FRACTURE BEHAVIOR OF 9% AND 14% Cr ODS STEELS PREPARED BY MECHANICAL ALLOYING

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1 Powder Metallurgy Progress, Vol.12 (2012), No FRACTURE BEHAVIOR OF 9% AND 14% Cr ODS STEELS PREPARED BY MECHANICAL ALLOYING H. Hadraba, L. Stratil, I. Dlouhý Abstract The paper describes the influence of the microstructure (coming from the chemical composition and extrusion shape) of 9 and 14% Cr-W(Mo)-Ti- Y 2 O 3 ODS steels prepared by mechanical alloying and subsequent hotextrusion on their impact properties. 9Cr-2W ferritic-martensitic steel present lower upper shelf energy (USE) about 7 J and a lower ductile to brittle transition temperature (T DBTT ) about -30 C compared to 14Cr-1W ferritic steel one. The transition energy curves of low-activation ferritic 14Cr-1W ODS steel was shifted by about 50 C toward lower temperatures and USE by about 4 J towards higher energies. The extrusion shape also plays a role on the impact properties. The texture caused by the forming process strongly influenced the impact behavior of the steels. The extensive splitting of the fracture surfaces of plate shaped steel comparing with steels extruded as bar was found. Keywords: ODS steel, low-activation steel, KLST mini Charpy INTRODUCTION The oxide dispersion strengthened steels (ODS) were developed as structural tubing material for fast breeder reactors and are nowadays structural materials of first choice for future nuclear power sources [1-3]. ODS steels contain small amounts (about 0.25 wt.%) of homogeneously dispersed nano-size yttria particles, to increase creep strength of the steel [4]. Moreover, the yttrium-rich nanoparticles effectively suppress softening annealing by blocking dislocations motion at elevated temperatures and subsequent recrystallization of ferritic matrix of the steel [5-7]. Nowadays two classes of ODS steels can be identified. First one is characterized by ferritic-martensitic microstructure and nominal chemical composition 9Cr-2Mo(1W)-0.5Ti-0.25%Y 2 O 3 [2]. The steel is known as Eurofer and was developed as evolution of ferritic steel HT91 (12Cr- 1Mo) used as a structural material in conventional fission reactors by lowering the Cr content to 9Cr-1W(V,Ta) due to minimisation of transmutational damage in fast neutron spectrum. The second class of current ODS steels is characterized by ferritic microstructure and nominal chemical composition 14Cr-0.5Mo(1W)-0.5Ti-Y 2 O 3 [1]. The steel is the direct successor of HT91 steel with slightly changed chemical composition [8]. From the point of view of the future use of the steel in the nuclear power industry the resistance of the steel against the catastrophic cleavage fracture is one of the important factors. It was found that the mechanical alloying of the atomized tempered ferritic-martensitic steel Eurofer 97 by Y 2 O 3 particles led to a significant degradation of the impact behavior of the steel [9]. This behavior of ODS steels is connected to the remaining porosity and morphology of Cr Hynek Hadraba, Luděk Stratil, Ivo Dlouhý CEITEC IPM, Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Brno, Czech Republic

2 Powder Metallurgy Progress, Vol.12 (2012), No precipitates caused by Hot Isostatic Pressure (HIP) consolidation process and could be partly overcome by subsequent thermo-mechanical treatment [10] or by hot extrusion process [11]. The ductile-brittle transition behavior of ODS steels is thus influenced both by the oxide dispersion and the bcc matrix microstructure. The addition of yttria particles in Fe-9Cr steel Eurofer 97 led to the shift of the USE and T DBTT towards lower energies and higher temperatures respectively [12]. Increase in the yttria content led to an additional shift of the T DBTT towards higher temperatures [13]. From the point of view of high-temperature application of the ODS steels the nature of yttria rich precipitates is partly controlled by the addition of Ti [14]. Solid solution saturation by oxygen, argon or hydrogen during the milling process of very fine powders can also affect the impact response of the ODS steel [15,16]. Another issue linked to the ODS steels is their pronounced anisotropy coming from the fabrication route of the steel by powder metallurgy and hot extrusion. The ODS steels typically contains grains elongated with a factor 1:10 in the uniaxial direction (in the extrusion direction). The plastic strain of ODS steels at fracture in the uni-axial direction is about twice higher than in the hoop direction and the fracture toughness master curve in the hoop direction is shifted towards higher temperatures by about 150 C compared to the fracture toughness master curve in uniaxial direction [17]. The extensively deformed hotextruded bars embodied better impact properties (lower T DBTT, higher USE) than those consolidated by HIPing process [11]. The present study is focused on the fracture properties of the ODS Cr-W(Mo)-Ti- Y 2 O 3 steels. ODS steels containing different amount of Cr (9 and 14 wt.%), Ti (up to 1 wt%), Y 2 O 3 (0.3 wt.%) and W (up to 2 wt.%) or Mo (0.3 wt.%), hot-extruded as a plate or bar were prepared by powder metallurgy. The fracture behavior of these ODS steels was studied by means of instrumented Charpy impact testing. EXPERIMENTAL Three ODS steels of type Fe-Cr-W(Mo)-Ti-Y 2 O 3 were prepared by mechanical alloying process and are summarized in Table 1. The steels were prepared by alloying a pre-alloyed metal powders and an yttria powder either using an attritor or a ball milling equipment. The resulting powder was sealed into a mild steel can, degassed and consolidated into a bar or a plate by hot extrusion at 1100 C and then air-cooled. Sub sized Charpy KLST specimens of 3 4 mm cross section and length of 27 mm was machined and the V notch of 1 mm depth was machined according to the DIN standard. The impact energy was studied in conventional longitudinal sample orientation (i.e. the long axis of the specimens was oriented in the rolling/extrusion direction). Tab.1. Composition (in wt.%), alloying process and extrusion shape of ODS alloys. alloy chemical composition mechanical Cr Mo W Ti Y2O3 alloying final shape CEA attritor plate, tk mm ODM ball mill bar, diam. 30 mm CEA attritor bar, diam. 16 mm The impact tests were performed using instrumented pendulum impact tester (Zwick/Roell B ) with a 15 J capacity released with a velocity of 3.85 m. s -1 in the temperature region between -180 C to +325 C according to the standards EN and EN ISO The impact energy transition curves (the impact energy KV [J] as a function of the temperature T [ C]) were constructed from impact energies recorded. Ductile to brittle transition temperature T DBTT [ C] was evaluated as midpoint between LSE

