Method of Magnetic Control of Percentage of Ferrite and Strain-Induced Martensite in Stainless Steels

The aim of this work is to non-destructively characterize two-phase austenitic chromium-nickel stainless steels using developed magnetic technique to control the percentage of ferromagnetic phases in relatively weak magnetic fields lower than 600 A/cm. By room-temperature rolling austenitic steel specimens, the microstructure consisting of different volume fractions of martensite have been obtained. Using magnetic-force microscopy a significant difference in strain-induced martensite and ferrite morphology has been demonstrated which has a decisive effect on magnetic properties of two-phase austenitic-ferritic and austenitic-martensitic steels. From the experiments, a clear correlation between strain-induced martensite and ferrite volume content and the measured phase-sensitive magnetic parameters of the maximum differential magnetic susceptibility and the area under the field dependence of differential magnetic susceptibility has been established. It has been found that the magnetic field amplitude corresponding to the maximum of the differential magnetic susceptibility dependence allows for unambiguous determination of the type of the ferromagnetic phase in the austenitic steel. The sensitivity of the proposed small-sized clamp-on measuring transducer for phase composition control of the material of finished two-phase steel products without cutting out samples is discussed.