Effect of cooling medium on the κ carbide precipitation behavior, microstructure and impact properties of FeMnAlC low-density steel

The effect of four different cooling mediums, namely ice brine, water, oil, and air on the κ carbide precipitation behavior, microstructure, and impact properties of FeMnAlC low-density steel is thoroughly investigated by using OM (Optical Microscope), SEM (Scanning Electron Microscope), TEM (Transmission Electron Microscope), XRD (X-Ray Diffraction), VSM (Vibrating Sample Magnetometer), Rockwell hardness tester and in conjunction with low-temperature impact experiments. The results indicate that after cooling with different mediums, the austenite structure exhibits a mixed crystalline phenomenon and has a significant presence of twins. The average grain size of austenite increases as the cooling rate decreases, with the largest size observed in air-cooled conditions. Concurrently, the volume fraction and size of intragranular κ carbides gradually increase with a decreasing cooling rate. This phenomenon contributes to an increase in Rockwell hardness, with the highest value recorded in the air-cooled state at 36.8 HRC. Furthermore, the impact toughness initially increases and then decreases as the cooling rate decreases. Notably, water cooling demonstrates the highest impact energy at 68 J. The formation of intergranular κ carbides in oil and air-cooled conditions is identified as the primary reason for the decline in impact toughness.