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MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AUSTENITIC STAINLESS STEEL AISI-321 AFTER RADIAL SHEAR ROLLING

Abdrakhman Nayzabekov1, Sergey Lezhnev1, Oleg Maksimkin2, Kira Tsai2,Eugeniy Panin3, Alexandr Arbuz3

ABSTRACT

In this work, an experiment was conducted for obtaining ultrafine-grained state of long bars of steel AISI-321, in which the workpiece while the minimum technically possible the temperature of 800ᴼС were rolled by a radial-displacement rolling with a diameter of 30 mm and diameter 13 mm on the mill SVP-08. Microstructure analysis showed the presence of equiaxial ultrafine-grained structure in the peripheral areas of the workpiece and the pres-ence of elongated fibrous texture in the axial zone. The strength of the workpiece has increased more than 2 times, while the plasticity has decreased not so much. The study of microhardness distribution showed a smooth decrease of 10.2 % from the periphery to the center of the produced sample, with an overall growth of microhardness of 2 times relative to the original.

Keywords: radial-shear rolling, ultrafine-grained structure, stainless steel, mechanical properties, microhard-ness, intensive plastic deformation, transmission electron microscopy.

Received 20 Septenber 2017Accepted 15 March 2018

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1 Rudny Industrial Institute, 50 years of October str. 38 Rudny, Kazakhstan2 Institute of Nuclear Physics of the National Nuclear Center of the Republic of Kazakhstan, Ibragimova str. 1 Almaty, Kazakhstan3 Karaganda State Industrial University, Republic str. 30 Temirtau, Kazakhstan E-mail: zubra_kz@mail.ru

INTRODUCTION

At present, progress in obtaining new metal materi-als is aimed at improving their internal structure. The most effective way to improve the internal structure of the metal, and therefore its properties, is the intense plastic deformation. Bringing the microstructure of the metal to ultra fine-grained (UFG) state opens exceptional prospects in the field of material engineering [1 - 2].

At the same time, with an increase in the share of grain boundaries in the volume of the workpiece, both the mechanical properties of the material and other properties such as electrical resistance or radiation re-sistance are changed [3]. The latter phenomenon is due to the fact that the surface of grain boundaries serves as a drain surface for radiation defects, and the state of the

UFG increases the number of such surfaces. The use of intensive plastic deformation should lead to a new class of materials for nuclear power, which would have not only an increased level of mechanical properties, but also resistance to neutron irradiation. This problem is very relevant, since a large share in the cost of operation of nuclear power plants is the replacement and recycling of structures working in the core. Transition to new more resistant materials will allow to save at once essentially.

However, difficulties begin here that are associated, first of all, with the lack of a reliable method of produc-ing a sufficient number of UFG materials of sufficient overall dimensions. The most commonly used and studied methods, such as equal-channel angular pressing (ECAP) with good microstructure quality, have signifi-cant limitations on the length of the resulting preform,

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which greatly complicates industrial application [1, 4]. The method of multiaxial forging allows to obtain rela-tively large blanks, but it is labor-intensive, inefficient and has its limitations [5 - 6].

Radial shear rolling (RSR) allows to obtain an UFG structure, but has problems with the homogeneity of the structure and properties over the cross section of the bil-let. The difference from cross-helical rolling, which is used, for example, for piercing pipes, is that the rolling of a solid rod is carried out along a three-roll scheme with large feed angles contributing to the intensification of shear deformation [7].

However, in the case of the production of articles having a tube shape (for example, the fuel element tube of the reactor), these drawbacks may not be significant. This gives topicality to the issue of obtaining an UFG structure by radial shearing rolling on materials used in nuclear power engineering.

Features of radial-shear rolling and equipment for its realization

Radial-shear rolling, as one of the types of cross-helical rolling, was developed in The National Univer-sity of Science and Technology “Moscow Institute of Steel and Alloys” (NUST “MISA”) [7 - 10]. The main difference and especially the particular type of rolling is that, in contrast to the stress-strain state with high tensile stresses in the case of classical helical rolling with pipe piercing, there are vice versa, compressive stresses with the same intense deformation of the peripheral metal layers. This combination is the key in the production of UFG materials by the methods of intense plastic de-formation. A similar scheme of the stress-strain state is realized by the use of three rolls arranged with respect to each other with an increased (up to 20 degrees or more) feed angle [7].

Such a scheme is very complicated in the theoretical description, since non-linear deformations occur with the interaction between each part of the workpiece with each roller in the volume of the workpiece, with mutu-ally intersecting gradients in the velocities and directions of deformation along the radius [11 - 12]. This causes nonmonotonicity and turbulence of the deformation, which favorably affects the formation of the UFG structure [12 - 15]. On the other hand, each layer of the metal of the billet along its radius receives its special combination of stresses, deformations, flow velocities

and flow directions, which leads to a difference in the resulting structure and hence properties along the cross section of the billet.

