• Journal of the Korean Society for Heat Treatment
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• v.16 no.4
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• pp.191-196
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• 2003

For improvement of the wear performance of Ti alloy, vacuum-carburizing technique was tried for the first time using propane atmosphere. During the low pressure carburizing carbide was formed at the surface and carbon transfer was occurred from the carbide to the matrix. It was found that: (i) surface hardness increased with the reduction of operating pressure and time; (ii) optimum hardness distribution could be obtained with the proper choice of temperature and carbon flux control; and, (iii) case depth was largely influenced not by time but by temperature. The two steps process was recommended for obtaining thick case depth and high surface hardness of Ti alloy. For the low oxygen partial pressure, it was necessary to introduce additional CO gas to the atmosphere.Grain boundary oxidation and non-uniformity could be prevented.

• Journal of the Korean institute of surface engineering
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• v.46 no.5
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• pp.192-196
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• 2013

Stainless steels (AISI 316L) are carburized by Inductively coupled plasma using $CH_4$ and Ar gas. The ${\gamma}_c$ phase(S-phase) is formed on the surface of stainless steel after carburizing process. The XRD peak of carburized samples is shifted to lower diffracting angle due to lattice expansion. Overall, the thickness of ${\gamma}_c$ phase showed a linear dependence with respect to increasing temperature due to the faster rate of diffusion of carbon. However, at temperatures above 500, the thickness data deviated from the linear trend. It is expected that the deviation was caused from atomic diffusion as well as other reactions that occurred at high temperatures. The interfacial contact resistance (ICR) and corrosion resistance are measured in a simulated proton exchange membrane fuel cell (PEMFC) environment. The ICR value of the carburized samples decreased from 130 $m{\Omega}cm^2$ (AISI 316L) to about 20 $m{\Omega}cm^2$. The sample carburized at 200 showed the best corrosion current density (6 ${\mu}Acm^{-2}$).

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2012.05a
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• pp.177-178
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• 2012

In this experiment, post-nitriding treatment has been performed at $400^{\circ}C$ on AISI 316 stainless steel which is plasma carburized previously at $430^{\circ}C$ for 15 hours. Plasma nitriding was implemented on AISI 316 stainless steel at various gas compositions (25% N2, 50% N2 and 75% N2) for 4 hours. Additionally, during post nitriding Ar gas was used with H2 and N2 to observe the improvement of treatment. After treatment, the behavior of the hybrid layer was investigated by optical microscopy, X-ray diffraction, and micro-hardness testing. Potentiodynamic polarization test was also used to evaluate the corrosion resistance of the samples. Meanwhile, it was found that the surface hardness increased with increasing the nitrogen gas content. Also small percentage of Ar gas was introduced in the post nitriding process which improved the hardness of the hardened layer but reduces the corrosion resistance compared with the carburized sample. The experiment revealed that AISI 316L stainless steel showed better hardness and excellent corrosion resistance compared with the carburized sample, when 75% N2 gas was used during the post nitriding treatment. Also addition of Ar gas during post nitriding treatment were degraded the corrosion resistance of the sample compared with the carburized sample.

• Korean Journal of Metals and Materials
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• v.46 no.6
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• pp.357-362
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• 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.

• Korean Journal of Metals and Materials
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• v.50 no.7
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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.

• Journal of the Korean Society for Heat Treatment
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• v.35 no.3
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• pp.139-149
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• 2022

In the automobile and shipbuilding industries, various materials and components require superior surface strength, excellent wear resistance and good resistance to repeated loads. To improve the surface properties of the materials, various surface heat treatment methods are used, which include carburizing, nitriding, and so on. Among them, carburizing treatment is widely used for structural steels containing carbon. The effective carburizing thickness required for materials depends on the service environment and the size of the components. In general, however, there is a limit in evaluation of the surface properties with a standardized mechanical test method because the thickness or cross-sectional area of the carburized layer is limited. In this regard, the nanoindentation technique has lots of advantages, which can measure the mechanical properties of the material surface at the nano and micro scale. It is possible to understand the relationship between the microstructural change in the hardened layer by carburizing treatment and the mechanical properties. To be spread to practical applications at the industrial level, in this paper, the principle of the nanoindentation method is described with a representative application for analyzing the mechanical properties of the carburized material.

• Journal of the Korean Society for Precision Engineering
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• v.14 no.5
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• pp.53-59
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• 1997

This study has been performed to inmvestigate MRR(metal removal rate), REW(relative electrode wear), surface roughness, heat transumutation layer and microhardness distribution in cross-section of the machined surface with various pulse-on duration and peak pulse current, using the copper and graphite electrode on the heat treated STD11 which is extensively used for metallic molding steel with the EDM. The results obtained are as follows; a) There exists critical pulse-on duration(If Ip equals 5A, .tau. on is 50 .mu. s) which shows the the maximum MRR in accordance with peak oulse current and the MRR decreases when the pulse-on duration exceeds the critical pulse-on during because of the abnormal electric discharge. b) Safe discharge is needed to make maximum of MRR and the metalic organization must be complicated for discharge induction. c) Graphite has much more benefits than copper electrode when rapid machining is done without electrode wear. d) The most external surface has the highest microhardness because of car- burizing from heat analysis of the dielectric fluid and the lower layar of the white covered layer has lower microhar dness than base matal because of softening.

• Journal of the Korean Society for Heat Treatment
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• v.37 no.3
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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.

• Korean Journal of Materials Research
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• v.3 no.6
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• pp.652-660
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• 1993

It was investigated to obtain the relationship between magnetic properties and conditions of forming $Fe_{3}C$ single phase from acicular goethite by heat treatment under the atmosphere of CO and $N_{2}$ mixed gas. X-ray analyses, TEM and VSM measurements were imployed for the characteristics of the carbide. Acicular goethite was sinthesized under proper process parameters. $Fe_{3}C$ single phase was obtained above $550^{\circ}C$, 60min. and $Fe_{5}C_2$ was formed with $Fe_{3}C$ below that temperature. The soturation magnetization of $Fe_{3}C$ single phase was about 100emu/g regardless of the reaction temperature. The coersive force and the ratio of Mr/Ms decreased respectively from 780 to 400 Oe. and from 0.35 to 0.13 with increasing reaction temperature.

• Korean Journal of Materials Research
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• v.12 no.10
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• pp.776-783
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• 2002

Austenitic stainless steel (STS304) has been carburized using glow discharge plasma and its microstructure, wear resistance and corrosion property have been investigated. A repeat boost-diffuse carburizing was used as an effective plasma carburizing method. The effective case depth of the plasma carburized specimens was increased with the carbon concentration at the surface area. The specimens prepared by 3 hours plasma carburizing under $600^{\circ}C$ did not have the standard hardness for the effective case depth, but the specimen prepared by 11 hours plasma carburizing at $500^{\circ}C$ had nearly the same hardness with the specimen plasma carburized for 3 hours at $800^{\circ}C$. The wear resistance increased with temperature but the corrosion properties of the specimens prepared over $600^{\circ}C$ decreased rapidly due to the grain boundary sensitization. However, the specimen plasma carburized for 11 hours at $500^{\circ}C$ had nearly the same wear resistance with the specimen plasma carburized for 3 hours at $800^{\circ}C$ without deterioration of corrosion property. This could be resulted from the fact that the microstructure of the specimen plasma carburized for 11 hours at $500^{\circ}C$ was composed of martensite and austenite, because a partial martensite transformation was occurred only in the specimen plasma carburized for 11 hours at 50$0^{\circ}C$.