INFLUENCING PARAMETERS ON THE DISTORTION OF DISK-SHAPED GEAR WHEEL BASE BODY DERIVED FROM CARBURIZATION

M. Steinbacher, B. Clausen, F. Hoffmann, H.-W. Zoch

Foundation Institute of Materials Science, Badgasteiner Straße 3, D-28359 Bremen, Germany The distortion of components made of steel and the distortion management are key factors for a more economic production. The distortion is a versatile attendant to each manufacturing step. Therefore it is of common understanding that the final dimensional change of a steel component has to be considered as an accumulative result derived from the entire production chain. The approach of “Distortion Engineering” is applied to disks, which are used as an archetype for gear wheel base bodies. An in-extenso investigation of different manufacturing parameters throughout the process was performed using Design of Experiment (DoE) methods to identify major influences. Particular attention was paid to the interaction of the main parameters influencing distortion during heat treatment. Therefore disks with different history were case hardened using varying heat treatment parameters. Disk dishing and dimensional changes were examined. Keywords Carburizing, SAE 5120 (20MnCr5), disk, shape changes, size changes, DOE

1. INTRODUCTION The heat treatment of parts made of steel is in most cases accompanied by distortion. The elimination of the distortion related changes of size and shape of parts is in most cases indispensable. In the course of the heat treating process, the distorted parts are hardened and therefore must be machined using expensive manufacturing techniques like grinding. Hence the control of distortion is the key to cost and time reduction in the production chain. The Collaborative Research Center “Distortion Engineering” (SFB570) is scientifically dealing with the aim of distortion control. Its approach is to understand the mechanisms of distortion and then reduce size and shape changes either by avoiding the release of so-called distortion potentials or by applying compensation strategies. The distortion mechanisms of typical heat treated parts made of steel are examined on archetypal sample geometries. This paper deals with the carburization-related distortion of disks, which are used as archetypes of gear wheels.

2. BACKGROUND The distortion introduced during the heat treatment of steel parts made of transforming steel grades is strongly related to the local development of transformation and temperature gradients, the sources of stresses. The volume changes and the related stresses can lead to plastic deformation and eventually result in size and shape changes. The extent is heavily dependent on the hardenability of the treated steel, on the quenching conditions, and on the dimensions of the treated parts. A paradigm for typical distortion behavior of different steels was already given by Wyss [1] (see figure 1).

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Figure 1: Influence of dimension, hardenability and quenching media on cylinder distortion

(according to [1]) During carburization, the chemical composition of the surface layer is altered by the addition of carbon. The increased carbon content affects the transformation behavior of the carburized layer. Thus most diffusion controlled transformation mechanisms are delayed in time and martensite start as well as martensite finish temperatures are shifted to lower temperatures. The carburization therefore produces a transformation behavior and timing of the carburized layer different from the core. In figure 2, the effect of the transformation diversity is shown. It can be seen that there are compressive stresses in the surface and tensile stresses in the core after the quenching process. Regarding the distortion of carburized parts, it can be assumed that a different behavior compared to a non carburized part is produced during quenching.

Figure 2: Effects of differences in the TTT diagram of surface and core on the stress development during

cooling (according to [2])

3. EXPERIMENTAL The investigations were carried out using 4 different measures of disks with a hole. The disks are shown in Figure 3. The 120 mm diameter disk is the standard shape for most investigations in the SFB570. Supplemental investigation of the influence of the ratio of CHD

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(case hardening depth) to wall thickness was carried out using Ø100 mm disks with different heights. The disk type used for the particular investigation is marked in the diagrams.

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Figure 3: Geometry of investigated disk samples

All samples were taken from one melt of SAE 5120 (EN20MnCr5) steel. The average chemical composition of the used steel was determined by optical emission spectroscopy (S-OES) as given in Table 1.

Table 1: Chemical composition of the used standard steel C Si Mn P S Cr Mo Ni Cu Al N 0.20 0.23 1.35 0.011 0.02 1.02 0.03 0.10 0.12 0.04 0.0154 In order to receive adequate reliable results, every heat treatment experiment was carried out with 4 to 8 samples per batch. The number of samples per batch is marked in the diagrams. The low pressure carburizing processes were carried out in a two chamber vacuum furnace at 940 °C. The samples were quenched after 20 minutes holding time at 840 °C with high pressure gas quenching utilizing 10 bars of nitrogen. The surface carbon content of the investigated disks was targeted to 0.7 Mass.-%.

4. RESULTS 4.1 Size changes of disks with a hole (Ø120 x Ø45 x 15) In figure 4 the average relative change of height related to the carburizing depth of the carburized and high pressure gas quenched disks are shown. Each case hardening batch was equally configured and contained 8 disks. It can be seen that there is a direct linear relation between the carburizing depth and the relative change of height. Furthermore there is a localized shape change of the average relative change of height, which is shown in figure 4 on the right side. With increasing radius the height grows and then shrinks to a minimum.

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Figure 4: Relative change of height of case hardened disks related to carburizing depth (left) and radii

(right) (CS = 0.7 %; disk Ø120 x Ø45)

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In figure 5 the relative changes of the inner and outer radii against the carburizing depth are shown. Again a linear relation of the radii changes with the carburization depth can be observed.

