• 열처리공학회지
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• 제37권3호
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• pp.128-137
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• 2024

In this study, the carburizing heat treatment process used in aircraft gear manufacturing was compared with the general carburizing heat treatment process using AISI 9310 steel. The process of carburizing followed by slow cooling, and then quenching after austenitizing(Process A) showed less compressive residual stress and less retained austenite in the surface layer compared to the process of quenching directly after carburizing(Process B). In prpcess B, there was a large amount of retained austenite when quenched immediately after carburization, and when treated with subzero, martensite rapidly increased and the compressive residual stress increased significantly, but at the same time, there is a risk of cracking due to severe expansion in volume. Therefore, in the case of aviation parts, it is believed that a step-by-step heat treatment cycle was adopted to ensure stability against heat treatment cracks. As a result of the final tempering after sub-zero treatment, the A process specimen showed a deeper effective case depth and HV700 depth and a higher hardness value above HV700 than the B process specimen.

• 열처리공학회지
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• 제5권1호
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• pp.32-40
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• 1992

This content is a part of the results of the study on the development of the vacuum carburizing technology. In this study the vacuum carburizing furnace being used was the furnace that developed through the joint project between KIMM and Kyung-Pook Heat Treating Co. from June 1988 to Nov. 1990. And the used carburizing gas was the propane gas and the introducing methods of the gas applied two methods such as pulse and constant pressure. By this study we established the basis of the furnace manufacturing technology and of the processing technology in the vacuum carburizing. Above all in this work there are notable meanings in a viewpoint of the foremost research in home. Hereafter, we are going to industrialize the vacuum carburizing technology by improving the results of the present work and by developing the process for the mass production.

• 대한금속재료학회지
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• 제50권7호
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• pp.517-522
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• 2012

In this study, a super carburizing treatment was applied to improve roller pitting fatigue life. It produced excellent properties of surface hardness and temper softening resistance by forming precipitation of fine and spherodized carbides on a tempered marstensite matrix through the repeated process of carburization and diffusion after high temperature carburizing step 1. The cycle II performed two times carburizing/diffusion cycle (process) after super carburization at $1,000^{\circ}C$ had fine and spherodized carbides to subsurface $200{\mu}m$. In this case, the carbide was $(Fe,Cr)_3C$ and there was not any massive carbides. In the case of Cycle II, the roller pitting fatigue life had a 6.15 million cycles. It was improved 48% compared to normal gas carburizing treatment.

• 열처리공학회지
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• 제13권5호
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• pp.337-345
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• 2000

As a case hardening process for stainless steels, nitriding is more preferred and widely used than carburizing which deterioates corrosion resistance severely. In order to add the nitrogen into the stainless steels, passive film on the surface must be removed effectively before nitriding. Conventional gas nitriding process is performed in the temperature range of 500 to $600^{\circ}C$ with $NH_3$ gas, which often leads to sensitization of stainless steels. In this study, we tried to activate passive film of austenitic stainless steels by heating at low pressure. ($900^{\circ}C$, $5{\times}10^{-2}$ Torr.) Nitriding was performed at the solution treatment temperature of $1100^{\circ}C$ with nitrogen molecules instead of $NH_3$ gas. An attainable nitrogen content in a case depends on the nitrogen gas pressure at constant nitriding temperature. A case depth is proportional to the square root of solution time, which suggests that inward diffusion of nitrogen follows the Fick's 2nd law. Surface nitrogen atoms are dissolved as interstitial solutes, or precipitated in the form of MN, $M_2N$ nitrides, which increase the case hardeness. Dissolved nitrogen in the case enhances the cavitation resistance of austenitic stainless steels dramatically.

• 열처리공학회지
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• 제32권5호
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• pp.201-211
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• 2019

Low carbon steel (S20C steel) and SPCC steel sheet have been austenitic nitrided at $700^{\circ}C$ 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 $700^{\circ}C$ for 1 hour. In this condition, the thickness of the surface layer without pores appeared about $60{\mu}m$ in S20C steel and $30{\mu}m$ 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 $1^{st}$ transformed surface layer for S20C steel, maintaining 10~12% residual ammonia condition via austenitic nitriding process resulted in ${\varepsilon}$ phase with relatively high nitrogen concentration (just below 4.23 wt.%N) among the mixed phases of ${\varepsilon}+{\gamma}$. The ${\varepsilon}$ phase was formed a specific orientation perpendicular to the surface. For $2^{nd}$ transformed layer for S20C steel, ${\gamma}$ phase was rather dominant (just above 2.63 wt.%N). For SPCC steel sheet, there appeared three phases, ${\gamma}$, ${\alpha}(M)$ and weak ${\varepsilon}$ phase. The nitrogen concentration would be approximately 2.6 wt.% in these phases condition.