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Univerzitet u Zenici
  Doctoral theses defended at the University in Zenica

Date and place of thesis defense:
12.11.2010. Faculty of Metallurgy and Materials Science
Candidate:
mr. sc. Almaida Gigović-Gekić
Mentor:
prof. dr. Mirsada Oruč

Thesis title:

Quantification Of The Effects Of Alpha-Gamma Elements On Mechanical Properties And Presence Of Delta Ferrite In Austenitic Stainless Steel Nitronic 60

Summary:
Nitronic 60 is a highly alloyed austenitic stainless steels for using at elevated temperatures. This steel has good mechanical properties, corrosion resistance and resistance to wear and adhesive wear. In comparison to conventional austenitic stainless steels, Nitronic 60 has an increased content of manganese and nitrogen to reduce the percentage of nickel content, because its price is unstable in the market. The effect of alloying elements in this steel is very important due to their influence to the formation of austenite and ferrite microstructure. Austenite stabilizing elements such as nickel, manganese, nitrogen and carbon need to ensure the formation of austenitic microstructure, until chromium, silicon and possibly molybdenum, titanium and niobium have a tendency to form ferrite microstructure, ie delta ferrite. The delta ferrite has a negative impact on mechanical properties and resistance to corrosion, so its content seeks to reduce in the steel. The studies that were carried out to examine the properties of austenitic stainless steel Nitronic 60 can be classified in several cycles. Before testing of samples, it should be defined technology of solution annealing that will provide austenitic microstructure without any type of the precipitates in the matrix. In the next cycle, the mechanical properties of rolled and annealed materials were tested. To test the stability of austenite microstructure, a dilatometric analysis of the material in the annealed condition was performed and program of precipitation annealing defined. On the base of the obtained results, a temperature of beginning of precipitation was defined as like as a temperature of maximum intensity of precipitation. Due to the application of this steel at elevated temperatures, a high temperature corrosion was tested too. Regression analysis on the base of results showed a correlation between chemical composition and mechanical properties of steel Nitronic 60. As a result of the analysis, the regression equations were obtained which are used for predicting the mechanical properties on the base of the content of alloying elements as Cr, Si, Ni and Mn at room and elevated temperatures.

Key words:
austenitic stainless steel, Nitronic 60, austenite, delta ferrite, mechanical properties, high temperature resistance, regression analysis


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