Thermomechanical processing of microalloyed bar steels for induction hardened components

Abstract

Thermomechanical processing (TMP) of microalloyed steels has been shown to produce refined microstructures by austenite grain boundary pinning and elevation of the austenite no-recrystallization temperature. This project investigated the effects of chemical composition (specifically, Al, V, and Nb), thermal and thermomechanical processing variations in preconditioning 1045 steel bars for subsequent induction hardening in order to achieve refined prior austenite grain sizes and improved mechanical performance. Multistep hot torsion on a Gleeble® 3500 was utilized to simulate hot rolling and TMP bar rolling schedules. A method for analyzing microstructural development (specifically focused on austenite recrystallization and strain accumulation) during hot torsion deformation was developed and utilized to assess microalloying and processing effects on strain retention in austenite. Subsequent induction hardening simulations were carried out using resistive heating and water quenching. Microstructures developed during rolling, intermediate heat treatment, and simulated induction hardening were examined using prior austenite grain boundary etching, light optical microscopy, and scanning electron microscopy. Nano-scale precipitates were characterized using transmission electron microscopy and small-angle X-ray scattering. Results show beneficial effects of both microalloying and thermomechanical processing in achieving refined austenite after rolling. Refined pre-induction microstructures resulted in finer post-induction prior austenite grain sizes, highlighting the advantage of refined as-rolled structures for induction hardening applications. Additionally, simulated induction hardened conditions with a range in prior austenite grain sizes (from 10-60 μm) were tested to failure in 3-point bending. The most refined post-induction prior austenite grain size resulted in a fracture load of approximately double that of the coarsest prior austenite grain size condition, demonstrating a benefit of refinement in prior austenite grain size for induction hardened steels. The austenite grain size that developed during simulated induction heating was not well correlated with precipitate size. While the influence of Zener pinning is thus uncertain, the results suggest a more prominent effect of austenite nucleation on the austenite grain size distribution after short-time austenitizing. Microalloy additions contributed to refinement of simulated induction hardened austenite grain size primarily by facilitating refinement of the pre-induction microstructures and to a much smaller extent from precipitate pinning of austenite grains.