• Journal of Advanced Marine Engineering and Technology
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• 제41권3호
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• pp.238-244
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• 2017

플라즈마 이온질화 기술은 특히 스테인리스강의 표면경도 향상을 통한 기계적 성질 개선을 위해 산업 전반에서 널리 사용되고 있다. 또한 저온처리가 가능할 뿐만 아니라 담금질 강, 가스 질화 또는 침탄에 비해 변형이 적으며, 높은 표면 경도와 부식 저항성을 향상시키는 이점이 있다. 많은 연구자들에 의해 $450^{\circ}C$ 이하의 온도에서 플라즈마 이온질화 처리 시 expanded austenite(S-상)에 의해 부식 저항성이 향상되는 것으로 나타났다. 이때 대부분의 실험은 HCl 또는 NaCl과 같은 염화물 용액에서 실시되었다. 그러나 전기화학적인 요인으로서 염화물 용액과 천연해수 사이에는 차이가 있다. 따라서 본 연구에서는 304 스테인리스강에 대하여 다양한 온도에서 플라즈마 이온질화 처리 후 천연해수 용액에서 전기화학적 특성 분석을 통해 결과적으로 내식성이 가장 우수한 최적의 플라즈마 이온질화 처리 온도 조건을 규명하였다.

• 한국해양공학회지
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• 제25권6호
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• pp.60-65
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• 2011

A low temperature plasma carburizing process was performed on AISI 316L austenitic stainless steel to achieve an enhancement of the surface hardness without degradation of its corrosion resistance. Attempts were made to investigate the influence of the processing temperatures on the surface hardened layer during low temperature plasma carburizing in order to obtain the optimum processing conditions. The expanded austenite (${\gamma}_c$) phase, which contains a high saturation of carbon (S phase), was formed on all of the treated surfaces. Precipitates of chromium carbides were detected in the hardened layer (C-enriched layer) only for the specimen treated at $550^{\circ}C$. The hardened layer thickness of ${\gamma}_c$ increased up to about $65{\mu}m$ with increasing treatment temperature. The surface hardness reached about 900 $HK_{0.05}$, which is about 4 times higher than that of the untreated sample (250 $HK_{0.05}$). A minor loss in corrosion resistance was observed for the specimens treated at temperatures of $300^{\circ}C{\sim}450^{\circ}C$ compared with untreated austenitic stainless steel. In particular, the precipitation of chromium carbides at $550^{\circ}C$ led to a significant decrease in the corrosion resistance. A diamond-like carbon (DLC) film coating was applied to improve the wear and friction properties of the S phase layer. The DLC film showed a low and stable friction coefficient value of about 0.1 compared with that of the carburized surface (about 0.45). The hardness and corrosion resistance of the S phase layer were further improved by the application of such a DLC film.

• 한국표면공학회:학술대회논문집
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• 한국표면공학회 2013년도 춘계학술대회 논문집
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• pp.92-94
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• 2013

우수한 기계적 강도 및 내식성을 가지고 있는 F51(Duplex Stainless Steel)에 저온 plasma Carburizing처리를 적용하여 온도 및 Ar농도에 따른 표면특성을 조사하였다. 플라즈마 처리된 시편은 표면에 탄소가 과포화된 S-phase 또는 Expanded austenite가 형성되어 미처리 시편에 비해 경도 및 내식성 모두 증가하였다.

• 대한금속재료학회지
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• 제47권10호
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• pp.629-634
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• 2009

The effects of processing parameters on the surface properties of the hardened layers processed by the low temperature plasma nitrocarburizing and the low temperature two-step plama treatment (carburizing+nitriding) were investigated. The nitrogen-enriched expanded austenite structure (${\gamma}_N$) or S phase was formed on all of the treated surface. The surface hardness reached up to 1200 $HV_{0.025}$, which is about 5 times higher than that of untreated sample (250 $HV_{0.1}$). The thickness of hardened layer of the low temperature plasma nitrocarburized layer treated at $400^{\circ}C$ for 40 hour was only $15{\mu}m$, while the layer thicknesss in the two-step plama treatment for the 30 hour treatment increased up to about $30{\mu}m$. The surface thickness and hardness increased with increasing treatment temperature and time. In addition, the corrosion resistance was enhanced than untreated samples due to a high concentration of N on the surface. However, higher treatment temperature and longer treatment time resulted in the formation of $Cr_2N$ precipitates, which causes the degradation of corrosion resistance.

