The Effects of Heating Rate on Austenite Grain Growth in a Ti-modified SAE 8620 Steel with Controlled Niobium Additions

Abstract

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⁻¹ to 1050 and 1100°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 1050°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.