Ring-Rolling Design of Yaw Ring for Wind Turbines

Met. Mater. Int., Vol. 0, No. 3 (014), pp. 51~56 doi: 10.1007/s1540-014-3017- Ring-Rolling Design of Yaw Ring for Wind Turbines Jong-Taek Yeom 1, *, Jeoung Han Kim 1, Jae-Keun Hong 1, Jin Mo Lee, Kook Joo Kim, Tae Ok Kim, Nam Yong Kim, and Hi Sang Chang 1 Korea Institute of Materials Science, Titanium Department, 797 Changwondaero, Changwon 641-831, Korea Taewoong Co., 146-1 Songjeong-dong, Gangseo-gu, Busan 618-817, Korea (received date: 19 October 01 / accepted date: 8 September 013) In this work, a ring-rolling process to formulate ring-shaped components for a wind turbine is designed by means of a simulation and in an experimental approach. The target of the ring-rolling design is a yaw ring with an outer diameter of approximately 3,130 mm. The ring-rolling design includes the design of the geometry and the optimization of the process variables. A calculation method was used for the geometry design, in this case for the initial billet and the pre-form (or blank) sizes, and for the final rolled ring shape. Also, a deformation map-based approach was utilized to determine the initial ring-rolling temperature and feed rate of the mandrel. A three-dimensional finite element method was used to predict the formation of rolling defects and the deformed shape in the ring-rolled components. The design criteria are to achieve uniform distributions of the strains and temperatures as well as defect-free ring-shaped components. Finally, an optimum process design to obtain a sound large-scale yaw ring without defects is proposed. It is validated by comparisons between the experimental data and the FE analysis results. Key words: metals, hot working, defects, recrystallization, rolling 1. INTRODUCTION At present, a rapidly growing market segment is the group of seamless ring-shaped components, including bearings and tower connector flanges, for wind power generation. Generally, ring-shaped components for wind turbines have been produced by a ring-rolling process. The ring-rolling process is widely used to produce seamless rings with diverse outer diameters for power generation plants, aircraft engines and large cylindrical vessels [1]. In the conventional ring-rolling process [], large ring-type products with a complicated cross-sectional shape are manufactured by machining rings with a plain cross-section which are rolled by a ring-rolling mill. Figure 1 shows a schematic diagram of a radial-axial ring-rolling mill. The main driving parts of a ring-rolling mill are the main roll, the mandrel and the axial roll. In the first step of the ringrolling process, a donut-shaped pre-form (or blank) is created by a hot forming process, including a piercing process. The blank is placed on the ring roller over an un-driven mandrel, and the mandrel is forced under pressure toward a driven main roll. When the blank comes in contact with the main roll, the friction between the main roll, the blank and the mandrel leads to the rotation of the blank and mandrel in the direction the main roll is turning. Through the ring-rolling process, the wall thickness of the ring is progressively reduced and its diameter is simultaneously increased. During the rotation of the ring, the reduction of the height is controlled by an axial roll. Although ring rolling is the most commonly used ring manufacturing process, very few attempts have been made to create a design methodology for the ring-rolling process [3-5]. In this work, the ring-rolling design of a yaw ring with a diameter of approximately 3,130 mm is presented via a dimension method, a deformation map-based approach, and a FEM (finite ele- *Corresponding author: yjt96@kims.re.kr KIM and Springer Fig. 1. Schematic diagram showing a radial axial ring-rolling machine.

