Saturated low frequency eddy current technique applied to microstructure phase quantification in duplex stainless steel

Duplex stainless steel (DSS) presents a good combination of mechanical properties and high corrosion resistance. Its microstructure is composed of two phases with different physical properties: delta ferrite and gamma austenite. The best properties of DSS for in service applications are achieved when these phases are in equal proportions. However, exposition to high temperatures, may entail undesired consequences impairing the mechanical and corrosion properties of the material. Eddy current testing (ECT) has already been used for DSS inspection and microstructure characterization, mainly to detect the presence of deleterious phases, such as sigma. However, conventional ECT was not able to quantify different amounts of austenite and ferrite in the absence of deleterious phases. The present study focuses on the experimental and computational problem of DSS inspection using ECT in saturated magnetic conditions in order to identify different amounts of austenite and ferrite within the DSS microstructure. Samples containing ferrite to austenite ratio equal to 50/50, 70/30 and 80/20 respectively were brought to saturation as designed in the simulations and the ECT was applied presenting a very good result. Saturated low frequency ECT condition proved to be a reliable methodology in quantifying delta ferrite and gamma austenite phases in DSS.