Computational analysis of thermally loaded duplex stainless steels: the role of the free surfaces and the microstructure

Hot-forged ferritic-austenitic duplex stainless steels possess complicated structural and thermomechanical features which necessitate numerical analys is when modeling their deformational behavior. The microstructure of these steels consists of two phases with different temperature-dependent thermome chanical properties. As a consequence, purely thermal cycling in the temper ature interval between 900 degreesC and 20 degreesC can generate residual s trains/stresses even in the absence of an external mechanical load. Stress gradients, which are to be expected near traction-free surfaces of specimen s due to Saint-Venant's principle are responsible for considerable deviatio ns from the uniformity conditions and thus also influence the character of spatial distributions of strains and stresses. If one aims at the modeling of entire specimen, this excludes a utilization of traditional concepts (e. g., homogenization or representative volume elements (RVE)) based on the as sumption of the absence of macroscopic stress gradients. Three model repres entations for a cylindrical two-phase specimen with a low aspect ratio are introduced and analyzed by means of numerical (finite element) simulation i n order to study the effect of free surfaces on the macroscopic response of ferritic-austenitic duplex steels to purely thermal loading. The numerical analysis is carried out for two different types of matrix-inclusion morpho logy covering a simplified and a random distribution of the phase domains. The solution of a complementary problem provides a general information on t he effect of the geometrical features of both the whole specimen (e.g., its aspect ratio) and of the microstructure element on the length of the free- end zone. Such zones in vicinity of the specimen's end faces influence not only the stress distribution but also the level of the irreversible axial s train increment per thermal cycle. It is shown that this increment is very sensitive to the type of the matrix-inclusion topology. The perimeter surfa ce of cylindrical specimen, in contrast, does not affect the axial and the circumferential stress components. The width of the zone measured in radial direction of varying radial stress is extends only the layer of microscopi c elements closest to this surface. (C) 2000 Elsevier Science B.V. All righ ts reserved.