Non-destructive, Contactless and Real-Time Capable Determination of the α’-Martensite Content in Modified Subsurfaces of AISI 304

Abstract

Cryogenic turning can be used to produce deformation-induced martensite in metastable austenitic steels. Martensite exhibits a higher hardness than austenite and increases the wear resistance of the workpiece. In order to reliably induce a desired martensite content in the subsurface zone during the turning process, a non-destructive, contactless and real-time testing method is necessary. Eddy current testing is an electromagnetic method that is non-destructive, non-contact and real-time capable. Furthermore, eddy current testing has been integrated into production processes many times. Eddy current testing can be used to detect the transformation of paramagnetic austenite to ferromagnetic α′-martensite based on the change in magnetic and electrical properties. Thus, the newly formed subsurface can be characterized and the manufacturing process can be monitored. The objective of this study was to understand the correlation of eddy current testing signals with newly formed α′-martensite in the subsurface of AISI 304 and to quantify the amount formed. The measurements were performed within a machining center. Several methods for reference measurement of martensite content are known in the literature. However, depending on the method used, large discrepancies may occur between the determined contents. Therefore, different analytical methods were used for reference measurements to determine the total martensite content in the subsurface. Metallographic sections, magnetic etching, Mössbauer spectroscopy, and X-ray diffraction with two different analytical methods were employed. Based on the correlation between the eddy current testing signals and the α′-martensite content in the subsurface, process control of the manufacturing process can be achieved in the future.

An approach to interpreting metastable austenitic material sensors for fatigue analysis

“…A separate model could be added for this aspect as well. A calibration curve for correlating the eddy current probing signal and martensite content currently needs to be derived experimentally, as is e. g. done in [15] via x-ray diffraction.…”

“…When the evaluation depends on the difference between two readings, and not an absolute reading, eddy current sensor resolution is especially significant, as two sensor level readings separated by less than measurement uncertainty do not provide precise information but only imply loads below a certain threshold. With standard lab equipment, i. e. with a better measuring chain than currently available for machine element integration, martensite contents can reliably be absolutely measured via eddy current testing with percentage-point resolution [15]. Percentage-point resolution determines that at best, 100 blocks can be resolved until the sensor is used up.…”

In-Situ Characterization of Microstructural Changes in Alloy 718 during High-Temperature Low-Cycle Fatigue

“…Harmonic analysis of eddy current signals (EC-HA) is a non-destructive testing technique suitable for characterizing such changes in the magnetic material properties. This testing technique is mostly used for non-destructive characterization of steel alloys with ferromagnetic properties [15][16][17]. For detecting microstructural changes in nickel-based superalloys by EC-HA, the fact can be exploited that the chromium depletion correlates with the Curie temperature and the amplitude of the third harmonic at 25 • C [18][19][20].…”