Microstructural evolution during isothermal tempering of a highly supersaturated metastable austenitic state of a PM high-speed steel
This study investigates the microstructural evolution of the powder metallurgical high-speed steel PM HS 3-3-4 during isothermal holding within the transformation gap between pearlite and bainite formation. The focus is on the mechanism of carbide spheroidization and the proposed formation of a chemically graded carbide rim. The heat treatments were carried out in a quenching dilatometer, enabling precise control of the thermal cycles and isothermal holding conditions. Spatially resolved analytic microscopy techniques, including scanning transmission electron microscopy (STEM), Auger electron spectroscopy (AES) and atom probe tomography (APT), image analysis and complementary thermodynamic simulations (MatCalc) were used to characterize the carbide evolution. Isothermal holding increased carbide circularity, carbide volume fraction, and hardness, while MatCalc calculations predict a strong thermodynamic driving force for additional carbide precipitation. The analytic techniques revealed a compositional gradient of approximately 15 nm at the carbide-matrix interface in the isothermally treated condition, compared with a sharp transition of only 2-3 nm in the directly quenched state. This nanometer-scale gradient provides strong evidence for epitaxial, interface-controlled carbide growth and the formation of a chemically graded carbide rim. The associated solute depletion of the surrounding austenite provides a plausible explanation for its reduced stability, reflected by an increase in the martensite start temperature. The combined results demonstrate that isothermal holding promotes interfacial chemical reorganization, offering new insights into carbide spheroidization and providing a basis for tailoring carbide morphology and the resulting mechanical properties through heat treatment.