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The suitability of twin roll strip casting for Ag-27.5%Cu-20.5%Zn-2.5%Mn-0.5%Ni brazing alloy (known as HS-49D) was examined in the present work and the mechanical properties and microstructure of the strip were also investigated. The effect of annealing heat treatment on the properties was also studied. The new manufacturing process has applications in the production of the brazing alloy. XRD and microstructural analyses of the Ag-27.5%Cu-20.5%Zn-2.5%Mn-0.5%Ni strip revealed a eutectic microstructure of an Ag-rich matrix (FCC) and a Cu-rich phase (FCC) regardless of heat treatment. The results of mechanical tests showed tensile strength of 434 MPa and 80% elongation for the twin roll casted strip. Tensile results showed decreasing strengths and increasing elongation with annealing heat treatment. Microstructural evolution and fractography were also investigated and related to the mechanical properties.

keyword : twin roll strip casting, Ag-27.5%Cu-20.5%Zn-2.5%Mn-0.5%Ni, mechanical property, annealing

The Kachanov-Rabotnov (K-R) creep model was proposed to accurately model the long-term creep curves above 10(5) hours of Alloy 617. To this end, a series of creep data was obtained from creep tests conducted under different stress levels at 950℃. Using these data, the creep constants used in the K-R model and the modified K-R model were determined by a nonlinear least square fitting (NLSF) method, respectively. The K-R model yielded poor correspondence with the experimental curves, but the modified K-R model provided good agreement with the curves. Log-log plots of ε*-stress and ε*-time to rupture showed good linear relationships. Constants in the modified K-R model were obtained as λ=2.78, and κ=1.24, and they showed behavior close to stress independency. Using these constants, long-term creep curves above 10(5) hours obtained from short-term creep data can be modeled by implementing the modified K-R model.

keyword : creep, creep curve, nonlinear Least square fitting, NLSF, kachanov-rabotnov (K-R) model, Alloy 617

Thermomechanical processes were carried out to evaluate their effects on the mechanical and the microstructural property of a ferritic-martensitic steel. Modified 9Cr-1Mo steels were hot-rolled at a temperature of either 780℃ or 850℃ after normalizing at 1050℃ and then were air-cooled. Continuous annealing at 850℃ for 2 hours immediately after the hot rolling was also performed and they were compared to the specimens without thermomechanical process. The result showed that there were little differences between the hot rolled specimens in terms of the precipitation density and size. However, V content inside the MX precipitates increased in the case of the specimen rolled at 850℃. The application of the continuous annealing induced coarsening of the Nb-rich MX precipitation as well as an increase in the amount of V-rich MX precipitation, which is expected to enhance high temperature mechanical properties of the ferritic-martensitic steel.

keyword : ferritic-martensitic steel, thermomechanical process, hot rolling, annealing, precipitation

The effects of processing parameters on the surface properties of the hardened layers processed by the low temperature plasma nitrocarburizing and the low temperature two-step plama treatment (carburizing+nitriding) were investigated. The nitrogen-enriched expanded austenite structure (γΝ or S phase was formed on all of the treated surface. The surface hardness reached up to 1200 HV0.025, which is about 5 times higher than that of untreated sample (250 HV0.1). The thickness of hardened layer of the low temperature plasma nitrocarburized layer treated at 400℃ for 40 hour was only 15 μm, while the layer thickness in the two-step plama treatment for the 30 hour treatment increased up to about 30 μm. The surface thickness and hardness increased with increasing treatment temperature and time. In addition, the corrosion resistance was enhanced than untreated samples due to a high concentration of N on the surface. However, higher treatment temperature and longer treatment time resulted in the formation of Cr2N precipitates, which causes the degradation of corrosion resistance.

keyword : Low temperature plasma nitrocarburizing, expanded austenite, austenitic satinless steel, Low temperature plasma nitriding

The resistance to pitting corrosion of Fe-Cr alloys was evaluated by performing potentiodynamic polarization and critical pitting temperature (CPT) tests. For the potentiodynamic polarization tests, various standards were applied, i.e., KS D 0238 (wet, dry), ASTM G 61, and ISO 17475, showing different potentiodynamic polarization results including pitting potentials. ASTM G 61 and ISO 17475 standards presented relatively higher pitting potential while KS D 0238 (dry) indicated lower values than the others. Effects of surface roughness, scan rates, and exposure time to air before tests were also investigated. CPT tests were performed under two different applied potentials, 300 mV(SCE) and 200 mV(SCE) in deaerated 1 M NaCl aqueous solution. CPT values and the polarization test results showed a linear relationship.

