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The effect of annealing on the microstructural evolution of the matrix in low carbon manganese steels with different initial microstructures, including martensite, bainite and a mixture of ferrite and pearlite, was investigated in conjunction with the charge in mechanical properties. During annealing, steels with a deformed ferrite + pearlite microstructure underwent spheroidization of the lamellar cementite in the pearlite region and recovery or recrystallization in the ferrite region. Martensitic steels and bainitic steels experienced recovery and recrystallization of the matrix and the precipitation of cementite particles during annealing. Although the martensitic steels showed the highest strength up to the annealing temperature of 600 ℃, the large driving force from dislocations accumulated during cryogenic deformation, as well as dislocations inherited from the martensitic transformation, accelerated the softening rate during annealing. A small amount of equi-axed grains produced during annealing at 550 ℃ did not effectively improve ductility. The rapid softening and the improvement in ductility in martensitic steels and bainitic steels annealed at 600 ℃ could be attributed to the disappearance of elongated substructures, rather than the coarsening of cementite particles during annealing. The bainitic steel showed higher thermal stability and a better combination of strength and ductility than the other microstructures, such as the martensitic microstructure and a mixture of ferrite and pearlite. (Received April 16, 2018; Accepted July 2, 2018)

keyword : cryo-rolling, annealing, ultrafine grain, bainite, martensite

Nickel-based superalloys have excellent resistance to extreme environments such as high temperature and high stress and are used as materials for large gas turbines. In this paper, specimens were taken from a blade that had been used for a long time, and their life span was studied by analysis of microstructure. The microstructural analysis of the specimens was carried out using OM and SEM to observe coarsening, carbides and TCP phase on gamma prime. Low-cycle fatigue tests were performed on new material and an airfoil of long time-used blade. The test was conducted under various deformation conditions and temperature conditions of 760 ℃ and 870 ℃. The low cycle fatigue test was carried out using the Coffin- Manson equation and fatigue life was predicted. After the test, crack paths and fracture surfaces were analyzed using SEM. (Received March 27, 2018; Accepted June 15, 2018)

keyword : Ni-based superalloy, low cycle fatigue, microstructure, gamma prime, scanning electron microscopy (SEM)

Recently developed ultra-strong GIGA STEEL exhibites superior mechanical properties, with a high tensile strength of more than 1.5 GPa and good impact toughness. Nevertheless, its application to auto parts has been severely restricted, mainly due to a substantial reduction in the resistance to hydrogen embrittlement (HE) induced by aqueous corrosion. The susceptibility to this HE is closely associated with the carbon content, which leads essentially to the precipitation of iron carbides (Fe_2.4C / Fe₃C) with a low hydrogen overvoltage. This study focuses mainly on the effect of carbon content on the aqueous corrosion and hydrogen diffusion characteristics of ultra-strong steels. The hydrogen reduction reaction on the steel surface, and its diffusion kinetics in the steel matrix were evaluated using electrochemical polarization and hydrogen permeation tests, respectively. Furthermore, the HE indices of steels with different carbon contents were determined in a weakly acidic solution using the slow strain rate test (SSRT). This study demonstrates clearly that the hydrogen reduction rates were increased and its diffusion kinetics were decreased significantly, with increasing carbon content. Based on the present results, it is concluded that the critical technical issue for the development of ultra-strong automotive steels is the effective control of the carbide fractions in the microstructure. (Received May 30, 2018; Accepted July 2, 2018)

keyword : ultra-strong steel, carbide, corrosion, hydrogen diffusion, hydrogen embrittlement

This study evaluated the relationship between reheat cracking that occurs in the reheated weld zone formed by subsequent multi-pass welding and the preheat/interpass temperature. The multi-pass weldment was YP 690 MPa high-strength steel, which is used in mobile offshore platform units. The welding process was flux-cored arc welding (FCAW), which coupons were constrained by welding to simulate an actual constraint condition. Three preheat/interpass temperature ranges were employed. The characteristics of the weldment subjected to preheat/interpass temperatures higher than 80°C satisfied the relevant specifications. In the bending testing, a weldment subjected to a preheat/interpass temperature range of 50-60 °C was fractured. The results demonstrated reheat cracking in the reheated weld zone in multi-pass weldments. Reheating cracking (RHC) was initiated at carbides precipitated along the grain boundaries, induced by multipass heating; some cracks were arrested in the reheated zone and the others propagated to the columnar weld metal. The hardness of the cracked specimens was higher than that of the specimen subjected to a higher preheat/interpass temperature, confirming that a susceptible microstructure had formed due to the faster cooling rates. In the analysis of weld residual stress, the specimen with the lower preheating/interpass temperature range showed higher tensile residual stress than that of high preheating/interpass temperature range. Therefore RHC in the reheated zone of the weldment was mainly affected by grain boundary carbides, a higher hardness, and residual stresses. Based on the above discussion, the mechanism of RHC and assessment of structural integrity is presented. (Received June 1, 2018; Accepted June 20, 2018)

