발간논문

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In this study, the behavior of the fatigue fracture and the fatigue crack growth for 15-5 precipitation hardening stainless steels which were heat-treated at various temperatures were investigated. The acoustic emission signals were generated during the fatigue crack growth and they(the results) were evaluated by the time frequency analysis method. Shot peening was carried out on artificially degraded Type-A specimen by various methods, and the compressive residual stress of surface was also evaluated. As the heat treatment temperature was increased, the base structure, martensite, remained unchanged but the sub-structure was changed. The crack growth rate of the artificially degraded Type-A specimen was faster than that of Type-B specimen due to the coarse microsturcture. The crack growth of the Type-C specimen was hindered cause of the fine microstructre and the precipitates. The threshold area was a little bit different among three types of specimens due to the microstructure. The frequency ranges of acoustic emission signals have nothing to do with the types of specimens. That is, the fatigue crack length of 13 mm produced burst signals about 0.1-0.15 MH_Z while 18mm crack generated continuous signals about 0.1-0.3 MH_Z. Because the signals due to the interfacial fracture of martensite were too strong, the affection of extraction was indistinguishable. Introduction of compressive residual stress by stress double shot peening (SDSP) was an useful method to improve the fatigue strength of degraded specimen.

keyword : 15-5 Precipitation Hardening(PH) stainless steel, Fatigue crack propagation rate, Heat treatment condition, Degradation, Acoustic emission, Shot peening

The continuous indentation test, which measures indentation depth with applied load, has been used to evaluate the tensile properties whose accuracy can vary depending on such experimental conditions as maximum depth ratio, indenter radius, number of unloadings and unloading ratio. In this study, Taguchi method, one of the designs of experiment, was applied to quantify the effects of the experimental conditions effectively. Using a signal-to-noise ratio calculated from the error in the indentation tensile properties, the optimum values of the experimental conditions were presented. The indentation tensile properties from the load-depth curves with the optimum conditions showed better accuracy than those with existing conditions.

keyword : Continuous indentation test, Experimental conditions, Taguchi method, Design of experiment, Tensile properties

The effects of Cr and/or Zr additions on tensile and corrosion behaviors of Cu-6i-4Sn alloy were examined. It was observed that the changes in microstructure and tensile properties of Cu-6Ni-4Sn alloy were not significant with the addition of 0.1 to 0.3% Cr and/or Zr in the present study. The resistance to pitting corrosion was, however, improved in 3.5% NaCl solution with particularly the addition of Cr and Zr. The passive films observed in Cu-6Ni-4Sn alloys with Cr and/or Zr addition appeared to be more stable than non-Cr and/or Zr containing counterpart. The mechanism(s) of the improved resistance to pitting corrosion for Cu-6Ni-4Sn alloy with the addition of Cr and/or Zr is discussed based on the experimental observations.

keyword : Cu-6Ni-4Sn alloy, Tensile property, Corrosion property Cr and/or Zr

Low alloy steels for nuclear reactor are degraded progressively through microstructural changes due to high operation temperature and irradiation. Strength properties are the most important factors for the selection of materials and the determination of degradation degree. Interfacial properties affected by strengthening particles with the range of nm size and strength distribution in microstructures could not be tested by the conventional mechanical testing methods. Microindentation and nanoindentation technique have been studied as a alternative of conventional methods. Specially, nanoindentation technique with a μN load range and a nm residual indent size range has been highlightened as a tool which is very useful to determine those properties. Since each phase composing microstructure of steels has a characteristic hardness value, it is possible to characterize phase on the basis of that character. In this study, by using of microindentation and nanoindentation technique can be evaluated microstructural mechanical properties of low alloy steels manufactured under 7 different kinds of processes. Through the study, we verified potentialities whether or not indentation technique can be used as a method to determine degree of degradation.

keyword : Low-alloy steel, Microindentation, Nanoindentation Characteristic hardness, Phase characterization, Degree of degradation

Hardness and elastic modulus calculated from the Oliver and Pharr`s indentation curve analysis are overestimated for ductile metals comparing with the values measured from the direct observation of residual indentation impression. A dominant cause of the phenomenon is material pile-up around contact. However it is very difficult to consider the pile-up in analyzing the indentation curve without observing the residual indentation mark. Therefore, we propose an indirect analysis of the pile-up effect by estimating the elastic modulus independently based on the Hertzian contact theory. Pile-up corrected contact area is predicted from the conventional elastic recovery equation by inserting the pre-determined elastic modulus. The proposed analysis is applied to the nano-indentation tests on (100) tungsten single crystal and produced a reasonable hardness comparing with the direct observation result through an atomic force microscope.

