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Modeling of austenite decomposition in an AISI 4140 steel Chipalkatti, Jayprakash


The final mechanical properties and also the distribution of residual stresses in a heat treated product, depend upon the various microstructural constituents. In heat treatment operations, in order to predict the final properties, it has become increasingly important to be able to predict the final microstructure distribution for a given thermal history. Mathematical models of microstructure prediction are a useful tool for this purpose as they eliminate the need for experimentation. The characterization of the kinetics of austenite decomposition to equilibrium and/or non equilibrium phases in a low alloy steel, such as AISI 4140, by performing isothermal transformation experiments is time consuming and hence not cost effective. Further, to generate meaningful and accurate data with respect to the various stages of the progress of the austenite decomposition, many experiments are required. Kirkaldy's model, involving the transformation kinetics of the various possible reaction products, provides a very useful tool in this regard as this model is based on only the chemical composition and the austenite grain size of the steel. By observing certain limitations of this model, especially for low alloy steels, Li and co-workers have proposed modifications to this model. The Kirkaldy model along with the model proposed by Li and co-workers is assessed in the present work. The purpose of this study was to characterize the austenite decomposition kinetics under continuous cooling conditions in a AISI 4140 steel by applying Kirkaldy's model and Li's model and to test the results of these models by perforating GLEEBLE controlled continuous cooling experiments on this steel. The validity of the Kirkaldy model and the Li model was tested in two different ways. First, the calculated response was compared with the published TTT diagram for a given chemistry and y grain size for an AISI 4140 steel. The calculated response was subsequently tested using the experimentally measured CCT data for a different AISI 4140 steel. As the characterization of continuous cooling kinetics requires knowledge of the phase diagram, a mathematical model based on thermodynamic equations is used to derive the phase diagram for this steel. The model is based on the equality of chemical potentials for each of the alloying elements in the phases that are in equilibrium. The results of this model are then used in modeling the isothermal and continuous cooling transformation kinetics. It was observed that Kirkaldy's and Li's models both yield a reasonably good prediction of the TTT curve for this steel, when compared to a published TTT diagram for this steel. Under continuous cooling conditions, the microstructures predicted by the Li model are closer to the experimentally observed microstructures than are the Kirkaldy model predictions.

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