Application of electromechanical impedance technique in the monitoring of sigma phase embrittlement in duplex stainless steel

Abstract

Duplex stainless steels (DSS) have a structure composed of austenite and ferrite in approximately equal volume fractions which provides an excellent integration between mechanical properties and corrosion resistance. However, the formation of intermetallic phases, such as sigma (σ) phase could become a major problem, once a small percentage of this phase originates a drastic deterioration of toughness and ductility, phenomenon known as σ phase embrittlement. In the present work, the effect of the isothermal aging treatment at 800 °C (temperature in which σ phase can be formed) on the degradation of mechanical properties of UNS S31803 DSS was investigated. The results showed that σ phase precipitation occurred regularly and by eutectoid decomposition. The effect of sigma phase on the mechanical properties was analyzed by the microhardness and impact tests at room temperature. The assessment of DSS also includes an σ phase embrittlement monitoring alternative method based on the electromechanical impedance (EMI), using piezoelectric sensors. The basic idea of the σ phase embrittlement monitoring method is that the increase in the hardness and the decrease in the impact toughness will leads to the variation in the EMI signatures by modifying structural stiffness. The proposed methodology is evaluated using the impedance module (|Z|), resistance (R), reactance (X) and by the statistical indexes: Relative Variation of Natural Frequencies (RVNF) and Root Mean Square Deviation (RMSD). This method is considered a nondestructive test, presents accurate results with high sensitivity and reliability providing capacity of determining sigma phase embrittlement and has promising application potential in this and other materials.