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질소첨가 Type 347 및 316 스테인리스강의 저주기 피로저항성 및 균열생성 특성평가
황태복,오용준 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.5
Low cycle fatigue(LCF) properties of Nb-stabilized type 347 and unstabilized type 316N austenitic stainless steels(SS) having nitrogen content of about 0.1wt% were evaluated at ambient and nuclear power plant operating temperature of 330℃. The resistance to LCF in two steels was compared in view of the underlying microstructure. Type 316N showed longer fatigue life than type 347 at both ambient temperature and 330℃. Higher test temperature resulted in shorter fatigue life in type 316N but similar fatigue life in type 347. The similar life in type 347 was attributed to the different cyclic peak stress response with temperature-cyclic hardening at ambient temperature and cyclic softening at 330℃. Dislocation microstructure showed more homogeneous behavior in type 347 than 316N, which came from fine Nb carbide dispersion and higher planar slip property. From the fractography of the tested samples, we found different crack initiation mechanisms in the two types of steels. For type 316Nm initial fatigue cracks mostly formed intergranularly at high input strain range while transgranularly at low input strain range. But type 347 showed intergranular initiation regardless of input strain range. This seemed to be attributed to the fine carbide distribution and higher planar slip property which prevent persistent slip band development especially at low input strain range. Coarse Nb(CN) particles in type 347 accelerated fatigue crack growth through forming voids under the repeated small deformations. (Received February 2, 2004)
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μBGA 무연솔더 (Sn-3.5Ag-0.7Cu)와 전해 및 무전해 Ni 도금층 계면의 열적 안정성
오성룡,황태복,오용준 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.9
The morphology and thermal stability of the interfacial phases in the joints between lead free solder (Sn3.5AgO.7Cu) and electroless Ni-P (12 at%P) and electro Ni under bumper metallizations(UBM) were investigated as a function of thermal aging. Samples were prepared by reflowing at 250℃, N₂ atmosphere and aging in the range of 125 to 170℃ for up to 1000h. After reflow, in the electrolytic Ni/solder joint, the interfacial IMC was mostly Nirich (Ni,Cu)₃Sn4 with an addition of about 8 at% Cu while in electroless Ni-P/solder joint it showed both Cu-rich (Cu,Ni)_(6)Sny and Ni-rich (Ni,Cu)₃Sn4. After relatively short aging time, the interfacial IMC on electrolytic Ni layer has developed into the dual-layed structure of (Ni,Cu)₃Sn₄(bottom) and (Cu,Ni)_(6)Sny(top). But IMC on electroless Ni-P layer were (Cu,Ni)_(6)Sn_(6) and changed into the dual structure only after long aging time of 500h. The growth rate of IMC with aging was higher on electrolytic Ni layer than on electroless Ni-P. The different behavior of electroless Ni-P/SAC joint came from Ni₃P formation between IMC and electroless Ni-P layer which retards diffusion of Ni and Sn to the opposite direction. The thickness of Ni₃P layer was propositional to that of IMC. For longer aging time, NiSnP layer was found at the interface between IMC and Ni₃P layer. Interestingly, in spite of low consumption rate of electroless Ni-P than electrolytic Ni layer by suppressing IMC growth, the interface of Ni-P/IMC was locally hollowed by the accelerated reaction of the Ni-P layer through local thickness reduction of Ni₃P and NiSnP layers. This reduced the effective remaining thickness of Ni-P layer.
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경수로 원전 가압기 밀림배관의 저주기 피로 수명 및 파괴 특성평가
오용준(Yong Jun Oh),황태복(Tai Bok Hwang) 대한기계학회 2003 대한기계학회 춘추학술대회 Vol.2003 No.8
Low cycle fatigue(LCF) properties of SS 347 and 316N stainless steels used for PWR surgeline piping were evaluated at room temperature and plant operating temperature of 330℃ and the resistance to LCF in the two steels was compared in view of the underlying microstructures. LCF tests were performed for cylindrical round specimens with 8nun-gauge length and 7㎜-outer/4㎜-inner diameters using hydraulic dynamic testing machine. For SS347, thermal-mechanical fatigue properties were additionally evaluated in the temperature range of 100 to 330℃ using the induction heating and air cooling system linked to the LCF test machine. SS316N showed longer fatigue life than SS347 at both of RT and 330℃. Higher test temperature resulted in shorter fatigue life in SS316N while similar fatigue lives in SS347. The similar lives in SS347 were attributed to the different cyclic deformation behavior at RT and 330℃. At room temperature, it was consistently work-hardened throughout the fatigue life while at 330℃ cyclically softened to the end of fatigue life. But, SS316N showed cyclic-softening at both of RT and 330℃. These different cyclic responses only came from the fine NbCN precipitates in the matrix in SS347. From the fractography of the tested samples, we additionally found different crack initiation mechanisms in the two steels. For SS316N, initial fatigue cracks at high input strain range mostly formed intergranularly while at small input strain range the tendency toward transgranular initiation became higher. But SS347 showed intergranular initiation regardless of input strain range.
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