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This paper discusses a molybdenum-added alloy design for normalized pressure vessel steels, to reduce hydrogen induced cracking (HIC) and inhibit crack propagation. The change in microstructure produced by the modified alloy composition was analyzed to determine its effect on HIC characteristics. The microstructural change was observed by optical microscopy, hardness tests, scanning electron microscopy, and electron backscattered diffraction. The crack length ratio and crack thickness ratio were evaluated using the NACE TM 0284 standard, and ultrasonic testing was used for HIC analysis. The formation of polygonal ferrite and pearlite during the processing of the alloy creates localized areas of high stress concentration at the polygonal ferrite/pearlite interface, due to the expansion/contraction of various structures during the transformation. This results in the generation of potential hydrogen-trapping sites, subsequent HIC, and crack propagation. The addition of molybdenum leads to a decrease in the volume fraction of the pearlite structure in the steel in favor of a more beneficial bainitic ferrite microstructure, which is generated during the normalizing process. This bainitic transformation creates a more favorable expansion/contraction compatibility and reduces/breaks up the ferrite/pearlite banding. The combination of these two characteristics can result in an overall lower stress-intensity state, which can minimize hydrogen-trapping and crack propagation. This study demonstrates that the resistance of normalized pressure vessel steels to HIC can be significantly improved by incorporating molybdenum in the alloy design. (Received April 8, 2019; Accepted June 4, 2019)

keyword : sour service, molybdenum, hydrogen induced crack (HIC), normalizing steel, pressure vessel steel

Laser nitriding is a surface engineering technique that is being used on titanium alloys to improve their surface hardness and wear resistance. It is an attractive process because of its simplicity and the possibility of producing hard layers of substantial depth with minimum effect to the bulk of the material. The current study characterized and compared laser surface nitriding of Ti-6Al-4V alloys in a nitric acid solution and in a nitrogen gas atmosphere. The melt pool shape, microstructure, hardness, and cracking susceptibility of the nitride layers processed at various nitrogen concentrations were investigated over a wide range of processing variables. The results show that the hardness of the laser nitrided layer increases with increases in both the concentration of nitrogen and the energy density of the laser. The hardness levels and hardened areas were significantly different because nitrogen has a different diffusivity in the nitric acid solution compared to a nitrogen gas atmosphere. When nitriding in a nitrogen atmosphere, the more energetic thermal reaction and accelerated surface vaporization cause rougher nitrided surfaces. The laser surface nitrided in a nitric acid solution significantly enhanced the hardening efficiency and hardness level without processinduced cracks. The results of this study will be useful in process design for the laser nitriding of Ti-6Al-4V alloy, which significantly enhances the surface performance of laser-processed parts. (Received April 11, 2019; Accepted June 3, 2019)

keyword : laser surface nitriding, Ti-6Al-4V alloy, nitric acid, nitrogen, hardness, roughness

Alloy design of 9Cr-1Mo-V-Nb weld metals was carried out through thermodynamic simulation, and elevated temperature tensile testing was performed on the designed alloy. Based on the thermodynamic simulation of the 9Cr-1Mo-V-Nb weld metal, 0.09 to 0.10 wt% C was added to increase Cr-rich M₂₃C₆. Nitrogen was reduced from 0.04 to 0.02 wt% to suppress the formation of the Z-phase. Elevated tensile tests at 600 ℃ showed that the 0.10C specimen, in which a large amount of Cr-rich M₂₃C₆ was formed, produced a higher tensile strength and elongation than the 0.09C specimen. Below the fracture surfaces after the elevated temperature tensile test, the microvoids located in the 0.10C specimen were smaller than those in the 0.09C specimen. In the 0.10C and 0.09C specimens, the microvoids generated by oxide inclusions were mostly located in the grain boundaries. Cr-rich M₂₃C₆ precipitates in the weld metal were mainly located on the grain boundaries, and the (V, Nb)-rich MX precipitates were located in the grains. In the 0.10C weld metal, a large amount of Cr-rich M₂₃C₆ precipitates, which were coherent with the grain, were distributed at the grain boundaries. The coherent Cr-rich M₂₃C₆ precipitated along the grain boundary probably inhibited the mobility of dislocations and grain boundaries, and the propagation of microvoids generated by oxide inclusions, therefore increasing the high temperature strength and elongation. (Received April 8, 2019; Accepted May 24, 2019)

keyword : 9Cr-1Mo-V-Nb steel, weld metal, thermodynamic simulation, elevated temperature tensile properties, Crrich M23C6 precipitates

