This study investigated the effects of high-temperature solution nitriding conditions on the microstructure and hardness of 430 ferritic stainless steel. Specimens were solution-nitrided at 1,150-1,300℃ for 60-540 min under a nitrogen atmosphere, followed by oil quenching. At 1,150 and 1,220℃, nitrogen absorption promoted austenite formation during nitriding, followed by martensitic transformation during quenching, resulting in increased hardness. With increasing nitriding time, the transformed region extended toward the specimen center and the hardness gradient decreased. At 1,260℃, partial transformation occurred, whereas treatment at 1,300℃ mainly resulted in ferritic grain growth because of reduced nitrogen solubility. XRD and SEM analyses showed that no detectable nitride precipitates formed under the present conditions. These results indicate that the microstructure and hardness of 430 ferritic stainless steel can be controlled by adjusting the temperature and time of high-temperature solution nitriding.

The electroconsolidation (EC) process is a proprietary method (Superior Graphite Co.) for pressure assisted densification of powder preforms. Granular graphite was both the pressure-transmitting and heating medium as direct current was passed through. The present research was conducted to determine the temperature and the pressure distribution in the die. Alumina pellets and slices of Ni-C eutectic pellets were used as probes. The temperature distribution in the die was inferred by observing the melting behavior of the Ni-C eutectic slices using an X-ray imaging system. The pressure-assisted master sintering surface (PMSS) was used to infer the pressure distribution at a given the temperature distribution. in a granular graphite pressure-transmitting medium.

Controlled gas nitriding is a representative surface hardening process that enables precise control of the phase constitution and thickness of the compound layer by regulating the nitriding potential (Kn). In this study, controlled gas nitriding was performed on C01, C12, and C45 steels with different carbon contents under the same nitriding potential to investigate the effect of carbon content on the phase transformation behavior of the compound layer. The microstructures of the nitrided layers were characterized using optical microscopy (OM) and scanning electron microscopy (SEM), while phase analysis was conducted by X-ray diffraction (XRD) and CALPHAD thermodynamic calculations. In addition, the compound layer thickness and nitrogen concentration profiles were analyzed to evaluate their relationship with carbon content. The results showed that the surface hardness and compound layer thickness increased with increasing nitriding temperature and treatment time for all specimens. However, even under the same nitriding potential, the phase constitution of the compound layer varied depending on the carbon content. As the carbon content increased, the fraction of γ'-Fe₄N decreased, whereas that of ε-Fe₂-₃N increased, which was in good agreement with the CALPHAD simulation results. Furthermore, carbon redistribution during nitriding resulted in the formation of a decarburized layer and a carbon-rich layer. These findings demonstrate that the carbon content of steel is a key factor governing the phase formation and growth behavior of compound layers during controlled gas nitriding.

This study examined cementite morphology and interior hardness during spheroidization of hypereutectoid pearlitic wire rod (0.90 wt.% C, 5.5 mm diameter). Specimens were held at 640-740℃ for 1, 2, and 4 h. Hardness profiles and optical micrographs indicated surface softening consistent with decarburization at 720 and 740℃; thus, mean hardness at r/R ≤ 0.5 was used as the representative value. SEM analysis showed progressive fragmentation, shortening, and rounding of lamellar cementite with increasing temperature and holding time. At 740℃ for 4 h, aspect ratio decreased from 8.50 to 3.66 and circularity increased from 0.294 to 0.522. Below the estimated Ac1 (640-720℃), morphology metrics and interior hardness changed gradually. In contrast, at 740℃ near the estimated Ac1, greater morphological changes occurred from 1 h, and interior hardness fell below the subcritical temperature-time prediction. Interior hardness decreased from 416 to 275 HV and showed empirical linear correlations with decreasing aspect ratio and increasing circularity. A JMAK-type empirical analysis yielded an apparent activation energy of approximately 75 kJ/mol for the subcritical conditions. At 740℃, the principal difference was in time dependence rather than mean rate constant, supporting its separate treatment outside the subcritical calibration range.

This study investigated the effects of intermediate preheating time, aging time, and rolling temperature on the microstructure and mechanical properties of an Al-8Zn-2.5Mg-1.0Cu-0.2Sc-0.1Zr (wt%) alloy subjected to repetitive rolling. Increasing the intermediate preheating time from 15 to 25 min accelerated precipitation of η and T phases prior to aging, shortening the aging time to reach peak hardness from 28 to 12 h. The specimen preheated for 20 min exhibited the highest hardness owing to the combined effects of grain refinement and precipitation strengthening. Aging promoted recovery and partial recrystallization, resulting in a more homogeneous microstructure. Hardness and strength increased with decreasing rolling temperature. The cryogenically rolled specimen exhibited the highest tensile strength of 675 MPa and an elongation of 22.5%, demonstrating the best strength-ductility balance. These results indicate that intermediate preheating and cryogenic rolling are effective approaches for optimizing the microstructure and mechanical properties of Al-Zn-Mg-Cu-Sc-Zr alloys.

To clarify the effect of homogenization of Zn element on the corrosion behavior of Mg-6%Zn casting alloy, long-term solution treatment was conducted at 623 K for up to 96 hrs and the resulting microstructures and corrosion properties were investigated using optical microscopy (OM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), immersion, and electrochemical tests. In the as-cast state, the microstructure was characterized by eutectic compounds of Mg-Zn in α-(Mg) matrix, accompanied by localized Zn-rich regions. After 48 hrs of solution treatment, most Mg-Zn compound particles were dissolved into the matrix, though the Zn-rich regions persisted. A nearly homogenized microstructure was achieved when solution-treated for 96 hrs. The solution-treated microstructure exhibited better corrosion resistance than the as-cast one, and the homogenized microstructure resulted in lower corrosion rate than the solution-treated one. The microstructural examinations revealed that the enhanced corrosion resistance in the homogenized microstructure compared to the solution-treated one was mainly due to the formation of a more uniform and denser surface corrosion film with fewer cracks and defects in response to the increased ZnO content in the film.

The effects of solution and aging heat treatments on the evolution of γ' precipitates and the mechanical properties of cast IN738 nickel-based superalloy were systematically investigated. Solution treatments were performed at 1,150, 1,200, and 1,250℃ for 4 h, followed by furnace cooling, air cooling, or water quenching. Aging treatments were subsequently conducted at temperatures ranging from 650 to 1,150℃. The microstructural evolution of γ' precipitates was quantitatively characterized using field-emission scanning electron microscopy and image analysis, while mechanical properties were evaluated by Rockwell hardness and room-temperature tensile tests. Differential thermal analysis revealed that complete dissolution of γ' precipitates occurred at approximately 1,250℃. The cooling rate significantly influenced γ' precipitation behavior, with furnace cooling producing coarse γ' precipitates and rapid cooling promoting a fine and uniformly distributed secondary γ' structure. Increasing the aging temperature resulted in significant γ' coarsening through Ostwald ripening, accompanied by increases in precipitate size and area fraction. The highest hardness and ultimate tensile strength were obtained after aging at approximately 781℃ due to the formation of fine coherent γ' precipitates. At higher aging temperatures, precipitate coarsening reduced precipitation strengthening, whereas partial recovery of mechanical properties at 1,050℃ was attributed to changes in g/γ' lattice misfit and coherency strain. These results demonstrate that the mechanical performance of IN738 is governed by the combined effects of γ' morphology, distribution, and lattice coherency, providing valuable guidelines for optimizing heat-treatment processes of nickel-based superalloys for high-temperature turbine applications.