강구조물의 파괴방지와 국내기술개발과제

맹선재 한국발명진흥회 1979 特協 Vol.4 No.6

상이한 초기조직에서 얻은 이상조직강의 열처리에 따른 인장변형 및 파괴거동

맹선재,신동혁,김일영 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.6

The different starting structures of the present study were ferrite + coarse pearlite (F+ P₁), ferrit + fine pearlite(F+P₂) and martensite(M). True stress vs. true strain curves were analysed before and after tempering. When compared the work hardening rate before tempering, the DP steel with the M-starting structure has shown the highest value. Uniform true strain was decreased with increasing tempering temperatures up to 400℃, and then increased above 400℃. Void formation behavior beneath the fracture surfaces of the tensile specimens has been studied. Preferred sites of void formation are the ferrite-martensite interfaces, the elongated martensite ends and the precipitate particles. Voids are also formed by splitting of martensite phases in the case of tempering.

상이한 초기조직에서 얻은 이상조직강의 열처리에 따른 인장특성

맹선재,신동혁,김일영 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.6

The effect of tempering temperature on the tensile properties of dual phase steels, obtained from different microstrctures, were studied. The starting microstructures of the studied dual phase steels were three kinds ; ferrite+coarse pearlite(F+P₁), ferrite+fine pearlite(F+P₂) and martensite(M). By increasing the tempering temperature, tensile strength was decreased, independently of the starting microstructures. Yield strength of the F+P starting structures was increased at the tempering temperature around 200 C and then decreased with the increased tempering temperature. However, the DP steel of M-starting structure has shown progressive decrease in yield strength with increased tempering temperature. The tempering effect on the elongation was similar, independently of the starting structures. At the early stage of tempering, tensile properties were influenced predominantly by the segregation of carbon at dislocation sites. At the later stage of the tempering, softening effect by the decomposition of martensite is more effective.

7475 알루미늄합금의 중간가공열처리 공정변화에 따른 초소성

맹선재,신동혁,유재은 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.2

Effect of the RI-ITMT variations on grain refinement and the superplasticity of 7475 aluminum alloy has been studied. The increased number of warm rolling passes contributed for better grain refinement, that is the average grain diameter of 7.5㎛ was obtained. As the solution treatment time increased, the distribution and the size of secondary particles became more uniform and finer, respectively, and the superplasticity was enhanced. The maximum superplastic elongation of this study was 2000% at 516℃, and at the deformation rate of 1.7 × 10^(-3)/s. The attained superplasticity of this study is much higher than any of the previously published results on 7475 and 7075 alloys. Cold rolling process, instead of the warm rolling of the conventional RI-ITMT, has resulted a little decreased superplasticity. However the superplastic elongation was still quite high.

초소성 7475 Al 합금의 고온 변형

맹선재,신동혁,주연준,조종원,박경태 대한금속재료학회(대한금속학회) 1994 대한금속·재료학회지 Vol.32 No.11

High temperature deformation behavior was studied in a superplastic 7475 Al alloy having a grain size of 14㎛ at temperatures in the range of 723-789K. The objective of this work was to examine the possible rate controlling mechanisms that govern superplastic deformation. The values of the stress exponent and the activation energy, as well as microstructural observation, indicated that grain boundary sliding was the dominant mechanism for superplastic deformation but dislocation creep was rate-controlling. The specimens tested at a superplastic region of 2×10^(-4)∼1×10^(-2)s^(-1) and 753K ∼ 789K revealed dispersoid-free zones(DFZs) near grain boundaries. It was suggested that DFZs were due to the solute diffusion along dislocations piled-up at grain boundaries.

가공경화된 구리 다결정의 열적으로 활성화된 변형기구

맹선재,허성강,신현길 대한금속재료학회(대한금속학회) 1972 대한금속·재료학회지 Vol.10 No.1

이 연구에서는 실온에서 여러단계로 가공경화(prestrain hardening)시킨 구리 다결정을 저온에서 다시 변형시켜 flow stress 대 온도의 곡선을 두 가지 변형율에 대해 얻었다. flow stress와 온도는 서로 일차적인 관계에 있었으며 Adams와 Cottrell의 실험결과나 Seeger의 이론에서 기대되는 것과 같은 200°K 부근에서의 flow stress 값의 급격한 변화는 발견할 수 없었다. 변형기구는 주로 열적으로 활성화된 vacancy 형성에 의한다고 가정하고 Seeger의 이론을 이용하여 실험결과를 고찰하였으며 가공경화의 정도에 따른 activation volume, 전위선분의 길이, 전위 밀도 및 비열적응력(athermal stress) 등을 결정하였다. As a study of low temperature thermally activated deformation mechanism in polycrystalline copper, flow stress vs temperature curves of various prestrain hardened samples were determined for two different strain rates. Flow stress vs temperature curves showed linear relationship. Rapid change of the flow stress near 200°K reported by Adams and Cottrell, and Seeger, could not be observed. Assuming the deformation is controlled by thermally activated vacancy formation, experimental results were analysed according to Seeger's theory, and activation volumes, length of dislocation segments, dislocation densities and athermal stresses for various prestressed states were determined.