keyword : Fe-Cr alloys, potentiodynamic polarization, pitting potential, critical pitting temperature

The passivation (or deactivation) of a metal surface during oxide film formation has been quantitatively explored for a ferritic stainless steel by using dynamic electrochemical impedance spectroscopy (DEIS). For this purpose, the electrochemical impedance spectra were carefully examined as a function of applied potential in the active nose region of the potentiodynamic polarization curve, to separate the charge transfer resistance and oxide film resistance. From the discrepancy in the potential dependence between the experimental charge transfer resistance and the semi-empirically expected one, the degree of passivation could be quantitatively estimated. The sensitivity of passivation of the steel surface to anodic potential, which might be the measure of the quality of the oxide film formed under unit driving force or over-potential, decreased by 31% when 3.5 wt% NaCl was added to a 5 wt% H2SO4 solution.

keyword : dynamic electrochemical impedance spectroscopy, charge transfer resistance, ferritic stainless steel, corrosion, passive film

Many ancient and historical artifacts were made by copper and its alloys. In the case of outdoor exposure artifacts, the surface could be suffered from corrosion and tarnish by the reaction with its environment. In order to preserve the artifacts, surface treatment would be needed and BTA coating has been usually applied to tin-bronze. This paper dealt with the evaluation of the properties of BTA coated film using a linear polarization method and AC impedance measurement. On the base of corrosion rate and film resistance for the specimen formed by coating process, optimum coating conditions are as follows; 3 cycles brushing or 3 cycles spraying coatings for natural dried process and 1 cycle brushing coating or 5 cycles spraying coating for hot-air dried process.

keyword : benzotriazole, BTA, outdoor bronze, artifact, corrosion

The consolidation of mechanical alloyed Ti5Si3 powder by electro-discharge-sintering has been investigated. A single pulse of 2.5 to 8.0 kJ/0.34 g was applied to each powder mixture using 300 and 450 μF capacitors. A bulk-like solid with Ti5Si3 phase has been successfully fabricated by the discharge with an input energy of more than 2.5 kJ in less than 160 μsec. Micro-Vickers hardness was found to be higher than 1350, which is significantly higher than that of a conventional high temperature sintered sample. The formation of Ti5Si3 and consolidation occurred through a fast solid state diffusion reaction.

keyword : titanium-silicide, mechanical alloying, electro-discharge-sintering, vickers hardness, phase transformation

High temperature wear of STD 61 tool steels sliding against the Al-9%Si coated steels used for hot press forming has been studied in comparison with that of the tool steels sliding against the uncoated steels. Wear tests have been performed using a pin-on-disc configuration under an applied normal load of 50N for 20 min with heating the coated and uncoated steels up to 800℃. It was found on the worn surface of the STD 61 tool steels sliding against the Al-9%Si coated steels that the formation of the glazed layers containing Al transferred from the coated tribopair may contribute to a reduction of the coefficient of friction, and detachment in part occur due to delamination wear, resulting in higher specific wear rate. On the other hand the Fe-oxide wear debris entrapped on the softer surface of the uncoated steels can act as a tribosurface, leading to decreased adhesive wear of the STD 61 tool steels, resulting in a lower specific wear rate.

keyword : high temperature wear, glazed Layers, tribopair, friction, tribosurface

Effects of 85℃/85% Temperature/Humidity (T/H) treatment conditions on the peel strength of an electroless-plated Ni/polyimide system were investigated from a 180° peel test. Peel strength between electroless-plated Ni and polyimide monotonically decreased from 37.4±5.6 g/mm to 22.0±2.7 g/mm for variation of T/H treatment time from 0 to 1000 hrs. The interfacial bonding mechanism between Ni and polyimide appears to be closely related to Ni-O bonding at the Ni/polyimide interface. The decrease in peel strength due to T/H treatment appears to be related to polyimide degradation due to moisture penetration through the interface and the bulk polyimide itself.

keyword : adhesion, temperature/humidity, peel test, alkali surface pretreatment, electroless-plated Ni, polyimide