5 Jong-Taek Yeom et al. ment method) simulation. The proposed optimum design for the ring-rolling process of the yaw ring is verified in a comparison with an actual rolled yaw ring. 3.1. Dimension design of the ring-rolling process The calculation method for the dimension design of the ring-rolling process was based on the method suggested by Shivpuri [3]. The tolerance (TOL) for each of the ring surfaces is expressed by the following equation: TOL = F 1 + F + F 3 (). EXPERIMENTAL PROCEDURES The material for the yaw ring used in this work was lowalloy steel (KS SCM 440). The microstructure of the low-alloy steel was formed from typical low-alloy steel ingot with a dendrite structure. To analyze the high-temperature deformation behavior of the material with state variables such as the strain, strain rate and temperature, hot compression tests were performed in a temperature range of 850 C and 150 C at 50 C intervals with strain rates between 10 3 and 10 s 1. Compression test specimens 1 mm in height and 8 mm in diameter were machined. All tests were carried out in a vacuum (~10 torr) up to a strain of approximately 0.7 using a computer-controlled servo-hydraulic testing machine. The specimens were quenched by purged N gas as soon as the compression tests were completed. The flow stress curves obtained from the compression tests were corrected for the deformation heating effects using the following expression [4]: α σdε 0 ΔT = ----------------- C p ρ ε Here, α is the fraction of plastic work converted to heat, considered here to be 0.9. r is the density and C p is the heat capacity. The flow curves are then plotted against T 0 +DT, where T 0 is the nominal test temperature, and the isothermal flow stress data are obtained by interpolating these plots [6]. Finally, the corrected flow curve data were used to generate a deformation processing map of the yaw ring material. 3. RESULTS AND DISCUSSION (1) In this equation, F 1 is the allowance for unavoidable defects, F is the growth factor for the ring diameter and F 3 is the growth factor for the ring height or wall thickness. In particular, unavoidable defects in the F1 allowance are typically surface defects such as flanking and pitting. The initial dimensions are obtained from the final product by adding the tolerance (TOL) and are checked from the size and weight limitations of the mill system. The tolerances differ depending on the materials, the product weight, and the size. For carbon steel materials, the applied tolerance considered an additional volume of 10% [3]. The final yaw ring size in this work was determined as an OD (outer diameter) of 3,130 mm, an ID (inner diameter) of,70 mm and an H value (height) of 185 mm. In the ring-rolling process, non-uniform shape defects such as fishtail, washer- and sleeve-type defects, as shown in Fig., often occur through a poor design of the ring-rolling process. To minimize the non-uniform shape defects, especially fishtail defects, the relationship between the wall thickness and the ring height, expressed using Eq. (3), was used. h 1 b 1 = h h 1 h b h b 1 h b ---- = 1 ---- + ---- Here, the subscripts 1 and denote the blank and finished ring dimensions, h is the height, b is the wall thickness, and d is the diameter. This relationship shows that the formation of ring-rolling defects can be avoided when the ratio of the radial to the axial feed per revolution is kept equal to the ratio of the axial to the radial dimensions. In general, a mill system has fixed punches, and the punch size determines the inner diameter of the blank (d 1i ). Therefore, the blank geometry was obtained in this case by calculating the outer diameter (d 1o ) and the height of the blank (h 1 ). Assuming volume constancy during the rolling process, d 1i determined by the punch size, and h and b given by the desired final ring dimen- (3) Fig.. Several types of the non-uniform shape defects: (a) fishtail, (b) washer-type shape defect and (c) sleeve-type shape defect.

sions, Eq. (5) can be formulated. It is solved for b 1 numerically as follows: d 1o d 1i h A 1 h 1 A h = ---- = ----------------- h 1 d o d i Ring-Rolling Design of Yaw Ring for Wind Turbines 53 (4) b 1 d o + d ----------------------- i ---- 1 ( d 1i + b 1 ) b b ---- b = h + ---- 1 h (5) From Eqs. (4) and (5), the blank height (h 1 ) and outer diameter (d 1o ) can be obtained by substituting the previously defined values of the height (h ) and wall thickness (b ) of the finished ring and the inner diameter of the blank (d 1i ) [4]. On the basis of the calculation method, the dimension design of the yaw ring was calculated. The blank size was determined as an OD (outer diameter) of 1,0 mm, an ID (inner diameter) of 450 mm and an H value (height) of 370 mm. 3.. Method of evaluating the hot workability of the yaw ring material A dynamic materials model (DMM) [7-1] was developed on the basis of the fundamental principles of the continuum mechanics of a large plastic flow, physical systems modeling, and irreversible thermodynamics. The DMM includes an important parameter called the efficiency of power dissipation. The parameter h is a dimensionless parameter, representing how efficiently the material dissipates energy by microstructural changes. Therefore, this parameter is an important parameter for determining the optimum conditions for thermo-mechanical processing. That is, the parameter h indicates the dissipating ability of the workpiece as normalized by the total power absorbed by the system. For an ideal linear dissipater, m = 1 and η = 1. Details can be obtained from the work of Malas and Seetharaman [13]. η = m/ ( m + 1) m = ( logσ/ logε ) T, ε However, the DMM is limited in terms of its ability to express unstable conditions. In order to resolve the limitations of the DMM, the instability criterion developed by Ziegler was added to the DMM [14]. According to Ziegler s criterion, deformation would be unstable when a dimensionless instability parameter x becomes negative, as shown in Eq. (7). ln( m/ ( m + 1) ) ξ = ----------------------------------- + m < 0 (7) lnε 3.3. Deformation process map of yaw ring materials and its analysis To analyze the high-temperature deformation behavior and establish the deformation processing map of the yaw ring materials, hot compression tests were conducted. The yaw ring (6) Fig. 3. Deformation processing maps for (a) a low-alloy steel (KS SCM440) at a strain of 0.7 and (b) mild steel at a strain of 1.0 as reported by Prasad and Sasidhara [16]. material selected in this work was a low-carbon steel (KS SCM440). On the basis of analyses of the flow curves and microstructures obtained from the hot compression tests, the processing map for the low-alloy steel at a true strain of 0.7 was established, as shown in Fig. 3. In this case, the sensitivity values (m) of the strain rate at different temperatures and the strain rates required to obtain the power dissipation parameter (h) were determined from a plot of the Log(s) versus Log( ε ) at a given temperature with the cubic spline interpolation method. The processing map of the low-carbon steel was compared with that of mild steel at a strain level of 1.0 by

