Martensite Content and Magnetic Permeability Measurement in Austenitic Stainless Steel Using Eddy Current Array Analytical Solutions

During the manufacturing and service process of 304 austenitic stainless-steel pressure vessels, pipelines, and other industrial equipment, it is easy to generate magnetic microstructures such as martensite, which will not only accelerate the corrosion failure of the equipment but also cause local overheating failure of key components due to its high magnetic permeability characteristics. In this study, the analytical solution of eddy current array (ECA) in the detection of an infinite metal plate and pipe under different operation modes and coil configurations was obtained through theoretical derivation. Based on these analytical solutions, combined with the formation mechanism of strain-induced martensite and the magnetic permeability measurement standard of weak magnetic materials, the transmit–receive coil array elements are optimally selected, and a set of sweep-frequency eddy current nondestructive measurement method of magnetic structure content and its approximate permeability considering the curvature and thickness of components was developed. Algorithm effectiveness is verified through comparative testing of metal plate and rod tensile specimens at different strain levels. The experimental results show that the measurement errors of martensite content and relative permeability are less than 5% when the specimens’ strain $\epsilon \le 35$ %.