20Kh13 STEEL 313 Fig. 4. Uniaxial tensile stress strain diagrams of 20Kh13 steel in vacuum (1), lead (2), and the Pb Bi eutectic (3) at temperatures of 350 C (a), 400 C (b), and 500 C (c). specimens fracture at smaller strains (Figs. 4а and b). Note that the abrupt break of the tensile curves under these conditions testifies to the insignificant energy intensity of the development of the crack in the indicated temperature interval and, therefore, to the combined ductile brittle fracture mechanism. At temperatures above 450 С, the plastic deformability of the steel in the eutectic melt is restored (Fig. 4с). The fractographic analysis of the fracture surfaces after fracture in the lead bismuth eutectic melt confirms the ductile brittle transition in the character of fracture. In the near-surface zone of fractures of specimens fractured at temperatures of 350 С and 400 С, smooth facets (Fig. 5а) and a fan-like pattern formed by the cleavage surfaces (Fig. 5b) are seen. This indicates that, in this medium, the character of fracture is brittle or combined ductile brittle. With increase in temperature up to 500 С, the character of fracture changes from brittle to ductile, and numerous deep dimples emerge on fracture surfaces (Fig. 5с). CONCLUSIONS The study of the regularities of the influence of the lead melt and lead bismuth eutectic melt on the mechanical properties of 20Kh13 ferritic martensitic steel in the temperature range С shows that the Pb

7 314 O. І. YAS KIV AND V. M. FEDIRKO Fig. 5. SEM images of a fracture surface of 20Kh13 steel after fracture in the Pb Bi eutectic melt at temperatures of 350 C (a); 400 C (b), and 500 C (c). melt and Pb Bi melt facilitate a decrease in the ultimate strength of the steel by 10 20% against that in vacuum, and this effect is more substantial at higher temperatures. Steel 20Kh13 is susceptible to liquid-metal embrittlement in the temperature range С, which manifests itself to a smaller degree in lead and to a greater degree in the lead bismuth eutectic. In comparison with the percentage elongation of the steel in vacuum, its percentage elongation in lead at a temperature of 450 С and in the lead bismuth eutectic melt in the temperature range С decrease by 11% and 15 30%, respectively. REFERENCES 1. N. M. Bezkorovainyi and A. G. Ioltukhovskii, Structural Materials and Liquid-Metal Heat-Transfer Agents [in Russian], Énergoatomizdat, Moscow (1983). 2. G. М. Gryaznov, V. А. Evtikhin, L. P. Zavyal skii, et al., Materials Science of Liquid-Metal Systems of Thermonuclear Reactors [in Russian], Énergoatomizdat, Moscow (1989). 3. K. Ehrlich, E. E. Bloom, and T. Kondo, International strategy for fusion materials development, J. Nucl. Mater., 283, (2000). 4. K. Ehrlich, Materials research towards a fusion reactor, Fusion Eng. Des., 56, (2001). 5. R. L. Klueh, D. S. Gelles, S. Jitsukawa, et al., Ferritic/martensitic steels overview of recent results, J. Nucl. Mater., , (2002). 6. R. L. Klueh, N. Hashimoto, R. F. Buck, and M. A. Sokolov, A potential new ferritic/martensitic steel for fusion applications, J. Nucl. Mater., , (2000). 7. Comparative Assessment of Thermophysical and Thermohydraulic Characteristics of Lead, Lead-Bismuth and Sodium Coolants for Fast Reactors, Preprint IAEA-TECDOC-1289, Vienna (2002).

8 INFLUENCE OF LEAD AND Pb Bi EUTECTIC MELTS ON THE MECHANICAL PROPERTIES OF 20Kh13 STEEL Design of an Actinide Burning, Lead or Lead-Bismuth Cooled Reactor That Produces Low Cost Electricity, INEEL/EXT MIT-ANP-PR-083. FY-01 Annual Report, Idaho Falls (2001). 9. J. U. Knebel, X. Cheng, G. Muller, et al., Thermalhydraulic and corrosion challenges for the target module of an accelerator-driven system (ADS), in: Proc. of the 3rd Int. Topical Meeting on Nuclear Application of Accelerator Technology AccApp 99 (November 14 18, USA, Long Beach) (1999), pp M. I. Chaevskii and A. L. Bichuya, On elimination of the softening effect under the action of the Pb Bi eutectic by decreasing the strain rate, Fiz.-Khim. Mekh. Mater., 5, No. 5, (1969). 11. M. I. Chaevskii, V. F. Shatinskii, and V. V. Popovich, Role of oxygen in the adsorption decrease in the workability of steel specimens contacting with melt, Fiz.-Khim. Mekh. Mater., 1, No. 6, (1965). 12. V. V. Popovich and I. G. Dmukhovskaya, Liquid-Metal Embrittlement of Deformable Metals [in Russian], Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv (1983). 13. I. G. Dmukhovskaya and V. V. Popovich, Phenomenological model of embrittlement of metals under the adsorption action of liquid-metal media, Fiz.-Khim. Mekh. Mater., 18, No. 6, 5 13 (1982). 14. W. Kraus and G. Nolze, Powder Cell a program for the representation and manipulation of crystal structures and calculation of the resulting X-ray powder patterns, J. Appl. Cryst., 29, (1996). 15. Powder Diffraction File 1974: Search Manual Alphabetical Listing and Search Section of Frequently Uncounted Phases. Inorganic, Philadelphia (1974).