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 analysis when modeling their deformational behavior. The microstructure of these steels consists of two phases with different temperature-dependent thermomechanical properties. As a consequence, purely thermal cycling in the temperature interval between 900°C and 20°C can generate residual strains/stresses even in the absence of an external mechanical load. Stress gradients, which are to be expected near traction-free surfaces of specimens due to Saint-Venant's principle are responsible for considerable deviations 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 assumption of the absence of macroscopic stress gradients. Three model representations for a cylindrical two-phase specimen with a low aspect ratio are introduced and analyzed by means of numerical (finite element) simulation in 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 morphology covering a simplified and a random distribution of the phase domains. The solution of a complementary problem provides a general information on the 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 strain increment per thermal cycle. It is shown that this increment is very sensitive to the type of the matrix-inclusion topology. The perimeter surface 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 microscopic elements closest to this surface.