To avoid grain coarsening resulting from high-temperature carburizing, the effects of cyclic quenching and tempering on the microstructure and mechanical properties of 18CrNiMo7-6 gear steel were investigated. Three groups of samples were compared, which went through 0/1/3 times of quenching–tempering cycles after initial pseudo-carburizing. The variations in grain size, hardness, tensile strength, and toughness were systematically assessed using a series of experimental techniques. The experimental results indicate that the austenite grain size decreases from 14.8 μm to 5.0 μm as the number of cycles increases, accompanied by improved grain uniformity, which is beneficial to fine-grain strengthening mechanisms. During the phase transition, defects in the original martensite structure are transferred to the newly formed austenite, with the energy stored during the martensitic-to-austenitic transformation driving the grain refinement process. However, after several cycles of quenching and tempering, the release of some residual stresses and dislocations reduces the driving force for recrystallization, limiting further grain refinement. Although the strength decreased slightly after three cycles due to a reduction in dislocation density, toughness increased to a maximum of 172 J/cm², primarily due to the enhancement of grain refinement and grain boundary density, which effectively hindered crack propagation. This study confirms the efficacy of cyclic heat treatment in refining grain structure and improving both strength and toughness, thereby contributing valuable insights to the research and development of high-performance gear steels.

The hydrogen embrittlement susceptibility of a newly developed 1700 MPa-grade ultra-high-strength steel with a primary austenite grain size of 4 μm was studied and the mechanical properties and microstructure were characterized. The results show that the hydrogen content in the steel increases with the extension of charging time: the value reached 0.35 wppm with a charging time of 96 h. On the contrary, the fracture mode of the experimental steel remained ductile after hydrogen charging, and the elongation and the section shrinkage showed little difference, indicating an excellent resistance to hydrogen embrittlement, which could be ascribed to the refined microstructure and good cleanliness of the experimental steel.

This study investigates the effects of tempering temperature on the microstructural evolution and mechanical properties of Cr-Ni-Mo-V steel designed for pressure vessel applications. The microstructure was characterized via scanning electron microscopy (SEM), transmission electron microscopy (TEM, Thermo Fisher Talos F200X), electron backscatter diffraction (EBSD), and physicochemical phase analysis. Mechanical performance was evaluated through tensile and impact tests, followed by a detailed discussion of the underlying strengthening mechanisms. The results demonstrate that the microstructure after tempering is fully tempered martensite. Samples tempered between 425 °C and 525 °C exhibit significant tempering resistance, maintaining a tensile strength of approximately 1300 MPa. This is primarily attributed to the synergistic effect of dislocation strengthening and the precipitation of MC-type carbides. As the tempering temperature increases to 625 °C, the dislocation density decreases sharply from 3.71 × 10¹¹ cm⁻² to 1.18 × 10¹¹ cm⁻², leading to a decline in strength. Concurrently, the impact energy increases significantly from 71 J to 132 J. The improvement in toughness is mainly attributed to the significant elevation of the crack initiation threshold, which is dominated by the reduction in matrix dislocation density, the coarsening and spheroidization of carbides, and the alleviation of local stress concentration. The relative proportion of high-angle grain boundaries (HAGBs, misorientation > 15°) increases from 51.9% to 57.7% during tempering, which is a result of the massive elimination of low-angle grain boundaries rather than an increase in the absolute length per unit area of HAGBs.