The Effect of Cyclic Heat Treatment on the Microstructure and Mechanical Properties of 18CrNiMo7-6 Gear Steel.

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/cm, 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.