The workpiece along the section, according to the trajectory-velocity features of the metal flow, can be con-ditionally divided into 2 characteristic zones. The outer peripheral zone - receives a high shear strain. Elements of the structural structure of the metal subjected to the expanding flow with two-sided compression (along the trajectory and along the radius) take the form of isotropic isolated particles of high dispersity [7]. Axial area gets a high linear deformation with a reduction of turbu-lence deformation. The speed of particles in the axial fiber and its length as well as in the longitudinal rolling increases in proportion to the coefficient of extraction. The elaboration of the structure of the metal acts on the type of longitudinal rolling or pressing. The structural elements of the structure are extracted and refined with the formation of classical linear rolling texture. These features are described in detail and illustrated in the works of S.P. Galkin [7 - 10, 12].

Based on these data, NUST “MISA” was designed family of radial-shear rolling mills – RSP-10-30 [9, 10, 16], RSP 14-40 (also produced under the label of SVP-08), MISA -100, MISA -130T. These units are intended for pressure treatment by hot radial-shear solid billets of circular cross section and the finished rolled products with high requirements for precision and surface finish of alloy, high-alloy steels and non-ferrous metals and alloys, includ-ing alloys based on titanium, nickel, copper, zirconium, etc. Mills were exported to the enterprises and scientific organizations of Russia, Kazakhstan, South Korea, Poland. Among them, the mill SVP-08 set in Rudny Industrial In-stitute. The appearance of the machine is shown in Fig. 1.

Fig. 1. Radial-shear rolling mill SVP-08.

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The SVP-08 mill is designed for hot deformation of bars of solid round profile from practically any metal materials, including low-plastic, continuous cast and powder. Rolling of bars with a diameter of 8 - 35 mm is carried out in a three-roll stand of a special rigid construction from the initial blanks with a diameter of 15 - 40 mm by crimping them in diameter in one or several passes using rolls of special calibration and, if necessary, with intermediate heaters. Diameter of rolls 76 mm; the draw ratio reaches: 1.1 - 5.0; Mill capacity: 0.1 - 0.3 t/h; the power of the main drives is 3 × 7.5 kW.

A special feature of the mill’s design is the stand that provides higher stiffness of the stand, which is a single cylinder with turned grooves for roll cassettes inserted into a cylinder of larger diameter rigidly connected to the base of the mill. The synchronous movement of the rollers is ensured by a special design in front of the work stand.

EXPERIMENTAL As the material for the experiment was chosen as

one of the most used in nuclear power engineering steel grades - AISI-321 (0,08 % C, 17 – 19 % Cr; 9 - 11 % Ni; 2 % Mn; 0.8 % of Si; 0.5 to 0.7 % Ti). In particular, it is used to make tubes of fuel cells for fast neutron nuclear reactors, the most promising type of nuclear reactors. Also this brand is applied to production of the equipment working in environments of the increased aggressiveness (heat exchangers, pipes, details of furnace and reactor fittings, electrodes of spark incendiary candles).

For the experiment, we used a rod with a diameter of 30 mm and a length of 300 mm, the rolling temperature was chosen constant and equal to 800ᴼС. The choice of temperature is a compromise of the desire to use the lowest temperature to reduce the proportion of static recrystallization and the actual energy-power capabili-ties of the mill to produce the workpiece with a lower temperature and, consequently, with increased resist-ance to deformation. In addition, a similar temperature regime for obtaining the structure of stainless steels by equidistant angular compression was used in the work [4], which makes it possible to compare the results of the impact of different severe plastic deformation (SPD) processes on the metal.

For several passes the workpiece was rolled from 30 mm to 13 mm, then immediately cooled with water. After rolling out of the bar, cylinders with a length of 30 mm were cut on a high-precision Struers AccuTom-5

cutting machine along the bar, samples for tensile testing were cut, in the form of strips 30x3x0.3 mm (Fig. 2 (a) the lower strip on the right). The Central (largest) section was used for the preparation of ТEM-objects to study the fine structure. The places of taking samples for micro-structure research are shown in Fig. 2 (b). The remaining half of the solid (Fig. 2 (a), left) was used to measure the microhardness over the cross section of the rod.

RESULTS AND DISCUSSION Samples for fine structure studies were prepared

electrolytically in solution (600 ml methanol, 360 ml butyl cellosolve, 60 ml perchloric acid) and examined on a transmission electron microscope JEM-2100 (JEOL, Japan) at an accelerating voltage of 200 kV. Photos of typical microstructure species in the center and on the periphery of the rod are shown in Figs. 3 and 4.

