Microstructural evolution of 42CrMo steel during isothermal transformation

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The isothermal phase-transformation behaviour of commercial rolled 42CrMo medium-carbon Cr-Mo alloy steel was systematically investigated over a broad temperature range of 350–725 °C using Gleeble-3800 thermomechanical simulation, combined with optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back-scatter diffraction (EBSD), and X-ray diffraction (XRD). The ferrite-pearlite microstructure forms at 625–675 °C, exhibiting polygonal ferrite and lamellar pearlite. Within the bainite transformation regime (350–475 °C), typical morphological transition from lower bainite to upper bainite was observed: specimens isothermally held at 400 °C show fine acicular bainite with carbide-like features inside ferrite laths, whereas samples transformed at 450 °C develop sheaf-type upper bainite with carbide-like precipitates distributed along lath boundaries. EBSD characterization reveals non-monotonic evolution of ferritic orientation-domain size and grain-boundary misorientation distribution across the bainite transformation temperature window. The area-weighted equivalent circular diameter (ECD) of BCC orientation domains reaches maximum near 450 °C, while the fraction of 2–15° low-angle grain boundaries achieves its maximum value at 425 °C. TEM-EDS characterisation confirms ubiquitous existence of TiN inclusions; MnS particles are frequently observed adjacent to TiN in lower-bainite specimens, and these particles are inherited from steel-making solidification process rather than precipitated during isothermal bainite transformation. XRD phase screening demonstrates that bcc/bct ferritic matrix dominates the microstructure for samples treated between 400 and 450 °C, and retained austenite is only present as a minor constituent below the reliable quantitative detection limit of the current experimental setup. This work quantitatively clarifies the temperature-dependent microstructural evolution law of isothermally transformed 42CrMo steel, provides experimental reference for heat-treatment process optimisation of this widely used engineering steel.