발간논문

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The effects of cooling rate, forming method and alloying on the suppression of ordering reaction have been studied in high Si steels. Four kinds of cooling were used : air cooling, oil quenching, water quenching and ice brine quenching. An order-disorder transition diagram was obtained in terms of Si content and cooling rate. It was confirmed that the cooling rate was a more important factor to suppress the ordered phase than annealing conditions. When 0.15 wt% Mn was added to 6.5 wt% Si steel and fabricated by spray casting, the ordering did not take place.

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Creep crack growth (CCG) behavior of type 316LN stainless steel was investigated using C^* and C_t as characterizing fracture parameters under elevated temperatures The CCG tests were conducted under various applied load levels at 600℃. For determining material constants required for calculating the fracture parameters, tension and creep tests were also conducted at 600℃. Results slowed good agreements between the estimated values and measured ones for both of the C^* and C_t. Since the C_t was applicable to transient creep condition as well as extensive creep condition, the C_t was employed in determining a CCG model for 316 LN. Fracture mode near the crack tip was a typical intergranular fracture as shown in the creep tests. The model determined in this study can be used to evaluate creep crack growth life of components made of 316LN steel.

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Flaws nucleated during operation of structures such as nuclear power plants, petroleum facilities and gas equipment raise many safety problems, and thus the integrity assessment of crack-like flaws in structures has been very important. Since large-scale structures include weldments with complex micro-structures and inhomogeneous material properties, it is essential to consider both metallurgical and mechanical factors. In this study, a methodology for integrity assessment in welded structures was developed based on API 5L X65 grade natural gas pipeline (an approximately 2400 ㎞ pipeline in Korea). A failure assessment diagram(FAD), which includes all failure modes from linear elastic fracture to plastic collapse, was used for integrity and fitness-for-service(FFS) assessment of the pipeline. Since tensile properties and fracture toughness of base metal, weld metal and heat affected zone(HAZ) are required for the FAD, tensile tests and fracture toughness tests were performed on each region. In particular, the evaluation of representative properties of HAZ make it possible to assess FFS accurately by using the HAZ-based FAD when crack-like flaws exist in HAZ. This HAZ-based FAD is different from current codes, which assess integrity by the material properties of weld metal or base metal even when flaws exist in HAZ.

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Apparent mechanical properties of welded structural components are deviated from the initially designed values due to the residual stress. Thus, exact assessment of the residual stress using a nondestructive method became an important issue in reliability researches. A nanoindentation model for evaluating the equi-biaxial thin film residual stress was proposed in the previous study. However, the surface residual stress in the welded joint is a non-equi-biaxial state and complex elastic/plastic deformations are involved in the macroindentation. Therefore, the previous model was modified to evaluate the real non-equi-biaxial residual stress by considering a new stress-proportional factor and a significant effect of the plasticity. The newly modified stress-analyzing model was verified empirically by comparing the evaluated uniaxial stress with the already known residual stress of 4-point bended specimen.

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In order to investigate the effects of TiC and Cr_23C_6 carbides on the creep-fatigue behaviors, total strain range controlled creep-fatigue tests were conducted at 600℃ in the TiC and Cr_23C_6 aged AISI 321 stainless steels with various carbide densities at grain boundary. It is observed that creep-fatigue lives of small TiC and TiC aged alloys are longer than that of Cr_23C_6 aged alloy in the same test conditions. To verify differences of creep-fatigue life between TiC and Cr_23C_6 aged alloys, microstructural observations are conducted by scanning electron microscope(SEM) and transmission electron microscope(TEM). It is expected that cavitation resistance of TiC aged alloy is stronger than that of Cr_23C_6 aged alloy. From the microstructural observations, it is verified that formation and growth of grain boundary cavities in TiC carbides are more retarded than in Cr_23C_6 carbides. Therefore, it is suggested that the types of carbides are a more prominent factor than the density of carbides for grain boundary cavitation in austenitic stainless steels.

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Effects of Sn morphology on the friction and wear characteristics of Al-Sn alloys were investigated by using the tribology tester, optical microscope and scanning electron microscope. Three different types of Sn morphologies named elongated, small network and large network structure were obtained by cladding and heat treatment. Wear resistance of the specimen having large network structure exhibited better anti-friction characteristics compare to that of the elongated structure. Also specific wear rate and friction coefficient decreased markedly with change of Sn morphology from elongated to large network structure. Abrasive and adhesive wear occurred for the elongated structure, while oxidation wear was observed for the network structure.

