Growth Detection of Martensite-Austenite Interface by Magnetic Barkhausen Noise Technique

When a variable magnetic field is applied to a ferromagnetic material, the change in magnetic induction occurs discontinuously as a result of the movement of magnetic domain walls. These discrete and stochastic jumps can induce, in a probe coil near the material surface, signals known as Magnetic Barkhausen Noise (MBN). MBN are sensitive to microstructural change, plastic deformation, and stress of the material. As a consequence, it can be used as a Non-Destructive Test in a large number of applications, for example to find unwanted phases, grain size control, quality control of surface treatments and degradation, detect residual stresses, etc. In this work, the on-line characterization of the transformation process of the austenite-martensite phase by deformation of stainless steel (SS) AISI 304 was studied applying the MBN technique. For this purpose, the specimens were subjected to uniaxial monotonic tensile tests up to failure. As a consequence of the plastic deformation of the SS austenitic phase, a ferromagnetic phase appears which produces MBN. The specimens were cut in two different directions considering the rolling direction (0 o and 90 o ). In order to characterize the MBN evolution with the strain, it was measured during the process of increase of the load applied to the specimen in the tensile test. To characterize the transformation process, various statistical parameters were estimated which were evaluated based on the applied deformation. Finally, the temporal evolution and the Root Mean Square (RMS) function of the MBN signals were evaluated during the load increase. The MBN allowed to detect that the rolling direction of the specimens strongly influences the amount of martensite phase produced. Furthermore, it was also revealed that the MBN technique is appropriate for evaluating the initiation and evolution of the austenite-martensite phase transformation