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The combination of nickel and molybdenum additions is widely used to enhance hardenability and low‑temperature toughness in quenched and tempered thick plates. However, the interaction between alloying content, cooling rate during quenching, and reaustenitization strategy remains critical for achieving uniform martensitic microstructures through the plate thickness. In this work, two low‑carbon Ni–Mo-B alloy designs (0.5% and 1%Ni) were evaluated to quantify the effect of cooling rate and reaustenitization conditions (single quench at 910 °C or 950 °C, and double quench at 910 °C) on tensile and toughness properties.
Plane strain compression tests were used to replicate industrial thermomechanical schedules, followed by controlled quenching and tempering. Tensile testing, Charpy impact testing, and extensive microstructural characterization (OM, FEG‑SEM and EBSD) were performed to relate microstructure to mechanical response.
For both alloys, increasing the cooling rate promoted higher tensile strength, consistent with the progressive suppression of softer transformation products. In the 0.5%Ni-Mo-B steel, low cooling rates (1–2 °C/s) led to mixed microstructures containing ferritic and bainitic regions embedded in martensite, resulting in inferior toughness. In contrast, the 1%Ni-Mo-B grade formed fully martensitic microstructures across all cooling rates, displaying more stable toughness levels and confirming the beneficial role of nickel in improving through‑thickness hardenability. Reaustenitization at 950 °C and the application of a double quenching at 910 °C produced more homogeneous martensite, improving both tensile and toughness performance. The toughness improvement associated with double quenching was particularly pronounced for the 0.5%Ni-Mo-B steel, where mixed microstructures were effectively eliminated.
Overall, the results highlight the combined effectiveness of Mo and Ni alloying, cooling rate, and reaustenitization strategy in controlling microstructural homogeneity and optimizing the strength–toughness balance in heavy‑gauge quenched and tempered Ni–Mo-B steels.