University of Oulu

Ali, M., Porter, D., Kömi, J. et al. Effect of cooling rate and composition on microstructure and mechanical properties of ultrahigh-strength steels. J. Iron Steel Res. Int. 26, 1350–1365 (2019) doi:10.1007/s42243-019-00276-0

Effect of cooling rate and composition on microstructure and mechanical properties of ultrahigh-strength steels

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Author: Ali, Mohammed1,2; Porter, David1; Kömi, Jukka1;
Organizations: 1Materials and Mechanical Engineering, Centre for Advanced Steels Research, University of Oulu, Oulu 90014, Finland
2Steel Technology Department, Central Metallurgical Research and Development Institute, Helwan, Cairo 11421, Egypt
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 5.8 MB)
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Language: English
Published: Springer Nature, 2019
Publish Date: 2020-01-14


The influence of cooling rate on the microstructure and mechanical properties of two new ultrahigh-strength steels (UHSSs) with different levels of C, Cr and Ni has been evaluated for the as-cooled and untempered condition. One UHSS had higher contents of C and Cr, while the other one had a higher Ni content. On the basis of dilatation curves, microstructures, macrohardness and microhardness, continuous cooling transformation diagrams were constructed as a guide to heat treatment possibilities. Cooling rates (CRs) of 60, 1 and 0.01 °C/s were selected for more detailed investigations. Microstructural characterization was made by laser scanning confocal microscopy, field emission scanning electron microscopy combined with electron backscatter diffraction, electron probe microanalysis and X-ray diffraction. Mechanical properties were characterized using macrohardness, tensile and Charpy V-notch impact tests. UHSS with the higher C and Cr contents showed lower transformation temperatures and slower bainite formation kinetics than that with the higher Ni content. Higher cooling rates led to lower volume fractions and carbon contents of retained austenite together with finer prior austenite grain size, as well as effective final grain size and lath size. These changes were accompanied by higher yield and tensile strengths. The best combinations of strength and toughness were obtained with martensitic microstructures and by avoiding the formation of granular bainite accompanied by proeutectoid carbides at low CR. For the cooling rates studied, UHSS with the higher C and Cr contents showed the higher hardness and strength but at the cost of toughness.

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Series: Journal of iron and steel research international
ISSN: 1006-706X
ISSN-E: 2210-3988
ISSN-L: 1006-706X
Volume: 26
Pages: 1350 - 1365
DOI: 10.1007/s42243-019-00276-0
Type of Publication: A1 Journal article – refereed
Field of Science: 216 Materials engineering
Funding: Open access funding provided by University of Oulu including Oulu University Hospital. The authors acknowledge the Egyptian Ministry of Higher Education (Cultural Affairs and Missions Sector) for financial support during this work.
Copyright information: © The Author(s) 2019. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (, which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.