Acta Polytechnica https://doi.org/10.14311/AP.2021.61.0762 Acta Polytechnica 61(6):762–767, 2021 © 2021 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague CAVITATION WEAR OF EUROFER 97, CR18NI10TI AND 42HNM ALLOYS Hanna Rostovaa, ∗, Victor Voyevodina, b, Ruslan Vasilenkoa, Igor Kolodiya, Vladimir Kovalenkoa, Vladimir Marinina, Valeriy Zuyoka, Alexander Kuprina a National Science Center Kharkiv Institute of Physics and Technology, Institute of Solid State Physics, Materials Science and Technologies NAS of Ukraine, 1 Akademichna Str., 61108 Kharkiv, Ukraine b V.N. Karazin National University, Physics and Technology Faculty, Department of Reactor Materials and Physical Technologies, 4 Svobody Sq., 61022 Kharkiv, Ukraine ∗ corresponding author: rostova@kipt.kharkov.ua Abstract. The microstructure, hardness and cavitation wear of Eurofer 97, Cr18Ni10Ti and 42HNM have been investigated. It was revealed that the cavitation resistance of the 42HNM alloy is by an order of magnitude higher than that of the Cr18Ni10Ti steel and 16 times higher than that of the Eurofer 97 steel. Alloy 42HNM has the highest microhardness (249 kg/mm2) of all the investigated materials, which explains its high cavitation resistance. The microhardness values of the Cr18Ni10Ti steel and the Eurofer 97 were 196.2 kg/mm2 and 207.2 kg/mm2, respectively. The rate of cavitation wear of the austenitic steel Cr18Ni10Ti is 2.6 times lower than that of the martensitic Eurofer 97. Keywords: Cavitation erosion, wear, steel, hardness, structure, resistance. 1. Introduction Realization of ambitious programs of development and construction of nuclear power plants of a new generation (GEN IV, Terra Power Wave reactor etc.) will be possible only after solutions of problems of nu- clear material science are found. Promising materials for future generations of reactors, in addition to high radiation and corrosion resistance, high mechanical characteristics, should also have an increased cavita- tion resistance to the coolant (supercritical water or liquid metals) [1]. Among the main promising materials for future generations of reactors, the ferrite-martensitic steel Eurofer 97 and the Cr-Ni-Mo alloy 42HNM stand out. The Eurofer 97 is a European reference material within the framework of the European Fusion Develop- ment Agreement (EFDA)–Structural Materials. In Eu- rope, EUROFER97 has been recognized as a prospec- tive material [2] for first walls, divertors, blanket and vessels of fast breeder reactors [3–8]. One of the main reason for its selection are the high mechanical prop- erties at service temperatures coupled with the low or reduced activation characteristic under radiation with the result of a very low loss of mechanical properties of the Eurofer 97 steel. This material behaviour has been reported in many studies and important initiatives are still ongoing [9–11]. Nickel superalloys are selected for their usage in nuclear reactor core systems [12–17], in particular, nuclear power plants with a molten salt coolant [18– 20] and Advanced Ultra Super Critical (AUSC) power plants [21, 22]. It is due to their advantage over the austenitic steels in terms of radiation and corrosion resistance (including molten salts) [12, 13] at relatively low neutron irradiation temperatures. In particular, the 42HNM alloy is considered as a candidate material for accident-resistant fuel (ATF) claddings [23]. In a moving fluid flow under certain hydrodynamic conditions, the continuity of the flow is disrupted, and cavities, caverns and bubbles are formed, which then collapse [24]. This phenomenon, occurring in the liquid flow, causes a cavitation erosion of the mate- rial [25]. Depending on the intensity of the cavitation and the time of exposure, the destruction of the metal surface can be fractions of a square millimetre, and sometimes even several square meters. The depth of the destruction of materials and products made from them is also different – up to a complete destruction. Cavitation erosion can carry away an amount of metal no lesser than corrosion; hence, the importance of studies on cavitation resistance, which will reduce metal losses and increase the durability and reliability of parts and devices, is obvious. It is known that the cavitation resistance of a material is determined by its composition and structure [26]. In this regard, in this work, we studied the cavi- tation wear of promising reactor materials with dif- ferent crystal structures – Eurofer 97 and 42HNM. The Cr18Ni10Ti steel, widely used in nuclear power engineering, was chosen for a comparison. 2. Materials and methods of investigation The chemical composition of the materials under study (wt. %): Eurofer 97 (C – 0.11, W – 1.4, Mn – 0.6, V – 0.25, Cr – 9.7, Ta – 0.3, Fe – balanced), 42HNM (Cr 762 https://doi.org/10.14311/AP.2021.61.0762 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 61 no. 6/2021 Cavitation wear of Eurofer 97, Cr18Ni10Ti and . . . (a). (b). (c). (d). (e). (f). Figure 1. Microstructure of investigated materials: Eurofer 97 – (A, B); Cr18Ni10Ti – (C, D); 42HNM – (E, F). Optical microscopy – (A, C, E) and SEM – (B, D, F) images. – 42, Mo – 1.4, Ni – balanced) and Cr18Ni10Ti (Cr –18.7, Mn – 1.1, Ni – 10.5, Ti – 0.6, Fe – balanced). The size of the samples was 20 × 20 × 0.5 mm. The investigated materials are reactor-grade and were studied in the initial state; heat treatment modes: EUROFER97 – normalization (980 °C/27’) plus tem- pering (760 °C/90’/air-cooled), Cr18Ni10Ti – water quenching at 1050 °C, 42HNM – austenisation at 1130 °C. Microstructural studies were carried out on metal- lographic inversion microscope Olympus GX51 and on scanning electron microscope Jeol 7001-F. The specimens for metallographic studies were preliminary encapsulated into bakelite and then grinded on SiC paper (graininess from P120 to P1200) and polished on diamond suspensions with a fraction size of 1 and 0.05 µm. Etching of all samples was carried out on a Tenupol 5 setup with a reagent of 88 % ethanol + 6 % perchloric acid + 6 % glycerol at a voltage of 39 V at a room temperature. The XRD analysis was performed on DRON-2.0 X- ray diffractometer in cobalt Co-Kα radiation, using Fe selectively-absorbing filter. The diffracted radiation was detected by a scintillation detector. Microhard- ness of the materials was measured on a LM 700 AT tester with a Vickers diamond indenter at a load of 2 N, with a holding time – 14 s. Studies of the cavitation wear of the samples were carried out on a facility described in detail in the work [27, 28]. The cavitation zone was created by ul- trasonic waves under the end face of the concentrator installed in a vessel with distilled water. The oscil- lation amplitude of the end face of the concentrator was 30 ± 2 µm at a frequency of 19.5 kHz [29]. The sample was mounted at a distance of 0.50 mm from the concentrator surface. The erosion of the samples was measured gravimetrically with an accuracy of ± 0.015 mg. The dependence of the weight loss on the time of exposure to the cavitation was measured, and from these data, kinetic curves of destruction of the samples were plotted. The average cavitation wear rate of the materials was determined in the quasilinear sections of the cavitation wear rate curves. 3. Results and discussion The general view of the microstructure of the materials is shown in Fig. 1. The initial structure of Eurofer 97 is tempered martensite, with prior austenite grain boundaries pres- ence, with an average size of 6 µm (Carbon mainly in M23C6 and MX precipitates). The microstructure of Cr18Ni10Ti steel is austenitic with the presence of twins with an average grain size of 7.5 µm. 42HNM 763 H. Rostova, V. Voyevodin, R. Vasilenko et al. Acta Polytechnica Figure 2. Diffraction patterns of the investigated samples: a) Eurofer 97; b) Cr18Ni10Ti; c) 42HNM. (a). (b). Figure 3. Cavitation wear mass loss (A) and cavitation wear rate (B) for Eurofer 97, Cr18Ni10Ti and 42HNM alloys. alloy has FCC structure and an average grain size ∼ 25 µm. Diffraction studies have shown that all samples are single-phase, the diffraction lines in the diffractograms are narrow (Fig. 2), meaning that the samples are in a coarse-crystalline state (grain size ≥ 1 µm). The sample Eurofer 97 consists of Fe-α fer- rite/martensite with a lattice parameter a = 2.8726 Å. The line intensity distribution corresponds to the (110) texture. The Cr18Ni10Ti steel consists of Fe-γ austen- ite with a lattice parameter a = 3.5894 Å. The inten- sity distribution of the austenite lines corresponds to the (220) texture. The 42HNM alloy is also single- phase and consists of an FCC phase (solid solution based on nickel and chromium) with a lattice parame- ter a = 3.5903 Å. In the diffractogram of the sample, the intensity of the lines (200) and (220) are overes- timated, which indicates a more complex texture as compared to the previous samples. The results of the cavitation erosion experiments are shown in the form of curves of a sample mass loss depending on the test time (Fig. 3a) and curves of the rate of cavitation erosion (Fig. 3b). From the obtained data, it can be seen that the 42HNM alloy has the highest resistance to cavitation wear of the studied