Abstract
In order to quantify ferrite content, three techniques, XRD, ferritoscope and optical metallography, were applied to a duplex stainless steel UNS S31803 solution-treated for 30 min at 1,000, 1,100 and 1,200 °C, and then compared to equilibrium of phases predicted by ThermoCalc® simulation. As expected, the microstructure is composed only by austenite and ferrite phases, and ferrite content increases as the solution treatment temperature increases. The microstructure presents preferred grains orientation along the rolling directions even for a sample solution treated for 30 min at 1,200 °C. For all solution treatment temperatures, the ferrite volume fractions obtained by XRD measurements were higher than those achieved by the other two techniques and ThermoCalc® simulation, probably due to texturing effect of previous rolling process. Values obtained by quantitative metallography look more assertive as it is a direct measurement method but the ferritoscope technique should be considered mainly for in loco measurement.
Highlights
Duplex stainless steels (DSS) are characterized ideally by equal amounts of ferrite and austenite, which provides increased mechanical resistance (680 to 880 MPa ultimate tensile strength) due to the fine grain size, typical of these steels (Sedriks 1996; Souza et al 2005)
The main objective of this research is to compare different methodologies of ferrite phase quantification: quantitative optical metallography, magnetic measurement by ferritoscope and analysis of the peak intensities of the phases by X-ray diffraction, comparing them to the equilibrium of phases predicted by ThermoCalc® simulation
Assuming that the grains are randomly oriented, the integrated intensity of a given phase i is proportional to the volume fraction of that phase, Vi, as shown in Eq 1: So, it is possible to analyze the measured ferrite contents by the three described methods and the expected equilibrium ferrite fraction from ThermoCalc® simulation
Summary
Duplex stainless steels (DSS) are characterized ideally by equal amounts of ferrite and austenite, which provides increased mechanical resistance (680 to 880 MPa ultimate tensile strength) due to the fine grain size, typical of these steels (Sedriks 1996; Souza et al 2005) They present higher corrosion resistance when compared to ferritic stainless steels, promoted by high content of chromium, nitrogen and molybdenum, and, due to the presence of austenite, the DSS present good ductility and toughness (250 J impact toughness) (Young et al 2007). The results showed considerable discrepancies between the values obtained by these techniques (Magnabosco and Spomberg 2011) Another technique to evaluate the volume fraction of the phases is by analysis of the peak intensity of X-ray diffraction (XRD) patterns. The main objective of this research is to compare different methodologies of ferrite phase quantification: quantitative optical metallography, magnetic measurement by ferritoscope and analysis of the peak intensities of the phases by X-ray diffraction, comparing them to the equilibrium of phases predicted by ThermoCalc® simulation
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