Constitutive modelling and identification of parameters of the plastic strain-induced martensitic transformation in 316L stainless steel at cryogenic temperatures

The present paper is focused on constitutive modelling and identification of parameters of the relevant model of plastic strain-induced martensitic transformation in austenitic stainless steels at low temperatures. The model used to describe the FCC → BCC phase transformation in austenitic stainless steels is based on the assumption of linearization of the most intensive part of the transformation curve. The kinetics of phase transformation is described by three parameters: transformation threshold (pξ), slope (A) and saturation level (ξL). It is assumed that the phase transformation is driven by the accumulated plastic strain p. In addition, the intensity of plastic deformation is strongly coupled to the phase transformation via the description of mixed kinematic/isotropic linear plastic hardening based on the Mori–Tanaka homogenization. The theory of small strains is applied. Small strain fields, corresponding to phase transformation, are decomposed into the volumic and the shear parts. The grade AISI 316L, stainless steel often used in cryogenic applications, has been chosen as a good example of the austenitic structure. The magnetic permeability of fine gauge stainless steel sheets (thickness 0.15–0.25 mm) subjected to monotonic straining was measured as a function of strain. The detailed methodology of relevant measurements is presented in the paper. Tuning of the constitutive model is described and the relevant parameters are identified. The model has been applied in the design of thin-walled bellows expansion joints for the large Hadron Collider (LHC), at present under construction at CERN.