• 한국표면공학회지
• /
• 제37권1호
• /
• pp.53-57
• /
• 2004

Austenitic stainless steel AISI 304 was nitrided in a low-pressure RF plasma using pure nitrogen. With a treatment of time of 4.0h at $400^{\circ}C$, the nitrogen-rich layer on the sample was $3\mu\textrm{m}$thick and had a hardness of approximately 4.4 times higher than that of untreated material. XRD data showed that as the process temperature rose from 350∼$450^{\circ}C$, the expanded austenite peaks became more prominent while the austenite peaks became weaker. Expanded austenite was transformed to ferrite and CrN at the treatment of$ 500^{\circ}C$. Langmuir probe measurements showed that electron density decreased above $450^{\circ}C$.

• Journal of Advanced Marine Engineering and Technology
• /
• 제41권3호
• /
• pp.238-244
• /
• 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
• /
• 제25권1호
• /
• pp.56-60
• /
• 2011

A low temperature plasma carburizing process was performed to AISI 304L austenitic stainless steel to achieve the enhancement of surface hardness without a compromise in their corrosion resistance. Attempts were made to investigate the influence of the processing temperatures on the surface-hardened layer during low temperature plasma carburizng in order to obtain the optimum processing conditions. The expanded austenite (${\gamma}C$) was formed on all the treated surfaces. Precipitates of chromium carbides were detected in the hardened layer (C-enriched layer) only for the specimen treated at $500^{\circ}C$. The hardened layer thickness of ${\gamma}C$ increased up to about $35\;{\mu}m$, with increasing treatment temperature. The surface hardness reached about 1000 $HK_{0.05}$, which is about 4 times higher than that of the untreated sample (250 $HK_{0.05}$). Minor loss in corrosion resistance was observed for the specimens treated at temperatures of $310^{\circ}C-450^{\circ}C$ compared with untreated austenitic stainless steel. Particularly, the precipitation of chromium carbides at $500^{\circ}C$ led to a significant decrease in the corrosion resistance.

• 대한금속재료학회지
• /
• 제46권6호
• /
• pp.357-362
• /
• 2008

The 2-step low temperature plasma processes (the combined carburizing and post-nitriding) were carried out for improving both the surface hardness and corrosion resistance of AISI 316L stainless steel. The effects of processing time and temperature on the surface properties during nitriding step were investigated. The expanded austenite (${\gamma}_N$) was formed on all of the treated surface. The thickness of ${\gamma}_N$ was increased up to about $20{\mu}m$ and the thickness of entire hardened layer was determined to be about $40{\mu}m$. The surface hardness reached up to $1,200HV_{0.1}$ which is about 5 times higher than that of untreated sample ($250HV_{0.1}$). The thickness of ${\gamma}_N$ and concentration of N on the surface were increased with increasing processing time and temperature. The corrosion resistance in 2-step low temperature plasma processed austenitic stainless steels was enhanced more than that in the untreated austenitic stainless steels due to a high concentration of N on the surface.

• 대한금속재료학회지
• /
• 제47권10호
• /
• pp.629-634
• /
• 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
• /
• 한국표면공학회 2013년도 춘계학술대회 논문집
• /
• pp.92-94
• /
• 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.

• 한국해양공학회지
• /
• 제25권6호
• /
• pp.60-65
• /
• 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.

• 한국표면공학회지
• /
• 제42권3호
• /
• pp.116-121
• /
• 2009

The 2-step low temperature plasma processes (the combined carburizing and post-nitriding) offer the increase of both surface hardness and thickness of hardened layer and corrosion resistance than the individually processed low temperature nitriding and low temperature carburizing techniques. The 2-step low temperature plasma processes were carried out for improving both the surface hardness and corrosion resistance of AISI 316L stainless steel. The influence of gas compositions on the surface properties during nitriding step were investigated. The expanded austenite (${\gamma}_N$) was formed on all of the treated surface. The thickness of ${\gamma}_N$ and concentration of N on the surface increased with increasing both nitrogen gas and Ar gas levels in the atmosphere. The thickness of ${\gamma}_N$ increased up to about $20{\mu}m$ and the thickness of entire hardened layer was determined to be about $40{\mu}m$. The surface hardness was independent of nitrogen and Ar gas contents and reached up to about 1200 $HV_{0.1}$ which is about 5 times higher than that of untreated sample (250 $HV_{0.1}$). The corrosion resistance in 2-step low temperature plasma processed austenitic stainless steels was also much enhanced than that in the untreated austenitic stainless steels due to a high concentration of N on the surface.

• 대한금속재료학회지
• /
• 제46권3호
• /
• pp.125-130
• /
• 2008

Conventional plasma carburizing or nitriding for austenitic stainless steels results in a degradation of corrosion resistance. However, a low temperature plasma surface treatment can improve surface hardness without deteriorating the corrosion resistance. The 2-step low temperature plasma processes (the combined carburizing and post nitriding) offers the increase of both surface hardness and thickness of hardened layer and corrosion resistance than the individually processed low temperature nitriding and low temperature carburizing techniques. In the present paper, attempts have been made to investigate the influence of the introduction of Ar gas (0~20%) in nitriding atmosphere during low temperature plasma nitriding at $370^{\circ}C$ after low temperature plasma carburizing at $470^{\circ}C$. All treated specimens exhibited the increase of the surface hardness with increasing Ar level in the atmosphere and the surface hardness value reached up to 1050 HV0.1, greater than 750 $HV_{0.1}$ in the carburized state. The expanded austenite phase (${\gamma}_N$) was observed on the most of the treated surfaces. The thickness of the ${\gamma}_N$ layer reached about $7{\mu}m$ for the specimen treated in the nitriding atmosphere containing 20% Ar. In case of 10% Ar containing atmosphere, the corrosion resistance was significantly enhanced than untreated austenitic stainless steels, whilst 20% Ar level in the atmosphere caused to form CrN in the N-enriched layer (${\gamma}_N$), which led to the degradation of corrosion resistance compared with untreated austenitic stainless steels.

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