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.
Study on deformation uniformity of mandrel forging process and control measures of inner hole eccentricity
Abstract The deformation uniformity and the control measures of the eccentricity of the inner hole in actual mandrel forging are researched by numerical simulation combined with experimental investigation. In one-step pressing, taking the width of the upper platform and the inverted cone angle of the large deformation area in one-step pressing as the characteristic parameters, the influence law of the pressing reduction of the anvil on the distribution of the mandrel forging deformation was obtained. In sequential overturned pressing, the influence of the 45° and 60° billet overturning schemes commonly used in the actual production on the deformation distribution and its uniformity was simulated. The result analysis shows that the deformation uniformity of the 45°overturning scheme is better than that of the 60°. The difference of the actual pressing reduction at each step in the overturning makes the angle of the large deformation area and the overturning angle in actual operation mismatch leads to the uneven deformation distribution. Based on the above research results, a kind of symmetrical overturning mandrel forging process is proposed to replace the existing sequential turning process, which greatly improved the deformation uniformity. Then, two kinds of hole eccentricity correction methods are put forward, which are stepped reduction mandrel forging combined with symmetrical overturning scheme (SO-Stepped Reduction) and changing the initial position in sequential overturning scheme (CIPSO-Stepped Reduction). Simulation analysis and experimental research show that the two kinds of hole eccentricity correction methods can effectively correct the hole eccentricity in the mandrel forging process. The methods of CIPSO-Stepped Reduction is easy to operate and more efficient.
Relationship of microstructure and mechanical properties of TRIP-aided steel processed by press forging
Abstract The main emphasis of this study has been placed on thermomechanical processing simulations performed using press forging. In order to develop a comprehensive understanding of the effect of large deformation of bulk specimens, introduced in the austenite recrystallized, nonrecrystallized and intercritical temperature regions, on the microstructure evolution and mechanical properties, the C–Si–Mn transformation induced plasticity (TRIP)-aided steel was used for experimental. The choice of applied strain and thermal parameters had strong impact on austenite transformation kinetics and obtained final multiphase structure, which clearly modified the mechanical properties of TRIP steel. To rationalize the retained austenite volume fraction employing different thermomechanical schedules, the effect of austenite conditioning on ferrite transformation was considered at the first step and the subsequent isothermal bainite transformation treatment was optimized in the second step. The microstructures of treated samples were characterized by optical metallography and scanning electron microscopy. The tensile tests were conducted to characterize the development of TRIP effect with respect to volume fraction of retained austenite in multiphase structure. The volume fraction of retained austenite was measured using X-ray and neutron diffraction methods. The specific design of thermomechanical schedule with varying of strain and temperature values affected multiphase structure development in TRIP bulk steel and yielded in good combination of high tensile strength over 900 MPa and elongation over 20% nearly at all experimental schedules. The complex relationship among the volume fraction of the retained austenite, the morphology and distribution of phases present in the microstructure, and mechanical properties of TRIP steel were revealed. These findings suggested that the intercritical treatment conditions prior austenite transformation appeared to have decisive effect on steel refinement and forming of convenient multiphase structure, which should provide both, the high strength and sufficient ductility of bulky products.