발간논문

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In a welded joint, the formation of residual stress is inevitable. The stress can result in decrease of fatigue life in a welded structure under superimposed external stress. On this account it is important to evaluate the residual stress in use of structural steel. But conventional residual stress evaluating methods have difficulties of the derivation of residual stress free state in the field and destructive or lab-scale experiments. In this study, new residual stress evaluating model was suggested using continuous indentation technique. We theoretically induced load-depth curve for residual stress free state from the hardness and strength of the material. From the curve, we evaluated the residual stress in a welded joint using the difference from residual stress state. And we used invariant contact area because it is uniform value regardless of residual stress in the material or plastic deformation at the indentation test. The residual stress results from continuous indentation technique showed good agreement with conventional methods such as hole-drilling and saw cutting methods for API X65 and SS400. Therefore, this method has possibilities in actual application of industrial field.

keyword : Continuous indentation technique, Load-depth curve for residual stress free state from the hardness and strength of the material, Invariant contact area

Small angle neutron scattering (SANS) has been used to investigate the influence of Cu on radiation hardening and embrittlement in neutron irradiated reactor pressure vessel steels. In this study, the Cu precipitate of thermally aged Fe-1.0 wt%Cu alloys was transformed from BCC to FCC, when the mean radius of precipitates attains to about 5 nm. The precipitation has continued beyond peak hardness. The size distribution, composition, shape, and volume fraction of Cu precipitates were obtained from SANS and high-resolution transmission electron microscopy (HR-TEM). Indirect transformation method developed by O. Glatter was used to obtain the size distribution of Cu precipitates from SANS data.

keyword : Fe-Cu alloy, Cu Precipitate, Small angle neutron scattering, Reactor pressure vessel steel, Micro-hardness, Indirect transformation method

This study was performed to investigate the surface properties and cell toxicity of the anodized and hydrothermally treated Ti, Ti-6Al-4V and Ti-6Al-7Nb alloys. Bioactivity was evaluated from surface activation layer formed on the surfaces in a Hanks` solution for 30 days. Cell toxicity was evaluated from optical density of survival cell. The porous oxide films were formed on all of Ti alloys by anodic oxidation. The anodic oxide films of Ti-6Al-7Nb alloy was composed of strong anatase peaks and weak rutile peaks and Ti was composed of strong anatase peaks without rutile peak and Ti-6Al-4V alloy was composed of weak anatase peaks and weak rutile peaks. The oxide films of all of specimens showed the increase in intensity of anatase peaks after hydrothermal treatment. The surface activity layers were formed only on the oxide films of Ti-6Al-7Nb alloy. Ti-6Al-7Nb alloy shown was significantly higher optical density than Ti and Ti-6Al-4V alloy on the MTT assay for cell toxicity evaluation.

keyword : Bioactivity, Anodizing, Hydrothermal treatment, Cell toxicity, Ti Alloys

Atmospheric Plasma Cleaning (APC) was studied for the residual rolling oil contaminants on the surface of cold rolled Stainless Steel after optimizing APC conditions. Electrodes were placed at a distance of 10 mm, with an RF power of 220W, and line velocity of 0.5 m/min. The residue left by APC was 0.78 mg/㎡, which was less than that of the 1 mg/㎡ approximately by other classical cleaning methods. Especially, APC combined with pre-cleaning by 550℃ Nitrogen gas left a residue of only 0.01 mg/㎡. Cleaning effect are also improved when oxygen gas is added to the atmospheric plasma. It was confirmed by the investigation of the official Korea Chemical Analysis Center that the APC-hot nitrogen method is superior to other methods. The residual contaminants for different methods were as follows: the classical method: about 1 mg/㎡, APC : 0.78 mg/㎡, and APC-hot nitrogen: 0.10 mg/㎡.

keyword : Atmospheric Plasma Cleaning, APC, Rolling oil, Cold rolled stainless steel, Micro cleaning

Si incorporated diamond-like carbon (Si-DLC) films ranging from 0 to 2 at.% contents were deposited on STS 316L substrates for orthopedic implants by means of r.f. plasma-assisted chemical vapor deposition (r.f. PACVD) technique, using mixtures of benzene (C6H6) and silane (SiH4) as the precursor gases. This study provides the reliable and quantitative data for assessment of the effect of Si incorporation on wear-corrosion properties in the simulated body fluid environment through the tribological and electrochemical test. It was found that wear and corrosion to resistance of Si-DLC coatings with increasing Si content were improved owing to high Sp³ bonding.

keyword : Diamond-Like carbon, Wear, Corrosion

One alternative method to estimation of the remaining life of pipelines containing active corrosion defects is presented. This reliability assessment is carried out using extreme value distribution of the corroded defects instead of already published failure pressure model like NG18 or ASME B31G. The failure probability of pipelines depends on the number of corroded defects, and it could be calculated directly as the area exceeded a defined LV (Limited Value of corrosion depth). The remaining life of pressurized pipelines can also be estimated by the PDF of extreme value distribution as calculating the exceeded area with a defined failure probability.

keyword : Probability density function, Extreme value distribution, Remained Life, Pipeline, Failure probability, Corrosion depth

Ta and Ta(N) thin films were deposited on Si (100) substrate by an ion beam deposition method at various substrate bias voltage under Ar/N2 atmosphere in the different pressure ratio of Ar and N2. The effect of nitrogen concentration in Ta(N) on the surface morphology, crystalline microstructure, electrical resistivity was investigated. Furthermore, to examine the diffusion barrier property of Ta(N) films, Cu(100 nm)/Ta(30 nm)(with or without N2)/ Si structures were annealed at 550~700℃ for 30 min in H2 atmosphere. It was found that by adding nitrogen gas a metastable β-phase of Ta(N) films diminished, while a-phase stabilized. As a result, the Ta(N) films deposited at the substrate bias voltage of -50V under Ar/N2 atmosphere showed a good surface morphology and low resistivity. Concerning the thermal stability, the resistivity of Cu/Ta/Si structure considerably increased at 650℃. On the other hand, no change in resistivity and no evidence of a reaction between Cu and Si were observed for Cu/Ta(N)/Si structures annealed up to 650℃. This means that the thermal stability of Cu/Ta(N)/Si structures is improved by an addition of nitrogen.

keyword : Ta(N), α-Ta, Diffusion barrier, Ion beam deposition, Resistivity, Substrate bias voltage, Thermal stability