keyword : flux cored arc welding, reheating cracking(RHC), preheat/interpass temperature, quenched-tempered(QT), residual stress, structural integrity

Martensitic 10Cr1MoW steels are widely used for high temperature components in turbine engines because they have good mechanical properties and corrosion resistance. Delta-ferrite can form locally in large ingots and the ferrite phase degrades the metal’s high-temperature properties. To decrease the fraction of delta-ferrite the solidification process should be controlled. In this paper, directional solidification experiments were carried out with martensitic 10Cr1MoW steel at various growth velocities, 5 μm/s - 50 μm/s. The fraction and size of the delta ferrite declined with increasing growth velocity, and also declined with increasing distance from the solid/liquid interface of the directionally solidified sample. These results are discussed in relation to the dendrite spacing of the delta primary phase, and precipitation of the gamma phase with solidification rates. As the solidification velocity decreased and the distance from the solid/liquid interface increased, the fraction of delta ferrite increased. (Received May 9, 2018; Accepted June 14, 2018)

keyword : directional solidification, delta ferrite, austenite, segregation

This study designed a counter current extraction process for comparing the separation efficiencies that are obtained using Cyanex 572 and PC88A for the elements in a Pr and Nd mixed solution. This process involves multiple extraction and scrubbing processes using a mixer settler. The process parameters are calculated using an equation proposed by Xu Guangxian. The separation factor for the extraction and scrubbing process was 0.09 and 0.08 higher when using Cyanex 572 compared to the experimental values obtained using PC88A. The optimized extraction factors such as the stage numbers, feeding solution flow rate, organic phase flow rate, and scrubbing solution flow rate were calculated using the optimal extraction ratio equation. In order to derive optimum extraction conditions, the target purity and recovery ratio of Nd was set to 99.9% and 99% respectively. When using Cyanex 572 for the counter current extraction process, the total number of extraction and scrubbing stages decreased by 11 in comparison with PC88A. A comparison between the extraction, scrubbing stages, and other parameters indicate that Cyanex 572 is preferable over PC88A from the viewpoint of economic efficiency. (Received June 7, 2018; Accepted June 25, 2018)

keyword : counter current extraction, cyanex 572, PC88A, rare earth metal, separation factor

Severe resistivity size effect in Cu interconnects is attributed to the relatively long bulk electron mean free path (39 nm at 298 K), which is inherently determined by phonon scattering. In this regard, Ru and Co have been recently considered as attractive alternatives for next-generation interconnect materials because the significantly shorter electron mean free paths for Ru (6.6 nm) and Co (11.8 nm) have been predicted to lead to a dramatically reduced resistivity size effect, which may allow lower resistivity of these materials than Cu at sufficiently reduced interconnect dimensions despite their relatively high bulk resistivies. In this study, the impact of surface and grain boundary scattering for Ru and Co was assessed based on the Fuchs-Sondheimer surface scattering model and the Mayadas-Shatzkes grain boundary scattering model, first by fitting these models to the resistivity data reported in the literature and second by artificially varying the scattering parameters in the models. The results predict that the wire resistivities of Ru and Co will cross below that of Cu at wire-widths of 10-15 nm. (Received June 4, 2018; Accepted June 25, 2018)

keyword : ruthenium, cobalt, interconnects, resistivity size effect, electron mean free path

