In the master’s thesis, we characterized niobium nitrides and carbonitrides in the Fe-Cr-Ni system. The influence of the nickel content and the cooling rate on particle precipitation was analyzed. Niobium microalloying improves the mechanical properties of stainless steels by solution hardening and the formation of fine precipitates. Steels lose their resistance to high-temperature creep, their impact toughness deteriorates, and grain boundaries and dislocations migrate more easily, with poor distribution or the presence of primary particles in the microstructure.
The main aim of this work was to study and characterize niobium particels in the cast microstructure of steels at different nickel concentrations and cooling rates. For this purpose, we developed thermodynamic computational models to estimate the solubility product of niobium nitrides, the segregation of elements during solidification of the steel melt, with the results being additionally supported by ThermoCalc calculations. We made three steel batches with different nickel contents in an induction furnace, which were cast into sand molds. Every casting had a few different diameters to achieve different cooling rates. The solidification and cooling of the melt was monitored by a simple thermal analysis and a thermal camera. The cooled pieces were cut and prepared for further processing.
We chemically analyzed the samples. The polished and etched samples were examined with an optical microscope. The phases and the size of the secondary dendritic arm spacings were determined from the microstructure. A more detailed examination of the microstructure was performed with a scanning electron microscope, a microchemical EDS analysis was performed, and the karbonitrides and inclusions in the samples were characterized. We also performed an automatic analysis of the niobium particles by scanning electron microscope images. The content of ferrite in the microstructure was evaluated using a ferritscope.
We determined the solubility product of niobium nitrides, the temperature influence and the influence of the addition of chromium and nickel on the solubility product using computational models. Using segregation models, we predicted at what proportions of the fraction solid phase the crystalization of niobium carbonitrides occurs. Primary niobium carbonitrides, niobium carbonitride eutectic phases and heterogenous nucleations on MnS inclusions were observed in steel with 10 wt. % Ni. As the proportion of nickel in the solution decreases, the solubility of nitrogen in the melt increases, which is manifested by a lower formation of primary carbonitrides and eutectic niobium carbonitrides. MnS non-metallic inclusions play an important role in the heterogeneous nucleation of carbonitrides. As the proportion of nickel in the solution decreases, the precipitation of niobium carbonitrides in the solid increases.
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