발간논문

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The corrosion experiments of iron were performed in boiling 55% Ca(NO₃)₂solution (115℃) to elucidate kinetics of intergranular corrosion (IGC) of iron. Four kinds of iron specimens were used in this study. Three kinds of specimens with the following different chemical compositions were prepared from a starting material with 20 to 40 ppm C, 17 ppm N, and 20 ppm P. One batch (A) of this material was normalized only, a second batch (B) was also decarburized up to under 10 ppm C prior to normalizing, and a third batch (C) was decarburizbd, and then nitrided up to 140 to 220 ppm N prior to normalizing. The fourth specimen (O) contained 200 ppm C, 60 ppm N, and 110 ppm P (Armco iron). All normalized specimens (A, B, C, O) were annealed (A₁,B₁,C₁,O₁) and tempered (A₂,B₂,C₂,O₂). After the specimen was exposed for 15 min to 24 h to the IG Ctest solution at 1000 mV_SHE (passive region) and the time-current curve was recorded, the specimen was then examined for IGC by optical microscopy. The time-current curves and optical micrographic observations suggested that IGC occurred after passivation of specimen and passivation is also necessary for occurrence of IGC of iron. All normalized (A, B, C, O), the annealed (A₁,B₁,C₁,O₁) and the tempered (A₂,B₂,C₂) specimens show no intergranular attack. Only the annealed (O₁) and tempered (O₂) Armco iron specimens were markedly susceptible to IGC. The excess dissolved nitrogen or nitride at the grain boundaries (gbs) does not lead to a definite IGC. The presence of fine carbides at the gbs may cause IGC, but a critical amount of carbon at the gbs may be necessary for occurrence of IGC.

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By using the quasi-steady state solution of the diffusion equation, we discussed the time evolution of a perturbation on the flat solid-liquid interface during solidification of dilute binary alloy or pure materials and the interaction between solute diffusion effect and interface curvature effect. As the results, we showed that the perturbation is propagated to the lateral directions with cell spacing, λ_s (=2π√(3D)_o) irrespective of its initial morphology and at this spacing, the interaction between diffusion and curvature effect reached equilibrium state. Also, appling these results to the secondary arm branching of dendrite, we showed that the branching mechanism can be expressed by the perturbation propagation process.

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The stress corrosion cracking (SCC) of AA 7075 in an aqueous 3.5 wt.% NaCl solution acidified with HCl to PH = 1 was studied under constant load as a function of the applied potential, ageing stage and temperature. Activation energy due to SCC was found to be 57 kJ mole^(-1) for peakaged specimens and 65.8 kJ mole^(-1) for overaged specimens. Time to failure due to SCC, t_f, increased with decreasing potential until -966 mV_(SHE) and then decreased. Stress corrosion crack propagation rate was found to be 0.85-1.05×10^(-5) ㎜ sec^(-1) (24.6-30.4 ㎃ ㎝^(-2)) at the open circuit potential, -566mV_(SHE), and 1.2-1.5×10^(-4) ㎜ sec^(-1) (347.7-434.6 ㎃ ㎝^(-2)) at the anodic overvoltage of ＋200mV. Time to failure, t_f, increased as ageing time increased at the open circuit potential. The similar ageing time dependence of failure time was also observed at the applied anodic potential of ＋200mV. It is suggested on the basis of observations of SCC fracture surfaces and SCC activation energy, and results of t_f variations with applied potential that SCC is caused by hydrogen embrittlement. The results of ageing stage dependence of t_f indicate that SCC behaviour is mainly controlled by matrix precipitate.

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The change of mechanical properties of Fe-33%Cr-12-15%Co alloys was studied as a function of the degree of spinodal decomposition. Step aging method of the solution treated specimens from 650℃ to 590℃ in 20℃ interval was employed to simulate the change of degree of spinodal decomposition, Yield strength of the spinodally decomposed specimens increased rapidly as increasing the degree of decomposition while elongation behaved the opposite. Room temperature deformation mode of the specimen step aged to 630℃ was mainly slip mode while that of the specimen step aged to 590℃ was mainly twin mode for 12% Co alloys. Electron micrograph of dislocation substructure is suggesting that this transition may be related to the Peierl`s force change of α₂particles as the degree of spinodal decomposition increases. These and other behaviors such as fracture were also discussed.

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The morphology of reduced layer of iron ore was thoroughly investigated by means of optical microscopic technique. Between the grain size of Wu¨stite and the retardation of reduction process are closely related. The results withdrawn are as follows; 1. The rate retardation is caused by sintered Wu¨stite grains surrounded by sintered reduced iron layers. 2. All Wu¨stite grains are measured by means of Martin method and the total area of Wu¨stite is calculated. With this calculated Wu¨stite area, the reduction degree is deduced and the result is well conformed with chemically analysed data. 3. Experimental results and the data which calculated by simulation model of Soma are acceptably conformed. 4. The effective diffusion coefficient, C_sD_o (1.27×10^(-12), at 750℃) is calculated from the experimental results.

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Precipitation hardening behavior in 18%Ni cobalt free managing steel was studied by X-ray diffraction test, replica electron microscopic observation and hardness test. Precipitation hardening rate in early stage of aging was decreased with increasing deformation degree. This is because the precipitation occurred during rolling at room temperature. The precipitated phase in 18%Ni cobalt free managing steel was confirmed to be intermetallic compounds such as Ni₃Mo, FeMo and Fe₂Mo by X-ray diffraction test. At aging of 20 hours at 482℃ in 70%-cold rolled managing steel, small amount of the decomposed austenite was detected by the X-ray diffraction test.

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Microstructure analysis and tensile test were performed to study the effects of microstructure and drawing on the flat rolling of high carbon steel wires. Critical reduction ratio in flat rolling increases due to the developed fibrous microstructure as drawing ratio increases, and is roughly proportional to the area reduction ratio in tensile test. Edge crack is formed at the angle of 45 degree to the rolling direction due to tensile stress. Critical width to thickness ratio of Stelmor treated wire is superior to that of the lead patenting treated wire. Sorbite created in the lead patenting treatment is considered to be deterious to flat rolling of wire.

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