Material flow analysis for hot-forming of 20MnCr5 gear wheel blanks

IDE 2008, Bremen, Germany, September 17 th 19 th, 2008 77 Material flow analysis for hot-forming of 20MnCr5 gear wheel blanks Rüdiger Rentsch Foundation Institute of Materials Science (IWT), Badgasteinerstr. 3, 28359 Bremen, Germany, rentsch@lfm.uni-bremen.de Abstract During the manufacture of disk-like gearwheel blanks made of 20MnCr6 (SAE 5120) a characteristic distortion behavior was observed, which has been related to the forging process. The steel showed a typical banded structure and a distribution of chemical elements (segregations) in rolling direction. When analyzing the micrographs of the forged gearwheel blanks, a significant variation in the material flow was found. In order to clarify the cause of the observed material flow variations and to determine its impact on the distortion potential of the disks, a twofold approach has been applied: a) to determine and to describe the material flow characteristics on basis of micrographs of forged disks and b) to carry out FE-analyses on the theoretical material flow and its parameters. In this contribution results of the material flow analysis are presented and compared with FE-analyses of the forming process. Keywords 20MnCr5, material flow, forging, disk, FEM 1 Introduction 20MnCr5 (SAE 5120) is a high performance steel for gear wheels in automotive industry. The material is produced by continuous casting. For gear wheel applications usually the material goes through two major hot-forming sequences after casting, hot-rolling at the steel manufacturer and die forging at the gear wheel blank manufacturer. During hot rolling, the blooms will be reduced in several steps to steel bars with diameters between 25 to 100 mm. The bars were cut into billets and forged to their final shape. The disks-like gear wheel blanks showed a characteristic distortion behavior, which has been related to the forging process [Clausen 2008]. Besides the chosen forging strategy, which determines the material flow, the material structure and segregations are possible parameter of the found distortion behavior. While in casting the material structure and segregations are generated, they will be deformed significantly during forging, which will then partially be cut off, depending on the applied cutting strategy. In addition the microstructure analysis of the forged disks revealed a significant variation of the material flow. Towards determining the impact of the forging process on the distortion potential of disks, this work describes results of their material flow analysis. 2 Forging of the gearwheel blanks The forging of the disks, the gearwheel blanks respectively, took place in a 3-step process (cf. Figure 1). In the first step billets (Ø 73 mm, height 59.7 mm) were upset with a pressure of 100 t to a height of 22.5 mm and an outer diameter of about 117 mm (see dotted line in Figure 1b). In the second step the resulting disks were pre-punched, i.e. forming a blind hole in its center, and sized with a main pressure of approx. 750 t (cf. cross-section in Figure 1b). In the final step the punching of a central hole was carried out with a pressure of 20-30 t. The forged disks had a diameter of 127 mm, a height of 22.5 mm and a center hole of 40 mm (cf. cross-section in Figure 1c). The final disk height of 22.5 mm corresponds to a logarithmic strain of 0.98. The

78 IDE 2008, Bremen, Germany, September 17 th 19 th, 2008 disks were forged at a temperature of 1250 C and then annealed to a ferrite-pearlite structure (930 C 1h / 3 min 650 700 C 2 h / air cooling). The micrographs in Figure 1 show a characteristic banded structure of rolled 20MnCr5 with a ferrite-pearlite structure which is more pronounced towards the part center (for details about the chemical composition see [Clausen 2008]). The banded structure results from different etching behavior of ferrite (bright) and pearlite (dark) in a saltpeter and hydrochloric etchant. When micro probing the billets a higher concentration of Chromium and Manganese was found in the brighter ferrite regions [Prinz 2008]. Figure 1: Structures of a billet cross-section (a), a disk cross-section after the 2 nd forging step (b) and of the disk cross-section of a final forging blank (c) The banded structure in Figure 1a is aligned parallel to the main direction of forming, i.e. to the rolling direction of the steel bars. Since the banded structure is related to the distribution of the chemical elements in the material, i.e. the ferrite and pearlite as well as the segregations, its deformation is directly governed by the forming process and the resulting material flow. On the other hand the local material properties at forging temperature determine the material flow as well. Thus the banded structure provides information of the local material flow. In Figure 1b a forged, intermediate state of the disk is shown, which is close to its final shape as it is already pre-punched and sized to the final dimensions of the forging blanks. Here the banded structure is deformed most in the pre-punched center, where major curvatures of the banded structure can be observed. From here this region of major curvature extends radially (to the left) by falling gradually towards the horizontal center line of the cross-section. In opposite to this, the micrograph of the disk in Figure 1c shows a quite different material flow characteristic. Starting at the separation plane of punching, here the main flow in radial direction is steeply directed downwards first. Then the material flow turns before it rises after about one third of the length of the cross-section. The punching tool moved relative to the disk cross-section in Figure 1c in upward direction, from bottom to top.

