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.

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.

The effect of a nitride layer on the forming behavior of CrN and (Cr, Fe)₇C₃ multilayers for thermoreactive deposition (TRD) was investigated. Plasma nitriding followed by TRD (PN-TRD) produced a larger coating thickness than the case of direct TRD with no plasma nitriding. For PN-TRD, an Fe_2-3N layer of 10 μm in thickness was produced on AISI 52100 steels using plasma nitriding, followed by TRD using a mixed powder composed of 30 wt % Cr, 2 wt % NH₄Cl, and 68 wt % Al₂O₃. During TRD at 800 °C, a CrN layer of 2 μm in thickness was formed along with a thin layer of mixed carbide (Cr₇C₃) and nitride (CrN) on top. As the deposition temperature was increased to 950 °C, a new layer of Cr₇C₃ was formed underneath the outermost layer composed of mixed Cr₇C₃ and CrN. At 950 °C, a Cr-rich zone indicated a thickness of ~7 μm. As the deposition time increased to 3 h at 950 °C, a new layer of (Cr, Fe)₇C₃ was produced at the interface between the CrN formed at 800 °C and the base metal. This layer formed because of the abundant resources of Cr and C provided from the TRD powder and base metal, respectively. The multilayer and interface were concretely filled without the formation of voids as the TRD time increased to 6 h at 950 °C. The TRD process on a pre-nitrided layer was successfully applied to produce multilayers of CrN and Cr₇C₃.

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 the free radicals in the carburization. The free radicals can remove the natural oxide film by the strong reaction of the hydrocarbons, and then thermodynamically activity can be increased. When the gas ratio was below 1, the supersaturation expanded austenite layers were formed on the surface of the AISI 316L stainless steel, which had the maximum carbon solubility up to 11.5 at.% at 743 K, were formed. On the other hand, when the gas ratio was above 1, the carbon concentration of them remained low even if the process time was enough increased 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 to total mixture gases to form the expanded austenite layer with the high carbon concentration in the direct low-temperature vacuum carburization.