초소성 7475Al 합금의 고온 변형 특성

맹선재,신동혁,주연준,하종신 대한금속재료학회(대한금속학회) 1992 대한금속·재료학회지 Vol.30 No.2

The superplastic properties of fine-grained 7475 Al alloy were investigated at 516℃ over the strain-rate range of 2.8 × 10^(-4)s : to 1.7 × 10^(-2)s^(-1). A significant improvement in superplastic elongation was achieved by adopting two-stage strain-rate deformation compared to a usual single strain-rate test. The initially higher strain-rate deformation in the first stage caused rapid microstructural changes including the conversion of subboundaries to high angle grain boundaries, The optimum stain-rate in the first stage was found to be 8.3 × 10^(-3)s^(-1). The specimens tested at lower strain-rates, revealed the dispersoid-free zones(DFZs) near grain boundaries normal to the stress axis. When a higher strain-rate was applied to the specimens with DFZs, the grain refining was not observed. This absence of grain refining was due to the concentration of plastic deformation in the weak DFZs.

신라범종에 대한 금상학적 연구

맹선재 대한금속재료학회(대한금속학회) 1975 대한금속·재료학회지 Vol.13 No.4

중탄소 (0.25C) Ni-Cr-Mo-V 강의 변태조직과 지연파괴특성 (遲延破壞特性)

맹선재,강찬형 대한금속재료학회(대한금속학회) 1980 대한금속·재료학회지 Vol.18 No.5

0.25C-2.5Ni-2.5Cr-0.5Mo-0.1V鋼의 變態組織과 遲延破壞擧動 사이의 關係를 調査하였다. 즉 Martensite 組織, lower bainite 組織 및 이들의 1對 1 比率의 混合組織을 만들어 同一한 引張强度水準(143㎏/㎟)이 되도록 tempering (450℃,40分) 시킨 後 25℃의 蒸溜水 雰圍氣 속에서 외팔보 굽힘試驗機로 遲延破壞試驗을 實施하였다. K_(ISCC) 값으로 評價할 때 lower bainite 組織이 가장 優秀하였고 martensite 組織은 이보다 약간 低下하였으며 混合組織에서 가장 낮은 값을 나타내었다. Martensite 組織과 混合組織의 破面樣相은 모두 粒界破面이었다. 그러나 martensite組織에서는 粒界에 ductile tearing의 量이 顯著히 많았다. Lower bainite 組織의 破面은 microplastic tearing과 microvoid coalescence가 混在한 樣相이다. 위 實驗結果들을 Oriani의 水素脆性에 관한 decohesion說과 關聯시켜 論議하였다. 특히 混合組織의 遲延破壞特性이 單一組織의 경우보다 낮은 것은 martensite와 lower bainite의 境界面에서 水素에 依한 decohesion이 促進된 때문이라고 說明된다. Delayed failure behaviour of the different transformation structures of 0.25C-2.5Ni-2.5Cr-0.5Mo-0.1V low alloy steel has been studied. The studied microstructures are martensite, lower bainite, and mixed structure of 50% martensite and 50% lower bainite. All these structures have been tempered at 450℃ for 40 min to have the same tensile strength level of 143㎏/㎟. Delayed failure testing has been carried out with cantilever bend tester, in distilled water at 25℃. By comparing K_(ISCC) values, lower bainitic structure has shown the highest value, although it is only slightly higher than that of the martensitic structure. Mixed structure has the lowest resistance to delayed failure. The fracture modes of both martensitic and mixed structures have been observed as intergranular. In the martensitic structure, however, it is noticeable that there is a larger amount of ductile tearing between intergranular facets. The fracture mode of lower bainitic structure is the mixed topography of microplastic tearing and microvoid coalescence. The above experimental results are discussed in terms of Oriani's decohesion theory of hydrogen embrittlement. The lowest resistance of the mixed structure to delayed failure may be due to the enhanced decohesion by hydrogen at the phase boundaries of martensite and lower bainite.

저탄소 2% Mn 강 및 2% Mn-Nb-V 강의 냉간압연후(冷間壓延後)의 시효처리효과 : Ⅱ. Splitting 현상

맹선재,강찬형,현도빈,정순효 대한금속재료학회(대한금속학회) 1982 대한금속·재료학회지 Vol.20 No.10

Splitting behavior of tensile specimens of low carbon 2% Mn steel and 2% Mn-Nb-V steel that had been cold rolled and aging treated were studied. The occurrence of splitting depends on steel composition, degree of cold reduction and aging treatment conditions. The susceptibility to splitting is closely related to the weakness of ferrite grain boundaries This weakness is attributed to elongation of the ferrite grains in the direction of cold rolling and also to precipitation and growth of carbides at the grain boundaries during the aging treatment.