Tervo, H. & Pallaspuro, S. & Kaijalainen, A. & Porter, D. & Kömi, J. & Mehtonen, S. & Pikkarainen, T. (2018) Detrimental effect of coarse titanium-niobium nitrides on the fracture toughness of the CGHAZ in a 500 MPa offshore steel for cold climate conditions. In: Proceedings of the 10th International conference on Clean steel. 18-20 September 2018 Budapest, Hungary, 1-11.
Detrimental effect of coarse titanium-niobium nitrides on the fracture toughness of the CGHAZ in a 500 MPa offshore steel for cold climate conditions
|Author:||Tervo, H.1; Pallaspuro, S.1; Kaijalainen, A.1;|
1University of Oulu, Finland
2SSAB Europe Oy, Finland
|Online Access:||PDF Full Text (PDF, 1.2 MB)|
|Persistent link:|| http://urn.fi/urn:nbn:fi-fe2018101037962
Hungarian Mining and Metallurgical Society (OMBKE),
|Publish Date:|| 2018-10-10
Offshore steels for cold climate conditions require not only relatively high strength to improve the cost-efficiency of the structures, but also excellent toughness at low temperatures to guarantee the safety of the structures in harsh environments. The most challenging locations to fulfil both requirements are in the welded joints of these steels because the weld thermal cycles cause detrimental changes to the steel microstructure. Titanium and niobium are often used in these steels to form nanoscale precipitates and reduce austenite grain growth during the thermal cycle. However, there is a risk that in addition to the nanoscale precipitates formed from the solid state, microscale coarse inclusions are also formed from the melt. These inclusions are hard and brittle and thus likely to deteriorate mechanical properties such as fracture toughness and impact toughness. In this study, two experimental 500 MPa offshore steels with differing inclusion structures were studied to find out if titanium based nitrides significantly deteriorate the fracture toughness of the coarse grained heat-affected zone (CGHAZ) at -40 °C. A Gleeble 3800 was used to simulate the CGHAZ, and the fracture toughness was determined by three point bending testing. Inclusions were characterized using FESEM-EDS. It was found that fracture toughness of the steel with coarse nitrides was lower and the scatter higher than in the steel with a higher calcium based inclusion content and without the coarse nitrides. Fractographic examination showed that the failure in the samples with the lowest toughness was initiated by coarse titanium-niobium nitrides. To avoid toughness problems caused by the CGHAZ under cold conditions, it is necessary to prevent the formation of coarse nitrides during steel production.
|Pages:||1 - 11|
Proceedings of the 10th International conference on Clean steel. 18-20 September 2018 Budapest, Hungary
|Type of Publication:||
D3 Professional conference proceedings
|Field of Science:||
216 Materials engineering
The authors are grateful to the Finnish Funding Agency for Technology and Innovation (Tekes) for financing this work as a part of the research projects DIMECC SIMP – System Integrated Metals Processing and FLEX – Flexible and Adaptive Operations in Metal Production.
© 2018 The Authors and Hungarian Mining and Metallurgical Society (OMBKE).