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1 Journal of Achievements in Materials and Manufacturing Engineering

1 2015

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Etching techniques for duplex steels

Martínek P., Podaný P.

Journal of Achievements in Materials and Manufacturing Engineering

2015

Vol. 68, nr 1

11--16

Purpose: This contribution describes a search for an optimum etching technique for duplex ferritic-austenitic steels which would enable metallographers to find the fractions of major phases by image analysis and determine the amounts, distributions and types of intermetallic phases. Design/methodology/approach: The microstructures were revealed by etching with seven different reagents. The phase compositions were evaluated using either image analysis or test grid-based quantitative analysis. The experimental materials were X2CrNiMoN 22-5-3 steel and its cerium-doped variant. Each of them was examined in two conditions: upon opendie forging and subsequent solution annealing and upon long-time annealing, where the latter led to extensive precipitation of intermetallic phases. The fractions of major and intermetallic phases were also determined using EBSD. Both quantitative and qualitative EBSD data were then compared to the values obtained using optical microscopy. Findings: In X2CrNiMoN 22-5-3 duplex steel, the microstructure can be revealed using various reagents and both chemical and electrochemical etching. The differences between the reagents, when used for evaluating the amounts of major phases (austenite + ferrite), were not substantial. The fraction of sigma phase in long-time-annealed samples can be evaluated using image analysis only if etched with NaOH solution or NH4OH. These etchants also effectively reveal carbides on grain boundaries. However, the values obtained with NaOH are overestimated. When the other reagents are used, the evaluation must be done using another method (e.g. grid-based quantitative analysis). Sigma phase proportions found by optical microscopy are higher than those measured using EBSD. In order to identify microstructural variations across the forged parts, specimens were taken from three locations (centre, ¼ , edge). The sigma phase amounts found in all three testing locations of the sample of the cerium-doped duplex steel were higher than the corresponding amounts in the cerium-free sample. In both materials, the amounts of sigma phase are higher in the centre of the sample than near the edge. This difference is more significant in the ceriumfree material. Originality/value: The article is devoted to phase content evaluation in duplex steels by different methods. EBSD and image analysis of micrographs were compared. Micrographs were acquired by light microscopy after microstructure revelation by various etchants. The content of ferrite, austenite and intermetallic phases was evaluated.