During vacuum carburizing, coarse reticulated carbides tend to precipitate along grain boundaries due to high-carbon-potential conditions. This phenomenon is often one of the main factors in the failure of conventional gear steels. In this paper, a novel solid-solution carburizing process was proposed to achieve nano-carbide formation in the surface of the carburizing layer, and the conventional carburizing process and material thermodynamic calculations were combined to study the carburized layer by changing the parameters of the carburizing process, and to optimize the microstructure and properties of the carburized layer. The results showed that the high carbon potential or the long-time boost carburizing process could easily cause the enrichment of many carbon atoms in the traditional carburization, thus forming a carbide network and decreasing the carburization efficiency. The minor increase in large-sized M₇C₃ carbides did not significantly improve the surface hardness and wear resistance. However, the presence of small and dispersed M₂C carbides was the main factor in improving the microhardness and mechanical properties. The novel solid-solution carburizing process could improve the carburizing efficiency and transform reticulated carbides into nano-dispersed M₂C carbides. The surface carbon content and microhardness of 1.07% and 875 HV, respectively, increased 17.7 and 2.4% compared to conventional carburizing processes at 1100 °C. On the other hand, the surface’s ultimate tensile strength was found to be 1900 MPa by mini-tensile testing, and the core had a good match of strength and toughness. It was concluded that the novel solid-solution carburizing process could dissolve the carbon network and thus effectively increase the surface carbon content, achieving fully nanosized carbide on the surface. Modifying the size, morphology, and distribution of the nano-M₂C carbides dispersed within the lath-martensite after tempering the test steel was found to be the main factor in improving the mechanical properties.

Cyclic heat treatment is an effective approach for enhancing the mechanical properties of 18Ni(C250) maraging steel, and the selection of cyclic heat treatment temperature is a key factor. In this study, a cyclic heat treatment process with a two-step solution treatment is employed to investigate the influence of cyclic heat treatment temperature, specifically the first solution treatment temperature (920 °C, 950 °C, and 980 °C), on the microstructure and mechanical properties of 18Ni(C250) maraging steel. The results indicate that with an increase in the cyclic heat treatment temperature, the average grain size of the 18Ni(C250) maraging steel decreases initially and then increases. When the cyclic heat treatment temperature reaches 950 °C, the grain size is at its minimum, exhibiting optimal grain uniformity. Additionally, the increase in cyclic heat treatment temperature results in a reduction in the size of martensitic lath with the same orientation inside the grains, along with an increase in the relative quantity of low-angle grain boundaries. Furthermore, the volume fraction and size of retained austenite show a monotonous increase with the rise in the temperature of the cyclic heat treatment, and the rate of increase becomes notably larger when the temperature is raised from 950 °C to 980 °C. Based on the observed microstructural changes, the variation in the mechanical properties of the 18Ni(C250) maraging steel was analyzed. Specifically, as the cyclic heat treatment temperature increases, the tensile strength of the 18Ni(C250) maraging steel initially increases and then stabilizes, while the elongation and fracture toughness exhibit a monotonic increase.

Owing to the continuous increasing of steel strength, mechanical properties including toughness and fatigue performance are becoming increasingly sensitive to inclusions in ultra-high strength steel. Rare-earth treatment is considered as an effective method to reduce the harmful effects of inclusions, but is rarely applied in secondary-hardening steel. In the present study, different amounts of cerium were added in a secondary-hardening steel to investigate the modification effect of Ce on non-metallic inclusions in steel. The characteristics of inclusions were observed experimentally using SEM-EDS and the modification mechanism was analyzed based on thermodynamic calculations. The results indicated that the main inclusions in Ce-free steel are Mg-Al-O + MgS. Thermodynamic calculation indicated that MgAl₂O₄ is firstly formed in liquid steel and then successively transformed into MgO and MgS during cooling process. When the Ce content is 0.0030%, the typical inclusions in steel were individual Ce₂O₂S and MgO + Ce₂O₂S complex inclusions. When the Ce content was increased to 0.0071%, the typical inclusions in steel were individual Ce₂O₂S- and Mg-containing inclusions. Ce treatment modifies the angular magnesium aluminum spinel inclusions into spherical and ellipsoidal Ce-containing inclusions, thus reducing the harmful effect of inclusion on steel properties.

Maraging steel is a prominent category of ultrahigh-strength steel (UHSS) characterized by excellent comprehensive properties, and it finds wide applications in manufacturing load-bearing structural components. In this study, a novel tungsten-containing maraging steel, C-250W, was designed. The effects of aging treatments on the mechanical properties, microstructure, precipitations, and reverted austenite of C-250W steel were investigated. The results revealed that the optimal combination of strength and toughness could be achieved through an aging treatment of C-250W steel carried out for 5 h at 480 °C after solution treatment at 1000 °C for 1 h. As the aging temperature increased, the proportion of dimples in the impact fracture gradually decreased while that of quasi-cleavage increased, leading to a reduction in Charpy impact energy. The boundary of martensitic lath decomposed gradually as the aging temperature increased, and it disappeared entirely at temperatures higher than 550 °C. Moreover, the aging process resulted in the formation of phases, including spherical Fe₂M (M represents Mo, W) and thin strip-shaped Ni₃N (N represents Mo, Ti) precipitates. These precipitates coarsened from 5 nm to 50–200 nm with increasing aging temperature. Additionally, the content of reverted austenite increased with the aging temperature. Within the temperature range of 400 °C to 500 °C for aging treatment, the content of film-shaped reverted austenite was approximately 3%, primarily distributed at the boundary of martensite lath. When the aging temperature exceeded 550 °C, the content of reverted austenite reached 20.2%, and its morphology changed from film-shaped to block-shaped, resulting in a decline in strength and toughness.