Date of Award
Master of Engineering (ME)
Professor J.S. Kirkaldy
The austenite-niobium carbonitride equilibrium has been accurately characterized between 1000°C and 1250°C for commercial steel compositions. A closed capsule technique using hydrogen as carrier gas has been employed to equilibrate Fe-Nb alloys with Fe-C alloys and determine the Υ/Υ+NbCx phase boundary in the temperature range 1000-1250°C. The results have been used to define the solubility of niobium carbide in austenite with improved accuracy. Solubility limits have also been determined by chemical analysis of carbides extracted from a laboratory melted steel. A defect carbide NbC₀.₉ and a carbonitride NbC₀.₉₁N₀.₀₄ have been identified as the equilibrium precipitate compositions at 1000°C in two steels.
To test the data for thermodynamic consistency detailed analysis of the thermochemical data of the defect carbides of niobium has been made. The thermodynamics of the binary Fe-C, Fe-Nb and Nb-C systems have been utilized to determine the Υ/Υ+NbCx equilibrium over relevant composition and temperature ranges and this is in good agreement with the experiments. Solubility product relations are given for the carbide NbC0.87 as this carbide composition has been shown to be a justifiable average equilibrium composition over the temperature range in this study. The carbide solubility relations have been used along with the reported carbonitride solubilities, to estimate the solubility relation for a metastable cubic nitride of composition NbN₀.₈₇.
An accurate method for calculating the composition and solubility of nonstoichiometric carbonitride precipitates in steel has been devised using the above carbide and nitride solubility relations and treating the carbonitrides as compounds with a fixed nonstoichiometry NbCxN(₀.₈₇-x). The effects of Mn, Si, Cr, Ni and Mo on the solubilities of carbonitrides have also been evaluated and it is shown that the carbonitride solubility in a multi-component austenite can be satisfactory predicted. It is therefore possible to calculate, for example, precipitate fractions and theoretical maximum temperatures required in slab reheating to completely dissolve precipitates.
Lakshmanan, V.K., "Thermodynamics of the Interstitial Phases of Niobium in Iron Austenite" (1977). Open Access Dissertations and Theses. Paper 364.