Study of Alumina in Austenitic Stainless Steels
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The purpose of this work is to optimize the chemical composition as well as heat treatment for the creation and improvement of the mechanical performance in alumina forming austenitic stainless steel. Alumina forming austenitic stainless steel consists of a typical microstructure of austenitic stainless steels, austenitic matrix, with a key exception of an alumina oxide formation on the outer most layer of the alloy. The contribution of aluminum for oxidation resistance is similar to that of chromium; while in a corrosive environment both form a stable oxide layer to prevent critical loss of mass of the parent material. However, alumina scales have a much higher thermodynamic stability in addition to the orders of magnitude slower in growth rate when compare to chromia scales. Austenite contributes directly to twinning and the mechanical performance of the alloy due to its transformation to martensite under external stress. In order to stabilize austenite against martensitic transformation through temperature, as opposed to mechanical stresses, the martensite start temperature is calculated through the Ishida model. The addition of aluminum to an austenitic stainless steel composition can alloy for the growth of alumina oxide; however it also promotes a BCC microstructure and too much aluminum addition allows for the destabilization of the austenitic FCC matrix. In order to create an acceptable balance for alumina forming austenitic stainless steel, thermodynamic and kinetic models are used to predict properties of the steel alloy. In this work, a theoretical approach is coupled to a Genetic Algorithm based optimization procedure to design and find the optimal chemical composition for an alloy to form the superior alumina oxide while maintaining the properties of austenitic steel. In particular, the effective valence and third element phenomena is used in conjunction in order to accurately map out possible trends and predict alloy oxide behavior. The findings from such a model supports the major conclusion that the combination of third element phenomena, oxide transport properties, material thermodynamics and kinetics can be used to provide accurate insight on the formation of alumina for austenitic stainless steel.