저 탄소강의 오스테나이트 질화 시 암모니아 가스첨가 조건변화가 표면층 조직 및 기공변화에 미치는 영향 (Effect of Changes in Condition of Ammonia Gas Addition on the Surface Layer Microstructure and Porosity during Austenitic Nitriding of Low Carbon Steels)

서지정보

· 발행기관 : 한국열처리공학회
· 수록지 정보 : 열처리공학회지 / 32권 / 5호 / 201 ~ 211페이지
· 저자명 : 이제원, 노용식, 성장현, 임수근

초록

Low carbon steel (S20C steel) and SPCC steel sheet have been austenitic nitrided at 700oC 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 700oC 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 1st 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 2nd 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.

영어초록

Low carbon steel (S20C steel) and SPCC steel sheet have been austenitic nitrided at 700oC 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 700oC 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 1st 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 2nd 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.

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