University of Oulu

Pashangeh, S., Ghasemi Banadkouki, S. S., Besharati, F., Mehrabi, F., Somani, M., & Kömi, J. (2021). Color Light Metallography Versus Electron Microscopy for Detecting and Estimating Various Phases in a High-Strength Multiphase Steel. Metals, 11(6), 855. doi:10.3390/met11060855

Color light metallography versus electron microscopy for detecting and estimating various phases in a high-strength multiphase steel

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Author: Pashangeh, Shima1; Ghasemi Banadkouki, Seyyed Sadegh1; Besharati, Fatemeh1;
Organizations: 1Department of Mining and Metallurgical Engineering, Faculty of Engineering, Yazd University, Yazd 89195-741, Iran
2Materials and Mechanical Engineering, Centre for Advanced Steels Research, University of Oulu, 90014 Oulu, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 6.8 MB)
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Language: English
Published: Multidisciplinary Digital Publishing Institute, 2021
Publish Date: 2021-07-08


In this study, fresh attempts have been made to identify and estimate the phase constituents of a high-silicon, medium carbon multiphase steel (DIN 1.5025 grade) subjected to austenitization at 900 °C for 5 min, followed by quenching and low-temperature bainitizing (Q&B) at 350 °C for 200 s. Several techniques were employed using different chemical etching reagents either individually (single-step) or in combination of two or more etchants in succession (multiple-step) for conducting color metallography. The results showed that the complex multiphase microstructures comprising a fine mixture of bainite, martensite and retained austenite phase constituents were selectivity stained/tinted with good contrasting resolution, as observed via conventional light optical microscopy observations. While the carbon-enriched martensite-retained austenite (M/RA) islands were revealed as cream-colored areas by using a double-step etching technique comprising etching with 10% ammonium persulfate followed by etching with Marble’s reagent, the dark gray-colored bainite packets were easily distinguishable from the brown-colored martensite regions. However, the high-carbon martensite and retained austenite in M/RA islands could be differentiated only after resorting to a triple-step etching technique comprising etching in succession with 2% nital, 10% ammonium persulfate solution and then warm Marble’s reagent at 30 °C. This revealed orange-colored martensite in contrast to cream-colored retained austenite in M/RA constituents, besides the presence of brown-colored martensite laths in the dark gray-colored bainitic matrix. A quadruple-step technique involving successive etching with 2% nital, 10% ammonium persulfate solution, Marble’s reagent and finally Klemm’s Ι reagent at 40 °C revealed even better contrast in comparison to the triple-step etching technique, particularly in distinguishing the RA from martensite. Observations using advanced techniques like field emission scanning electron microscopy (FE-SEM) and electron back scatter diffraction (EBSD) failed to differentiate untempered, high-carbon martensite from retained austenite in the M/RA islands and martensite laths from bainitic matrix, respectively. Transmission electron microscopy (TEM) studies successfully distinguished the RA from high-carbon martensite, as noticed in M/RA islands. The volume fraction of retained austenite estimated by EBSD, XRD and a point counting method on color micrographs of quadruple-step etched samples showed good agreement.

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Series: Metals
ISSN: 2075-4701
ISSN-E: 2075-4701
ISSN-L: 2075-4701
Volume: 11
Issue: 6
Article number: 855
DOI: 10.3390/met11060855
Type of Publication: A1 Journal article – refereed
Field of Science: 216 Materials engineering
Funding: This research was funded by the Genome of Steel (Profi3) by the Academy of Finland, grant number 311934.
Copyright information: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (