Thesis defence in the University of Oulu

Doctoral Candidate

Master of Science (Technology) Aleksi Laukka

Faculty and research unit

University of Oulu Graduate School, Faculty of Technology, Process Metallurgy Research Group

Field of study

Process metallurgy

Date and time of the thesis defence

24.1.2020 12:00

Place of the thesis defence

Linnanmaa, auditorium L10

Topic of the dissertation

The effects of microalloying on the scale formation of AISI 304 stainless steel in walking beam furnace conditions

Opponent

Associate Professor Daniel Lindberg, Aalto University

Custos

Professor Timo Fabritius, University of Oulu

Using small additions of alloying elements to affect the corrosion that occurs during the manufacturing of stainless steel

Stains in stainless steel? Barring the passive oxide layer, a definite maybe when accounting for the operating environment and the successfulness of the product design. In manufacturing, stainless steel “rusts” in many stages. This research focused on the oxidation, or high temperature corrosion, happening during the production of stainless steels.

When talking about stainless steels, usually the topic is on austenitic stainless steels that have been alloyed with chromium and nickel. The other popular option is ferritic stainless steels with high chromium alloying amounts. Adding more alloying elements increases the steel’s capability to resist corrosion, or rusting, in challenging environments. This plethora of alloying elements also causes and increase in the complexity of the oxide, or scale, layer that forms on the steel during the manufacturing. Applying the poem from Edgar Allan Poe: this layer also has layers.

How many layers is enough? A construction company’s answer would be as many as the city’s construction supervision admits plus two. According to the oxide scale layer forming on the steel, this is a trick question because it depends on the atmosphere and time in how many can be achieved. Studying the amount of the layers can be used to see the point in which the steel’s ability to resist corrosion has been hindered considerably. When the time period it takes for a certain oxide layer to form in certain atmosphere, boundaries to minimize material loss during manufacturing can be mapped. Preventing the oxidation to progress too far also helps to preserve the properties and the surface quality of the steel in next production stages as well as those of the final product.

In the distant past, the doctorand applied to study architecture. Prowess in calculus and constructing miniatures appeased the referees, the ability draw did not. The electron microscope has proven itself to be the better artist. With it, the effect between multiple parameters and the scale layer’s growth was found in a certain steel. As an example, alloying small amounts of boron to one of the most produced stainless steels leads to an oxidation behavior that decreases the material loss and increases later processing stages. Applying this information to use has a direct economical effect in causing less material to be wasted and an indirect effect to the end product’s quality.

 

Dissertation

Last updated: 13.1.2020