The irreversible deformation of a duplex stainless steel under thermal cycling

In duplex stainless steels the constituent phases austenite and ferrite have different coefficients of thermal expansion. On varying the temperature a remarkable stress state develops which changes in sign from phase to phase. On pure thermal cycling the yield strength of the phases can be exceeded. Specimens of a forged duplex steel with uniaxial anisotropic microstructure were investigated in dilatometer experiments. Under repeated thermal cycles specimens deformed irreversibly with a monotonic accumulation of strains. In longitudinal specimens an increase in length was observed. Transverse specimens, however, decrease in length. A micromechanical model applying the finite element method is used to calculate the thermal properties of the duplex steel from the properties of its constituent phases. A mechanism driving the ratchet effect is proposed. The temperature dependent yield strength of the phases is responsible for the irreversible deformations. The model points out the importance of correct representation of the spatial distribution of the constituent phases in the micromechanical model. A systematic quantitative investigation of the effects of microstructure on the overall material behavior is based on a description of real and model microstructures by the same stereological parameters. Results from model calculations are in good agreement with results from experiments.