Research on Filling Limit of Profile Ring Rolling on Circumferential Surface

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1 Research on Filling Limit of Profile Ring Rolling on Circumferential Surface J. H Kang R&D Center Director, JAC Coupling, 720-1, Hakjang-dong, Sasang-gu, Busan, Korea Abstract In the present study, we designed a ring rolling preform for producing a ringed product with a grooved outer circumferential surface. ne of the most common defects in the profile of a ring rolling process is unfilling, which occurs when the deformation amount of product is small or the depth of the groove in the outer surface is large. To prevent unfilling, the deformation amount should be increased, or the preform should be designed to be very similar to the final profile. A filling limit equation for eliminating defects from products is proposed based on the aspect ratio and forging ratio. The filling limit equation was derived using the results of finite element analyses for four cases. The validity of the proposed method was tested by comparing the predication of the filling limit equation with the results of a full-size ring rolling operation, and the production stability was verified with mass production test. Keywords Finite Element Analysis, Filling limit equation, Preform design, Profile ring rolling, Cost saving. Figure.1 Schematic illustration of general radial-axial ring rolling method and cross section I. INTRDUCTIN Ring rolling is widely used for ringed products owing to its high dimensional precision, high productivity, low production cost, and the uniformity of the circumferential metal circulation. Ring rolling is used for producing a wide range of ringed products including gear rims, slowing rings, rail wheel bearings, rings used for jet engines and spacecraft, nuclear reactor parts, connection flanges, wheels and valves. Generally, the facilities used for ring rolling described in Figure.1 can produce rings with rectangular cross sections by extending the outer diameter surface. The ring extending mechanism is pressing a doughnut shaped preform in the radial direction with a main roll and a mandrel, and a pair of axial rolls is used to prevent unfilling in the direction of the thickness. [1] The most widely used ring products in industries are assembled by bolting or welding with other components. The ring products with rectangular section to be machined for bolting flange or welding neck have a lower material utilization ratio than the profiled ring product due to machined out volumes as shown in Figure.2. (a) Bolt connection (b) Welding connection Figure.2 Shape comparison of rectangular section ring and profiled ring Regarding ring rolling, Johnson and Needham [2] explained the flow in rectangular-shaped products during the ring rolling process using the upper bound method and experiments. Mamalis [3] experimentally validated the flow characteristics in T-shaped products. 40

2 Yang and Kim [4] analyzed ring rolling process on steady state, plain strain conditions and Kim et al. [5] performed 3 dimensional finite element analysis with spatial mesh system for rectangular section ring and T shaped ring. Shivpuri and Eruc [6] proposed a preform design method for improving the precision of the crosssectional dimensions of the final product using rectangular cross-sectional rings with ERCRNGRL program. Joun et al. [7] introduced axisymmetric finite element analysis method to optimize preform shape filling the profile of hub races and Davey and Ward [8] applied successive preconditioned conjugate gradient method to ring rolling analysis. Recently researches related to ring rolling process are focused on profile ring rolling technique for cost savings. Kim et al.[9] analyzed the ring rolling process for large slewing ring by finite element analysis, and also presented experimental validations of profiled products. Yeom et al.[10] investigated rolling defects in a large scale ringed product of alloy steel. Hua et al.[11] conducted analytical and experimental studies to generalize the preform of profile ring rolling and establish a preform relationship for obtaining the final profile. Wang et al.[12] also studied stable profile ring rolling for the mass production of ring products. Kim et al.[13] applied FE analysis and physical modeling to optimize blank dimension. Parvizi and Abrinia[14] applied upper bound method to ring rolling process for the feasibility of industry and verified the result comparing FE analysis results. Rectangular cross section rings are often characterized by overlaps on their inner and outer circumferential surfaces, and their upper and lower surfaces, whereas profile cross section rings often have unfilling defects in their profile. To produce ringed products with profile cross sections, the change in the profile and deformation amount to the final product should be taken into consideration in designing the profile and dimensions of the preform. Although the unfilling can be estimated by finite element analysis, the long time required does not make it feasible in industry. In this study, we derived a filling limit equation that can be used to determine the proper filling of the final product using the dimension change rate and the aspect ratios of the final product and the preform. The validity of the proposed equation was also tested by comparing its predictions with the results of actual production. II. FILLING LIMIT EQUATIN F PRFILE RING RLLING A. Definition of parameters for profile ring rolling Figure.3 shows representative profiles of a ring product with a profiled outer circumferential surface and its preform. 41 Figure.3a shows the final profile to be produced by ring rolling. Figure.3b shows the profile of the rectangular cross section preform for producing the ringed product, and Figure.3c shows another preform that can be used to reduce deformation amount to be forged. (a) General shape of profiled ring (b) Rectangular section preform (c) Shaped section preform Figure.3 General descrption of profiled ring and deformation amount from preform to final shape The rectangular cross section and profile cross section preforms can both be used to forge the profile shown in Figure.3a. However, unfilling may occur when using the rectangular preform shown in Figure.3b if the deformation amount to be ring rolled is large.

