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Abstract
Duplex stainless steels have a structure normally composed of austenite and ferrite in approximately equal proportions. In order to attain control of its fabrication processes and performance, it is important to understand its microstructural evolution, due to the formation of intermetallic phases such as sigma (σ) and chi (χ), which may cause a severe deterioration of mechanical properties. In the present study, the evolution of sigma phase during heat treatments at temperatures in which intermetallic phases can be formed (800oC-900°C) was studied using magnetic analyses on a SAF2205 (DIN 1.4462/UNS S31803) steel. A significant reduction of the intensity of Magnetic Barkhausen Noise (MBN) was observed with the increase of heat treatment time, indicating a decrease in the quantity of ferromagnetic phases. For 24-hour-long treatments, the Barkhausen Noise signal is almost completely enclosed by the background noise, indicating the existence of a very small volume fraction of ferrite. If proper calibration samples are to be produced, this technique may be a viable method for non-destructive evaluation of field components working under thermal conditions that may cause the formation of intermetallic phases.
Highlights
Duplex Stainless Steels (DSS) usually combines corrosion resistance with interesting mechanical properties
At 900°C, the reaction is not nearly as advanced after 24 hours as it is at 800°C, indicating that the “nose” of the C-curve that characterizes sigma formation kinetics is below 900°C
The results presented confirm the observations of Normando et al.[24], that sigma phase formation can be accurately followed using only non-destructive techniques, which allows for inspections of facilities in the field
Summary
Duplex Stainless Steels (DSS) usually combines corrosion resistance with interesting mechanical properties Their microstructure is normally composed of approximately equal fractions of austenite and ferrite. Grain boundaries, dislocations and stress fields are effective barriers for the movement of the domain walls. For this reason, MBN is sensitive to microstructure, plastic and elastic strains in ferromagnetic materials[18,19,20,21]. Ginsztler et al.[25], compared potentiostatic etching and magnetic methods, and reported that Barkhausen noise measurements are accurate tools to evaluate the amount of ferromagnetic phase in superferritic stainless steels. Dobránszky et al.[26] showed the efficiency of Barkhausen noise as a method to evaluate the decrease in ferrite fraction in a superduplex stainless steel
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