Characterization of the Microstructure and Precipitates Formed During the Thermomechanical Processing of a CrNiMoWMnV Ultrahigh-Strength Steel

The effect of total applied strain (TAS), finish forging temperature (FFT) on the microstructure, and precipitation kinetics of a newly developed low-cost, low-alloy CrNiMoWMnV ultrahigh-strength steel has been investigated. A Gleeble 3800 thermomechanical simulator is used to simulate the hot forging process and its influence on the precipitation kinetics. Field-emission scanning electron microscopy combined with electron-backscattered diffraction is used to characterize the final overall microstructures, whereas transmission electron microscopy on carbon extraction replicas is used to characterize the precipitates in terms of morphology, size distribution, mean equivalent circle diameter, the 90th percentile in the cumulative diameter distribution (D90%_ppt), chemical composition, and crystallography. Thermo-Calc software is used to predict the precipitates expected in austenite at equilibrium. The final microstructure consists of lath martensite and a small fraction of the precipitates AlN, TiN, and composite TiN–AlN. Differences in the degree of strain-induced precipitation caused by variations in TAS and FFT have been shown to greatly influence precipitate size distributions. Variations in the degree of precipitate dissolution and coarsening cause variations in the prior austenite grain size, which subsequently cause variations in the effective grain size of the final microstructure, i.e., that are defined by high-angle grain boundaries.