Microstructural evolution and mechanical properties of Fe–Cr–Mo–Mn alloy under IQ and IQT heat treatments

This study systematically explores the microstructural evolution and mechanical performance of Fe–Cr–Mo–Mn alloy subjected to isothermal quenching (IQ), isothermal quenching + tempering (IQT), and conventional quenching + tempering (QT) heat treatment processes. Compared with the tempered sorbite microstructure produced by conventional QT, IQ-250 consists of a mixture of martensite and lower bainite, achieving ultra-high tensile strength (~ 1861.8 MPa) but exhibiting limited ductility and poor low-temperature impact toughness. Compared with the tempered sorbite microstructure produced by conventional QT, IQ-250 consists of a mixture of martensite and lower bainite, achieving ultra-high tensile strength (~ 1861.8 MPa) but exhibiting limited ductility and poor low-temperature impact toughness. In contrast, the IQT-250 process, derived from IQ treatment, effectively preserves the deformation sub-structure, and produces a refined tempered sorbite microstructure with finer grains and higher dislocation density. In addition, unlike the coarse, grain-boundary-enriched carbides in QT specimens, IQT-250 produces fine, equiaxed, and uniformly dispersed M₃C carbides, which effectively impede dislocation motion, reduce grain-boundary stress concentrations. The IQT-250 treatment markedly enhances mechanical performance, achieving 913.7 MPa UTS, 23.2% elongation, and 141.6 J/cm² impact toughness, demonstrating superior strength–ductility–toughness synergy compared with conventional QT.