keyword : Nano-indentation, Hardness, Elastic modulus, Hertzian contact, File-up behavior

The characteristics of upper nozzle clogging was investigated using upper nozzle clogging index(CI_Ar) in a continuous casting of steel. CI_Ar was derived from the modified value of argon back pressure, which was dependent on the argon flow rate in upper nozzle, liquid steel throughput, and weight of liquid steel in a tundish. Inclusion attachment to and its dislodging from upper nozzle, the phenomena of which could be detected from the slow increase and the following sudden decrease of CI_Ar, was found to occur continuously for all sequences of cast in case of severe nozzle clogging.

keyword : Upper nozzle, Clogging, Ar back pressure, Dislodging

The microstructure of Cu-16 at%Ag filamentary nanocomposite fabricated by thermo-mechanical process has been characterized by the combined use of a high resolution electron microscopy and a computer-aided image processing. The silver filaments with a diameter of 1.5-8 nm were observed along the 〈111〉_Cu directions in the copper matrix, and the orientation relationship between the copper matrix and the silver filament was determined to be cube on cube; (100)_Cu ∥ (100)_Ag, ［010］_Cu ∥ ［010］_Ag. The interfaces between the copper matrix and the silver filament were parallel to the 〈111〉_Cu directions and the misfit dislocations were detected at interphase interfaces. In the lamellar structure region, the twin bands with a width of 6--8 nm were observed and silver filaments within the twin band also appeared twinned. A ｛111｝twin orientation relationship was found between the copper matrix and silver filament and the twin interface was parallel to the ｛111｝ twin plane.

keyword : Cu-16 at%Ag filamentary nanocomposite, High resolution electron microscopy, Image processing, Interphase interface structure, Misfit dislocation, Twin structure

The present study is concerned with the microstructural analysis and the evaluation of hardness and wear properties of (TiC,TiB)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron beam irradiation. The mixtures of Ti+C, TiC+TiB_2, and Ti+B_4C powders and CaF_2 flux were deposited on a Ti-6Al-4V substrate, and then electron beam was irradiated on these mixtures using an electron beam accelerator. The surface-alloyed layers of 0.9∼1.6mm in thickness were formed without defects, and contained a large amount (30∼44vol.%) of precipitates such as TiC and TiB in the martensitic matrix. This microstructural modification including the formation of hard precipitates and hardened matrix in the surface-alloyed layers resulted in the improvement hardness and wear resistance. Particularly in the surface-alloyed material fabricated with Ti+B_4C powders, the wear resistance was greatly enhanced by nine times higher than that of the Ti alloy substrate because a 44 vol.% of TiB and TiC was precipitated homogeneously in the martensitic matrix. These findings suggested that the surface-alloying using high-energy electron beam irradiation was useful for the development of titanium-base surface-alloyed materials with increased surface hardness and improved wear properties.

keyword : Surface alloying, Electron beam irradiation, Ti-6A1-4V, TiC, TiB, Flux, Wear resistance, Hardness

12% Cr steel used for a nozzle in steam turbine was paste-boronized and its surface properties at high temperature were evaluated. Typical tooth structure, which consisted of outer layer of FeB and inner layer of Fe_2B, was observed at the boronized layer of the specimen and its activation energy was 286.7 kJ/mole. Cracks due to the difference of thermal expansion coefficient and brittleness of FeB layer were observed. Voids were also observed at tip area of tooth structure. Weight of the specimen exposed at 740℃ increased rapidly from the initial stage of oxidation. Iron oxide was mixed in chromium oxide at outer layer of the oxidized specimen, but chromium oxide and molybdenum were mixed in iron oxide at inner layer. Boronized specimen has shown excellent erosion resistance up to 20 times higher than that of non-treated specimen.

keyword : Boronizing　treatment,　12%Cr steel, Steam turbine nozzle, High temperature oxidation,　Erosion　behavior

The effects of the grain size on the magnetic properties in IF steel after strain annealing were investigated. A 92.5% cold rolled and annealed IF steel sheet was temper rolled by 0.4% reduction in thickness and subsequently strain annealed at the temperature ranging between 670℃ and 950℃ for various time periods. The grains after strain annealing became more coarse than those after primary annealing. The grains were coarsened due to the strain induced grain boundary migration(SIGM). It was found that the permeability increased and coercivity decreased with the increase of grain size. The optimum magnetic properties was achieved after strain annealing at 850℃ for 30 min. Under this condition, the coercivity was measured to be 0.6Oe and the maximum permeability was 13000, also sound that the optimum grain size was 83 μm.

keyword : Permeability, Coercivity, Texture, Strain annealing, Grain coarsening, SIGM