Role of Mo in Enhancing the Elevated‐Temperature Strength of Steel Through Dislocation and Grain‐Boundary Stabilization

Fire‐resistant structural steels must retain load‐bearing capacity under fire exposure, and their elevated‐temperature performance is governed not only by alloy design but also by the thermal stability of the microstructure. This work investigates the effects of Mo on the microstructural evolution, mechanical properties, and fire resistance of steels subjected to air‐cooling and water‐cooling. SEM observations reveal that Mo promotes a limited fraction of bainite under air‐cooling and refines the bainitic/martensitic lath structure under water‐cooling. Mechanical testing demonstrates that Mo enhances room‐temperature strength under both cooling conditions; however, its contribution to fire resistance is strongly microstructure‐dependent. In the air‐cooled steel, the yield‐strength retention at 600 °C remains essentially unchanged upon Mo addition, whereas in the bainitic/martensitic steel, the yield strength at 600 °C increases markedly from ~162.9 to ~254.4 MPa with the retention ratio rising from 0.29 to 0.38. TEM analysis after holding at 600 °C identifies coarse alloyed cementite along lath boundaries, without a high number density of fine Mo‐rich precipitates, indicating that precipitation strengthening is unlikely to dominate. EBSD characterization, prior austenite reconstruction, and boundary statistics confirm that Mo mitigates dislocation annihilation and preserves block/sub‐block boundary density during elevated‐temperature exposure, thereby enhancing resistance to recovery, coarsening, and softening.