Effect of Cr/V ratio on the microstructure evolution and tensile behavior of a high-vanadium high-speed steel

This study systematically investigates the influence of Cr/V ratio (0.2, 0.4, 0.6, 0.8, and 1.0) on the microstructure and tensile behavior of high-vanadium high-speed steel (HVHSS) using SEM, EBSD, and TEM. The results indicate that lower Cr/V ratios promote the formation of coarse blocky MC-type carbides, which progressively refine and undergo spheroidization as the Cr/V ratio increases. Concurrently, M₇C₃-type carbides, initially absent at low Cr/V ratios, develop a networked distribution with significant coarsening at higher ratios. The matrix phase evolution reveals a non-monotonic trend in austenite content, peaking at approximately 40 % for a Cr/V ratio of 0.6, while martensite content follows an inverse relationship. Tensile tests demonstrate an optimal strength-ductility balance at a Cr/V ratio of 0.6, exhibiting a peak ultimate tensile strength of 485.7 MPa alongside superior ductility. This enhancement is attributed to the synergistic effects of (i) finely dispersed spherical MC carbides impeding dislocation motion via Orowan strengthening and (ii) strain-induced austenite-to-martensite transformation (TRIP effect) absorbing deformation energy and delaying fracture. Fractography analysis reveals a transition from cleavage (Cr/V = 0.2) to quasi-cleavage (Cr/V = 0.6), reverting to cleavage-dominated failure at higher ratios (>0.6). TEM observations near the fracture surface of the 0.6 Cr/V sample reveal dense dislocation arrays and stacking faults within austenite, improving plastic compatibility. The optimized mechanical performance stems from dual strengthening mechanisms: Orowan strengthening by nano-scale MC carbides and TRIP-assisted strain hardening. These findings provide a theoretical framework for designing multiphase steels with tailored carbide distributions and phase stability, enabling superior strength-ductility synergy through dislocation engineering and crack suppression.