In recent years, significant progress has been made in preparing conformal cooling dies for die casting using additive manufacturing technology. Among these advancements, 18Ni300 maraging steel has been widely applied because of its excellent forming characteristics. Currently, most research on selective laser melting (SLM) manufacturing of 18Ni300 maraging steel has primarily focused on the changes in microstructure after a heat treatment and the influence of precipitate phases on the strength, with limited emphasis on the impact of toughness. However, toughness plays a crucial role in determining the service life and safety of the molds.
Although previous studies have explored reverse-austenite, systematic research on the toughness of 18Ni300 is currently lacking. Therefore, this study aims to systematically investigate the impact of the aging and solution temperatures on the microstructure and mechanical properties of 18Ni300 maraging steel. Additionally, it will specifically analyze the influence of reverse-austenite on the strength and plasticity of 18Ni300 maraging steel prepared using SLM technology. This study clarifies the relationship between the process, structure, and performance of 18Ni300 maraging steel, and proposes an optimal heat-treatment system. These findings offer valuable guidance for the practical application of this steel in various industries.
In this study, 18Ni300 powder was used as the raw material. Experimental samples were obtained through selective laser melting (SLM) using an appropriate method. Following the formation, the samples were subjected to various heat treatments. The bulk samples were ground and polished with sandpaper, followed by etching with a 4% nitric-acid solution in alcohol. The microstructure was examined using optical microscopy (OM) and scanning electron microscopy (SEM). The mechanically polished samples were additionally polished with SiO2 and the crystal structure of the material was analyzed using electron backscatter diffraction (EBSD). X-ray diffraction (XRD) was utilized to analyze the phase composition and determine its content. Finally, tensile tests were conducted at room temperature using a universal testing machine and the corresponding fracture surfaces were observed.
The morphologies of the tested samples are shown in Figure 3. The printed sample displays distinct fish-scale-like fusion pools and lath martensite structures, whereas the honeycomb-like microstructure is not discernible in the SEM image. Following the aging treatment, the boundaries of the fusion pools in the samples become indistinct, and the boundaries of the honeycomb-like microstructure in the SEM image begin to dissolve. In the solution and aging-treated samples, the boundaries of the fusion pools vanish completely, and the martensite is transformed into a more refined structure. Additionally, the honeycomb-like microstructure observed in the SEM image also completely disappears.
The XRD analysis of the samples reveals that the phase composition of the as-printed sample comprises martensite and residual austenite, whereas the aged sample consists of martensite, residual austenite, and reverse-austenite. Almost the entire microstructure of the solution- and aging-treated sample is composed of martensite. Figure 5 shows that the highest amount of reverse-austenite is observed in the aged sample. Furthermore, Table 3 indicates that the sample aged at 490 °C exhibits the highest content of reverse-austenite.The mechanical properties of the sample are closely correlated with the reverse-austenite content, as depicted in Figure 8. Notably, the sample aged at 490 °C exhibits greater toughness with only a marginal reduction in strength. However, the relationship between austenite and the strength toughness of 18Ni300 is not a simple linear correlation because of factors such as precipitates and the martensite morphology. Overall, it is evident that reverse-austenite significantly enhances the toughness and marginally decreases the strength. With an increase in the reverse-austenite content from 0.1% to 6.9%, the elongation after fracture improves by 72.5%, whereas the tensile strength decreases by 2.3%.
The printed samples of 18Ni300 maraging steel manufactured by SLM display a distinct molten pool and a microstructure comprised of coarse martensite and a small proportion of residual austenite. Following the aging treatment, a ductile phase called reverse-austenite is generated. After the post-solution and aging treatments, the microstructure exhibits uniform and dense plate-like martensite with no notable presence of the austenite phase. A direct aging treatment at 490 °C is considered the optimal heat-treatment process for achieving an ideal balance between strength and toughness. At this temperature, the microstructure exhibits the highest reverse-austenite content (volume fraction: 7.7%). The ultimate tensile strength is 2012.8 MPa, and the elongation after fracture reaches a peak value of 6.9%. Therefore, a direct aging treatment at 490 °C is regarded as the most optimal heat-treatment process.
The fine reverse-austenite within the maraging steel manufactured via SLM serves as a toughening phase, enhancing the toughness without significantly compromising the strength. With an increase in the reverse-austenite volume fraction from 0.1% to 6.9%, the elongation after fracture experiences a 72.5% improvement, albeit at the expense of a 2.3% decrease in the ultimate tensile strength. Thus, the reverse-austenite is advantageous for achieving exceptional overall mechanical properties in maraging steel manufactured via SLM. The fine reverse-austenite plays a pivotal role in enhancing themaraging steel. However, in the maraging steel manufactured via SLM using 18Ni300, precipitation strengthening constitutes the primary mechanism with a limited effective range of precipitation temperatures. Further research is necessary to increase the reverse-austenite content, while maintaining adequate precipitation strengthening.
