The properties of duplex stainless steels (DSSs) depend on the ferrite-austenite ratio and on the amount of secondary phases. The actual phase content depends on the chemical composition of the steel and the technology of rolling and heat treatment of the final product. Therefore, it is necessary to control the volume fractions of all phases, their size and the distribution pattern. The existing physical methods for assessing the volume fraction of phases, for example, magnetometry or X-ray diffraction (XRD) quantitative phase analysis, work either with reference to certain databases to interpret the results, or in very narrow determination ranges. These methods also require the manufacture of additional samples, while metallographic assessment can be carried out on samples after mechanical tests. Therefore, in this work, a metallographic technique for assessing the phases in DSSs, based on selective etching and subsequent analysis according to ASTM E 1234, was developed.
Using thermodynamic modeling and scanning electron microscopy, the phases in the samples were identified. Studied samples were obtained after different heat treatments, including solution annealing and quenching from 1050-1200°C to obtain different amounts of ferrite, and annealing at 600-800°C to precipitate sigma-phase. By selecting reagents and etching technique, images of microstructure were obtained. The obtained images are suitable for measuring the volume fraction of ferrite and sigma-phase in the manual mode using the grid intercept method according to ASTM E 562 and in the automatic mode according to the ASTM E 1245. It is shown that chemical etching makes it possible to obtain more contrasting images for automatic analysis; however, the chemical method is very sensitive to changes in the conditions of sample preparation. Electrolytic etching makes it possible to obtain a stable high etching quality, but it requires selection of techniques for each sample using expensive equipment.
It was shown that the developed methodology for quantitative analysis based on selective etching and metallographic assessment according to ASTM E 1245 allows obtaining a much more accurate result compared to the proposed ASTM E 562 method, which well correlates with the XRD quantitative phase analysis. The accumulation of the results of a quantitative assessment the DSS’s structure according to the developed methodology will be a reliable basis for creating new and improved production technology of existing steels.
