발간논문

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A kinetic study on the electrodeposition of Ni-Cr-P alloys onto a steel has been performed using a rotating disk geometry. Activation energies of nickel and phosphorus in the temperature range between 8℃ and 45 C were 12.1㎉/mole and 9.6㎉/mole, respectively, and that of chromium between 8℃ and 25℃ was 5.4㎉/mole. The rates of electrodeposition of nickel and phosphorus seems to be controlled partly by electrochemical reaction and partly by mass transport, namely mixed controlled. However, the rate of chromium deposition seem to be controlled by mass transport. As the stirring speed increased, the concentration of nickel in the deposit decreased, but that of chromium increased and phosphorus was not affected. On the other hand, as the temperature was increased, the concentration of nickel in the deposit increased, that of chromium decreased and phosphorus did not vary. A model was proposed to evaluate composition of nickel, chromium and phosphorus in the Ni-Cr-P alloys. The model included the influence of diffusion, forced convection and equilibrium concentration considering the activity coefficient of chemical species in the electrolytic solutions of Ni-Cr-P system. The composition in the coating layer predicted by the model agreed well with the measured composition at various experimental conditions, such as disc rotating speed, applied potential and temperature.

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This paper includes a basic study of the applicability and feasibility of the laser surface alloying technique for enhancing the surface properties of materials, which are used in severe corrosion and erosion environments. Further, the effects of laser processing parameters(such as beam power, beam size, scanning speed, powder feed rate, gas flow rate) on the dimension and shape of the alloyed layer were also included. The dimension and shape of the alloyed layer were found to have more or less exponential relationship with processing parameters, especially with the power intensity, powder feed rate and scanning speed. Microstructures of the laser alloyed layer consisted of three different layers. In the layer I, adjacent to the bond line, applied Cr₃C₄ completely dissolved and mixed with the melted substrate Which transformed to C saturated austenite matrix and carbides (Cr_7C₃ and Cr_(23)C_6) along dendrite boundaries during rapid cooling. In the layer II which is a mid-region of the alloyed layer, Cr₃C₂ also dissolved but did not appear to mix with the substrate completely and transformed as nonequilibrium phase of M₃C₂ during cooling. In the layer III, some of the Cr₃C₂ were not dissolved and embedded in the C, Fe enriched austenitic solid solution.

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Stainless steels in general has passive film having strong corrosion resistance on surface. Therefore it must be mesessarily removed by etching in mixing solution of sulfuric and chloric acid before nitriding treatment. But in the ion nitriding, nitride layer was easily formed because passive film was removed without difficult by sputtering effect. The removal extent of these passive films was greatly effected by gas mixing ratios and pressure and holding times of presputtering factors in presputtering stage. Thus we have examined the effect of the pre sputtering factors in contrast to ion nitriding behviour of next stage. As a results of experiment it has been known that presputtering pressure and holding time was not nearly effective on the formation behavior of nitride layer. But when Ar/H₂ gas mixing ratios was ½(vol%) was the most effective of the all presputtering conditions. It was resulted from the combination of mechanical reaction by argon bombardment and chemical reaction by reduction of hydregen on the passive film. For the ion nitriding of the STS304 having artificial oxide layers which was grown more thick than natural passive film, first of all, the oxide layer must be removed to the appropriate thickness which could easily diffuse nitrogen into matrix by bombardment of Argon ion.

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Laser surface-alloying method was applied on steel in order to develop a functional gradient Inconel/Steel material. Inconel 690 sheet was placed on steel substrate, and then a series of high power CO₂: laser beam was irradiated on surface in order to produce homogeneous alloyed layer. Simultaneous compositional gradients of two different kinds of alloying elements (Cr and Ni) were investigated in this material. In order to determine the microstructure, phase and compositional profiles in this material, SEM, XRD and EDS were used. Sequential repetition of laser surface-alloying treatments up to 4 times resulted in about 3 mm layer of fair compositional gradients from 21℃ to 0% for Cr, from 40% to 0% for Ni and from 39% to 99% for Fe in this material. Since compressive stress was formed in heat-affected region due to martensitic transformation while relative tensile stress was developed in alloyed region, cracks were formed at the boundary region between alloyed region and substrate region.

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The microstructures of PTA (Plasma Transferred Arc) weld-overlayers were evaluated for those of open arc weldoverlayer and spray & fused coating. DSRW (Dry Sand Rubber Wheel) abrasive wear and potentiostatic corrosion tests in 3% NaCl solution were conducted. The Stellite 6 weld-overlayer by PTA method contains more eutectic and hence shows a little higher hardness than that by open arc method. The 16C weld-overlayer by PTA method contains coarser borides and carbides, but shows a little lower hardness than that by spray & fuse method. The borides and carbides in the spray & fused 16C coating are fine and more or less needle shape, and porosities are still present after fusing treatment. PTA weld-surfaced Stellite 6 overlayer shows twice more wear resistant than that of open arc method largely due to higher eutectic content. PTA weld-surfaced 16C overlayer also shows more wear resistant than that of spray & fuse method due to the size difference in borides and carbides. The results of potentiostatic corrosion tests indicate that both overlayers of PTA are superior to those of open arc and spray & fuse processes.

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