The initial structure of the steel AISI-321 in the state of delivery has a coarse-grained structure with a grain size of about 40 - 60 µm. After deformation with a total yield of 4, after reaching the degree of accumu-lated deformation equal (by approximate estimates) to 11 - 13 at a favorable stress state, an ultra-fine-grained microstructure in the peripheral part of the rod with a grain size (300 - 600 nm) was obtained comparable to

Fig. 2. Preparation of a sample for tensile testing (A) and samples for TEM (B).

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the results of known studies [4, 13 - 15]. Thus, it should be noted that grains of peripheral part have equilibrium character. The structure of the central zone of the bar is elongated in the direction of rolling long and narrow grains resembling the texture of rolling.

The mechanical properties were determined by the tensile testing of flat specimens on the Instron-1195 test machine. For testing were selected strips, which central line passes at a distance of 0.5 of the diameter of the rod that will allow to obtain the most objective results, tak-ing into account the heterogeneity of the microstructure in the cross section of the rod. According to the results of testing of 5 samples, the average value of the tensile strength was equal to 1073 MPa, which is approximately 2 times higher than the original value of 580 MPa. The elongation was 21 %, which is approximately 2 times

less than the initial value of 40 %. Reduction of plasticity in this case is within the norm for materials that have undergone such treatment. One of the tensile curves is shown in Fig. 5.

The microhardness measurements were carried out on hardness testing HVS-1000B Vickers with a force of 9.87 N at shutter speeds of 15 seconds. Given the hetero-geneity of the microstructure it was decided to investigate the change of the microhardness over the cross section of the rod. For this purpose measurements were carried out through each millimeter of sample cross-section 3 times at each point of the graph. The results for each point were averaged. Schedule changes of microhardness in the cross section are shown in Fig. 6. The initial micro-hardness is 160 HV. After radial shear rolling, the level of microhardness rose to 288 - 321 HV and generally

Fig. 3. Microstructure of the peripheral zone of the AISI-321 steel rod after radial shear rolling.

Fig. 4. The microstructure of the axial part of the rod in AISI-321 after radial-shear rolling.

Fig. 5. Tensile curve of AISI-321 steel sample after radial shear rolling.

Fig. 6. Change in microhardness along the section of AISI-321 steel rod after radial-sliding rolling.

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increased by 2 times compared to the original. At the same time, due to the structural inhomogeneity of the rod section, there is a smooth drop in the microhard-ness level of the central zone of the rod by 10.2 % (33 HV). A greater strengthening of the periphery is due to a significant intensification of shear deformation under conditions of coughing in contact with the rolls, and consequently, a decrease in the effect of static recrys-tallization. The decrease in microhardness at the center of the rod can be due to both the presence of another type of microstructure and the influence of the return processes, since the central part of the rod must have a high temperature and stay hot longer. Slight drop of hard-ness at the surface of the rods due to the presence of the decarburized layer formed during hot plastic working.

CONCLUSIONS

Thus, radial-shear rolling with a total drawing of 4, in one rod was obtained microstructure of two dif-ferent types. On the periphery of the rod was relatively equiaxed ultrafine-grained structure with a grain size of 300 to 600 nm, while in the axial zone of the rod was oriented stripe texture is similar to a rental. The study of the distribution of microhardness on the rod section also revealed the difference between the central and peripheral sections by 10.2 %. At the same time, the drop in the level of microhardness from the edges to the center was smooth. The overall level of the microhard-ness increased almost 2 times from HV 160 to HV 288 - 321. Also approximately 2 times increased mechanical properties - the tensile strength from 580 MPa to 1073 MPA. Plasticity is thus decreased.

Since the lowest possible technically possible tem-perature for radial shear rolling of this material was used, it can be asserted that in this type of mill, it is possible to produce stainless steel bars with an UFG structure with a limit of up to 200 nm. Further improvement of the microstructure can be achieved by a combination of temperature-strain treatment.

The results are in good agreement with the data of the studies [4, 7 - 15]. It is also important to note that obtaining such a structure by one of the most common SPD methods - equal channel angular pressing, requires at least 6 - 8 pressing cycles [1 - 2, 4] and is possible only for short blanks, while at the mill of the radial - Shear rolling is obtained in 3 - 5 passes for blanks of

unlimited length. The problem is the heterogeneity of the structure and properties in the axial and peripheral zones of the rod, however, for tubular parts this defect may be insignificant.

