Comparative study of ferrite quantification methods applied to duplex stainless steels

Abstract

Duplex stainless steels (DSS) present approximately the same quantities of ferrite (magnetic phase) and austenite. These steels are replacing austenitic stainless steels in some applications due to its high mechanical and corrosion resistance. However, changes in the volumetric percentage of those phases can modify material properties, justifying the development of this study, which has as principal aim a comparison between three different ferrite quantification methods. Samples of two different DSS (UNS S31803 or UNS S32750), originally supplied as 20mm round bars, were solution-treated between 1000°C and 1250°C, in order to obtain different ferrite/austenite ratios. Ferrite quantification was performed in three sections: longitudinal, transverse and diagonally oriented to the rolling direction. The volume fraction of ferrite was determined using three distinct methods: quantitative stereology with an optical microscope, magnetic measurements using a ferritscope, and X-ray analysis by comparison of the relative intensities of peaks corresponding to the planes that generate maximum intensity of diffraction for single-phase samples. Results are discussed, analysing the reliability of the studied methods.