발간논문

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Effect of controlled rolling conditions (such as reheating temperature, reduction amount in nonrecrystallization region, and finish rolling temperature ) on the separation phenomena was studied in C-Mn-Nb-V and low C-Mn-Nb-Mo HSLA steels. Change of the maximum separation length, microstructural features, texture, and strength with controlled rolling conditions were examined and their interrelationship was analysed. Major factors governing the change of separation length were i) solution and precipitation of Nb ii) Martensite-Austenite constituent and banded structure iii) strain induced precipitation and [100] texture.

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Effects of cold working on the damping capacity in Mn-30Cu, Mn-40Cu wt.% and one of multielement Mn-Cu alloy aged at 425℃ for 2hr after quenching from 850℃ were studied by varying the amount of cold working. Measurements of damping capacity were carried out by torsional pendulum method. Cold working by rolling caused a decrease in damping capacity but subsequent 425℃ reaging could increase the damping capacity. Also by cold working, the mechanical properties such as hardness and tensile strength were increased. Temperature dependence of damping capacity was also investigated. Most of high damping capacity peaks were found in the range of 40 to 140℃ on Mn-30Cu and Mn-40Cu, but in multielement sample the damping capacity was decreased by raising temperature. The activation energies for twin boundary relaxation process were 1.44-1.94×10⁴J/g·atom in Mn-30Cu and 1.80-3.66×10⁴J/g·atom in Mn-40Cu alloy.

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The anomalous codepositions for nickel-zinc alloys have been studied to evaluate the fact that the more electronegative zinc than nickel preferentially deposits from the simple salt solution of both metal ions. Experimental results showed that the concentration ratio of nickel and zinc ions and the current densities gave a minor effects for the anomalous codepositions, but the effects of bath temperatures largely acted to convert to normal depositions by depressing the deposition overpotentials, as the temperature approach to boiling point of the solutions.

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An investigation was carried out to develop the abrasion-and heat resistant P/M alloy steels for an application to valve seat of diesel engine. The alloy powder obtained by mixing the individual elemental powders was compacted and sintered by conventional P/M technique and the sintered specimen was subjected to the usual heat treatment for hardening. Under the optimal sintering-and heat treatment condition; (sintered at 1200℃ for 100 min., oil quenched from the austenitizing temperature of 850-900℃ and tempered at 550℃ for 60 min.) the ultimate tensile strength of the sintered alloy steel was more than 1300 N/㎟. It has also shown the appropriate hardness and elongation, as 40.5 HRC and 3%, respectively.

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When the fatigue crack growth rate is given by the Paris equation, dc/dN=A(ΔK)^m, the a-m correlation may be expressed as log A=a+bm, where a=-6.74, b=-1.04 at R=0 and a=-7.09, b=-0.85 at R=0.3 for aluminum alloys indifferent to their compositions and thermo-mechanical treatments. Even though A or m did not vary as a single valued function of the yield strength or hardness of the alloys, A increased and m decreased with increasing yield strength or hardness due to cold working of an alloy. Therefore the fatigue crack growth behavior of the alloy may sometimes be predicted using its tensile or hardness test results.

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A study has been made of the effects of aging treatment and the third additional elements (Ti, Cr, V) on the phase change in the Al-Zr alloy splats produced by Atomizing-Splat Quenching method. In the Al-Zr alloy aged for 1 hour, discontinuous precipitation occurred from grain boundaries at 350℃, microhardness decreased rapidly above 400℃ and coarser plate-like Al₃Zr appeared at 500℃. Al-Zr-Cr and Al-Zr-V alloys showed an aging peak at 450℃ and Al-Zr-Ti alloy at 400℃. Al-Zr-Cr alloy had the highest peak hardness and Al-Zr-V alloy showed the best thermal stability.

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Al diffusion to Inconel 713C by pack commentation in Ar gas with FeAl₂, Al＋Fe and Al powders has been investigated for the purpose of improvement of hot corrosion resistance. The main results are as follows. 1. Al diffusion coating was possible with only Al powder without activator (NH₄Cl). 2. The thickness of diffused layer increased with increasing of holding time, but the coated layer was the thickest when it was treated for 15 hours at 1000℃ in Ar. 3. The diffused layer by FeAl₂(or Al＋Fe) powder was thicker than that by Al powder, and the thickness of coated layer by Al powder was thicker than that by FeAl₂(or Al＋Fe) powder under the same condition. 4. Hot corrosion resistance of the coated layer was good, for example, the specimens by FeAl₂( or Al＋Fe) powders endured 3-4 cycles, and those by Al powders endured more than 15 cycles in salt mixture of 75%Na₂SO₄＋25%NaCl for 20 hours at 900℃ per cycle. 5. Hot sulfuric corrosion attack of the coated specimens were progressed quickly when the coated layer was cracked from cyclic thermal shock, because of infiltration of melted salt through it.

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