ON RING DEFORMATION BY INTERNAL PRESSURE

Large-sized rings, manufactured by various methods of metal forming, are used in many industries. For the power industry, it is relevant to manufacture of retaining rings made of non-magnetic austenitic steel in order to strengthen the winding frontal parts of the rotors of turbine-type generators of a large unit capacity. In the process of generator operating, the retaining ring is one of the most loaded elements. As a result, material of retaining rings should have high strength properties, sufficient plasticity and good magnetic inductivity. Deformation of rings by internal pressure is the most promising and effective way of their cold hardening, providing a favorable and uniform stress-strain state of the metal in the manufacture of non-magnetic retaining rings for powerful turbine-type generators. Since the finished ring must acquire specific dimensions and a specified deformation degree in the process of cold hardening, the urgent task is calculation of the billet dimensions. The existing calculation procedure relies heavily on experimental manufacture data and is applicable only to a narrow range of rings, which reduces the accuracy of calculation and, ultimately, leads to an increase in ring allowances and a decrease in the metal utilization factor. In this research work a new technique for calculating the initial dimensions of rings, which is based on the incompressibility condition was developed and proposed. Taking into account the assumed boundary conditions, a system of two equations with three terms is compiled. To solve an incomplete equation system, it was suggested to introduce additional equations – in first version of the technique, the well-known solution of Nadai was used. In the second version – the condition of constancy of relative thickness of the ring wall permissible from the experimental data of deformation of rings of different sizes was used. The results of calculating the rings initial dimensions for both proposed techniques were compared with the experimental data. The maximum deviation from experimental data does not exceed 4 % and the deviation average value does not exceed 1 %, which indicates a sufficiently high accuracy of the proposed calculation techniques and the possibility of using them in manufacturing practice.

1. Gotlib B.M., Vakalyuk A.A. Manufacture of retaining ring for powerful turbo-generators: technology and control. Fundamental’nye issledovaniya. 2011, no. 12, pp. 96–101. (In Russ.).

2. Gotlib B.M., Vakalyuk A.A. Automation of the hydrostretching process of large diameter retaining rings. Vestnik Ural’skogo gosudarstvennogo universiteta putei soobshcheniya. 2013, no. 3 (19), pp. 18–33. (In Russ.).

3. Tokarev A.G., Savchinskii I.G., Sivak R.I. Strain hardening of billets of turbogenerator retaining rings with a capacity of 500 MW. Obrabotka metallov davleniem. 2010, no. 4 (25), pp. 94–98. (In Russ.).

4. Kolupitskii K.A. Stages of design modernization of turbogenerator rotor bandages. Izvestiya SPBGETU LETI. 2016, no. 9, pp. 38–41. (In Russ.).

5. Wang Zh., Ning X., Meng Q. etc. A new insight into manufacturing fine-grained heavy retaining rings. Materials and design. 2016, vol. 103, pp. 152–159.

6. Surzhenko I., Glavatska N., Berns H. Texture formation and anisotropy of mechanical properties of retaining rings made of austenitic CrMnN steel. Mat. – wiss. U. Werkstofftech. 2005, no. 2 (36), pp. 51–55.

7. Gotlib B. M., Vakalyuk A. A. Ill-defined regulation of hydrostretching process of large diameter retaining rings on a hydraulic press with a force of 300 MN. Vestnik UGATU. 2012, vol. 16, no. 2 (48), pp. 70–75. (In Russ.).

8. Li F., Zhang H., He W. etc. Compression and tensile consecutive deformation behavior of Mn18Cr18N austenite stainless steel. Jinshu Xuebao /Acta Metallurgica Sinica. 2016, vol. 52 (8), pp. 956–964.

9. Wang Z.H., Sun S.H., Wang B. etc. Importance and role of grain size in free surface cracking prediction of heavy forgings. Material Science Engineering A. 2015, vol. 625, pp. 321–330.

10. Wang Zh., Fu W., Sun Sh. Effect of preheating temperature on surface cracking of high nitrogen CrMn austenitic stainless steel. Journal of Materials Science & Technology. 2010, vol. 26(9), pp. 798–802.

11. Olmi G., Freddi A. LCF on turbogenerator rotors and coil retaining rings: material characterization and sensitivity analyses. The European Physical Journal Conferences. 2010, vol. 6, pp. 1–9.

12. Ren Y.L., Niu L.J., Ren J., Qi Z.Y. Dimension and property prediction of retaining ring in hydraulic expansion. Suxing gongcheng xuebao. 2014, vol. 21(6), pp. 1–6.

13. Olmi G., Freddi A. Reliability assessment of a turbogenerator coil retaining ring based on low cycle fatigue data. Archive of mechanical engineering. 2014, no. 1, pp. 5–34.

14. Balyts’kyi O.I. Corrosion-mechanical characteristics of the mate rials of nonmagnetic shroud rings of turbogenerators. I. 8Mn-8Ni-4Cr and 18Mn-4Cr steels. Materials Science. 1997, vol. 33(4), pp. 539–552.

15. Balyts’kyi O.I. Corrosion-mechanical characteristics of the materials of nonmagnetic retaining rings of turbogenerators. II. Highnitrogen 18Mn−18Cr steels. Materials Science. 1998, vol. 34(1), pp. 97–109.

16. Balyts’kyi O.I. Corrosion-mechanical characteristics of the materials of nonmagnetic retaining rings of turbogenerators. III. Crack formation in the course of service. Materials Science. 1998, vol. 34(2), pp. 279–287.

17. Sherlock T.P., Jirinec M.J. Failure of Non-Magnetic Retaining Ring in a High-Speed Generator Rotor. Handbook of Case Histories in Failure Analysis. Esakul K.A. ed. Vol. 2. ASM International, 1992, 525 p.

18. Orita K., Ikeda Ya., Iwadate T., Ishizaka J. Development and production of 18Mn-18Cr non-magnetic retaining ring with high yield strength. ISIJ International. 1990, vol. 30(8), pp. 587–593.

19. Unksov E.P., Johnson W., Kolmogorov V.L. Teoriya plasticheskikh deformatsii metallov [Theory of metals plastic deformation]. Unksov E.P., Ovchinnikov A.G. eds. Moscow: Mashinostroenie, 1983, 598 p. (In Russ.).

20. Nadai A.L. Theory of flow and fracture of solids. Vol. 1. New York: McGraw-Hill, 1950. (Russ. ed.: Nadai A. Plastichnost’ i razrushenie tverdykh tel. T. 1. Moscow: Mir, 1969, 864 p.).