IDE 2008, Bremen, Germany, September 17 th 19 th, 2008 79 In order to clarify the cause of the observed material flow variations and to determine its impact on the distortion potential of the disks, a twofold approach has been applied: a) to determine and describe the material flow characteristics on basis of the investigation of forged disks and b) to carry out FE-analyses on the theoretical material flow. Since the disks were manufactured for a more comprehensive analysis on distortion phenomena in the manufacturing chain of gear wheel blanks (for more details see [Clausen 2008]), in the following all the disks were cut and casehardened after forging. The disks had a final diameter of 120 mm, a height of 15 mm and a center hole of 45 mm. In Figure 1c) the dashed line shows the cross-section of the disks after cutting, due to the applied cutting parameters. 3 Experimental analysis of the material flow In a first screening the material flow of 16 disks was analyzed at one cross-section each. Figure 2 shows the cross-section of two disks representing the two major flow patterns in a pronounced form (Figure 2a corresponds to Figure 1c and Figure 2b to Figure 1b). a) flow pattern 1 (disk No. 1) b) flow pattern 2 (disk No. 5) Figure 2: Variations of the material flow and their characteristics at the cross-section of forged and cut disks As criteria for grouping, the angle α and the y-value were chosen (cf. Figure 2). The angle α is measured between the direction of main material flow and the horizontal center line of the cut disks. The y-value is related to the starting point of the main flow. It is determined at the disk hole (R i ), relative to the horizontal center line. Since in some cases angle α is about 0, also the average distance between main flow line and center line was analyzed (y α ). Further the angles β and γ were measured as they provide, in principle, information on differences in the local flow resistance during forging which is stored in the finished disks. The above described material flow data were determined for the disks at 1 cross-section each (see Table 1). The mean values of α, y and y α have all negative signs, while the standard deviations are large, for angle α in particular. Using these criteria, two groups with different flow patterns were built, for which Figure 2 shows examples of extreme types. For 12 out of 16 disks a flow pattern according to pattern 1 was found (Figure 2a). Half of all disks showed a variation of pattern 1 for which the main material flow angle α was around 0, with a main material flow that starts closer or at the horizontal center line of the disks (small y-value) and run then below it. The number of disks according to pattern 2 were 4 out of 16 (Figure 2b). Disks with the flow pattern 2 have a main material flow which starts at or above their horizontal center line and falls then continuously, even across the center line (positive angle α). The mean values of both groups differ significantly from the mean values over all disks and the standard deviations are partially much smaller as well, in addition to the opposite signs for α, y and y α, which provide a clear distinction of the flow patterns on basis of the chosen values.

