Evolution Mechanism of Multi-Precipitation Regulates Mechanical Properties and High-Temperature Strength in Medium-Alloy Structural Steel

Precipitation strengthening is one of the fundamental factors occurring at high temperatures in medium-alloy structural steels, which offer greater durability under service conditions. This research employed transmission electron microscopy (TEM) via carbon replicas combined with scanning electron microscopy (SEM) to analyze carbide evolution and its influence on both mechanical properties and high-temperature strength. During the tempering process, ε-carbides precipitate at 200 °C and subsequently transform into M₃C at 400 °C and coarser M₇C₃ at 600 °C. Coarser carbides (M₇C₃ and M₃C) and metastable carbides (ε-carbides) are not sufficient to make steel strong at high temperatures. Moreover, nucleating and growing at interfaces, rod-shaped M₃C diminishes the toughness of the steel. Under tempering at 600 °C, a substantial amount of nanoscale M₂C carbides precipitate. This improvement not only elevate the material's toughness but also leads to an enhancement of yield strength (from 1237 ± 12 MPa to 1340 ± 8 MPa) along with a rise in high-temperature strength (from 388 ± 8 MPa to 421 ± 4 MPa). Combined with high toughness, nanoscale M₂C with high thermal stability promoted both yield strength at room temperature and high-temperature strength. The type and size of carbides serve as key determinants for yield strength while being decisive parameters for high-temperature strength.