Quenching and partitioning of plate steel: development of design methodologies

Abstract

Quenching and partitioning (Q&P), a new heat steel treatment concept to develop high strength martensitic microstructures with retained austenite, has been implemented industrially to make sheet products. This process is also of interest for thicker plate products, to employ transformation-induced plasticity of microstructures containing austenite to enhance toughness and/or wear resistance. The applicability of the Q&P process to plate steel has been explored in this work, considering through-thickness thermal profiles and associated microstructural gradients. Thermal gradients which develop in thick plate are much greater than in thinner sheet products, and are expected to have a profound influence on microstructure development of Q&P plate. A numerical model was used to simulate temperature evolution in a flat plate during cooling and heating. Cooling behaviors were examined across a range of thicknesses (6 to 50 mm) for a range of quenching processes. Partitioning behaviors during off-line plate heat treating, involving the transfer of a quenched plate and reheating in a furnace, were also examined and modeled. The important processing parameters for Q&P plate were identified as: plate thickness, quench medium, initial quench time, transfer time, furnace residence time and furnace ambient temperature. Design methodologies were developed for both the quenching and partitioning steps of plate processing. The quenching design methodology was based on the ‘optimal’ QT method used in sheet Q&P processing. The partitioning design methodology employed the Holloman-Jaffe tempering parameter, applied to non-isothermal partitioning. The design methodologies were experimentally validated through dilatometry simulations of plate Q&P processing, according to numerically simulated profiles. An 18 mm plate of a 0.4 wt pct C-1.5 wt pct Si alloy (300M), severe water quenched and partitioned during air cooling and furnace reheating, was simulated using dilatometry. Q&P microstructures were successfully obtained through the thickness of a simulated 18 mm plate and RA fractions comparable to sheet Q&P studies were achieved in some cases. Non-isothermal partitioning treatments were successful in retaining austenite, and substantial decomposition of RA was avoided during furnace residence times up to 25 minutes. The final RA fraction was shown to be less sensitive to initial quench time (i.e. initial quench temperature) than expected. This was important for Q&P plate processing: thermal gradients induced by quenching do not lead to large differences in RA fraction. Selected Q&P plate microstructures were assessed for abrasive wear resistance using dry-sand rubber-wheel testing. Results of preliminary wear testing were inconclusive. The conceptual development achieved in this work is an important step towards commercial Q&P plate processing.