80 IDE 2008, Bremen, Germany, September 17 th 19 th, 2008 main flow value y (at hole) angle α of main flow line all disks average distance to main flow line y α angle β angle γ mean -0.19 mm -1.18-1.53 mm -44.5 48 std.div. 1.34 mm 3.40 1.34 mm 5.9 5.6 pattern 1 (12 disks) mean -0.73 mm -2.60-2.23 mm -48.4 47 std.div. 0.98 mm 2.42 0.44 mm 5.1 4.1 pattern 2 (4 disks) mean 1.43 mm 3.06 0.58 mm -37.0 46.7 std.div. 0.90 mm 2.15 0.85 mm 0.6 7.7 Table 1: Material flow data of 16 disks derived from 1 cross-section each. Since all disks were analyzed at only one cross-section, the material flow variation over the circumference needed to be determined as well. Further a possible source for material flow variations was identified. Initially the billets were upset to a height of 22.5 mm which resulted in an outer diameter of about 117 mm. On shifting the upset billets from the upsetting stage to the prepunching and sizing die (diameter of 127 mm), an eccentric forging of up to 10 mm was possible. It is assumed that, if eccentric forging took place, it would show up as material flow variation within single disks, in particular in disks which already showed an extreme material flow. In order to determine the variation of the material flow within the disks, 5 of the 16 disks of the first screening with a pronounced flow pattern were selected. At these disks, the material flow was analyzed at 3 additional cross-sections: at 90, 180 and 270 relative to the first cross-section. The above described material flow data for the 5 selected disks are listed in Table 2. disk No. main flow value y (at hole) angle α of main flow line average distance to main flow line y α angle β angle γ 1-0.9 +/-0.9 mm -3.8 +/-1.7-2.2 +/-0.3 mm -46 +/-4 49 +/-7 2-0.9 +/-0.4 mm -3.6 +/-1.2-2.1 +/-0.1 mm -49 +/-2 47 +/-3 3 1.4 +/-0.8 mm 3.1 +/-0.6 0.0 +/-0.5 mm -36 +/-1 48 +/-5 4 1.9 +/-0.7 mm 5.6 +/-1.9 0.7 +/-0.5 mm -31 +/-5 47 +/-4 5 2.3 +/-0.4 mm 5.5 +/-1.4 1.2 +/-0.2 mm -36 +/-2 53 +/-5 Table 2: Comparison of the material flow data of 5 disks derived from 4 cross-sections each. For the chosen disks, the mean values of the first three material flow criteria in Table 2 (y-value, angle α, y α ) allow for a clear assignment of the specific material flow pattern (flow pattern 1 for disks 1 and 2; flow pattern 2 for disks 3 to 5; cf. Figure 2). The deviations of the flow data are in the range of their respective group and are partially significantly smaller. The values of angle γ are about the same for all disks, while in case of angle β significantly smaller values were measured for the disks with flow pattern 2 (cf. Figure 2b) as the main material flow arose here above the center line of the cut disks. In order to access the quality of the forming process and to evaluate its deviations, the theoretical material flow can be identified on basis of FE-analysis. 4 Calculation of the material flow For the calculation of the material flow the finite element (FE) program DEFORM 3D has been employed. The numerical simulation of the forging process was carried out closely to the

IDE 2008, Bremen, Germany, September 17 th 19 th, 2008 81 description in chapter 2 (cf. Figure 1). The material data, like flow curves and others, were taken from literature [Doege 1986]. A half cylinder model was used und a visco-plastic material behavior with thermal-coupling was applied to the tetrahedron elements. Further remeshing was activated and a shear stress friction law with a coefficient μ=0.25 applied to the die surfaces with an average temperature of 300 C. For visualizing internal deformation and material flow, regular grids were inserted at symmetry planes of the initially cylindrical billet model (cf. Figure 1a). The grids deformed and displaced according to the local material flow and were carried on, from the first forming step (upsetting) to the final one. In the following figures, the results of the ideal, concentric forging are presented together with the results of forging with maximum eccentricity of the upset model (after first step) in the pre-punching and sizing die (second forming step). In Figure 4 views of half models after the second forming step are shown (cut in x/y plane). The horizontal grids (x/z plane) were placed at half height of the billet models, thus they also appear at about half height of the deformed models. In case of concentric forming (Figure 4a) the deformed grid shows a perfect symmetry relative to the y-axis. In y-direction the main deformation of the grid occurred in the center, because of pre-punching for the center hole, while the outer grid region warped slightly due to the asymmetric disk cross-section. a) b) Figure 4: Calculated material flow in the disks with the deformed central x/z plane of a cylindrical billet ( a) for a billet in the die center; b) for a billet eccentrically positioned in the die) The eccentric positioning (10 mm) of the upset billet model was applied in positive x-direction. The effect of eccentric forging in the pre-punching and sizing die on the deformation of the grid in x/z-plane shows the comparison of Figure 4a and Figure 4b. Here major differences arise only at the rim of the forged blind hole (pre-punching). The eccentric positioning leads mainly to a change of the material flow around the hole (see arrows in Figure 4b). The deformation of the grid is stronger in direction of eccentricity (+x-direction) and less strong in opposite direction than in case of concentric forming. The spike in the grid at the rim of the forged blind hole is generated during the very end of this forming step. When applying a vertical grid to the central x/y plane of the billet, the material flow in forging direction can be studied (see Figure 5). The material flow in the x/y plane as well as the deformed grid are symmetric for centric forging, because of y-axis symmetry. The overall material flow in this plane is similar for centric and eccentric forging, although for this plane the eccentricity of the upset billet was maximum (10 mm to the right side of the forging die axis). In both cases the main material flow follows along the center line of the disk cross-section in radial

