University of Oulu

Hannula, J., Kaijalainen, A., Porter, D.A., Somani, M.C. and Kömi, J. (2020), Evaluation of Mechanical Properties and Microstructures of Direct‐Quenched and Direct‐Quenched and Tempered Microalloyed Ultrahigh‐Strength Steels. steel research int.. doi:10.1002/srin.202000451

Evaluation of mechanical properties and microstructures of direct‐quenched and direct‐quenched and tempered microalloyed ultrahigh‐strength steels

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Author: Hannula, Jaakko1; Kaijalainen, Antti1; Porter, David A.1;
Organizations: 1Faculty of Technology, Materials and Mechanical Engineering, University of Oulu, P.O. Box 4200, 90014 Oulu, Finland
Format: article
Version: accepted version
Access: embargoed
Persistent link: http://urn.fi/urn:nbn:fi-fe2020110389112
Language: English
Published: John Wiley & Sons, 2020
Publish Date: 2021-10-23
Description:

Abstract

Herein the effects of molybdenum and niobium on the microstructures and mechanical properties of laboratory‐rolled and direct‐quenched and direct‐quenched and tempered steels are revealed. The microstructures are martensitic with yield strength of 766–1119 MPa in direct‐quenched condition and 632–1011 MPa in direct‐quenched and tempered condition. Mo and Nb additions lead to a fine martensitic microstructure that imparts a good combination of strength and toughness. Steel with 0.5 wt% molybdenum has a high yield strength of 1119 MPa combined with low 28 J transition temperature of −95 °C in direct‐quenched condition. Molybdenum and niobium increase the strength significantly during tempering due to enhanced solute drag and precipitation hardening. Addition of 0.25 wt% molybdenum increases yield strength from 632 to 813 MPa after tempering. However, the combination of niobium and molybdenum results in even greater increase in yield strength during tempering compared to the nonalloyed version, producing almost 400 MPa increase in yield strength. Transmission electron microscopy reveals that only niobium forms stable precipitates during tempering, indicating that molybdenum largely remains in solution. X‐ray diffraction analysis elucidates that molybdenum and molybdenum–niobium alloying prevents annihilation of dislocations, leading to the presence of high densities of dislocations after tempering.

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Series: Steel research international
ISSN: 1611-3683
ISSN-E: 1869-344X
ISSN-L: 1611-3683
Volume: In press
DOI: 10.1002/srin.202000451
OADOI: https://oadoi.org/10.1002/srin.202000451
Type of Publication: A1 Journal article – refereed
Field of Science: 216 Materials engineering
Subjects:
Funding: The authors gratefully acknowledge the financial support of the IMOA (International Molybdenum Association) and SSAB Europe Oy.
Copyright information: © 2020 Wiley‐VCH GmbH. This is the peer reviewed version of the following article: Hannula, J., Kaijalainen, A., Porter, D.A., Somani, M.C. and Kömi, J. (2020), Evaluation of Mechanical Properties and Microstructures of Direct‐Quenched and Direct‐Quenched and Tempered Microalloyed Ultrahigh‐Strength Steels. steel research int.. doi:10.1002/srin.202000451, which has been published in final form at https://doi.org/10.1002/srin.202000451. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.