3 Powder Metallurgy Progress, Vol.12 (2012), No (lower shelf energy value) and USE (upper shelf energy value) of impact energy. The microstructure of the steels was observed by means of optical microscopy (Olympus GX51), scanning electron microscopy (JEOL 6460) and transmission electron microscopy (Philips CM12). Fracture surfaces were studied by means of confocal optical microscopy (Olympus LEXT 3100) and scanning electron microscopy enabling to identify fracture propagation mechanism. RESULTS AND DISCUSSION Pronounced anisotropy of microstructure of the ODS steels prepared coming from fabrication route is obvious. Morphologic textures obtained for the steel of 9%Cr-2%W type rolled as a plate (denominated as CEA9-2) and for the steels of type 14%Cr-0.3%Mo (denominated as ODM401) and of type 14%Cr-1%W (denominated as CEA14-1) hotextruded as a rod are given in the Fig.1. Fig.1 The grain size and 3D reconstruction of texture of CEA9-2 ODS steel (left), ODM401 ODS steel (middle) and CEA14-1 ODS steel (right). Microstructure of the extruded and rolled (plate-shaped) CEA9-2 ODS alloy is made of bands consisting of pancake shaped grains. The average grain size in the transverse plane is about μm and the grains are elongated with a factor 1:2 in the extrusion direction. The microstructure of the rod-shaped 14%Cr ODS steels generally consists of cigar shaped grains elongated along the extrusion direction. Average grain size of ODM401 ODS steel in the transverse plane is about 0.3 μm and the grains are elongated with factor 1:10. The grains of CEA14-1 alloy have diameter about 0.55 μm and the grains are elongated with a factor 1:2.5 in the direction of extrusion. Bending KLST specimens taken from the ODS steels were identically oriented and notched perpendicularly to the extrusion direction. The notch of KLST specimens is indicated in the 3D reconstruction of the textures of the steels prepared in the Fig.1. Due to notch orientation regarding to the microstructural features and due to chemical composition differences, the

4 Powder Metallurgy Progress, Vol.12 (2012), No corresponding fracture appearance of 9%Cr and 14%Cr ODS steels completely differs. (see Fig.2). Fig.2 The 3D reconstruction of fracture surfaces of CEA9-2 ODS steel (left), ODM401 ODS steel (middle) and CEA14-1 ODS steel (right) fractured at -100 C. The 3D reconstruction of the fracture surface of plate-shaped CEA9-2 ODS steel shows that the fracture consists of transverse cracks and splitting perpendicular to the main cracking plane (see Fig.2 - left). The specimen behaved as a set of bonded thin samples each cracked in the plain stress condition. The edges of splits then formed pairs of shear lips of those thin samples. The stress state ahead of the notch is characterized by a high degree of triaxiality. If the stress in the plastic zone ahead of the notch is in any direction above the fracture stress, then the fracture proceeds preceding the cracking in the direction of the notch. The splitted parts of the fracture surfaces failed particularly by dimple micro mechanism and the transversal parts of fracture surfaces were formed by cleavage transcrystalline micro mechanism. The fracture surfaces of rod-shaped ODM401 and CEA14-1 ODS steels seem comparable with wrought steels: the fracture initiated ahead of the notch and spread along the extrusion direction (see Fig.2 middle and right). The uniaxially elongated grains led to the crack deviation along the grain boundaries. The microstructural features also affected the appearance of the fracture micro-mechanism (see Fig.3). The cleavage facets of the plate-shaped CEA9-2 ODS steel were elongated in accordance with pancake-shaped grains of the steel (see Fig.3 left). In the case of the rodshaped ODM401 and CEA14-1 ODS steels the cleavage facets are rather equiaxed which is consistent with the cigar shape grains mentioned above (see Fig.3 - right).

