Finite Element Simulation of Steel Quench Distortion- Parametric Analysis of Processing Variables
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Finite Element Simulation of Steel Quench Distortion- Parametric Analysis of Processing Variables F. A. García-Pastor, R.D. López-García, E. Alfaro-López, M. J. Castro-Román 1 Cinvestav Unidad Saltillo, Carretera Saltillo-Monterrey Km 13, Ramos Arizpe, Coahuila, Mexico 2 San Luis Rassini, Puerto Arturo No. 803, Piedras Negras, Coahuila, Mexico ABSTRACT Steel quenching from the austenite region is a widely used industrial process to increase strength and hardness through the martensitic transformation. It is well known, however, that it is very likely that macroscopic distortion occurs during the quenching process. This distortion is caused by the rapidly varying internal stress fields, which may change sign between tension and compression several times during quenching. If the maximum internal stress is greater than the yield stress at given processing temperature, plastic deformation will occur and, depending on its magnitude, macroscopic distortion may become apparent. The complex interaction between thermal contraction and the expansion resulting from the martensitic transformation is behind the sign changes in the internal stress fields. Variations in the steel composition and cooling rate will result in a number of different paths, which the internal stresses will follow during processing. Depending on the route followed, the martensitic transformation may hinder the thermal stresses evolution to the point where the stress fields throughout the component may actually be reverted. A different path may support the thermal stresses evolution further increasing their magnitude. The cross-sectional area also affects the internal stresses magnitude, since smaller areas will have further trouble to accommodate stress, thus increasing the distortion. Additionally, the bainitic transformation occurring during relatively slow cooling rates may have an important effect in the final stress field state. A finite-element (FE) model of steel quenching has been developed in the DEFORM 3D simulation environment. This model has taken into account the kinetics of both austenite-bainite and austenite-martensite transformations in a simplified leaf spring geometry. The results are discussed in terms of the optimal processing parameters obtained by the simulation against the limitations in current industrial practice. INTRODUCTION Quenching from the austenite phase field to improve steel hardness and strength through the martensitic transformation is a widely used process. However, macroscopic distortion is a common problem, which can appear during such processing. Macroscopic distortion is a result of the complex and rapidly variable stress field inside the component. There are two types of internal stresses during quenching: thermal and transformation-related stresses [1]. Thermal stresses are the result of contraction due to the rapidly decreasing temperature. The surface temperature drops faster than the core temperature, thus creating a compression-tension stress system which is eventually reverted, once the core temperature is
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