On the role of chemical heterogeneity in carbon diffusion during quenching and partitioning

Carbon partitioning is the core design of quenching and partitioning (Q&P) process, which significantly improves the stability of retained austenite (RA). However, the inevitable precipitation of transition carbides leads to a substantial consumption of carbon atoms. Conversely, in the present study, we have realized the full inhibition of transition carbides in martensitic lath and, in turn, improved the carbon utilization efficiency to stabilize austenite. During fast austenitization from Mn-partitioned pearlite, Mn-heterogeneous high-temperature austenite is produced and more carbon atoms are trapped in the Mn-enriched region. Following Q&P, the alternative film RA and lath martensite in nanoscale is obtained, in which RA is enriched with Mn and C while the lath martensite is depleted with Mn and C. On one hand, the depletion of Mn and C in lath martensite strongly reduces the driving force and nucleation rate for carbide precipitation. On the other hand, the nanoscale microstructure and heterogeneous Mn distribution effectively accelerate carbon diffusion from lath martensite into austenite. Therefore, carbide precipitation is substantially inhibited in lath martensite due to the kinetic mismatch between fast carbon diffusion and sluggish carbide precipitation. This study demonstrates that chemical heterogeneity provides a novel pathway to enhance carbon partitioning efficiency and tensile properties in Q&P steels.