Eddy current testing of martensitic transformation and surface microcracks in austenitic stainless steels with different hydrogen charging methods

抄録

This study focuses on hydrogen embrittlement of austenitic stainless steels. Among the many studies on hydrogen embrittlement of austenitic stainless steels, two phenomena, martensitic transformation and microcrack formation, are important to discuss the susceptibility to hydrogen embrittlement. This study focuses on eddy current testing, a non-destructive test used to evaluate the electromagnetic properties of materials, as a method to evaluate these phenomena. The objective of this study is to evaluate two phenomena, martensitic transformation and microcrack formation, using eddy current testing. Tensile tests were performed on AISI 304 specimens of metastable austenitic stainless steel, followed by hydrogen charging. Two types of hydrogen charging methods, electrolytic cathodic charging and high-pressure hydrogen gas exposure were used to prepare specimens with different hydrogen penetration depths. Several specimens were prepared with and without hydrogen charging and with different residual strain values. The impedance of each specimen was measured by eddy current testing. The relative permeability and equivalent conductivity of the specimens were estimated by comparing the measured results with an analytical solution for the coil impedance. The results showed that the relative permeability of the specimens increases, and the equivalent conductivity decreases with increasing strain for hydrogen-charged specimens. The increase in permeability is correlated with martensitic transformation. It is suggested that the phase transformation is caused by the application of hydrogen and strain. The decrease in conductivity was more significant for specimens with microcracks on the specimen surface. A correlation between the decrease in conductivity and microcracking was suggested.