In this study, the post-weld heat treatment (PWHT) cracking susceptibility in the heat affected zone (HAZ) of martensitic 10Cr steel was evaluated. The specimens taken from the 10Cr steel were thermally cycled using a Gleeble to simulate HAZ 1 (δ-ferrite) and HAZ 2 (single phase, γ). HAZ 2 had martensite and some M₂₃C₆-type carbides were present. On the other hand, HAZ 1 had martensite and the carbides were completely dissolved. In addition, δ-ferrite is observed in HAZ 1. The simulated HAZ specimens were stress rupture tested at a temperature range of 688-788 ℃ under stress of 150-450 MPa (21, 750-65, 250 psi). After the stress rupture test, a stress rupture parameter (SRP) was obtained using the rupture strength and ductility. In this way, the PWHT cracking susceptibility was evaluated through the SRP. Both HAZ 1 and HAZ 2 tended to increase rupture strength with decreasing PWHT temperature, but with a decrease in ductility. Also, it was noteworthy that the HAZ 1 had a lower PWHT cracking susceptibility than the HAZ 2 due to its higher rupture strength, despite its lower ductility. Because the precipitation strengthening occurs due to reprecipitation of MC-type carbide during PWHT. (Received May 3, 2019; Accepted May 28, 2019)

keyword : martensitic 10cr steel, heat-affected zone, post-weld heat treatment, stress rupture test, δ-ferrite

We investigated the effect of rapid thermal annealing (RTA) temperature report on oxygendeficient rutile TiO_2-x thin-film transistors deposited by RF magnetron sputtering. Amorphous TFTs that use TiO_2-x semiconductors as an active layer can be fabricated by low-temperature process and show remarkable electrical performance. The RTA post-annealing process provides greater production and development flexibility, and a fast preparation method. Structural analyses using X-ray diffraction suggested that when the TiO2-x film was annealed at different temperatures (400 °C, 500 °C, 600 °C, and 700 °C) it changed from an amorphous to a rutile phase. The oxygen vacancies in the TiO2-x region acted as traps for electrons and led to carrier transport behavior. The TFT based on a TiO_2-x channel layer annealed at 700 °C showed strongly saturated output characteristics, a much higher on/off current ratio of 7.2 × 10³ A, electron mobility of 0.15 cm²/Vs, a threshold voltage of 0.4 V, and a subthreshold swing of 0.31 V/dec. However, when the temperature of the RTA was 700 °C, the stability and reliability of the TFT was reduced and surface roughness increased, thereby reducing the mobility of the element charges, as well as leakage current. (Received March 12, 2019; Accepted May 17, 2019)

keyword : thin films, sputtering, surface, X-ray diffraction, rapid thermal annealing

In this study, a CuO buffer layer was deposited on a fluorine-doped tin oxide (FTO) substrate using a spin coating method, and then a CuO photoelectrode was grown on the CuO buffer layer using a modifiedchemical bath deposition (M-CBD) method. We investigated the morphological, structural, optical, electrical and photoelectrochemical properties of the CuO photoelectrode grown according to the number of deposited CuO buffer layers, using field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), UVvisible spectroscopy (UV-vis), electrochemical impedance spectroscopy (EIS) and three-electrode potentiostat, respectively. From the study, we found that the root mean square (RMS), surface area, optical energy bandgap, flat-band potential, acceptor density, and photoelectrochemical properties of the CuO photoelectrode were greatly influenced by changes in the CuO buffer layer, depending on the number of deposited buffer layers. It was also found that growth in the (11 ) and (111) direction planes of the CuO photoelectrode affected the sheetshape structural growth of the CuO photoelectrode. In addition, it was observed that the CuO photoelectrode using the CuO buffer layer grew uniformly and nano-sized. As a result, the highest photocurrent density value of -2.22 mA/cm² (at -0.55 V vs. SCE) was obtained from the CuO photoelectrode fabricated with 3 cycles of CuO buffer layer deposition. It was the thickest, had the lowest optical energy band gap, the highest RMS value, surface area, preferential growth on the (11 ) plane, flat-band potential and acceptor density. (Received March 12, 2019; Accepted May 13, 2019)

keyword : CuO, buffer layer, photoelectrochemical (PEC), photocurrent density, modified-chemical bath deposition(M-CBD)