5 Powder Metallurgy Progress, Vol.12 (2012), No Fig.3 Fracture surface of CEA9-2 ODS steel extruded as plate (left) and CEA14-1 ODS steel extruded as bar (right) fractured at -100 C. The temperature dependences of impact energy of the ODS steels prepared are given in the Fig.4. The impact properties of 9%Cr CEA9-2 ODS steel are much better than those obtained with usual HIPed ODS steels and are comparable with impact properties of 2. generation of ODS Eurofer steel (see comparison in the Fig.4 - left). The ODS Eurofer steel was compacted using HIP and subsequent thermo-mechanical treatments (TMT) was used for removing porosity and changing morphology of Cr precipitates. On the other hand the CEA9-2 ODS steel was prepared by two step hot working (extrusion and rolling) and subsequent cooling in air. The 14%Cr ODS steels ODM401 and CEA14-1 presents a USE and a TDBTT comparable with those obtained in the former commercial Cr-Mo-Al MA957 ODS steel. Fig.4 Temperature dependence of the impact energy of 9%Cr ODS steel CEA9-2 (left) and 14%Cr ODS steels ODM401 and CEA14-1 (right). CONCLUSIONS The present study was focused on the impact fracture properties of Fe-(9-14)Cr-(1-2)W/Mo-(0.2-1)Ti-0.3Y 2 O 3 ODS steels prepared by mechanical alloying and subsequent hot-extrusion and rolling. The microstructure of ODS steels was strengthened both by solid solution (by Cr and W) and by dispersion of nanosized yttria particles. The microstructure of as-extruded rod-shaped ODS steels contained uni-axially oriented grains elongated in the extrusion direction. In the case of subsequently rolled plate-shaped ODS steel, the grains

6 Powder Metallurgy Progress, Vol.12 (2012), No were two-axially oriented formed flat grains. The fracture appearance was affected both by chemical composition (9% or 14%Cr) and by microstructural features (grain shape, texture). Hot extruded materials present much better longitudinal T DBTT and USE than usually as-hiped ODS steels. The non-recrystallized 14%Cr-1%W steel presents much better impact properties than recrystallized 9%Cr-1%W ODS steel. The pronounced morphologic and crystallographic texture leads to a strong anisotropy in impact properties. Extensive splitting of fracture surface was evidenced and the orientation of the fracture plane towards the extrusion direction significantly modifies the impact response. Acknowledgement This work was realised in CEITEC - Central European Institute of Technology with research infrastructure supported by the project CZ.1.05/1.1.00/ financed from European Regional Development Fund. REFERENCES [1] Fischer, JJ.: U.S. Patent 4,075,010, issued 21 February 1978 [2] Lambard, V.: Method of manufacturing a ferritic-martensitic, Oxide Dispersion Strengthened alloy. US Patent 6,485,584 B1, 2002 [3] Ott, EA. et al.: Method for preparing an article having a dispersoid distributed in a metallic matrix. US Patent 6,974,506 B2, 2005 [4] Hayashi, T. et al.: Acta Materialia, vol. 56, 2008, p [5] Hoelzer, DT. et al.: Journal of Nuclear Materials, vols , 2007, p. 166 [6] Klueh, RL. et al.: Journal of Nuclear Materials, vols , 2002, p. 773 [7] Klueh, RL. et al.: Journal of Nuclear Materials, vol. 341, 2005, p. 103 [8] Benjamin, JS.: Composite metal powder. US Patent 3,591,362, 1971 [9] Lindau, R., Möslang, A., Schirra, M., Schlossmacher, P., Klimenkov, M.: J. Nucl. Mater., vols , 2002, p. 769 [10] Klimiankou, M., Lindau, R., Möslang, A.: J. Nucl. Mater., vols , 2007, p. 173 [11] Olier, P., Bougault, A., Alamo, A., de Carlan, Y.: J. Nucl. Mater., vols , 2009, p. 561 [12] Lucon, E.: Fusion. Eng. Des., vols , 2002, p. 683 [13] Preininger, D.: J. Nucl. Mater., vols , 2002, p. 514 [14] Sakasegawa, H., Chaffron, L., Legendre, F., Boulanger, L., Cozzika, T., Brocq, M., de Carlan, Y.: J. Nucl. Mater., vol. 384, 2009, p. 115 [15] de Castro, V., Leguey, T., Muñoz, A., Monge, MA., Fernández, P., Lancha, AM., Pareja, R.: J. Nucl. Mater., vols , 2007, p. 196 [16] Oksiuta, Z., Baluc, N.: J. Nucl. Mater., vols , 2009, p. 426 [17] Alinger, MJ., Odette, GR., Lucas, GE.: J. Nucl. Mater., vols , 2002, p. 484