# Anti-Galling Cold, Dry Forging of Pure Titanium by Plasma-Carburized AISI420J2 Dies
## 4 Discussion  
The formation of a relatively thick carburized layer at 673 K for 14.4 ks is due to the carbon solute diffusion process and carbon supersaturation. After [10; 12; 13; 14], the carbon and nitrogen supersaturation processes are common to plasma carburizing and nitriding treatments at 673 K. This supersaturation and diffusion of carbon and nitrogen solutes into stainless steel is recognized by the iron lattice expansion and high solute content depth profile in the carburized and nitrided layers. In addition, the grain refinement takes place with this carbon supersaturation process to sustain the carbon solute diffusion through the fine zone and grain boundaries in a manner similar to the nitrogen supersaturation in [10; 14]. A solid material with a two-dimensional structure has been identified as a typical solid lubricant; e.g., MoS\({}_{2}\) particles and coatings [19] and graphite [20] are frequently employed in industries to reduce the friction and galling in hot forming. In particular, a graphite mold provides a solution for the mold stamping of oxide glasses to optical lenses [21]; the lens surfaces are often contaminated by the dust of graphite. By contrast, DLC-coated dies often suffer from the adhesion of those glasses as well as metallic titanium, as reported in [22]. As analyzed in Figures 8-12, the unbound, free carbon film formed in situ during cold forging works as a solid lubricant to protect the die surface from the galling of titanium. Since this solid lubrication film is mainly formed only at the high-pressure contact interface, there is little possibility of its contaminating other work surfaces not only in cold forging but also in warm and hot forging. Isolation of unbound, free carbon solutes from the highly stressed AISI420J2 matrix plays an essential role in the formation of the carbon tribofilm for in situ solid lubrication. Many studies have reported the ordering of carbon atoms in iron and steel under the supersaturated state only in the presence of inner strains [23]. As suggested in [24], the externally applied compressive stress has an influence on carbon-carbon and carbon-iron interactions and induces the separation of supersaturation carbon solutes to ordered carbon and free carbon in stressed steel. This free carbon solute diffuses to the surface by a steep stress gradient and in situ forms a thin film on the contact interface during forging. An intermediate titanium oxide (TiO\({}_{\rm x}\)) film is also formed in situ together with the in situ solid lubrication by the carbon film. A few TiO\({}_{\rm x}\) stripes were formed at the center of the contact interface under high compressive stress during forging. With increasing reduction in thickness, the applied stress gradually lowers and the carbon isolation process weakens, so that the TiO\({}_{\rm x}\) film is formed on the contact interface away from its center line. The plasma carburizing conditions have an influence on this carbon supersaturation into the AISI420J2 matrix. In previous studies on low-temperature DC plasma carburizing [9; 10], no iron and chromium carbides were synthesized as a precipitate into stainless steel matrices. However, no statements were found in those papers on the plasma diagnosis of the activated species in plasmas. As suggested in [11], the ratio of CH\({}_{4}\) to H\({}_{2}\) in the mixture gas of argon, CH\({}_{4}\), and H\({}_{2}\) has an influence on the CH radical density as well as the ion density of C\({}^{*}\) (C\({}^{+}\), C\({}^{2+}\), C\({}^{3+}\), and C\({}^{4+}\)) in the ignited plasmas for low-temperature plasma carburizing. Further study is needed to optimize this ratio for high densification of CH radicals to drive the plasma carburizing process. The plasma-carburized AISI420J2 at 673 K has anti-galling capacity in forging titanium and titanium alloys, enough to be applied to the net shaping of raw titanium workpieces into biomedical parts.