Influence of material inhomogeneity on the mechanical response of a tempered martensite steel

Failure in steel weldments operating at high temperatures often occurs in the heat-affected zone adjacent to the weld. Such failures can be a result of material inhomogeneity within the heat-affected zone and in the case of tempered martensite steels have been linked with regions of untransformed alpha (ferrite) phase or over-tempered martensite within the intercritical region of the heat-affected zone. In this work, two-dimensional Voronoi tessellation is used to construct polygonal Voronoi cells to represent the microstructure of the heat-affected zone of a weld in a tempered martensite steel. The Voronoi construction is treated as a representative volume element of the material and is discretised by 8-node linear brick elements, with periodic boundary conditions. The lattice orientation at each material point is specified by three Euler angles, which are assumed to be randomly distributed, to represent the initial lack of texture in the intercritical region of the heat-affected zone. The constitutive response is represented by a nonlinear, rate-dependent, finite-strain crystal plasticity model. The results indicate that small amounts of ferrite can induce significant enhancements in stress and inelastic deformation at the interface of the ferrite and martensite grains. This localisation of stress and strain may be critical for microcrack and or void formation and may be a contributory factor to Type IV cracking.