Improvement of the radial-shear rolling method in order to obtain a more homogeneous structure along the bar section and pipe products with an UFG structure will make it possible to obtain bulk UFG materials with less time and energy, which will allow us to talk about commercial profitability and cheaper production of UFG materials. This is especially important for such special areas as nuclear power, where not only the high mechani-cal properties inherent in the UFG state are claimed, but also the special properties of these materials, such as increased radiation resistance.

Acknowledgements Work is performed under the state budget-funded

theme № AP05131382 “Research and development of the technology for obtaining ultrafine-grained materi-als with improved mechanical properties and increased radiation resistance for their use as materials of the first wall of thermonuclear reactors and in nuclear power engineering” with the program “Grant financing of scientific researches on 2018-2020” (Customer - the Ministry of Education and Science of the Republic of Kazakhstan).

REFERENCES

1. R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Bulk nanostructured materials from severe plastic defor-mation, Progress in Materials Science, 45, 2, 2000, 103-189.

2. C.C. Koch, I.A. Ovid’ko, S. Seal, S. Veprek, Struc-tural nanocrystalline materials: Fundamentals and ap-plications, Cambridge, Cambridge University Press, 2007.

3. Patent № 2293619, Russian Federation, A method of helical rolling, S.P. Galkin, 2007, Bull. 5, (in Russian).

4. S.P. Galkin, Radial shear rolling as an optimal tech-nology for lean production, Steel in Translation, 44, 1, 2014, 61-64.

5. N.V. Lopatin, G.A. Salishchev, S.P. Galkin, Math-ematical modeling of radial-shear rolling of the VT6 titanium alloy under conditions of formation of a globular structure, Russian Journal of Non-Ferrous

Abdrakhman Nayzabekov, Sergey Lezhnev, Oleg Maksimkin, Kira Tsai, Eugeniy Panin, Alexandr Arbuz

611

Metals, 52, 5, 2011, 442-447.6. Patent № 2009737 Russian Federation, Three-roll

screw rolling mill and technological tool screw roll-ing, B.A. Romantcev, V.K. Mihaylov, S.P. Galkin, M.G. Degtyarev, B.V. Karpov, A.P. Tchistov, 1994, Bull. 3, (in Russian).

7. S.P. Galkin, Regulating radial-shear and screw rolling on the basis of the metal trajectory, Steel in Transla-tion, 34, 7, 2004, 57-60.

8. A.B. Naizabekov, S.N. Lezhnev, H. Dyja, T. Bajor, K. Tsay, A. Arbuz, N. Gusseynov, R. Nemkaeva, The effect of cross rolling on the microstructure of ferrous and non-ferrous metals and alloys, Metalurgija 56, 1-2, 2017, 199-202.

9. K. Tsay, A. Arbuz, N. Gusseynov, R. Nemkaeva, N. Ospanov, I. Krupenkin, Refinement of the micro-structure of steel by cross rolling, J. Chem. Technol. Metall., 51, 4, 2016, 385-392.

10. O.V. Rybalchenko, Ph. D. thesis, Influence of severe plastic deformation on the structure, mechanical and service properties of steel 08H18N10T, Moscow, 2014, (in Russian).

11. O.P. Maksimkin, M.N. Gusev, K.V. Tsai, A.V. Yarov-chuk, O.V. Rybalchenko, N.A. Enikeev, R. Z.Valiev,

S.V. Dobatkin, Effect of neutron irradiation on the microstructure and the mechanical and corrosion properties of the ultrafine-grained stainless Cr-Ni steel, 116, 12, 2015, 1270-1278.

12. B.A. Romantsev, M.M Skripalenko, G.P. Zhigulev, M.N. Skripalenko, Use of a numerical experiment to determine the trajectories of particles of metal undergo-ing deformation, Metallurgist, 55, 9-10, 2012, 753-757.

13. M. Diez, H.-E. Kim, V. Serebryany, S. Dobatkin, Y. Estrin, Improving the mechanical properties of pure magnesium by three-roll planetary milling, Materials science and engineering A, 612, 2014, 287-292.

14. B.A. Romantsev, S.P. Galkin, V.K. Mikhailov, V.N. Khloponin, A.N. Koryshev, Micro-microsection mill, Steel in translation, 25, 2, 1995, 55-58.

15. M. Kwapisz, M. Knapiński, H. Dyja, A. Kawałek, Analysis of the alternate extrusion and multiaxial compression process, Archives of Metallurgy and Materials, 60, 1, 2015, 149-152.

16. A. Kawałek, A. Gałkin, H. Dyja, K. Ozhmegov, M. Knapiński, B. Koczurkiewicz, Plastometric model-ling of the E635M zirconium alloy multistage forg-ing process, Solid State Phenomena, 220-221. 2015, 808-8012.