82 IDE 2008, Bremen, Germany, September 17 th 19 th, 2008 direction. Although there are certain local differences in the material flow, in this symmetry plane (for both cases) the grid deformation for the eccentric forming is mainly shifted by about half the amount of eccentricity and in direction of eccentricity. Thus, at otherwise constant conditions, the form filling determined the material flow in this plane, which is about the same for both cases. However, when looking at the 10.th outer line of the grid, which was applied to the cylindrical billet, the asymmetric, shifted flow in the billet can be identified. Figure 5: Calculated material flow grids for the cross-section of the disks in the x/y plane; (A) for a billet in the die center; B) for a billet eccentrically positioned in the die) Applying the same method for identifying the main material flow line like for the analysis of the experimental results (cf. Figure 2), the simulation results reveal a flow pattern of type 1 in principle. Figure 5 shows, that the changes in the course of the main material flow line, due to eccentric forging, are rather small if compared with the results of concentric forging. Somewhat more obvious differences appear closer to the forged blind hole, where the main material flow line runs slightly steeper, respectively less steep, than in case of concentric forging, depending on the direction of eccentricity. The differences in material flow show clearer for off-symmetry planes (Figure 6). Figure 6 shows the deformed y/z grids of the initially cylindrical billet for the symmetrical (a) and eccentric forging condition (b). For concentric forging the y/z grid stays perfectly flat throughout the 3D process simulation, because of its symmetry relative to the y-axis. Their are two aspects following because of eccentric positioning of the upset billet (after 1. forging step), first the central y/z grid of the billet is shifted relative to die center, second the material flow is only mirror-symmetrical to the central x/y plane of the die. Both aspects led to the warping of the shown y/z grid in Figure 6b. At the beginning of the 2. forming step, the material displacement in the center of the die for the pre-punching (blind hole forming) determines the material flow. Because of its off-symmetry position, the y/z grid deforms intensively in the center (cf. arrows at point 1 in Figure 6b). However, at this stage of the process the overall forming would be the same for symmetric and asymmetric forming. When the material reaches the outer die shell, first in direction of eccentricity, the material flow needs to be diverted, at initially to the sides and then even backward, in order to completely fill the die. The material contacts the outer die shell in its center first (about half height), before it is mold into the top and bottom edges of the shell and the blind hole. Thus the y/z grid is bent at the outer die shell (cf. point 3 in Figure 6b), because of contact friction and subsequent material back flow into the die edges. At this stage of the forming process, a small material back flow occurs also at the bottom of the formed blind hole (point 1 in Figure 6b), but the back flow is stronger around the rim of the blind hole (point 2 in Figure 6b).

IDE 2008, Bremen, Germany, September 17 th 19 th, 2008 83 Figure 6: Calculated material flow in the disks with deformed y/z grid of the former cylindrical billet ( a) for central positioning in the die; b) for an eccentric positioning in the die) 5 Discussion The analysis of the micrographs of the 16 disks revealed two different flow patterns, 12 disks showed a type 1 flow pattern, in which the main material flow developed in the lower part of the disk cross-sections, and 4 disks a type 2 flow pattern, in which it developed in the upper part. Correspondingly the mean values of the characteristic flow data describe the flow pattern 1 as standard. Finite element analyses of the forming process confirmed flow pattern 1 as the theoretically expected material flow pattern from the continuous mechanics point of view. For the FE analysis a homogeneous, one-phase material was considered using flow curves of 20MnCr5 at forging temperature. Besides the two different flow patterns, the analysis of the disk micrographs showed rather strong variations. While 8 of the disks showed a material flow close to the standard flow pattern, each 4 showed an extreme type of pattern 1 or the different type 2 pattern. The closer analysis of 5 disks with extreme type of flow patterns revealed variations within the disks itself, which are significantly smaller than those between different disks and are clearly within the range of their respective group. Thus the eccentric forging in the pre-punching and sizing die does not influence the material flow significantly and is, therefore, not responsible for the resulting material flow pattern or the existence