Implementation of an open-die forging process for large hollow shafts for wind power plants with respect to an optimized microstructure
To realize large wind power plants in an economically feasible way, it is necessary to identify potential for lightweight design of the generator hollow-shafts, which are commonly produced by casting. The weight of these shafts can significantly be reduced by producing them by open-die forging, since the forming of the material leads to a higher strength, which allows to reduce the wall thickness noticeable. This paper describes the development and implementation of a forging process for hollow shafts with respect to an optimized microstructure. To numerically investigate this process, a realistic finite element simulation model was developed in a first step. The kinematic of the tools has been implemented authentically to provide a realistic material flow and process conditions. Additionally, a material model for the steel 42CrMo4 was integrated into the simulation model to predict the resulting microstructure. Using the implemented FE model, the forging process was optimized manually to achieve a homogeneous and fine-grained microstructure. The optimization was based upon a variation of different forging parameters and the sequence of forging steps. In the next step, a forging on laboratorial scale was performed to validate the simulation model. For this purpose, after forging, specimens from the hollow shaft were evaluated by metallography to determine the final grain size. A comparison of the results with the numerical simulation showed a general agreement of the measured grain size with the numerically calculated grain size. Based upon these results, the process model was transferred to an FE model with an industrial scale. By this it was possible to analyze the transferability of the used FE model regarding the assumptions about the kinematics and the sequence of the forging steps. A numerical investigation of the industrial process proved the scalability of the process to an industrial relevant geometry.
Numerical simulation and experimental verification of microstructure evolution in a three-dimensional hot upsetting process
Abstract A coupled thermoviscoplastic finite-element software has been developed for modeling metal flow in hot metal-forming processes, especially for predicting the microstructural changes and mechanical properties of the products. Using this code, the dynamic recrystallization kinetics and recrystallized grain-size distributions during hot metal-forming processes can be simulated. In order to confirm the validity of the code developed, the results of an experiment into 3-dimensional hot upsetting (45 steel) are reported in the present paper, the recrystallized grain sizes being found to be in good agreement with the simulated results.
Static recrystallization behaviour of a wide range of austenite grain sizes in microalloyed steels
The static recrystallization kinetics of four steels microalloyed with Nb, Ti and Nb-Ti have been investigated, covering conventional and hot direct rolling conditions. The influence of the grain size on the recrystallization time has been studied for a wide range of grain sizes (20-1000 μm). For the same deformation conditions it has been observed that the quadratic dependence on grain size works well in the range of finer grain sizes (< 160 μm). In contrast, the recrystallization times measured at coarse grain sizes are significantly lower than those predicted by this dependence. However, when the whole grain size range analysed is considered, a linear dependence on grain size gives the best fit to experimental data. The effect of strain on the recrystallization kinetics has been also analysed for different initial grain sizes. It has been observed that the time to reach a 50% recrystallized fraction (t 0.5 ) shows a dependence on strain of the type t 0.5 e -p . the exponent being influenced by the initial grain size. A unique equation has been proposed to describe the static recrystallization kinetics of Nb, Ti and Nb-Ti microalloyed steels. The predictions of the equations have been compared to previously published data on similar steels.