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Influence of process parameters and variable dimensions on the distortion of case hardened discs of 20MnCr5 [SAE 5120]
Abstract
In the presented research paper the distortion behaviour of components is analysed. Therefore, a large number of discs with inner bores of 20MnCr5 steel was used as a model geometry for simplification of gears. Altogether, six different variations in dimensions were used, where two different heights and three different outer diameters were combined. Additionally, the influence of carburising depth, surface carbon content and batch structure were included in the analyses. Among gas carburised and oil quenched discs, some discs were low pressure carburised and gas quenched for comparison. Since the experiments were arranged with the aid of a design of experiment, it was possible to identify the significant and highly significant influencing factors on the change of dimensions and shapes. Furthermore, the influence of the disc dimensions on the distortion behaviour was analysed and based on simulations, reasons for certain changes in shapes were found.
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Studies have shown that the distortion of case-hardened gears can be reduced by minimizing the hardenability scatter band. In order to achieve a high production rate in a pusher furnace, the tray is loaded with as many components as possible. It is therefore important to ensure that the quenching of the charge is as homogeneous as possible, in order to obtain the desired uniform structure and thereby a small shape distortion.
Changes in shape and dimension of automotive parts are influenced by many parameters, which can be allied with the pretreatment stage, the design of the parts, the loading and the heat treatment itself. Using a standardized pre-treatment stage the influence of the heat treatment on the distortion could be estimated and the influence of quenching parameters on changes in shape and dimension could be systematically investigated. Different types of quenching oils were tested and their influence on the distortion was examined. The oil circulation rate and the oil temperature were varied and the influence of a delayed circulation of the oil bath was studied. It was found that these parameters and the material properties (chemical composition, hardenability) had less influence on the distortion than the actual design of the parts. For comparison of oil and gas quenching a low pressure carburizing and pressurized gas quenching were applied. The changes in shape and dimension resulting from the different processes have been compared. It was shown that gas quenching caused a significant reduction in out-of-flatness and conicity, whereas the out-of-roundness was predominantly influenced by the design of the gear wheels.
The uniform dimensional change in steel after heat treatment was discussed. The method of catering for controlling dimensional change was to account for movement in the dimensions prior to heat treatment which required the changes to be very consistent. The modeling developments show that for low alloy steel Jominy hardenability provided method of quantifying transformation which was produced for individual casts and the results were calculated from chemical composition. During the heat treatment quenching, introduction of differential thermal stresses and variation in cooling rates contributed to distortion and transformation at different times.