The Influence of Niobium Microalloying on Austenite Grain Coarsening Behavior of Ti-modified SAE 8620 Steel

Abstract

The potential for suppressing unacceptable austenite grain growth during carburizing by Nb microalloying additions in the range of 0.02 to 0.11 wt% to a Ti-modified SAE 8620 carburizing steel were evaluated. Alloys, were designed based on fundamental equilibrium thermodynamic analyses, as part of an extensive study on the effects of alloy composition, thermomechanical history, and pseudo-carburizing conditions on austenite grain coarsening behavior. Laboratory samples were produced to simulate both conventional hot rolling and controlled rolling practices designed to produce different initial precipitate distributions. Pseudo-carburizing heat treatments, i.e. without a carburizing atmosphere, were performed in the temperature range of 950 to 1100°C for holding times of 30 to 360 min. Precipitate distributions, including size, number density, morphology, distribution, and chemical composition in selected samples from the as-rolled and pseudo-carburized conditions were evaluated with transmission electron microscopy on extraction replicas. Results showed that increasing Nb additions to the Ti-modified SAE 8620 steel restrained austenite grain coarsening, and increased the grain coarsening time, especially at temperatures below 1050°C. The Nb-free (Ti-modified) steel yielded either severely duplex grain structures or pseudo-normal grain growth (with very large mean grain diameter). However, holding a Ti–Nb-modified steel (e.g. 0.06 Nb wt%) at 950°C for 6 h or at 1000°C for 4 h. produced fine and uniform austenite grain structures (with a mean grain diameter less than 20 μm). The finer grain sizes observed in the Ti–Nb-modified steels were due to the presence of Nb-rich precipitates that hinder austenite grain coarsening, and precipitate distributions and grain growth behaviors are also influenced by the steel rolling history. The results indicate that Nb can successfully be used to suppress grain growth in carburizing steels.