Identification of the Direction-Dependency of the Martensitic Transformation in Stainless Steel Using In Situ Magnetic Permeability Measurements

The evolution of the martensite content is monitored throughout uniaxial tensile experiments on anisotropic temper-rolled stainless steel 301LN. Several martensite content measurement techniques are discussed. It is found that micrography, basic X-ray diffraction and EBSD provide good qualitative results, but the absolute errors in the estimated absolute martensite content can be greater than 10%. Magnetic saturation induction measurements provide the spatial average of the martensite content over a large volume, which eliminates inaccuracies associated with metallographic surface preparation. Inverse magnetostriction of the ferromagnetic martensitic phase has a strong effect on the results from magnetic permeability measurements. It is critically important to remove all elastic strains before measuring the magnetic permeability. Neutron diffraction is used to quantify the residual lattice strains in the martensite after removing all macroscopic elastic strains. The results demonstrate that the linear relationship between the magnetic permeability and the martensite content holds true despite the presence of small residual strains. In situ measurements of the martensite content evolution during tensile tests along the rolling, the cross-rolling and the 45° direction of the anisotropic sheet material reveal that the transformation kinetics are independent of the loading direction in stainless steel 301LN under uniaxial tension.