Modelling of the evolution of micro-grain misorientations during creep of tempered martensite ferritic steels

Tempered martensite ferritic steels are prone to low-angle boundary (LAB) vanishing and micro-grain size increase during creep deformation. A physically based model of LAB vanishing during creep deformation is presented. The LABs are modelled by simple dislocation arrays following the Read and Shockley model. Depending on the activated slip systems, mobile edge/screw dislocations annihilate with LAB parallel dislocations of opposite sign. The LAB misorientation frequency evolution versus creep strain can be analytically computed. The material parameters are the fractions of edge/screw dislocations in the LABs and the critical edge/screw dislocation annihilation distances. Different LAB misorientation frequencies obtained by electron backscatter diffraction (EBSD) and available in the literature are used as input data of the simulations. The computed LAB misorientation frequencies are then compared with the experimental frequencies obtained at different creep strains. If the material parameters belong to physical ranges, their influence on the predictions remain rather weak. The model generally permits a reasonable prediction of misorientation and micro-grain size evolutions during creep. Finally, the effects of precipitate evolution, climb and internal stresses are discussed.