Magnetic characteristics and mechanism of 304 austenitic stainless steel under fatigue loading

Abstract

This study examines the magnetic signal and mechanism of 304 austenitic stainless steel under fatigue. Fluxgate sensor is used to detect the magnetic signal during fatigue. Scanning electron microscope (SEM), x-ray diffraction (XRD), and Lorentz transmission electron microscope (LTEM) are used to study the microstructure’s evolution. On the basis of the microstructure results, a mechanism of magnetic characteristics is discussed and demonstrated. In this mechanism, martensite accumulated by phase transformation is magnetized by stress and enhances the magnetic signal. The magnetic signal’s trend and mechanism vary in three fatigue stages. In the first stage, the magnetic signal increases rapidly due to the increased content of strain-induced martensite. In the second stage, the magnetic signal increases steadily, and the relative strain-induced martensite content is only 0.6%. The increase in magnetic signal is primarily caused by the accumulated martensite being magnetized by stress. In the third stage, the magnetic signal increases rapidly due to the rapid development of fatigue cracks. Lastly, a differential magnetic signal processing method for detecting fatigue damage failure in 304 austenitic stainless steel is proposed; the proposed method can distinguish each of the fatigue stages. A warning of fatigue failure can be obtained when the results of the differential increase exceed the corresponding threshold value.