TaF₅ and VCl₃ were chosen as additives to enhance the hydrogen uptake and release rates of Mg. The total content of the additives was not more than 10 wt% since too high content reduces the fraction of Mg and thus the hydrogen storage capacity of the alloys. Samples with compositions of Mg-x wt% TaF₅-x wt% VCl₃ (x=1.25, 2.5, and 5) were prepared by reactive mechanical grinding. The temperatures at which the asmilled Mg-xTaF₅-xVCl₃ (x=1.25, 2.5, and 5) began to release hydrogen quite rapidly were 538, 613, and 642 K, respectively. Activation of the samples was not needed. In the first cycle (n=1), Mg-2.5TaF₅-2.5VCl₃ had quite a high effective hydrogen storage capacity (the amount of hydrogen absorbed for 60 min) of 5.86 wt%. Among the three samples, Mg-1.25TaF₅-1.25VCl₃ had the best hydrogen release properties. In n=4, Mg-1.25TaF₅- 1.25VCl₃ had the largest quantity of hydrogen released for 60 min at 593 K in 1.0 bar H₂, releasing 0.23 wt% H for 5 min, 0.34 wt% H for 10 min, and 3.31 wt% H for 60 min. After hydrogen uptake-release cycling, Mg- 1.25TaF₅-1.25VCl₃ had the smallest particle size. In n=5, Mg-1.25TaF5-1.25VCl3 released 2.01 wt% H for 5 min, 3.78 wt% H for 10 min, and 4.89 wt% H for 60 min at 623 K in 1.0 bar H₂. (Received May 30, 2018; Accepted June 15, 2018)

keyword : hydrogen absorbing materials, mechanical milling, scanning electron microscopy (SEM), X-ray diffraction, TaF₅ and VCl₃ addition

In our previous work, samples with a composition of 95 wt% Mg + 5 wt% CMC (Carboxymethylcellulose, Sodium Salt, [C₆H₇O₂(OH)x(C₂H₂O₃Na)y]n) (named Mg-5 wt%CMC) were prepared through hydride-forming milling. Mg-5 wt%CMC had a very high hydrogenation rate but a low dehydrogenation rate. Addition of Ni to Mg is known to increase the hydrogenation and dehydrogenation rates of Mg. We chose Ni as an additive to increase the dehydrogenation rate of Mg-5 wt%CMC. In this study, samples with a composition of 90 wt% Mg + 5 wt% CMC + 5 wt% Ni (named Mg-5 wt%CMC-5 wt%Ni) were made through hydride-forming milling, and the hydrogenation and dehydrogenation properties of the prepared samples were investigated. The activation of Mg-5 wt%CMC-5 wt%Ni was completed at the 3^rd hydrogenation-dehydrogenation cycle (N=3). Mg-5 wt%CMC-5 wt%Ni had an effective hydrogen-storage capacity (the quantity of hydrogen stored for 60 min) of 5.83 wt% at 593 K in 12 bar hydrogen at N=3. Mg-5 wt%CMC-5 wt%Ni released hydrogen of 2.73 wt% for 10 min and 4.61 wt% for 60 min at 593 K in 1.0 bar hydrogen at N=3. Mg-5 wt%CMC-5 wt%Ni dehydrogenated at N=4 contained Mg and small amounts of MgO, β-MgH₂, Mg₂Ni, and Ni. Hydride-forming milling of Mg with CMC and Ni and Mg₂Ni formed during hydrogenation-dehydrogenation cycling are believed to have increased the dehydrogenation rate of Mg-5 wt%CMC. As far as we know, this study is the first in which a polymer CMC and Ni were added to Mg by hydride-forming milling to improve the hydrogenation and dehydrogenation properties of Mg. (Received June 14, 2018; Accepted June 22, 2018)

keyword : hydrogen absorbing materials, mechanical milling, scanning electron microscopy (SEM), X-ray diffraction, a polymer CMC (Carboxymethylcellulose, Sodium Salt) addition, Ni addition.

The impressed current cathodic protection (ICCP) is widely used for preventing galvanic corrosion and it strongly influences the underwater electromagnetic signals of a surface ship. For military use, the signals should be reduced to avoid threats such as modern naval weapons triggered by the signals. We analyzed the underwater electric field (UEF) and corrosion related magnetic field (CRM) of a surface ship with four pairs of ICCP anodes using the boundary element method and varying the anodes placement in a longitudinal direction. The currents applied on ICCP anode pairs was optimized to minimize the UEF under corrosion protection of the hull using sequential linear programming at each placement of the anodes. As a result, UEF and CRM decreased as the root mean square of the distances between ICCP anode pairs on the hull decreased, but they were not intimately related to the total amount of the currents from the ICCP anodes under the optimized condition. (Received April 26, 2018; Accepted June 25, 2018)

keyword : galvanic corrosion, cathodic protection, ICCP, UEMS, UEF, CRM