• Journal of Surface Science and Engineering
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• v.44 no.3
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• pp.75-81
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• 2011

Growth behaviors of iron-nitride on S45C steels at low pressure gas nitriding were examined. Surfaces of the steels covered with fine and porous oxide during the pre-oxidation using $N_2O$ gas. Well faceted particles connected with them were observed after 1 min nitriding. They grew steadily and filled inter-pores during additional nitriding process. From the X-ray diffraction analysis, ${\gamma}'$-iron nitride was dominantly formed at the initial stage but the amount of ${\varepsilon}$-iron nitride was rapidly increased as nitriding treatment time. The porous layer was formed on the particles and thickened up to half of nitride layer after 60 min nitriding. The observed growth behaviors were discussed in internal stress related with volume expansion involved in transforming from iron to iron-nitrides.

• Journal of the Korean Society for Heat Treatment
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• v.13 no.5
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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.

• Journal of the Korean Society for Heat Treatment
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• v.20 no.5
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• pp.237-242
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• 2007

Friction coefficient of SM45C steel was surprisingly reduced with $H_2S$ and $C_3H_8$ gas during plasma sulf-nitriding. During the plasma sulf-nitriding, 100-700 sccm of $H_2S$ gas and 100 sccm of $C_3H_8$ gas were added and working pressure and temperature were 2 torr, $500-550^{\circ}C$, respectively. As $H_2S$ gas amount increased over 500 sccm, flake-like structures were developed on top of the nitriding layer and grain size of the nitriding layer were about 100 nm. The friction coefficient for the sample treated plasma sulf-nitriding under $N_2-H_2S$ gas was 0.4 - 0.5. The structure became more finer and amorphous-like along with $N_2-H_2S-C_3H_8$ gas and the nano-sized surface microstructures resulted in high hardness and significantly low friction coefficient of 0.2.

• Journal of Advanced Marine Engineering and Technology
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• v.32 no.2
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• pp.262-267
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• 2008

Plasma immersion ion beam (PIIB) nitriding process is an environmentally benign and cost-effective process, and offers the potential of producing high dose of nitrogen ions in a way of simple, fast and economic technique for the high plasma flux treatment of large surface area with nitrogen ion source gas. In this report PIIB nitriding technique was used for nitriding on austenite stainless steel of AISI304 with plasma treatment at $250{\sim}500^{\circ}C$ for 4 hours, and with the working gas pressure of $2.67{\times}10^{-1}$ Pa in vacuum condition. This PIIB process might prove the advantage of the low energy high flux of ion bombardment and enhance the tribological or mechanical properties of austenite stainless steel by nitriding, Furthermore, PIIB showed a useful surface modification technique for the nitriding an irregularly shaped three dimensional workpiece of austenite stainless steel and for the improvement of surface properties of AISI 304, such as hardness and strength

• Journal of Surface Science and Engineering
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• v.52 no.4
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• pp.194-202
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• 2019

A systematic investigation was made on the influence of processing parameters such as gas composition and treatment temperature on the surface characteristics of hardened layers of low temperature plasma nitrided 316L Austenitic Stainless Steel. Various nitriding processes were conducted by changing temperature ($370^{\circ}C$ to $430^{\circ}C$) and changing $N_2$ percentage (10% to 25%) for 15 hours in the glow discharge environment of a gas mixture of $N_2$ and $H_2$ in a plasma nitriding system. In this process a constant pressure of 4 Torr was maintained. Increasing nitriding temperature from $370^{\circ}C$ to $430^{\circ}C$, increases the thickness of S phase layer and the surface hardness, and also makes an improvement in corrosion resistance, irrespective of nitrogen percent. On the other hand, increasing nitrogen percent from 10% to 25% at $430^{\circ}C$ decreases corrosion resistance although it increases the surface hardness and the thickness of S phase layer. Therefore, optimized condition was selected as nitriding temperature of $430^{\circ}C$ with 10% nitrogen, as at this condition, the treated sample showed better corrosion resistance. Moreover to further increase the thickness of S phase layer and surface hardness without compromising the corrosion behavior, further research was conducted by fixing the $N_2$ content at 10% with introducing various amount of $CH_4$ content from 0% to 5% in the nitriding atmosphere. The best treatment condition was determined as 10% $N_2$ and 5% $CH_4$ content at $430^{\circ}C$, where the thickness of S phase layer of about $17{\mu}m$ and a surface hardness of $980HV_{0.1}$ were obtained (before treatment $250HV_{0.1}$ hardness). This specimen also showed much higher pitting potential, i.e. better corrosion resistance, than specimens treated at different process conditions and the untreated one.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2011.05a
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• pp.131-132
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• 2011

저압가스질화법을 이용하여 SCM440강을 신속질화처리 하였다. 기존의 가스질화법과는 달리 낮은 암모니아가스 농도의 분위기와 아산화질소를 사용한 산화반응의 영향으로 5시간의 짧은 질화처리로서 0.6mm 이상의 질화깊이를 확보할 수 있었다.

