Microstructural evolution and mechanical property correlation in boron-added modified 9Cr-1Mo steel

Castable B-containing steels have immense manufacturing possibilities owing to their wide range of potential applications. This paper focuses on the microstructural evolution and its correlation with the mechanical properties of boron-added (0.5, 2.5, and 5 wt%) modified 9Cr-1Mo steel. A relatively lower quantity of B (0.5 wt%) results in a hypo-eutectic alloy with a ferrite matrix and borides at the prior austenite grain boundaries. Interestingly, this material exhibits even higher uniform ductility than the conventional 9Cr-1Mo martensitic steel. In contrast, the 2.5B system forms a near-eutectic microstructure, while the 5B has a hyper-eutectic microstructure with large boride phases, resulting in the formation of a ferrite-boride composite structure. The composite microstructure exhibits enhanced material strength with significant fracture resistance compared to the pure phase due to the presence of a relatively softer and ductile ferrite phase. The significant increase in uniform ductility of the 0.5B sample has been attributed to the local misorientation near the grain boundary gradient zone. Whereas, for the 2.5B and the 5B samples, the increased strength primarily stems from the second phase ((Fe,Cr)2B) strengthening. Such varied microstructures and properties can be tuned to suit desirable applications. [Display omitted] •Thermodynamic calculations predicted phase nucleation temperatures precisely which impact material properties.•Microstructure and microchemical study unveiled diverse eutectic phases based on composition and revealed the (Fe,Cr)2B phase with bct-Fe2B type crystal structure.•Mechanical property correlation demonstrated the impact of second-phase strengthening on alloy strength.•In 0.5B, significant KAM concentrations emerged in the near-boundary gradient zone.•Manufactured castable metal matrix composite with customized chemistry and mechanical properties through precise control of boron concentration.