A multi-phase model for transformation plasticity using thermodynamics-based metallurgical algorithm

The current work is aimed at establishing a flexible thermo-metallo-mechanical finite element (FE) model for steels. The flexibility is improved in the metallurgical analysis by adopting a thermodynamics-based algorithm, in which only the chemical composition and the thermal history are needed. On the other hand, the transformation plastic constitutive equations are extended for consideration of multi-phase transformation. An explicit consistent tangent modulus is derived for model implementation. The microstructural results are coupled with the constitutive equations using utility and user subroutines in the FE commercial software ABAQUS to create the complete model. The transformation kinematic and strain evolution equations are first validated using experimental data from the literature. The effect of the cut-off function on the transformation plasticity model is discussed. The discrepancy found at the initial stage of strain evolution is explained from a metallurgical point of view. The proposed model is further applied to a multi-pass welding process of more than twenty thermal cycles. A good agreement is observed in comparison between the metallurgical graphs and the predicted microstructural distribution. The result of stress distribution is improved by including the transformation plasticity in simulation. The influence of the cut-off function is analyzed again by comparing the X-ray measurements of stresses at external surface with the predicted ones.