Analysis of thermal residual stresses in duplex-type materials

Hot forged ferritic-austenitic α + γ duplex stainless steels possess a highly anisotropic microstructure consisting of phase domains aligned parallel to the axis of forging. Such microstructures are termed duplex microstructures. Together with the different thermal expansion of the two phases this special microstructure leads to plastic deformation even in the absence of an external load upon pure thermal cycling with the level of thermally induced stresses surpassing the elasticity limit either of one or of both phases. Thermal cycling is also accompanied by residual stress generation. A general description of the duplex-material response to such type of loading is based on the elaboration of an adequate thermoelastoplastic description for duplex-type materials with various distributions of their phases in sections perpendicular to the axis of anisotropy. An analytical solution for ferritic-austenitic two-phase cylindrical specimens with extreme 'matrix-inclusion' topologies for different volume fractions of the phases is obtained for the elastic case. The value of the characteristic temperature of plastification for such topologies is calculated from temperature dependent single-phase properties (Young's modulus, coefficient of thermal expansion and yield stress). Analytic estimates for the longitudinal and transverse coefficients of thermal expansion for the two-phase material are determined; the conditions for an asymptotic transition to a mixture-type rule form are formulated. The full thermoelastoplastic analysis of duplex materials under thermal cycling and the distribution of the residual stresses in specimens of varying topology of the phases is analyzed by means of finite-element (FEM) calculations.