Monitoring of Martensitic Transformation in Austenitic Stainless Steel 304 L by Eddy Currents

A nondestructive eddy current technique has been applied to cyclic-strain induced martensitic transformation (MT) monitoring in austenitic stainless steel 304 L. During plastic deformation, the austenite transforms locally in martensite. Transformed martensite distribution depends on the geometry of the specimen under test (rectangular section in this work): transformation occurs first in the corners, later close to the flat free surfaces, and finally in the bulk. The obtained value is then representative of a small volume of the sample. Local measurements with an eddy current probe proved to be sensitive to this kind of transformation and even quantitatively after calibration. This article proposes a procedure to calibrate the eddy current response in order to determine the fraction of martensite formed under the probe without the need of X-ray measurements.