54 Jong-Taek Yeom et al. Prasad and Sasidhara [16]. In the processing map, the contour indicates the efficiency lines for the power dissipation h and the crosshatched area presents the unstable hot-deformation region. The map shows relatively high efficiency values at two different domains, indicating that this regime is operating in an optimum hotworking condition. The first is a domain with peak efficiency in the temperature range of 850 to 900 C at a strain rate of 10 3 s 1. This domain occurs in relatively low temperature conditions and has a peak efficiency of about 0.41. The second is a domain with peak efficiency in the temperature range of 1150 to 150 C with a strain rate ranging from 10 - to 10 1 s 1. This domain has a peak efficiency of about 0.38. Prasad and Sasidhara [16] also found two maximum η domains with similar areas in the processing map of mild steel, as shown in Fig. (b). One is a domain with peak efficiency of 0.37 at a temperature of 950 C and a strain rate of 10 s 1, and the other was observed in the temperature range of 1000 to 100 C and strain rate range of 0.04 to 1 s 1 with a peak efficiency value of 0.4. In this study, the microstructures clearly indicated the formation of wedge cracking at a relatively low temperature region and at lower strain rate ranges as well as active dynamic recrystallization at relatively high temperature regions within a strain rate range of 0.04 to 1s 1. Figure 4 shows the microstructures observed at the optimum forming regime and at the unstable regions. The microstructure observed in the optimum forming regime indicates the formation of fine grains in the prior g phase grain due to the dynamic recrystallization behavior [16]. On the other hand, the microstructure observed in the unstable region shows evidence of flow localization. These results were in good agreement with the results of Prasad and Sasidhara [16]. Finally, from the analyses of the deformation processing map and various operating conditions, the initial ring-rolling process condition required to avoid forming defects is as follows: a heating temperature of 150 C considering the temperature drop during the transfer of the workpiece from the furnace to the press and a feed rate of the mandrel between 0.5 and 1 mm/s. Fig. 4. Optical microscopy images observed in (a) the optimum deformation condition (100 C, 0.1 s 1 ) and (b) in the unstable regime (950 C, 10 s 1 ). Fig. 5. FE modeling for the ring-rolling process of the yaw ring. 3.4. Process design of ring rolling and its validation The commercial FEM code FORGE was used for the ringrolling simulation of a yaw ring with a diameter of approximately 3,130 mm. The yaw ring was ring-rolled with the mandrel and main roll design shown in Fig. 5. Based on a previous simulation of the ring-rolling process [4], the friction coefficient and interface heat transfer coefficient were set to 0.5 and 11 kw/ Km, respectively. The initial heating temperature of the blank was specified as 150 C. The angular velocity of the main roll and the feed rate of the mandrel were 18 rpm and 0.5 mm/s, respectively. These process variables were selected with the optimum conditions of the ring-rolling process obtained from the deformation processing map of the yaw ring material. Figure 6 shows the simulation results at the final stage of the ring-rolling process. In the simulation results of the stain and temperature distributions for the ring-rolling process, the highest strain level was observed at the top corner of the surface area, which was in contact with the main roll. The strain level is higher than that at the mid-plane, but the temperature level at the surface area is lower than that at the mid-plane due to the heat transfer between the workpiece and the work roll. The deformed shape obtained from the FE simulation indicates a uniform shape. Figure 7 shows the flow instability map at a true strain of 0.7, which can be used to determine the deformation stability of typical node points during ring-rolling simulations. The average strain rate of the FE simulation shown in Fig. 7 was determined by the total effective strain divided by the current time. The simulation result indicates that most node points are in a stable region. This means that if the yaw ring is manufactured with the optimum conditions of the ring-rolling process as determined from the deformation processing map, a sound yaw ring without forming defects can be obtained. It also demonstrates that the optimum ring-rolling conditions for a yaw ring with an outer diameter of about 3,130 mm would be a heating temperature of 150 C and a feed rate of 0.5 mm/s. Figure 8 shows the actual ring-rolling process of the yaw

Ring-Rolling Design of Yaw Ring for Wind Turbines 55 Fig. 7. Flow instability maps at a true strain of 0.7 to determine the deformation stability at a temperature of 150 C and feed rate of 0.5 mm/s. Fig. 8. The actual ring-rolling process of the yaw ring. Fig. 6. FEM simulation results obtained from the final stage of the ring-rolling process (the B cross-section). ring performed with the optimum design conditions. First, a slab of low-alloy steel was forged by a hydraulic press. The final yaw ring was made through cogging, upsetting and punching, and ring-rolling processes. Figure 9 shows the ring-rolled yaw ring as well as images of the macrostructure and microstructure of the ring. The images show that the actual ring has a relatively uniform shape without forming defects such as fishtail, fold, or crack defects. From the microstructures observed on the surface and middle areas of the ring, a uniformly fine pearlite and Fig. 9. (a) Ring-rolled yaw ring, (b) macrostructure of the rolled yaw ring, (c) microstructure observed on the surface and (d) that in the middle areas.

