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. Pseudocarburizing heat treatments, i.e. without a carburizing atmosphere, were performed in the temperature range of 950 to 1 100°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 1 050°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 1 000°C for 4 h. produced fine and uniform austenite grain structures (with a mean grain diameter less than 20 mm). The finer grain sizes observed in the Ti-Nb-modified steels were due to the presence of Nbrich 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.KEY WORDS: niobium microalloying; carburizing; precipitation; abnormal grain growth. © 2007 ISIJ Alloy and Process Design 2.1. Thermodynamic Basis for Alloy SelectionExperimental alloys, intended to assess the ability of niobium additions to suppress austenite grain coarsening at elevated temperatures, were developed based on fundamental equilibrium thermodynamic analyses of niobium-carbide precipitate formation. Following the previous work of Davidson et al.,7) which showed that a maximum retardation effect on austenite grain coarsening at carburizing temperatures was observed in a Ti-modified SAE 8620 steel with a Ti : N ratio close to stoichiometric (3.42 by weight), the experimental alloys were developed based on the following assumptions: i) Titanium added stoichiometrically to the SAE 8620 steel will precipitate N as TiN; ii) In the Ti-modified SAE 8620 steel, added niobium will precipitate as NbC; and iii) Maximum temperatures experienced during solid-state processing are below about 1 300°C. With these assumptions, the desired niobium levels were determined based on solubility product ...

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The effects of heating rate on austenite grain growth and precipitate distribution in Ti-modified SAE 8620 steels with Nb additions of 0.02, 0.06 and 0.1 wt% were evaluated with pseudo-carburizing, i.e. without a carburizing gas, heat treatments characteristic of high temperature vacuum carburizing. Laboratory plates were produced to simulate conventional hot-rolling and controlled-rolling processes. Specimens were heated at rates between 10 and 145°C min Ϫ1 to 1 050 and 1 100°C, held at the desired austenitizing (carburizing) temperature for 60 min, and immediately quenched in iced-water. Austenite grain structures developed at the austenitizing temperatures were evaluated with light optical metallography, and precipitate dispersions were evaluated using extraction replicas in the transmission electron microscope. Abnormal grain growth was observed in all samples processed at the highest heating rate to 1 050°C, but was suppressed at the lower heating rates with additions of 0.06 and 0.1Nb. Suppression of abnormal grain growth was correlated with the development of a critical distribution of fine NbC precipitates, stable at the austenitizing temperature. The importance to industrial carburizing practice of heating rate effects on precipitates and austenite grain size evolution are discussed.

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