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조기섭,권훈,Cho, Ki-Sub,Kwon, Hoon 한국열처리공학회 2019 熱處理工學會誌 Vol.32 No.4
Impact toughness and fracture behavior were studied in five kinds of non-heat treating steels containing bainite; standard(0.25C-1.5Mn-0.5Cr-0.2Mo-0.15V), high V(0.3V), Ni(0.5Mn-2Ni), W(0.4W instead of Mo), and high C-Ni(0.35C-0.5Mn-2Ni) steels. The good hardness and impact toughness balance was exhibited in the $1100^{\circ}C$-rolled condition, while the impact toughness was deteriorated due to coarse grained microstructure in the $1200^{\circ}C$-rolled condition. The impact toughness decreased with increasing the hardness in all steels studied. The fracture behavior was also basically identical, that is, the fracture area was divided into 3 zones; shear and fibrous zone, fracture transition zone with ductile dimples and cleavage cracks, where the cracks initiate and grow to critical size, unstable cleavage fracture propagation zone. The energy absorbed for the critical crack formation through the plastic deformation inside the plastic zone in front of the notch root contributed to a mostly significant portion of the total impact energy.
W<sub>35</sub>Fe<sub>43</sub>C<sub>22</sub>비정질 합금분말의 결정화 거동
권영준 ( Young Jun Kwon ),유정선 ( Jung Sun Yoo ),박수근 ( Soo Keun Park ),이근효 ( Keun Hyo Lee ),조기섭 ( Ki Sub Cho ) 한국열처리공학회 2018 熱處理工學會誌 Vol.31 No.4
W, Fe, and carbon powders were mechanical alloyed to produce W<sub>35</sub>Fe<sub>43</sub>C<sub>22</sub> ternary alloy powders containing nanocrystal W embedded within amorphous matrix. When the powder samples were heated to the primary crystallization temperature of 735℃, most parts of their amorphous region were fully crystallized to [W,Fe]- rich M<sub>6</sub>C carbides. Interestingly, a little portion of the carbides changes to stoichiometric line compounds (M<sub>12</sub>C and W<sub>6</sub>Fe<sub>7</sub>) and a solution phase (Fe-rich bcc), and remaining parts of the crystallites were amorphized again. The resulting microstructure was retained even by cyclic heating between room temperature of 1,200oC, and thus we found that the amorphous structure can be irreversibly formed at above glass transition temperature. (Received June 11, 2018; Revised June 25, 2018; Accepted July 3, 2018)
조기섭 ( Ki Sub Cho ),권훈 ( Hoon Kwon ) 한국열처리공학회 2017 熱處理工學會誌 Vol.30 No.2
Transformation behavior and hardness change were studied in five kinds of self-control steels; standard, high V, modified Ni, W, and high C-Ni steels. In the cooling rates of 10-100oC/min, the primary ferrite and bainite were formed, and the amount of the former increased with decreasing cooling rate. The bainite transformation temperature, Bs, was measured as 570, 560, 590, 575, and 565oC in experimental steels, respectively, which was similar to the calculated temperature. The self-control, that is, the consistency in hardness, was observed, in which the hardness increased with the decrease in Bs. In the case of hot compression testing, the lower temperature deformation led to the enhancement in hardness. (Received March 6, 2017; Revised March 15, 2017; Accepted March 24, 2017)