The effect of subzero quenching-tempering (SQT) on the mechanical performance and fatigue crack growth behaviour of martensitic 9Cr18Mo steel was investigated through mechanical tests, including tensile testing, micro-Vickers hardness measurements, and fatigue crack growth (FCG) analysis. Microstructural characterisations using SEM with EDS elemental analysis and XRD were performed to correlate microstructural features with mechanical performance. The as-received steel in the annealed condition (AR) exhibits a relatively uniform ferrite matrix containing irregular, globular M 7 C 3 and M 23 C 6 carbides. The SQT specimen consists primarily of tempered martensite with a more uniform dispersion of Cr-rich carbides, such as M 23 C 6 and M 7 C 3 , which markedly improve the steel's mechanical properties. The yield strength increases from 528 MPa to 1229 MPa, the ultimate tensile strength from 699 MPa to 1697 MPa, and the hardness from 220 to 260 HV to 600–650 HV. The higher strain-hardening parameters ( K and n ) obtained for the SQT specimen reflect early-stage dislocation resistance within the tempered martensite matrix, rather than sustained plastic deformation, leading to a substantial decrease in ductility. Fractographic examinations reveal mixed ductile-fracture features in the AR specimen, whereas cleavage-dominated fracture with fine voids is observed in the SQT specimen. Overall, SQT treatment effectively improves the strength and hardness of 9Cr18Mo steel, but its effect on FCG resistance remains limited. Therefore, further investigation into the optimisation of SQT treatment parameters is required to achieve a more balanced combination of strength and fatigue crack resistance for critical engineering applications.
Putra et al. (Thu,) studied this question.