Microstructure Representation of Steels by Means of Ferrofluids

The metastable austenitic steels 1.4307 and 1.4404 are characterized by a fcc lattice with paramagnetic properties which can be transformed by cold forming to ferromagnetic bcc martensite with acicular structures via a defect-rich intermediate stage. However, using classical wetchemical etching methods, defects and bcc martensite can visually not be clearly distinguished. Usually, magnetic or X-ray diffraction methods are used to quantify the ferromagnetic fractions. However, they do not allow for imaging the bcc phase distribution in the microstructure. Even with EBSD measurements in defect-rich phases, which, for process-related reasons, involve high investigation costs and long measurement durations, this is often only possible to a limited extent. The present study presents fast and simple contrasting by means of ferrofluids. It allows for a precise and spatially resolved differentiation between (ferromagnetic) martensite and (paramagnetic) defect structures on metallographic cross-section by optical microscopy. Furthermore, it is shown that, after a ferrofluid treatment of metallographic cross-sections made of duplex steel, ferromagnetic ferrite areas already show evidence of grain orientations.