Tran Hoang Quan, Henri Champliaud, Zhengkun Feng, and Thien-My Dao
Roll bending process, Heat forming, FEM, Ansys Ls/dyna
Processing high strength steel is not easy, and it becomes a hard-to-solve problem when the part to produce is large, thick and quasi-unique. One example of a thick high strength steel axisymmetric part is the conical shape of the crown of a Francis turbine runner. Several processes can be envisaged for the manufacturing processes of such large parts (welding or casting…), but few processes can deliver one within a reasonable time and at competitive cost. Among them the roll bending process, causing plastic deformation of a plate around a linear axis with little or no change in surface thickness, is considered as an interesting alternative. It is well known that by heating a part in the deformation zone it may reduce the forming forces. The roll bending process is no exception to this rule and in order to study the relationships between temperature effects, applied forces and plate thickness, a computer aided simulation program has been built in the Ansys/LS-Dyna environment. The parametric finite element simulation involves thermal and structural computations for predicting the shape variation from the initial pass to the final pass of this process. The thickness of the plate directly influences the reaction forces on the rollers. Preheating the workpiece to reduce the large reaction forces highly affects the roll bending machine life, the property of the final product and determines the maximum deformation speed, which can be used for reducing the processing cost and time. Therefore, the results of the simulations yield to a better understanding of the phenomena taking place in the process, and provide an opportunity to design an efficient heating system to control the temperature applied during the roll bending process. In this paper, it is shown that a plate to be rolled can be locally heated during the roll bending process in order to reduce the bending forces. This a promising alternative to the costly and time consuming casting process that is usually selected for manufacturing large and thick axisymmetric parts made of high strength steel.
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