of two of them. In further FE process simulations the influence of asymmetric forging conditions was analyzed. Varying the die temperature and the die friction parameter led to a shifting of the material flow to some degree, but not as much as necessary for explaining the two different flow patterns, because of these parameters. As a direct influence on the material flow the eccentric billet positioning in the forging die was considered. The results confirmed the rather small affect on the homogeneity of the material flow within one disk. Hence the cause of the different flow patterns as well as its rather strong variations cannot be explained by these parameters alone, but they have at least an potential for intensifying the emerging flow patterns. From Figure 1 it can be derived, that the main material flow develops during upsetting and / or forging in the sizing die. During upsetting the billets experience the highest over-all logarithmic strain of 0.98. During the second forging step (pre-punching and sizing), a major forming takes place by forming the center blind hole, but the change of the disk diameter (area) leads to a total logarithmic strain of only 0.16, assuming a constant disk height of 22.5 mm. In Figure 1b the dotted line shows the cross-section of the billet after upsetting relative to its shape and size after pre-punching and sizing. In principle, the forging conditions in upsetting are rather simple and symmetric. Usually the material flow is axis symmetric to the direction of upsetting and often

84 IDE 2008, Bremen, Germany, September 17 th 19 th, 2008 mirror symmetric relative to the horizontal center plane. Since all disks went through the same machines and setup, the remaining parameter of interests are the effective, local workpiece temperatures and material properties. The forging temperature of 1250 C was applied by a coreless induction furnace. However deviations in heating were not identified. The banded structure of the rolled 20MnCr5 billets is related to the distribution of the chemical elements in the material, i.e. the ferrite and pearlite as well as the segregations (Chromium and Manganese). Thus the banded structure itself is not homogeneous, additionally it appears more pronounced towards the billet center as the micrographs showed. In order to clarify the influence of the distribution of the chemical elements in the material, segregation-free, spray-compacted disks are being analyzed. However, besides differences between the properties of the real disk material and the homogeneous material in the FE-model, the responsible mechanism or parameter leads still to a highly axis-symmetric deformation with smaller material flow variations in radial direction of each disk, but varies somehow over its height. 6 Conclusion During the manufacture of gearwheel blanks made of 20nCr5, a characteristic disk distortion has been observed, which has been related to the forging process. In order to determine the parameter of this distortion behavior and to describe the impact of forging on the distortion potential of the disks, a batch of 16 disks was analyzed and the forging process simulated by FE analysis. The analysis of the micrographs of forged disks revealed a significant variation in the material flow and allowed to identify two major flow patterns. Using the FE analysis, one of the material flow patterns was identified as the one, which is to be expected for homogeneous, nonsegregated material and homogeneous billet temperature. Further the FE analysis allowed to show, that asymmetries in forging of the billets, in friction or die temperature do not change the material flow enough to explain the occurrence of both flow patterns. This is in accordance with significantly smaller material flow variations observed experimentally within single disks, even in case of extreme material flow. Since all disks went through the same machine setup and external parameter showed only minor influences, the remaining parameters of interest are expected to be internal quantities like the effective, local workpiece temperature and the variation of material properties. The high axis symmetry of the material flow within each disk and a somewhat concentric distribution of the chemical elements in the billets, due to rolling of the steel, support this view. If this phenomenon is related to the rather high forging temperature, different flow patterns should not be observed at lower temperature. For this purpose the analysis of disks forged at lower temperature is under way. Further spray-compacted disks are being analyzed in order to determine the effect of the distribution of the chemical elements. Acknowledgement The authors thank the Deutsche Forschungsgemeinschaft (DFG) for the financial support of the project A3 in the Collaborative Research Center 570 Distortion Engineering. References Clausen, B.; Frerichs, F.; Klein, D.; et al.: Identification of Process Parameters Affecting Distortion of Disks for Gear Manufacture Part I: Casting, Forming and Machining. Submitted to proc. 2nd Int. Conf. on Distortion Engineering, 17.-19.09.08, Bremen, Germany, 2008. Doege, E.; Meyer-Nolkemper, H.; Saeed, I.: Fließkurvenatlas metallischer Werkstoffe. Hanser Verlag, München, Germany, 1986. Prinz, C.; Hunkel, M.; Clausen, B.; Hoffmann, F.; Zoch, H.-W.: Characterization of segregations and microstructure and their influence on distortion of low alloy SAE 5120 steel. Submitted to proc. 2nd Int. Conf. on Distortion Engineering, 17.-19.09.08, Bremen, Germany, 2008.