Role of Austenitization and Pre-Deformation on the Kinetics of the Isothermal Bainitic Transformation
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NTRODUCTION
STEEL is generally the material of choice when it comes to structural components that demand high strength and good ductility. Furthermore, properties of steels can be tailored by varying the process parameters, such as deformation stresses and strains, and temperature. In particular, temperature is one key parameter affecting the final microstructure, such that the mechanical properties of a steel strongly depend on the temperature-time path during processing.[1] Moreover, the degree of anisotropy is dictated by temperature, such that an anisotropic microstructure is induced on an initially isotropic one by an increasing temperature gradient during cooling. For instance, forging is mostly expected to lead to an anisotropic microstructure as variations in the local temperature-time-deformation paths accompanied by inhomogeneously dissolved carbide bands can lead to dimensional instability,[2] resulting in high postprocessing costs. However, the anisotropic microstructure evolution during forging may also be used to advantage, such that a functionally graded workpiece could be manufactured.[3] Producing a near-net-shape workpiece during forging would prevent additional costs since minor post processing, such as turning or milling, is needed. A nearnet-shape workpiece should not only guarantee the geometrical stability, but it is also expected to fulfil the required mechanical properties within the workpiece. H.-G. LAMBERS and S. TSCHUMAK, Research Assistants, and H.J. MAIER, Professor, are with Lehrstuhl fu¨r Werkstoffkunde (Materials Science), University of Paderborn, 33095 Paderborn, Germany. Contact e-mail: [email protected] D. CANADINC, Assistant Professor, is with the Department of Mechanical Engineering, Koc University, 34450 Istanbul, Turkey. Manuscript submitted August 27, 2008. Article published online April 8, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
Ideally, the chosen temperature-deformation-time path should allow for different microstructures within one workpiece, since the desired properties might vary with the location within the final workpiece.[3,4] Especially for products, such as gears, which are hard at the surface and tenacious in the inner part, one has to optimize the forging process. However, this requires a solid knowledge of factors dictating the geometrical stability, such as phase transformations, which are accompanied by volumetric changes that lead to distortion. In many cases, distortion can be minimized by reducing the temperature gradients and the resulting stresses, yet even small elastic stresses and temperature gradients can affect the phase transformation kinetics and the local microstructure.[5–7] It is well known that elastic stresses are capable of accelerating the phase transformation kinetics,[6,8,9] which has recently come into consideration in the prediction of the final microstructure of forged steel products.[10,11] Several models were forwarded,[12–17] which successfully explain the occurrence of transformation plasticity, i.e., the inhomogeneous p
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