Effect of partitioning time on microstructural evolution and mechanical performance of a medium-silicon low-alloy steel during double quenching and partitioning treatment

Abstract

A medium-silicon low-alloy steel was subjected to double quenching and partitioning (Q&P) with partitioning times of 2–60 min to study microstructural evolution and mechanical performance. Microstructural characterization was carried out using optical microscopy (OM), field-emission scanning electron microscopy (FE-SEM), backscatter diffraction (EBSD), and transmission electron microscopy (TEM) with selected area electron diffraction (SAED). Mechanical properties were evaluated through hardness, tensile, and Charpy impact tests, while X-ray diffraction (XRD) was employed to quantify the retained austenite fraction and lattice characteristics. Increasing partitioning time stabilized retained austenite and promoted carbide-free bainitic laths. Total elongation increased from 9.7 to 18.5%, and Charpy impact energy from 16.5 to 29 J. EBSD and TEM revealed thin film and blocky retained austenite among martensitic and bainitic laths, with no carbide precipitation. XRD quantified retained austenite fraction and carbon content, showing enhanced mechanical stability with longer partitioning. The improved toughness is attributed to stress-assisted martensitic transformation of retained austenite, increasing local plasticity and reducing crack propagation. Consequently, the product of ultimate tensile strength and elongation increased from 11.2 to 21.2 GPa·%. These findings demonstrate that the fraction, morphology, and mechanical stability of retained austenite, rather than carbon partitioning alone, govern the strength – ductility – toughness balance in Q&P steels.

Keywords

quenching and partitioning / medium-silicon steel / retained austenite / EBSD / TEM

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