Public defence 2.10.2020

Friday, October 2, 2020

Doctoral Candidate: Master of Science (Tech.) Timo Juuti

Date & time: 2.10.2020 13:00

Place: Linnanmaa,  L2 & zoom

Topic: The precipitation of Laves phase and its contribution to mechanical properties in novel high-Cr ferritic stainless steels

Opponent: PhD Niilo Suutala, Tornio

Second opponent: PhD Silvia Barella, University of Milan

Custos: Professor Jukka Kömi, University of Oulu

Dissertation: pdf



Contribution of phenomena occurring at high temperatures to mechanical properties in novel high-Cr ferritic stainless steels

Recently, it is becoming exceedingly important for the industry to consider the environment, global warming and air pollution. To protect the environment, exhaust manifold systems are being developed to improve fuel economy and to clean exhaust gas from materials that are highly recyclable. Conventional manifolds are made of cast iron, which has poor heat resistance and is relatively heavy weight. Furthermore, cast iron is lot less recyclable than for e.g. stainless steels. Ferritic stainless steels are considered to replace cast iron, to offer a cheaper and more environment friendly option.

In this thesis existing ferritic stainless steels were developed as well as completely new steel grade for high temperature applications, such as the exhaust systems at Wärtsilä, the catalytic converters at Ecocat and for the automotive industry. The new steel grade was developed in collaboration with Outokumpu OYJ and Companhia Brasileira de Metalurgia e Mineração (CBMM). Steels used in high temperature applications are required to have good mechanical- and corrosion resistance properties. The high temperature strength can be increased by adding Nb to increase the basic strength of the material and by controlling the precipitation occurring in the structure during the high temperature service. Further coarsening of the precipitates has a significant effect on the mechanical properties of the steel. In addition to Nb, precipitation of other elements such as Ti, Si and Mo and formation of secondary phases such as Laves-phase influence on the high temperature properties of the product. The precipitation of certain elements such as Mo may also have an effect on the corrosion resistance properties of the steels.

By understanding and controlling the precipitation phenomena in these steels, significant improvements can be done to steels properties. The studied materials are ferritic stainless steel products from Outokumpu OYJ and as well as new experimental melts. The research included a comprehensive experimental part, where the high temperature conditions were simulated with thermomechanic Gleeble simulator as well as with Thermo-calc software. Electron microscopes, x-ray diffraction and spectroscopy among many other methods were used to study the precipitation and Laves phase. Furthermore, the actual properties of the steels were measured with tensile tests, macro- and micro hardness measurements and corrosions tests. The thesis includes five peer reviewed articles focused on Laves phase and modelling of the material.

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Last updated: 1.10.2020