The photovoltaic properties of perovskite solar cells (PSCs) were investigated by adding graphene oxide (GO) to the electron transport layer of TiO₂. The PSCs were fabricated with a glass/FTO/BL-TiO₂/meso- TiO₂ + GO/perovskite/HTL/Au electrode structure by adding various contents of GO (0.0, 0.3, 0.5 and 0.7 wt%) to the TiO2 layer. To analyze the photovoltaic characteristics and transmittance of the TiO₂ layer of the PSCs according to the GO content, solar simulator and ultraviolet-visible-near-infrared spectroscopy were used, respectively. Atomic force microscopy and field emission scanning electron microscope were used to analyze the microstructures of both the TiO₂ layer and the perovskite layer. The results of the photovoltaic characteristics indicated that energy conversion efficiency (ECE) was gradually increased to 12.70%, 13.13%, 14.19%, and 12.63% for 0.0, 0.3, 0.5, and 0.7 wt% GO contents, respectively. The highest ECE was observed at a GO content of 0.5 wt%, and a decrease in the ECE was confirmed at higher levels. The increase in ECE was confirmed by measuring short circuit current density (J_sc) and shunt resistance which increased with the GO addition. However, when an excessive amount of GO content was added, a decrease in the ECE was observed, due to the decrease in Jsc by the reduction in transmittance. In particular, an increase in the perovskite crystal size caused an increase in the ECE, due to the increased rms of TiO₂ at 0.5 wt% GO. With the 0.7 wt% GO TiO₂ layer, the perovskite grain size was decreased by the reduction in rms, and this resulted in a decrease in J_sc and ECE. These results suggest that we might improve the efficiency of perovskite solar cells by adding the proper amount of GO to the TiO₂ layer. (Received April 22, 2019; Accepted June 2, 2019)

keyword : electron transport layer, graphene oxide, perovskite grain, AFM, perovskite solar cells

In this work, the angular distribution of the sputtered Cu flux in a long throw sputtering (LTS) system is extracted from the comparison of experimentally-measured profiles of deposited films with simulated profiles of films in overhang contact structure. And effects of the sputtering process parameters such as Ar pressure during sputtering, RF power on substrates, and DC power on Cu target are investigated for a DC magnetron sputtering system with LTS. The bottom step coverage in contact is enhanced with decreasing operating pressure and is increased with increasing substrate RF power up to 200 W. However, the bottom step-coverage was reduced with substrate RF power above 400 W, possibly due to the re-sputtering effect of the deposited Cu films. DC power on Cu target does not affect the angular distribution of Cu atoms while the overall deposition rate is increased. Based on the estimated angular distribution of sputtered Cu flux, the profile of Cu film is deposition on a deep via of aspect ratio 10 and compared to the simulation of the film profile that shows a good agreement. (Received February 27, 2019; Accepted May 21, 2019)

keyword : DC magnetron sputtering, flux distribution, step coverage, long throw sputter, simulation

The evolution of microstructure and mechanical characteristics during the rejuvenation of a hot gas pass component of a gas turbine was investigated with an actual service-exposed bucket in a power plant. Heat treatment and hot isostatic pressing processes were conducted to rejuvenate a hot gas pass component of a F class gas turbine which had been operated for 101.15% of its expected lifetime. In the three step heat treatment, the specimens were exposed to 1210 ℃, 1120 ℃, and 845 ℃ for periods of 2 h, 2 h, and 24 h, respectively, with rapid cooling. Then, the specimens were exposed to conditions of 1200 ℃ and 100 MPa for 4 h to enhance their integrity. Analysis of the microstructure determined that after heat treatment the average size of γ' decreased around 60 μm, while the area fraction of γ' increased around 20% compared to before heat treatment. With respect to mechanical characteristics, the samples’ stress-rupture time, yield strength, and ultimate strength improved with heat treatment and hot isostatic pressing, while hardness did not show any meaningful variation. These phenomenological results suggest that heat treatment and hot isostatic pressing play a critical role in the rejuvenation of a hot gas pass component, and can provide an operating and maintenance strategy to enhance the economic feasibility of a combined cycle power plant. (Received March 25, 2019; Accepted May 16, 2019)

keyword : heat treatment, gas turbine blade, hot gas pass component, hot isostatic pressing