Niobium Solubility in Austenite in the Presence of Niobium Carbides, Nitrides and Carbonitrides

Niobium is added to carbon steels in small amounts (< 0.1weight %), thus being called a microalloying element, to increase mechanical strength and toughness. When added to steel, niobium is partly soluble in the matrix and another part combines with carbon and nitrogen forming a family of Nb x C y N z precipitates (niobium carbides, nitrides or carbonitrides), where the values ​​of x, y, z depend on the temperature and the chemical composition of the steel. The solubility equations for niobium in austenite available in the literature only provide the niobium content that could be solubilized at a given temperature. But when niobium is added above the solubility limit, the excess niobium will not only form the Nb x C y N z family of precipitates. This is what the proposed model calculates. The proposed model is easy to apply and provided results are very close to those determined experimentally by different researchers, who used different techniques such as atom probe, or matrix dissolution with precipitate filtration, for example. Although the proposed model has been used to calculate niobium in solution in austenite, the same can be applied to any other precipitate, such as carbides, nitrides or carbonitrides of vanadium and titanium, for example.