Effect of Microstructure and Surface Diffusion on Crack Resistance of Low-Carbon Steel Using Oil-Based CNT Nanofluid Quenching

This study investigates the limitations of quenching media in resisting crack propagation in low-carbon steel, particularly in relation to microstructural features and surface diffusion effects. A novel quenching medium was developed by incorporating carbon nanotube (CNT) into oil-based due to their excellent cooling properties. Specimens of low-carbon steel were prepared and heated at austenitization temperature for 1 hour followed by immersing into oil and oil-based CNT nanofluid. Both treated samples were polished delicately for microstructure and surface diffusion analysis with the untreated specimen served as a benchmark specimen. Further, the crack growth tests were carried out using compact tension (CT) specimen under constant amplitude loading (CAL). The results indicate that specimen quenched in oil-based CNT nanofluid provided the highest crack resistance and fatigue life at threshold value of (ΔKth) 17.1 MPa√m and 34,464 cycles which is higher than those of the untreated and oil-based quenched specimens. This improvement is attributed to different formation of coarse and fine upper bainite, grain size boundaries dislocation, and the presence layer depth of surface diffusion at 34 µm. Overall, the CNT-infused oil enhances heat transfer efficiency during quenching, effectively retarding crack propagation and extending fatigue life.