Magnetic microstructure of the sensitized SUS 304 stainless steel

摘要

A magnetic force microscope (MFM) is applied to the quantitative evaluation of the chromium depletion extent of stainless steel (304 SS) and the percentage of depleted grain boundaries. These two quantities are the major factors of stress corrosion cracking. In this study, MFM observations and the etch tests are performed for some 304 SS specimens sensitized in various conditions. MFM images of the samples show the magnetized regions along grain boundaries corresponding to the chromium depleted regions observed by the etch tests. The dependencies of the amplitude of MFM signals and the total length of magnetized regions per one scan area on the heat treatment time conform to the expected ones from the behavior of chromium depletion. From this fact, we see that it is possible to measure the chromium depletion extent and the percentage of depleted grain boundaries by means of an MFM. This method is useful because MFM-imaging enable very high spatial resolution observation with minimal sample preparation and relatively easy operations.