• Journal of Surface Science and Engineering
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• v.51 no.4
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• pp.249-255
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• 2018

High temperature and low temperature gaseous nitriding was performed in order to study of the surface hardening and wear properties of the nitrided AISI 410 Martensitic stainless steels. High temperature gaseous nitiridng (HTGN) was carried out using partial pressure $N_2$ gas at $1,100^{\circ}C$ for 10 hour, and Low temperature gaseous nitiridng (LTGN) was conducted in a gas mixture of NH3 and N2 at $470^{\circ}C$ for 10 hour. The nitrided samples were characterized by microhardness measurements, optical microscopy and scanning electron microscopy. The phases were identified by X-ray diffraction and nitrogen concentration was analyzed by GD-OES. The HTGN specimen had a surface hardness of about $700HV_{0.1}$, $350{\mu}m$ of case depth. A ${\sim}50{\mu}m$ thick, $1,250HV_{0.1}$ hard nitrided case formed at the surface of the AISI 410 steel by LTGN, composed nitrogen supersaturated expanded martensite and ${\varepsilon}-Fe_{24}N_{10}$ iron nitrides. Additionally, the results of the wear tests, carried out LTGN specimen was low friction coefficient and high worn mass loss of ball. The increase in wear resistance can be mainly attributed to the increase in hardness and to the lattice distortion caused by higher nitrogen concentration.

• Journal of Surface Science and Engineering
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• v.50 no.4
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• pp.272-276
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• 2017

The response of AISI 310 type austenitic stainless steel to the novel low temperature plasma carburizing process has been investigated in this work. This grade of stainless steel shows better corrosion resistance and high temperature oxidation resistance due to its high chromium and nickel content. In this experiment, plasma carburizing was performed on AISI 310 stainless steel in a D.C. pulsed plasma ion nitriding system at different temperatures in $H_2-Ar-CH_4$ gas mixtures. The working pressure was 4 Torr (533Pa approx.) and the applied voltage was 600 V during the plasma carburizing treatment. The hardness of the samples was measured by using a Vickers micro hardness tester with the load of 100 g. The phase of carburized layer formed on the surface was confirmed by X-ray diffraction. The resultant carburized layer was found to be precipitation free and resulted in significantly improved hardness and corrosion resistance.

• Journal of Surface Science and Engineering
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• v.53 no.2
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• pp.43-52
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• 2020

Various aging treatments were conducted on AISI 630 martensitic precipitation hardening stainless steel in order to optimize aging condition. Aging treatment was carried out in the vacuum chamber of Ar gas with changing aging temperature from 380℃ to 430℃ and aging time from 2h to 8h at 400℃. After obtaining the optimized aging condition, several nitrocarburizing treatments were done without and with the aging treatment. Nitrocarburizing was performed on the samples with a gas mixture of H₂, N₂ and CH₄ for 15 h at vacuum pressure of 4.0 Torr and discharge voltage of 400V. The corrosion resistance was improved noticeably by combined process of aging and nitrocarburizing treatment, which is attributed to higher chromium and nitrogen content in the passive layer, as confirmed by XPS analysis. The optimized condition is finalized as, 4h aging at 400℃ and then subsequent nitrocarburizing at 400℃ with 25% nitrogen and 4% methane gas for 15h at vacuum pressure of 4.0 Torr and discharge voltage of 400V, resulting in the surface hardness of around 1300 HV_0.05 and α'_N layer thickness of around 11 ㎛ respectively.

• Journal of Surface Science and Engineering
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• v.44 no.5
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• pp.173-178
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• 2011

Separator of stack in polymer electrolyte membrane fuel cell (PEMFC) is high cost and heavy. If we make it low cost and lighter, it will have a great ripple. In this study, low carbon steel is used as base metal of separator because the cost of low carbon steel is very cheaper commercial metal material than stainless steels, which is widely used as separator. Low carbon steel has not a good corrosion resistance. In order to improve the corrosion resistance and electrolytic conductivity, low carbon steel needs to be surface treated. We made Chromium electroplated layer of $5{\mu}m$, $10{\mu}m$ thickness on the surface of low carbon steel and it was nitrided for 2 hours at $1000^{\circ}C$ in a furnace with 100 torr nitrogen gas pressure. Cross-sectional and surface microstructures of surface treated low carbon steel are investigated using SEM. And crystal structures are investigated by XRD. Interfacial contact resistance and corrosion tests were considered to simulate the internal operating conditions of PEMFC stack. The corrosion test was performed in 0.1 N $H_2SO_4$ + 2 ppm $F^-$ solution at $80^{\circ}C$. Throughout this research, we try to know that low carbon steel can be replaced stainless steel in separator of PEMFC.