The effect of the iron nitride phases, ε-Fe_2-3N and γ′-Fe₄N, on the fatigue strength was investigated. Pure iron was used to observe only the effect of nitride, excluding the effects of factors, such as residual stress, depending on the alloy composition and microstructural change according to working on the fatigue strength. In this work, ε and γ′ phases were respectively grown at a time on the surface of the pure iron specimens using the appropriate nitriding potential K_N, the mixture rates of ammonia and hydrogen gases, at same temperature of 570 °C according to the Fe-N Lehrer diagram. Another γ′ phase was prepared by first growing the ε phase and then transformed from ε phase into γ′ phase by changing the K_N at the same temperature of 570 °C in the 2-stage gas nitriding. The fatigue strengths of the iron nitride consisted of ε and γ′ phases, γ′ phase, and γ′ phase grown by the 2-stage gas nitriding were evaluated, respectively. As a result, first, it can be seen that the diffusion layer of ε phase was deeper than γ′ phase, but fatigue strength was lower. On the other hand, fatigue strength of both the γ′ phases are higher than that of the ε, and the fatigue strength of γ′ phase nitride grown by 2-stage gas nitriding was almost similar to that of γ′ phase nitride grown at a time, i.e., fatigue strength was not significantly related to diffusion depth and depended on nitride phases in this study. Secondly, we cannot clearly conclude that there was the difference in fatigue strength according to the thickness of nitride layer consisted of γ′ phase. However, it is clear that when ε phase was transformed to γ′ phase, fatigue strength had the same level as γ′ phase formed at one time.

Laser powder bed fusion (LPBF) is a promising additive-manufacturing process for metallic materials. It has the advantage of flexibility in product design, such that various mechanical parts can be fabricated. However, because metal parts are built-up in a layer-by-layer manner, the material fabricated by LPBF has an anisotropic microstructure, which is important for the design of materials. In this study, the corrosion resistance of 18Ni300 maraging steel (MS) fabricated by LPBF was explored considering the building direction. Furthermore, the effects of heat treatment and aging on the microstructure and corrosion resistance were investigated. Sub-grain cells formed by rapid cooling in LPBF improve the corrosion resistance of MS. As a result, the as-built MS has the highest corrosion resistance. However, the sub-grain cells are eliminated by heat treatment or aging, which causes the deterioration of corrosion resistance. In the case of 18Ni300 MS, the cylindrical sub-grain cells are formed and aligned along the heat dissipation direction, which is similar to the building direction; thus, a significant anisotropy in corrosion resistance is found in the as-built MS. However, such anisotropy in corrosion resistance is diminished by heat treatment and aging, which eliminates the sub-grain cells.

In this study, the carburization characteristics of cast and cold-rolled CoCrFeMnNi high-entropy alloys (HEAs) with various grain sizes were investigated. All specimens were prepared by vacuum carburization at 940 °C for 8 h. The carburized/diffused layer was mainly composed of face-centered cubic structures and Cr₇C₃ carbide precipitates. The carburized/diffused layer of the cold-rolled specimen with a fine grain size (~1 μm) was thicker (~400 μm) than that of the carburized cast specimen (~200 μm) with a coarse grain size (~1.1 mm). In all specimens, the carbides were formed primarily through grain boundaries, and their distribution varied with the grain sizes of the specimens. However, the carbide precipitates of the cast specimen were formed primarily at the grain boundaries and were unequally distributed in the specific grains. Owing to the non-uniform formation of carbides in the carburized cast specimen, the areas in the diffused layer exhibited various carbide densities and hardness distributions. Therefore, to improve the carburization efficiency of equiatomic CoCrFeMnNi HEAs, it is necessary to refine the grain sizes.

The effect of the acetylene and hydrogen gases mixture ratios in direct low-temperature vacuum carburization was investigated. The gas ratio is an important parameter for producing free radicals in carburization. The free radicals can remove the natural oxide film by strong reaction of the hydrocarbons, and then thermodynamic activity can be increased. When the gas ratio was below one, carbon-supersaturated expanded austenite layers were formed on the surface of the AISI 316L stainless steel, which had a maximum carbon solubility up to 11.5 at% at 743 K. On the other hand, when the gas ratio was above one, the carbon concentration of the layers was low even if the process time was increased enough to reach the maximum carbon solubility. As a result, the carbon concentration underneath the surface was determined to be highly dependent on the gas mixture ratio of acetylene and hydrogen. In conclusion, it is necessary to restrict the ratio of acetylene and hydrogen gases in the total mixture of gases to form an expanded austenite layer with high carbon concentration in direct low-temperature vacuum carburization.

The effect of plastic deformation applied to AISI 316L in low-temperature vacuum carburizing without surface activation was investigated. To create a difference in the deformation states of each specimen, solution and stress-relieving heat treatment were performed using plastically deformed AISI 316L, and the deformation structure and the carburized layer were observed with EBSD and OM. The change in lattice parameter was confirmed with XRD, and the natural oxide layers were analyzed through TEM and XPS. In this study, the carburized layer on the deformed AISI 316L was the thinnest and the dissolved carbon content of the layer was the lowest. The thickness and composition of the natural oxide layer on the surface were changed due to the deformed structure. The natural oxide layer on the deformed AISI 316L was the thickest, and the layer was formed with a bi-layer structure consisting of an upper Cr-rich layer and a lower Fe-rich layer. The thick and Cr-rich oxide layer was difficult to decompose due to the requirement for lower oxygen partial pressure. In conclusion, the oxide layer is the most influential factor, and its thickness and composition may determine carburizing efficiency in low-temperature vacuum carburizing without surface activation.