materials, and the Eurofer 97 steel has the lowest one (Fig. 3a). The cavitation wear rate curves are characterized by the presence of an initial section, when the destruction is low, so-called incubation period, and a section with a maximum quasi-constant rate. The cavitation wear rate becomes constant after 3 hours of testing in the case of the investigated materials (Fig. 3b). Mechanical properties and structural characteristics of the investigated materials are given in Table 1. Alloy 42HNM has the highest microhardness of the investigated materials, which explains its high cavi- tation resistance. Despite the close values of micro- hardness, the rate of cavitation wear of the austenitic steel Cr18Ni10Ti is 2.6 times lower than that of the Eurofer 97 (Table 1). Scanning electron microscopy (SEM) was used to observe the eroded surfaces of the samples after the 764 vol. 61 no. 6/2021 Cavitation wear of Eurofer 97, Cr18Ni10Ti and . . . Alloy Characteristics Crystal structure d, µm a, Å HV , kg/mm2 Vc, cm3/min Eurofer 97 BCT 6.0 2.8726 207.2±6.0 2.6 × 10−6 Cr18Ni10Ti FCC 7.5 3.5894 196.2±6.1 1 × 10−6 42HNM FCC 25.0 3.5903 249.0±8.5 1.6 × 10−7 Table 1. Crystal structure, average grain size d, lattice parameter a, microhardness HV , cavitation wear rate Vc of investigated materials. (a). (b). (c). (d). (e). (f). Figure 4. SEM images of the eroded surfaces for the investigated materials: Eurofer 97 – (A, B); Cr18Ni10Ti – (C, D) and 42HNM – (E, F) under different magnification (A, C, E – 500; B, D, F – 3500). 765 H. Rostova, V. Voyevodin, R. Vasilenko et al. Acta Polytechnica 4 hours of the cavitation tests. It was found that the morphologies of the eroded surfaces are different to each other in the case of Eurofer 97, Cr18Ni10Ti and 42HNM alloy (Fig. 4). It was found that the cavitation damage for steel samples Eurofer 97 and Cr18Ni10Ti is similar in shape and is characterized by the formation of craters, pits, cracks and protruding steps on the surface of the samples. However, the difference in the degree of de- formation and the size of defects is obvious for the two materials under study. The surface of Eurofer 97 steel is covered with large craters and many deep pits (Fig. 4a, 4b). The dimensions of craters and cracks are ∼ 10 µm. In the case of the Cr18Ni10Ti steel, pits and cracks were significantly smaller in size (Fig. 4c, 4d). The defect sizes are at the level of ∼ 5 µm. The com- parison of the SEM images clearly indicates a signifi- cantly smoother surface of the 42HNM alloy (Fig. 4e) as compared to the steel samples. In addition, the large craters observed for Eurofer 97 and Cr18Ni10Ti were not found in the case of 42HNM (Fig. 4f). The presence of small pits and cracks (< 5 µm) for the 42HNM alloy can be associated with its high work- hardening characteristics as well as high corrosion re- sistance. It is known that nickel alloys with chromium and molybdenum are highly resistant to cavitation wear [30]. Usually, the resistance to cavitation erosion of martensitic and austenitic stainless steels is higher than that of ferritic stainless steels [31]. The excellent erosion resistance of martensitic stainless steels can be attributed to the uniform strain distribution and shorter effective average free martensite laths [32]. In this case, the low cavitation resistance of Eurofer 97 steel can be caused by the presence of ferrite in the steel structure. The use of various thermomechanical treatments can significantly improve the mechanical properties of Eurofer 97 steel [33]. However, the effect of such treatments on the cavitation resistance of this steel requires further research. 4. Conclusions The present work investigated the cavitation resis- tance of materials with different crystal structures: Eurofer 97 (BCT) and Cr18Ni10Ti, 42HNM (FCC). It was shown that the cavitation wear rate in dis- tilled water for the 42HNM alloy is 1.6×10−7 cm3/min, ∼ 1 × 10−6 cm3/min for Cr18Ni10Ti and 2.6 × 10−6 cm3/min for Eurofer 97. It was found that after the cavitation tests, the morphology of eroded surfaces differs from each other for the alloy Eurofer 97, Cr18Ni10Ti and 42HNM and is in good agreement with the rate of cavitation wear. The surface of Eurofer 97 steel is covered with large craters and a large number of deep pits; for Cr18Ni10Ti steel, the size of these defects is 2 times smaller. 42HNM alloy has the smallest size of erosion defects. Further studies are required to determine the ef- fect of various thermomechanical treatments on the structure and cavitation resistance of the Eurofer 97 steel. 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