University of Oulu

Pashangeh, S., Karimi Zarchi, H. R., Ghasemi Banadkouki, S. S., & Somani, M. C. (2019). Detection and Estimation of Retained Austenite in a High Strength Si-Bearing Bainite-Martensite-Retained Austenite Micro-Composite Steel after Quenching and Bainitic Holding (Q&B). Metals, 9(5), 492. https://doi.org/10.3390/met9050492

Detection and estimation of retained austenite in a high strength Si-bearing bainite-martensite-retained austenite micro-composite steel after quenching and bainitic holding (Q&B)

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Author: Pashangeh, Shima1; Zarchi, Hamid Reza Karimi1; Banadkouki, Seyyed Sadegh Ghasemi1;
Organizations: 1Department of Mining and Metallurgical Engineering, Mining Technologies Research Center, Yazd University, Yazd 98195-741, Iran
2Materials and Mechanical Engineering, Centre for Advanced Steels Research, University of Oulu, 90014 Oulun Yliopisto, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 6 MB)
Persistent link: http://urn.fi/urn:nbn:fi-fe2019091928794
Language: English
Published: Multidisciplinary Digital Publishing Institute, 2019
Publish Date: 2019-09-19
Description:

Abstract

To develop an advanced high strength steel with reasonable ductility based on low alloying concept as well as micro-composite microstructure essentially consisting of bainite, martensite and retained austenite, a Si-bearing, low alloy medium carbon sheet steel (DIN1.5025 grade) was subjected to typical quenching and bainitic holding (Q&B) type isothermal treatment in the bainitic region close to martensite start temperature (Ms) for different durations in the range 5s to 1h. While the low temperature bainite has the potential to provide the required high strength, a small fraction of finely divided austenite stabilized between the bainitic laths is expected to provide the desired elongation and improved work hardening. Various materials characterization techniques including conventional light metallography, field emission scanning electron microscopy FE-SEM, electron backscatter diffraction (EBSD), differential thermal analysis, X-ray diffraction (XRD) and vibrating sample magnetometry (VSM), were used to detect and estimate the volume fraction, size and morphology and distribution of retained austenite in the micro-composite samples. The results showed that the color light metallography technique using LePera’s etching reagent could clearly reveal the retained austenite in the microstructures of the samples isothermally held for shorter than 30s, beyond which an unambiguous distinction between the retained austenite and martensite was imprecise. On the contrary, the electron microscopy using a FE-SEM was not capable of identifying clearly the retained austenite from bainite and martensite. However, the EBSD images could successfully distinguish between bainite, martensite and retained austenite microphases with good contrast. Although the volume fractions of retained austenite measured by EBSD are in accord with those obtained by XRD and color light metallography, the XRD measurements showed somewhat higher fractions owing to its ability to acquisition and analyze the diffracted X-rays from very finely divided retained austenite, too. The differential thermal analysis and vibrating sample magnetometry techniques also confirmed the stabilization of retained austenite finely divided in bainite/martensite micro-composite microstructures. In addition, the peak temperatures and intensities corresponding to the decomposition of retained austenite were correlated with the related volume fractions and carbon contents measured by the XRD analysis.

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Series: Metals
ISSN: 2075-4701
ISSN-E: 2075-4701
ISSN-L: 2075-4701
Volume: 9
Issue: 5
Article number: 492
DOI: 10.3390/met9050492
OADOI: https://oadoi.org/10.3390/met9050492
Type of Publication: A1 Journal article – refereed
Field of Science: 216 Materials engineering
Subjects:
Copyright information: © 2019 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 (http://creativecommons.org/licenses/by/4.0/).
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