3 If the thickness change in the radial direction is large, filling can be achieved, but if the displacement between the preform and the final profile is small, unfilling may occur. If the profile of the preform changes as shown in Figure.3c, unfilling can be solved with a small displacement by changing the forging quantity until the final profile is produced. In this study, the amount of rolling in the axial direction was not taken into consideration. This was because the volumetric movement in the vertical direction was generally designed to be minimized in the profile ring rolling process to prevent unfilling defect due to axial direction volume movement. The aspect ratio (SF) used to define the filling limitation is the ratio of the thickness (Si) to the depth (Li) at the point of the profile as shown in Figure.3b and 3c. The aspect ratio can be expressed by Eq.(1). This equation represents the difficulty of forging. Li SFi (1) Si L i : Forging displacement between the preform and the final profile S i : Thickness of the final product The forging ratio (DR) is the ratio of the preform thickness to the final product thickness as shown in Figure.3b and 3c. It is a measure of the displacement ratio in the radial direction and is expressed in Eq.(2). ( S0 Si ) DRi (2) S0 S 0 : Thickness of the preform S i : Thickness of the final forged product Class Preform Profiled Ring Analysis Result Production Test Posi tion SF DR Filling Case1 Not Tested Top X a Case2 Top Bot tom X b c Case3 Top d Case4 Top Bot tom e f Figure 4 Description of 4 Cases of ring rolling analysis condition and analysis result 42

4 B. Finite Element Analysis The main process parameters of ring rolling are the displacement in the radial direction caused by the main roll rotation and the mandrel movement, the displacement in the axial direction caused by the axial roll rotation and movement, and the profile of the final product. In this study, using the same process conditions as those of the production of rectangular-shaped products, finite element analysis was conducted using four conditions. Three profiles used in industry were randomly selected for the finite element analysis of the final product. Two preform designs were also used, namely rectangular cross section and profile cross section. The forging ratio and aspect ratio were calculated for the four conditions by changing the profiles of the final product and the preform, respectively. Figure.4 shows the product shape of four conditions used for the analysis. Table I shows ring rolling operation conditions for finite element analysis. TABLE I SIMULATIN CNDITIN F RING RLLING PRCESS Description Material Value EN10025 S355NL Initial temperature 1250 Tool Temperature 200 Main Roll Rotary Velocity Mandrel Feeding Velocity Axial Roll Feeding Velocity 40RPM 0 ~ 1.5 mm/sec 0 ~ 0.3mm/sec In Case 1, in which a rectangular preform was used, the aspect ratio and forging ratio were higher than those of Case 3, in which a profiled preform was used. Case 1 therefore had a high forging difficulty but low forging ratio, which resulted in the unfilling phenomenon. However, Case 3 completed the filling with a low aspect ratio and high forging ratio because of the design of the preform profile, which was close to that of the final profile. The profiles of cases 2 and 4 had two flanges where the aspect ratio and the forging ratio in the upper and lower parts were different. It was confirmed that there was unfilling in the upper part in cases 1 and 2 respectively, which both had high aspect ratio and low forging ratio. Figure.5 shows the graph of the filling and unfilling phenomenon obtained by the finite element analysis. For the four conditions, filling and unfilling were found at a total of six points: two locations in the upper part in cases 1 and 3 and two locations in the upper and lower parts in cases 2 and 4. To increase the reliability of the graph, the aspect ratio and the forging ratio at the start and end of the filling were obtained by the finite element analysis for the four conditions. The results are shown in the Figure.5 The trend line in Figure.5 was drawn using the results of the finite element analysis, with regard to the point at which filling was completed. It indicates that filling occurred above the trend line, whereas unfilling occurred below the line. The filling limit equation was formulated by linear interpolation, and is as expressed in Eq. (3). Fi = DRi 0.404SFi (3) Here ) Fi > 0 : Filling Fi < 0 : Unfilling The applied raw material was EN10025 S355NL, which is widely used in welded structures. The flow stress in elevated temperature was calculated using JMATPR. The finite element analysis was conducted using the DEFRM TM -3D Ring Rolling module, with the assumption that the material was rigid-visco-plastic and that the main roll, axial roll, and mandrel were rigid bodies. Figure.4 shows the preform, final product, aspect ratio, forging ratio, and results of the finite element analysis for the four conditions. C. Filling limit equation for profile ring rolling In Figure.4, which shows the results of the finite element analysis, the contact with the main roll changed with the profiles of the final product and the preform. In cases 1 and 3, only the top part had a profile, and the profile of the preform was different. Figure 5 Deformation diagram to predict underfilling 43

5 Class Preform Profiled Ring SF DR F Prediction Production Result Q ty Test Under filling 1 Test Filling 1 Mass Production Filling 100 III. VALIDATIN F THE FILLING LIMIT EQUATIN To validate the filling limit equation, a trial product with two flanges in its outer circumferential surface was produced. The operation conditions and the raw material were the same as those of the finite element analysis. The rectangular cross section preform with EN S355NL was forged after heating to 1250 C and then ring rolling process was followed after reheating the preform at 1250 C. The rectangular preform was used to forge trial products with outer diameters of 640mm and 795mm respectively, and their filling was examined. The filling and unfilling were estimated for the two conditions using Eq.(3) and deformation diagram, and the results are shown in Figure.6 and Figure.7. The products shape and operation conditions such as shape factor and deformation ratio were shown in Figure.6. In the product estimated as unfilled in Figure. 6 and Figure.7, it was observed that the upper and lower parts could not contact the mold, and the rolling was completed with the unfilling condition shown in Figure.6. In the product with an outer diameter of 795, which is shown to be filled in Fig. 6, it was confirmed that the entire outer surface contacted the main roll, which indicated filling as predicted. Figure 6 Verification test conditions and prediction with Filling Limit Equation To verify the repeatability of the proposed filling equation, mass production was applied for the product with the same shape of two flanges in the outer circumferential surface. Rolling conditions and dimension were described in Figure.6 and the products produced in mass production conditions are shown in Figure.8 respectively. No unfilling defects were found in 100 pieces and the prediction capability of proposed equation was validated. Fig.7 Filling prediction on Deformation Diagram 44

6 The filling limit equation formulated in this study can be used as a guide for selecting the preform profile in the design stage, and it also helps to reduce defects and maximize the use of raw materials, thereby ensuring cost effectiveness. Fig.8 Mass production product IV. CNCLUSIN To generalize the preform selection method for preventing unfilling, which is one of the main defects that results from the profile ring rolling process using a main roll, we developed a filling limit equation based on the aspect ratio and forging ratio of the preform and the final product. Based on our findings, we draw the following conclusions. Filling and unfilling defects may be produced even when the same mold is used for the same profile ring rolling process, depending on the profiles and dimensions of the preform and the final product. This was confirmed by finite element analysis. The differences between the dimensions of the preform and the final ring rolling product can be generalized by the aspect ratio (which is an indication of the forging difficulty) and the forging ratio (which is an indication of the displacement), and this was used to derive a filling limit equation for evaluating the filling by finite element analysis. To validate the filling limit equation, two products with different outer diameters that could be used to assess filling and unfilling were produced. Although the two products were produced using preforms with the same outer profile, their parameters were found to be different because of their different final dimensions. The products exhibited the estimations of the filling limit equation and thereby validated it. The production repeatability was assessed with 100 pieces of mass production. No filling phenomenon in mass production test verifies the stability of proposed filling limit equation. REFERENCES [1] S.L. Semiatan, Forming and Forging, ASM International, 2001 [2] Johnson W., Needham G., Experiments on ring rolling. International Journal of Mechanical Sciences, Vol.10, pp , 1968 [3] Mamalis A.G., Hawkyard J.B., Honson W., Spread and Flow patterns in ring rolling, International Journal of Mechanical Sciences, Vol.18, pp.11-16, 1976 [4] Yang D.Y., Kim K.H., Rigid-plastic finite element analysis of plain strain ring rolling, International Journal of Mechanical Sciences, Vol.30, No8, pp , 1988 [5] Kim N.S., Machida S., Kobayashi S., Ring rolling simulation by the three dimensional finite element method, International Journal of Mechanical Science, Vol.30, No.4, pp.569~577, 1990 [6] Shivpuri R., Eruc E., Planning and simulation of the ring rolling process for improved productivity, International Journal of Machine Tools and Manufacturing, Vol.33, No.2, pp , 1993 [7] Joun M.S., Chung J.H., Shivpuri R., An axisymmetric forging approach to perform design in ring rolling using a rigid-viscoplastic finite element method, International Journal of Machine Tools and Manufacture, Vol.38, No.10-11, pp , 1998 [8] Davey K., Ward M.J., The practicalities of ring rolling simulation for profiled rings, Journal of Materials Processing Technology, Vol , pp , 2002 [9] Kim K.H., Suk H.G., Huh M.Y., Development of the profile ring rolling process for large slewing rings of alloy steels, Journal of Materials Processing Technology, Vol , pp , 2007 [10] Yeom J.T., Kim J.H., Park N.K., Ring-rolling design for large-scale ring product of Ti-6Al-4V alloy, Journal of Materials Processing Technology, Vol , pp , 2007 [11] Hua L., Qian D.S., Pan L.B., Deformation behaviors and conditions in L-section profile cold ring rolling, Journal of Materials Processing Technology, Vol.209, pp , 2009 [12] Wang L.G., Zhang Y.F, Wu J.S., Simulation and Test on rolling pairs of large wind-tower flange, China International Forge Master, pp.69 73, 2010 [13] Kim, S.Y., Nam, Y.G., Jeon, C.H., Joo, B.D., Moon, Y.H., The analysis of defects in ring rolling process by physical modeling, 14th International Conference on Metal Forming, pp , 2012 [14] A.Parvizi, K.Abrinia, A two dimensional upper bound analysis of the ring rolling process with experimental and FEM verifications, International Journal of Mechanical Sciences, Vol.79, pp ,