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The present study is concerned with microstructure and mechanical properties of oxide dispersion strengthened(ODS) Cu-Al_2O_3 alloys fabricated by internal oxidation. Atomized Cu-Al powders mixed with Cu_2O oxidant powders were internally oxidized and then hot-extruded to form ODS Cu-Al_2O_3 alloys without defects. Here, a 30% excessive amount of oxidant powders over a stoichiometrically calculated amount was required in order to sufficiently oxidize Cu-Al powders. In the extruded Cu-Al_2O_3 alloys, very fine Al_2O_3 dispersoids of 10 nm in diameter were homogeneously distributed inside Cu grains of 1 ㎛ in size. Yield and tensile strengths increased with increasing volume fraction of Al_2O_3 dispersoids, while elongation and electrical conductivity decreased, and all the properties of the Cu-Al_2O_3 alloys were sufficiently satisfied with the required properties for applications. To understand the mechanism responsible for the improvement of yield strength of the ODS Cu-Al_2O_3 alloys, yield strength was analyzed using the Orowan mechanism, which was fairly consistent with the experimental results.

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The effects of microstructure, including the volume fraction, size and shape of primary solid phases, on the tensile and corrosion behaviors of thixoformed 357(Al-7%Si-0.6%Mg) alloy were examined. It was found that the tensile elongation of 357-T5 alloy increased exponentially from 3.1 to 8.6% with increasing the solid volume fraction from 55 to 68%. The yield and tensile strength, however, showed no notable dependency on the solid volume fraction. The resistance to pitting corrosion with high solid volume fraction was substantially lower than that with low solid volume fraction in thixoformed 357-T5 alloy, which is believed to be due to the area effect as related to the galvanic corrosion between Si particle and eutectic Al phase. The tensile and corrosion behaviors of thixoformed 357-T5 semisolid alloy were discussed on the basis of the detailed micrographic and fractographic observations.

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To improve the oxidation resistance of γ-based TiAl alloy, Al-Ti-Cr coatings of approximately 150 ㎛ thickness were deposited on Ti-48Al alloy using an air plasma spraying(APS) process. We used Al-Ti-Cr powders with the composition of Al-2lTi-23Cr(at.%). Isothermal and cyclic oxidation tests at 1000℃ in air were utilized to investigate the oxidation resistance of both Al-Ti-Cr coatings and of TiAl alloys with Al-Ti-Cr coatings. Phase identification of Al-Ti-Cr coatings was performed by XRD. After exposure at 1000℃ in air, rutile TiO_2, which resulted from the oxidation of Ti during spraying, was always found in addition to protective α-Al_2O_3 in the oxide scale. However, isothermal and cyclic oxidation tests of a Ti-48Al alloy coated with an Al-Ti-Cr coating showed that this coating improved the oxidation resistance of TiAl alloy at 1000℃ despite partial spalling in the Al-Ti-Cr coating layer. Moreover, cyclic oxidation tests of Al-Ti-Cr coatings showed stable oxidation behavior at 1000℃ up to 100h because of good adhesion between the coating layer and the oxide scale although the growth rate of oxide was relatively high. Therefore, it could be suggested that the Al-Ti-Cr coating was effective in enhancing the oxidation resistance of the TiAl alloy at 1000℃, although the improvement of adhesion between the coating layer and the TiAl substrate was required.

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Oxidation behavior of Fe-Ni alloys added by small amount of Ni was studied, under the exposure to the temperatures up to 1100℃, 1150℃, and 1200℃, respectively. The slope of mass change curve representing an oxidation rate was 1 or 2 up to 20 seconds and decreased to 1/2 after 20 seconds in a parabolic shape. The temperature of the initial stage of oxidation increased more quickly at the furnace inside than at the surface of the specimen, and its behavior was reversed due to oxidation heat after 20 seconds later. Oxide layer was mainly composed of FeO. And particle of Fe_3O_4 and Fe_2O_3 was distributed in FeO layer. The order of the composition ratio for the oxide was Fe_2O_3,·Fe_3O_4,·FeO at the oxide surface, FeO,·Fe_3O_4,·Fe_2O_3 at the internal oxide, and FeO,·Fe_3O_4,·Fe_2O_3 at the surface of metal after stripping oxide layer.

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