Characterization of primary niobium particles in stainless steels with Different nickel contents

Abstract

The presence of primary Nb-particles and their morphology affected by changes in nickel concentration in the as-cast microstructure of stainless steels with 18 wt% Cr were analysed. The formation of primary particles is due to the decreasing solubility during cooling and solidification of the steel melt, and additionally, segregation of alloying elements occurs during solidification, which contributes to the formation of particles. The coarse, sharp-edged nitrides formed during solidification reduce the mechanical properties of the material, highlighting a significant reduction in impact toughness and resistance to high temperature creep. According to the thermodynamic models the solubility of Nb and N was estimated. Solubility product of NbN decreases by lowering temperature and increasing the Ni content. With the Scheil-Gulliver model, it was predicted that, in a solution with (wt%) 18Cr, 0.5Nb and 0.03N precipitation of primary Nb-particles starts at solid fractions 0.91. The results were also confirmed using the Scheil solidification calculator with Thermo-Calc program. Three batches containing 9, 4.7 and 0.16 wt% Ni were prepared in an open induction furnace and cast into sand moulds. The presence of primary Nb-particles, eutectic phases and heterogeneous nucleation particles on MnS non-metallic inclusions were observed in the microstructure using an optical microscope and scanning electron microscope. By lowering the Ni content in the solution, the solubility product of NbN increases and the proportion of primary particles are lower. During cooling the solubility of Nb-particles decreases, small NbCN precipitate on grain boundaries and on MnS non-metallic inclusions.