Use of a Micromagnetic Nondestructive Test in the Evaluation of the α’-Martensitic Transformation Generated in the Mechanical Fatigue Process of the AISI 304L Stainless Steel

Abstract

The detection of the strain-induced martensitic transformation with the magnetic Barkhausen noise (MBN) was investigated in the high-cycle fatigue (HCF) of the AISI 304L stainless steel at different points of the surface of an hourglass-type specimen. Load-controlled fatigue tests with loads below and above the fatigue limit were performed. Each fatigue test was interrupted at certain numbers of cycles, prior to fracture, and MBN measurements were performed without applied loading. Comparing with the initial condition of the steel under study, no large changes in the MBN measurements were detected in the specimens subjected to loads near the fatigue limit, i.e., no $\alpha '$ -martensitic transformation was detected. In the specimen subjected to the greatest load, it was possible to detect an uncommon distribution of the MBN signals, which are correlated with the strain-induced $\alpha '$ -martensite and were verified with optical microscopy and X-ray diffraction (XRD). The results show that the MBN is a sensitive technique to detect the strain-induced $\alpha '$ -martensite in HCF tests, which can be used as an indirect parameter to show the fatigue damage of materials.