Pre-straining effect on ferrite/pearlite anisotropic transformation strain in steels

The effect of pre-straining on ferrite/pearlite phase transformation in high strength steels is investigated. Two steels differing mainly in their Nb content (0.03C 0.68Si 1.82Mn 0.00 or 0.04Nb in mass%) are either pre-tensioned or pre-compressed and the same-directional transformation strain is measured. A crystal plasticity fast Fourier transform numerical model with back stress effect is used for further investigation and the results are compared. Experimentally, it was found that 30% pre-tension reduces the transformation strain in the same direction by 0.09% (0.00Nb) and 0.13% (0.04Nb), while 30% pre-compression increases it by 0.16% (0.00Nb) and 0.02% (0.04Nb). The numerical simulation estimates the experimentally observed anisotropic transformation strain. In the numerical simulation, even though the anisotropy in transformation plasticity has been qualitatively reproduced, a larger anisotropy effect was observed. The quantitative discrepancies found in between experimental results and numerical results are due to the annihilation of dislocations during the interval between the pre-straining and the onset of phase transformation. The anisotropy in transformation strain found in the experiments is large enough to cause an unacceptable dimensional change in e.g. heat treated steel sheet products.