• 한국표면공학회:학술대회논문집
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• 한국표면공학회 2012년도 추계총회 및 학술대회 논문집
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• pp.175-177
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• 2012

A corrosion resistance and hard nitrocarburized layer was distinctly formed on 310 austenitic stainless steel substrate by DC plasma nitrocarburizing. Basically, 310L austenitic stainless steel has high chromium and nickel content which is applicable for high temperature applications. In this experiment, plasma nitrocarburizing was performed in a D.C. pulsed plasma ion nitriding system at different temperatures in $H_2-N_2-CH_4$ gas mixtures. After the experiment structural phases, micro-hardness and corrosion resistance were investigated by the optical microscopy, X-ray diffraction, scanning electron microscopy, micro-hardness testing and Potentiodynamic polarization tests. The hardness of the samples was measured by using a Vickers micro hardness tester with the load of 100 g. XRD indicated a single expanded austenite phase was formed at all treatment temperatures. Such a nitrogen and carbon supersaturated layer is precipitation free and possesses a high hardness and good corrosion resistance.

• 한국표면공학회지
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• 제51권1호
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• pp.54-61
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• 2018

Plasma Nitriding treatment was performed on STS 204Cu stainless steel samples at a temperature of $400^{\circ}C$ for 15 hours with varying $N_2$ content as 10%, 15% and 25%. Regardless of the content of $N_2$, S-Phase which is a hardened layer of Nitrogen (N) supersaturated phase, was formed in the surface of plasma treated samples. When $N_2$ content was 25%, the thickness of the hardened layer reached up to about $7{\mu}m$ and the surface hardness reached a value of $560Hv_{0.05}$, which is about 2.5 times higher than that of untreated sample (as received $220Hv_{0.05}$). From potentiodynamic polarization test, it was observed that compared to as received sample, the corrosion potential and the corrosion current density of the plasma treated samples were decreased regardless of the $N_2$ content, but the corrosion resistance was not increased much due to the precipitation of $Cr_2N$. On the other hand, pitting potential of the samples treated with 10% and 15% $N_2$ was higher than that of as received sample, however, the samples treated with 25% exhibited a lower pitting potential. Therefore, 10% $N_2$ content was selected as optimum plasma nitriding condition and to further increase both the thickness and surface hardness and the corrosion resistance of the hardened layer, different $CH_4$ content such as 1%, 3% and 5% was introduced into the plasma nitriding atmosphere. With 1% $CH_4$, the thickness of the hardened layer reached up to about $11{\mu}m$ and the surface hardness was measured as about $620Hv_{0.05}$, which is about 2.8 times that of as received sample. And the corrosion resistance of the plasma treated sample by using 1% $CH_4$ was improved significantly due to much higher pitting potential, and lower corrosion current density. When the $CH_4$ content was more than 1%, the thickness and surface hardness of the hardened layer decreased slightly and the corrosion resistance also decreased.

• 한국표면공학회:학술대회논문집
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• 한국표면공학회 2001년도 추계학술발표회 초록집
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• pp.69-69
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• 2001

We report a study of the nitriding of the martensitic grade of stainless steel AKSK 420 in a low-pressure rl discharge using pure nitrogen. Much studied samples of the austenitic grade AISI 304 were treated at the same time to provide a comparison. With a treatment time of 4.0 h at $400^{\circ}C$, the nitrogen-rich layer on MSK 420 is 20pm thick and has a hardness about 4.3 times higher than that of the untreated material. The layer thickness is much greater than that obtained on AISI 304 under identical treatment conditions, reflecting the different Cr content of the two alloys. The alloy AlISI 420 is more susceptible than AISI 304 to the formation of CrN and ferrite, and this has a deleterious effect on the hardnes, gain. Below the temperature at which CrN forms, the treated layer retains its martensitic structure, but with a larger lattice parameter than the bulk, a phase that we term expanded martensite, by analogy with the situation with austenitic stainless steel. The fact that the treated layer retains a martensitic structure is interesting in view of previous evidence that nitrogen is an austenite stabilizer.