Stress- and Strain Induced Phase Transformations in Pearlitic Steels

An overview of the mechanically driven phase transformations taking place in nanocrystalline pearlitic steels in conditions of the severe plastic deformation (SPD), i.e. combination of high pressure and strong shear strains will be given. Conditions of the discussed experiments (room temperature and moderate strain rates) exclude any thermal origin of the observed transformations. One of them is strain induced cementite decomposition, which is a well-documented phenomenon taking place at cold plastic deformation of pearlitic steels. We explain this process taking into account friction forces at the interface between the hard cementite and ferrite. Under the high pressures and stresses higher than the ferrite matrix yield stress, the later one behaves like a viscoelastic fluid. The friction at the precipitate/matrix interface leads to two effects. One is to induce high strains on the precipitates. This leads to shift of thermodynamic equilibrium towards dissolution of the cementite. The second is wear of the cementite phase due to friction at the ferrite/cementite interface and mechanically induced drag of carbon atoms by the ferrite. This had been recently confirmed in 3D AP experiments, which demonstrated that the process of cementite decomposition starts with depleting of carbides with carbon and formation of non-stoichiometric cementite. The existing theories of atom drag by moving dislocations (ballistic models) can be regarded as one of the many possible mechanism of wear discussed by the wear theory. In that respect the process can be called athermal, as temperature indirectly influences wear processes but is not their main cause. We observed also another strain driven transformation in nanocrystalline pearlitic steel during room temperature high pressure torsion. This is a stress induced α→γ transformation, which has never been observed at conventional deformation of coarse grained iron and carbon steels. This was concluded to have occurred due to a reverse martensitic transformation.