Assessment of martensite content in austenitic stainless steel specimens by eddy current testing

Martensitic transformation is frequently observed in some manufacturing processes of austenitic stainless steel components, such as embedding, drawing, grinding or burnishing. The presence of martensite in an austenitic stainless steel implies a decrease in the tensile strain rate, which affects its manufacturing and, especially, its resistance to stress corrosion cracking and hydrogen embrittlement. On the other hand, the increase of the mechanical properties due to the presence of martensite, is useful for some applications, such as spring manufacturing. Hence, to know the amount of martensite present in an austenitic stainless steel is very important, especially if the martensite is quantified through a nondestructive technique. Eddy current testing (ECT) is an applicable method, the signals displaying information about the properties of the material. In this work a technique was developed to extract that information from ECT signals obtained with a conventional equipment. This martensite content could be estimated from the impedance modelling (Z) of probes coupled to lightly magnetic materials, and through adequate calibration. For the development of this method, AISI 304, 316 and 347 austenitic stainless steel specimens were used with different contents of a' martensite, which were tested with an ECTMAD8D equipment. Voltages measured during testing were mathematically transformed into the impedance plane and were compared with Z theoretical curves, showing a good correspondence. A linear relationship between the Z experimental components and the α'content of the specimens was verified, which makes this procedure suitable for the assessment of the martensite content in this type of stainless steels if adequate calibration pieces are available.