Effects of Tempering Temperature on the Microstructure and Mechanical Properties of MIM-4605 High-Strength Low-Alloy Steel

This research focuses on the effects of different tempering temperatures (200°C, 300°C, 400°C, and 500°C) on the microstructure and mechanical properties of MIM-4605 steel. The results show that the matrix of as-sintered MIM-4605 steel is primarily composed of a nickel-rich phase, ferrite, and pearlite, with a large number of nodular iron oxide particles with a diameter of around 1 µm dispersed in the matrix. However, the matrices of the as-quenched MIM-4605 steel and of the as-tempered MIM-4605 steel tempered at the different temperatures are primarily composed of lath martensite, as well as some needle martensite structures. It is worth noting that after tempering of the as-quenched MIM-4605 steel at 200°C for 4 h, a large amount of fine nodular ε-carbide precipitates can be found in the martensite matrix. The ε-carbide precipitates are only 5–10 nm in size and form a semi-coherent interface with the matrix, resulting in a drastic improvement of tensile strength. However, as the tempering temperature increases, the growth of the ε-carbide precipitates becomes coarser, which gradually leads to a decrease in the tensile strength of the MIM-4605 steel.