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Figure 5: Relative change of radius of case hardened disks related to carburizing depth (CS = 0.7 %; disk

Ø120 x Ø45)

4.2 Shape changes of disks with a hole (Ø120 x Ø45 x 15) Dishing as kind of shape change is observed at quenched disks. The amount of this shape change is measured by the change of slope [3]. In figure 6 the medial change of slope versus carburizing depth is shown. It can be seen that with increasing carburizing depth starting from a blank-hardened disk the dishing decreases in the beginning then increases to a maximum at a carburizing depth of 0.8 mm and then decreases again to a minimum at a CD of 1.5 mm. Along with the dishing, an substantial scatter can be observed. The observed variety of dishing within each batch was the scope of further metallographic investigations.

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Figure 6: Average change of slope of case hardened disks related to carburizing depth (CS = 0.7 %; disk

Ø120 x Ø45)

In figure 7 micrographs of two disks with the same CHD but different dishing slopes are shown. It is visible that the positions of the segregations deviate within the disks. Taking the offset of the segregation centers from the disk center as reference (see fig. 8), a linear connection of the slope with the segregation offset can be found for the carburizing depths of 0.6 mm to 1.2 mm (see figure 8). The carburizing depth of 1.5 mm shows a different behavior.

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Figure 7: Etched micrographs of the cross section of two disks and corresponding change of slope

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Figure 8: Micrograph of a disks cross section with marked offset of the segregation and diagram of the

correlation between segregation offset and dishing slope

4.3 Size changes of disks with a hole (Ø100 x Ø37.5 x 15 / 5) In figure 9 the change of outer diameter over the ratio of CHD to wall thickness is given. The results can be described by a second order polynomial as in Bomas et al., where similar results have been obtained on rings Further investigations with 1.5 mm height disks will show whether the actual functional relationship deviates from the predicted trend.

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Figure 9: Influences of the ratio of CHD to disks height on the change of outer radius (CS = 0.7 %; disk

Ø100 x Ø37.5)

Slope 0.441 [µm/mm] Slope -0.199 [µm/mm]

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5. DISKUSSION The disks show an increasing volume with a positive change in the short dimension and a negative change in the longer dimension. These results are compliant with the scheme Wyss [1] had introduced for hardened samples For samples with phase transformations starting first in the core as in case hardened samples, a bulging form is predicted. The dimensional change is influenced by shape changes [4, 5]. The dimensional change of the diameter 120 mm shows a linear connection to the carburizing depth. The diameter 100 mm disks could be connected polynomial to the ratio of case hardening depth to disk height. This is basically in conformity with Bomas’ results where a parabola connection of the longer dimension over the carburizing depth to wall thickness is described. The shape change of the disks is more complicated to explain and must be regarded as a parameter of more than one influencing factor. It could be seen that the case hardening depth is influencing the amount of shape change. One important parameter is the segregation structure, whose offset from the sample center can be related linearly to the dishing slope. But the samples with a CD of 1.5 mm did not fit into the pattern. This question is currently being investigated in an in-extenso investigation taking other production chain influences into account.

6. SUMMARY The distortion of disks with different carburizing depths was analyzed. It was found that dimensional changes are connected to the ratio of the CHD to disks height. For a small ratio (small CHD, big height) the dimensional change is connected linear to the CHD. The volume increases - while outer and inner radius shrink the height is increased. With growing ratio (increasing CHD or decreasing disk height) the tendency of the disks to increase their volume and decrease the outer radius comes to a turning point. Due to Wyss the dimensional and shape change behaviour carburized disks with a small ratio of CHD to disk height is changing towards that of a through hardened disk (ratio >20%). The predominant shape change occurring at disks with a hole is dishing. The dishing behaviour does not show a linear dependency on the carburizing depth. Additional investigations taking the whole process chain into account might lead to a better understanding of the main influencing parameters. At least a better understanding could be achieved by the use of FEM simulation to reveal any single step of distortion and their influencing factors during quenching. Further work will increase the amount of knowledge and therefore will hopefully lead into a better awareness and understanding of the mechanisms of distortion and their connection to the carburization.

Acknowledgement This work has been funded by the German Research Foundation (DFG) through the collaborative research center SFB 570 “Distortion Engineering” at the University of Bremen. The authors wish to acknowledge the DFG for their support. We also wish to acknowledge our gratitude and appreciation to several project partners in the SFB 570 for their contribution to the experiments this paper is based on.

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REFERENCES [1] Wyss, U.: Die wichtigsten Gesetzmäßigkeiten des Verzuges bei der

Wärmebehandlung des Stahles. In: Die Wärmebehandlung der Bau- und Werkzeugstähle, BAZ-Buchverlag, Basel, 1978.

[2] Macherauch, E.; Vöhringer, O.: Residual stresses after quenching. In: Theory and technology of quenching. Springer-Verlag Berlin, Heidelberg (1992), p. 117-182.

[3] Clausen, B.; Frerichs, F.; Klein, D.; Kohlhoff, T.; Lübben, Th.; Prinz, C.; Rentsch, R.; Sölter, S.; Stöbener, D.; Surm, H.: Identification of Process Parameters Affecting Distortion of Disks for Gear Manufacturing – Part I: Casting, Forming and Machining. Proceedings of the 2nd International Conference on Distortion Engineering, 17.-19. September 2008, Bremen, Germany, p. 29-39.

[4] Bomas, H.; Lübben, Th.; Zoch, H.-W.; Mayr, P.: Die Beeinflussung des Verzuges einsatzgehärteter Bauteile durch Abschreckvorrichtungen. HTM Härterei-Techn. Mitt. 45 (1990) 3, p. 188.

[5] Bahnsen, C.; Clausen, B.; Hoffmann, F.; Zoch, H.-W.: Influence of carburising on distortion behaviour. Proc. 1st International Conference on Distortion Engineering, Bremen, Germany, 14.-16.09.2005, p. 235-242.

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