Quenching and Partitioning of Plate Steels: Partitioning Design Methodology
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INTRODUCTION
QUENCHING and partitioning (Q&P)[1] is a heat treatment most frequently applied to low-alloy coldrolled sheet steels, to develop high-strength martensitic microstructures with retained austenite (RA).[2] The austenite is stabilized by partitioning of carbon from supersaturated martensite. Such microstructures provide excellent combinations of strength, ductility, and formability, which are of interest to the automotive industry.[3] Q&P is also of interest for thicker plate products, to employ transformation-induced plasticity (TRIP) in microstructures containing austenite to enhance toughness and/or wear resistance for applications such as ground-engaging machinery.[4,5] Toughness of these steels is known to increase with the amount of retained austenite, due to volume expansion during TRIP, causing blunting of crack tips.[4–8] A schematic illustration of a generic Q&P process is shown in Figure 1. The process involves an initial quenching step following austenitization to a carefully controlled quench temperature (QT) where a desired fraction of martensite is formed. The QT is often selected according to a methodology which predicts an ‘‘optimum’’ QT where the most austenite could be retained, based on ‘‘ideal’’ partitioning.[9,10] The controlled mixture of martensite and austenite formed
R.A. STEWART, J.G. SPEER, B.G. THOMAS, E. DE MOOR, and A.J. CLARKE are with the Colorado School of Mines, Golden, CO. Contact e-mail: [email protected] Manuscript submitted December 23, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
during the quenching step undergoes partitioning during holding at a partitioning temperature (PT) for a partitioning time (Pt). During the partitioning step, a substantial portion of the carbon in the martensite diffuses into the austenite, thereby stabilizing it. Final cooling to room temperature completes the process and may result in formation of ‘‘fresh’’ martensite. The QT selection methodology is used widely in the Q&P sheet literature,[11–16] although measured RA fractions are typically less than final predicted RA fractions calculated based on the basic constrained carbon equilibrium model; final RA fractions have been shown to be less sensitive to the QT than expected, in both experimental and modeling studies of a 0.19C-1.59Mn-1.63Si (wt pct) over a range of 60 C,[15] when considering partitioning kinetics. Q&P steels are also industrially produced as a hotrolled sheet product,[19] where partitioning takes place during coil cooling. The process does not follow the schematic shown in Figure 1; slow coil cooling and partitioning begin at the end of the quench step and there is typically no subsequent reheating. The use of coil cooling as a partitioning step was first proposed by Thomas et al.,[20] who also suggested a method to estimate the ‘‘degree’’ of partitioning. A variety of other Q&P process variations have been studied,[21–23] but only a small number of studies have examined the applicability of Q&P processing to thicker sections.[7,24,25] Hong et al. wer
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