Effect of Initial Composition Distribution on the Phase Transformation Behavior in the Fe-Cr-Ni System
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ABSTRACT A finite-difference implicit numerical model was used to study the diffusion-controlled a -y solid-state phase transformation in the Fe-Cr-Ni system. The influence of a non-uniform initial composition distribution was examined in order to assess the impact that non-uniform solute profiles resulting from solidification may have on subsequent transformation behavior in weldments and castings. A significant impact on the transformation kinetics and transformation path was found in some cases. The factors that affect the degree of influence are discussed. INTRODUCTION Modeling phase transformation behavior is a rapidly growing field. Such modeling is particularly valuable and desirable for weldments because of the high temperature excursions which weldments experience and the difficulty in properly and accurately evaluating the elevated temperature phase transformation behavior by experimental means. In the case of austenitic stainless steels, solidification of primary ferrite is followed by the solid-state transformation of ferrite to austenite. The ferrite-to-austenite transformation during cooling is typically incomplete, leading to a two-phase microstructure that consists of austenite with 5 to 15% of residual ferrite. In most cases, the ferrite-to-austenite transformation is diffusion controlled. Thus, it is possible to model this transformation effectively, provided that the diffusion behavior is properly evaluated. For multicomponent systems, the diffusion-controlled transformation cannot be readily calculated by analytical means. However, numerical methods are ideally suited to tackle such involved problems. By considering the ternary Fe-Cr-Ni system as representative of the class of austenitic stainless steels, the ferrite-to-austenite transformation has been examined recently for both cast stainless steels and welded stainless steels [1-7]. Finite difference methods have been used to follow the transformation behavior in the ternary system during isothermal exposure [1-4] as well as during continuous cooling [5,6]. These studies have modeled the transformation behavior by assuming local equilibrium at the ferrite/austenite interface. Comparisons of the calculations with experimental work have shown very good agreement [1-3]. Most recently, the transformation behavior in multipass welds has been modeled by examining the influence of multiple thermal cycles [7]. All of these earlier studies have shown that the consideration of a ternary system has a strong influence on the transformation behavior. Unlike simple binary systems, where unique tielines exist at any given temperature, the consideration of ternary systems introduces a range of equilibrium tie-lines. This added degree of flexibility results in many interesting effects. For example, the approach to final equilibrium is often indirect in that ferrite growth may take place initially, followed by dissolution of the ferrite. The change in phase composition as a function of aging time can also be quite complicated, with non-monotonic variations bei
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