Estimation of stress corrosion cracking sensitivity of type 304 stainless steel by magnetic force microscope

An application of a magnetic force microscope (MFM) to the measurement of the chromium depleted regions of type 304 stainless steel is proposed to enable more effective evaluation of the material sensitization to stress corrosion cracking than the conventional methods. The MFM images of sensitized materials show that the magnetizations are induced along grain boundaries by the chromium depletion. The dependence of the magnetization on the sensitization condition conforms to the expected one from the behavior of chromium depletion. Furthermore, the phase identification was performed by electron backscattered pattern technique to reveal the magnetization mechanism due to sensitization. Then, it was found that the magnetization is caused by the transformation from austenite phase to martensite phase. From the discussion on the temperature at which martensitic transformation starts, we see that it seems to be possible to detect regions where the chromium concentration is under 14% by using an MFM.