Eddy current characterization of cold-worked AISI 321 stainless steel

Austenitic stainless steels are alloys with desirable features especially in corrosive environments. However, some processes, such as cold-work, can change their original properties owing to the formation of α′-martensite during a strain-induced process. Many techniques have been employed to detect and quantify this metallurgical phase and the non-destructive methods are particularly interesting for quality control in industrial plants. In this work, the use of impedance phase angles from the eddy current testing is proposed to quantify the martensite volume fraction. Methods such as optical microscopy, X-ray diffraction, and vibrating sample magnetometry were also employed to characterize the samples microstructure. Upon magnetic measurements, it was found that the phase angle and martensite volume fraction are related by an exponential fit.