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저 탄소강의 오스테나이트 질화 시 암모니아 가스첨가 조건변화가 표면층 조직 및 기공변화에 미치는 영향
이제원 ( Jewon Lee ),노용식 ( Y. S. Roh ),성장현 ( J. H. Sung ),임수근 ( S. G. Lim ) 한국열처리공학회 2019 熱處理工學會誌 Vol.32 No.5
Low carbon steel (S20C steel) and SPCC steel sheet have been austenitic nitrided at 700℃ in a closed pit type furnace by changing the flow rate of ammonia gas and heat treating time. When the flow rate of ammonia gas was low, the concentration of residual ammonia appeared low and the hardness value of transformed surface layer was high. The depth of the surface layer, however, was shallow. With increasing the concentration of residual ammonia by raising up the ammonia gas flow, both the depth of the surface layer and the pore depth increased, while the maximum hardness of the surface layer decreased. By introducing a large amount of ammonia gas in a short time, a deep surface layer with minimal pores on the outermost surface was obtained. In this experiment, while maintaining 10~12% of residual ammonia, the flow rate of inlet ammonia gas, 7 liter/min, was introduced at 700℃ for 1 hour. In this condition, the thickness of the surface layer without pores appeared about 60 μm in S20C steel and 30 μm in SPCC steel plate. Injecting additional methane gas (carburizing gas) to this condition played a deteriorating effect due to promoting the formation of vertical pores in the surface layer. For 1<sup>st</sup> transformed surface layer for S20C steel, maintaining 10~12% residual ammonia condition via austenitic nitriding process resulted in ε phase with relatively high nitrogen concentration (just below 4.23 wt.%N) among the mixed phases of ε+γ. The ε phase was formed a specific orientation perpendicular to the surface. For 2<sup>nd</sup> transformed layer for S20C steel, γ phase was rather dominant (just above 2.63 wt.%N). For SPCC steel sheet, there appeared three phases, γ, α(M) and weak ε phase. The nitrogen concentration would be approximately 2.6 wt.% in these phases condition.
15-5PH 스테인리스강의 시효열처리 조건변화가 상변태 및 기계적 성질에 미치는 영향
김태수 ( T. S. Kim ),이제원 ( Jewon Lee ),노용식 ( Y. S. Roh ),성장현 ( J. H. Sung ),임수근 ( S. G. Lim ) 한국열처리공학회 2019 熱處理工學會誌 Vol.32 No.5
This study is to investigate the relationship between microstructural factors and tensile properties after aging heat treatment of the 15-5PH stainless steel at the temperature range of 450℃, 500℃ and 550℃ for various time. For the aging time of 2 hours, hardness showed maximum at 450℃ and then decreased with increasing aging temperature. While, hardness decreased gradually during aging 450℃, 500℃ and 550℃ from 1 hour to 5 hours but the hardness nearly unchanged until the 100 hours after 5 hours aging. When aging at 450℃, Cu atoms preferentially aggregated at the prior austenite grain boundaries and martensite lath boundaries, and Cu concentration at those boundaries was nearly unchanged even after aging for 100 hours. Therefore it was suggested that the coherency is still maintained after 100 hours aging at 450℃. Aging at 500℃ and 550℃ results in an increase in the concentration of Ni at the martensite lath boundaries and prior austenite grain boundaries, resulting in the formation of reversed austenite. Especially, when aged at 550℃ for 100 hours, the concentration of Ni remarkably increased at those boundaries, and thus the microstructure of herring bone shape was appeared. Considering the migration of Ni atom to the lath boundaries and prior austenite grain boundaries, Ni atoms contributed greatly to the formation of reversed austenite. On the other hand, it was found that Cu atoms hardly moving to those boundaries may not be contributed to the formation of reversed austenite. When aging at 450℃, the coarsening of the precipitated Cu atoms proceeded very slowly with increasing aging time, therefore the decrease in strengths were small but the reduction area was considerably increased due to the softening of the matrix. At the aging temperature of 500℃ and 550℃, the strengths decreased and the elongation and reduction area increased due to the appearance of the reversed austenite. Especially, the increase of reduction area was remarkable.
Mn, Fe, Ni 첨가가 급속응고/분말야금법으로 제조된 Al-Zn-Mg-Cu 계 합금의 응력부식균열 거동에 미치는 영향
김상식,류재철,임수근,이명호 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.11
Stress corrosion cracking behaviors of RS/PM Al-9Zn-2.5Mg-1Cu alloys with the addition of 2%Fe, 2%Mn, and 1%Fe+1%Ni, respectively, were examined in the present study. Each alloy was exposed to 3.5% NaCl aqueous solution under an applied stress of 200MPa for 5 and 20 days, respectively, and tensile tested in air. The alloys containing Fe+Ni and Mn showed a significant reduction in ductility after exposure for 20 days. SEM fractographic studies indicated that the size and depth of dimples, as well as the sign of plastic deformation around dimples, were greatly reduced after exposure for those alloys susceptible to stress corrosion cracking. The present study strongly suggested that hydrogen played an important role in stress corrosion cracking behaviors of Al-9Zn-2.5Mg-1Cu alloys with the addition of 2%Mn and 1%Fe+1%Ni, respectively.