University of Oulu

Sakari Pallaspuro, Antti Kaijalainen, Saara Mehtonen, Jukka Kömi, Zhiliang Zhang, David Porter, Effect of microstructure on the impact toughness transition temperature of direct-quenched steels, Materials Science and Engineering: A, Volume 712, 2018, Pages 671-680, ISSN 0921-5093,

Effect of microstructure on the impact toughness transition temperature of direct-quenched steels

Saved in:
Author: Pallaspuro, Sakari1,2; Kaijalainen, Antti1; Mehtonen, Saara3;
Organizations: 1Materials and Production Engineering, Centre for Advanced Steels Research, University of Oulu, P.O. Box 4200, 90014 Oulu, Finland
2Department of Structural Engineering, Norwegian University of Science and Technology, Richard Birkelands vei 1 A, 7491 Trondheim, Norway
3SSAB, P.O. Box 93, 92101 Raahe, Finland
Format: article
Version: accepted version
Access: open
Online Access: PDF Full Text (PDF, 1.2 MB)
Persistent link:
Language: English
Published: Elsevier, 2018
Publish Date: 2019-12-12


A sufficient level of toughness at low temperatures is paramount for the use of structural steels intended for arctic applications. Therefore, it is important for the steel industry to identify the factors that control brittle fracture toughness. In this study, the quantitative effect of microstructure on the impact toughness transition temperature has been investigated with 18 different thermomechanically rolled and direct-quenched low-carbon ultra-high-strength steels with varying martensite and bainite contents. The steels were produced by altering their chemical composition, the finish rolling temperature and the total reduction of the prior austenite grains in the non-recrystallisation temperature regime, i.e. austenite pancaking, and characterised in terms of microstructural constituents, grain size distributions and texture as well as by using Charpy-V impact and tensile testing. It is shown for the first time that the impact toughness transition temperatures T28J and T50 closely follow a dynamic reference toughness, defined by yield strength and the size of the coarsest grains in the effective grain size distribution at 80th percentile. Decreasing the area fraction of {100} cleavage planes oriented within 15° of the macroscopic fracture plane by increasing austenite pancaking is also shown to improve T28J. The best toughness is achieved with the lowest finish rolling temperatures that are nevertheless high enough to avoid the subsequent formation of granular bainite, which weakens both the toughness and strength. The results show that it is perfectly possible to produce untempered ultra-high-strength martensitic and martensitic-bainitic structural steels with adequate low-temperature toughness when the grain size is properly controlled.

see all

Series: Materials science & engineering. A, Structural materials: properties, microstructure and processing
ISSN: 0921-5093
ISSN-E: 1873-4936
ISSN-L: 0921-5093
Volume: 712
Pages: 671 - 680
DOI: 10.1016/j.msea.2017.12.037
Type of Publication: A1 Journal article – refereed
Field of Science: 212 Civil and construction engineering
214 Mechanical engineering
216 Materials engineering
Funding: This work has been done as a part of a doctoral project within the BSA programme of DIMECC Ltd. Funding from the Finnish Funding Agency for Innovation (Tekes) and the participating companies is gratefully acknowledged. Sakari Pallaspuro would also like to thank the Association of Finnish Steel and Metal Producers, Tekniikan edistämissäätiö (TES) and the University of Oulu Graduate School for financially supporting the visit to NTNU, and SSAB for the support provided and the material studied.
Copyright information: © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license