56 Jong-Taek Yeom et al. ferrite structure exists on these areas. 4. CONCLUSIONS The process design for the ring rolling of a yaw ring was formulated by a FE simulation and a deformation process map approach based on a dynamic materials model. The optimum process design was validated through the creation of a yaw ring by the ring-rolling process. The following results were obtained. (1) In order to determine the optimum ring-rolling condition with which to manufacture a yaw ring, a deformation processing map of the dynamic materials model, including instability criteria, was used. The processing map of the yaw ring material generated from hot compression test results indicated that the highest efficiency of power dissipation (h) is obtained at a temperature of 150 C with a strain rate condition corresponding to the feed rate of the mandrel, 0.5 mm/s (~0.1 s 1 ). () From a finite element simulation of the ring-rolling design, it was noted that the ring-rolling process performed under the optimum condition led to a sound shape of the yaw ring without forming defects. (3) The actual ring rolling of the yaw ring carried out by the optimum process condition resulted in a relatively uniform shape, with the microstructure of the yaw ring absent of defects. ACKNOWLEDGMENTS This work was financially supported by the Ministry of Knowledge and Economy, Korea, under the program (R000106) entitled Leading Industry Development for the Dongnam Economic Region. REFERENCES 1. E. Eruc and R. Shivpuri, Int. J. Mech. Tools Manufact. 3, 379 (199).. S. L. Semiatin, ASM Handbook: Vol. 14A Metalworking: Bulk Forming, pp.136-155, ASM International Materials Park (005). 3. W. Wang, T. Lee, and M. A. Reed, Phys. Rev. B 68, 035416 (003). 4. R. Shivpuri and E. Eruc, Int. J. Mach. Tools Manufact. 33, 153 (1993). 5. J. T. Yeom, J. H. Kim, N. K. Park, S. S. Choi, and C. S. Lee, J. Mater. Process. Tech. 187, 747 (007). 6. B. S. Kang and S. Kobayashi, Int. J. Mach. Tools Manufact. 31, 139 (1991). 7. I. C. Yoo, J. J. Park, and S. J. Choe, J. Korean Inst. of Met & Mater. 34, 973 (1996). 8. Y. V. R. K. Prasad, H. L. Gegel, S. M. Doraivelu, J. C. Malas, J. T. Morgan, K. A. Lark, and D. R. Barker, Metall. Trans. A, 15A, 1883 (1984). 9. N. Srinivasan and Y. V. R. K. Prasad, Metal. Trans. A, 5A, 75 (1994). 10. M. C. Somani, M. C. Somani, K. Muraleedharan, Y. V. R. K. Prasad, and V. Singh, Mater. Sci. Eng. A 45, 88 (1998). 11. B. C. Ko and Y. C. Yoo, Met. Mater. Int. 5, 555 (1999). 1. J. T. Yeom, E. J. Jung, J. H. Kim, D. G. Lee, N. K. Park, S. S. Choi, and C. S. Lee, Key Eng. Mat. 345, 1557 (007). 13. J. C. Malas and V. Seetharaman, JOM 6, 8 (199). 14. H. Ziegler, Progress in Solid Mechanics, pp.93-193, John Willey and Sons, New York (1963). 15. S. H. Park, H. Yu, H. S. Kim, J. H. Bae, C. D. Yim, and B. S. You, Korean J. Met. Mater. 51, 169 (013). 16. Y. V. R. K. Prasad and S. Sasidhara, Hot Working Guide: A Compendium of Processing Maps, pp.6-34, ASM International Materials Park (1997).