• Transactions of the Korean Society of Machine Tool Engineers
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• v.14 no.2
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• pp.75-82
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• 2005

Recently the needs of high reliable substances of high strength and high ductility are gradually increased with the development of aerospace industry. The characteristics of maraging steel has high ductililty, formability, corrosion resistant and high temperature strength and is easy to fabricate, weld and treat with heat, and maintain an invariable size even after heat treatment. e steels are furnished in the solution annealed condition and they achieve full properties through martensitic precipitation aging a relatively simple, low temperature heat treatment. As is true of the heat treating procedures, aging is a time/temperature dependent reaction. Therefore, the objective of this stud)'was consideration of fatigue characteristics according as Nb(niobium) content and time/temperature of heat treatment change. Also the stress analysis, fatigue lift, and stress intensity factor were compared with experiment results and FEA(finite element analysis) result. The maximum ftresses of)( Y, and Z axis direction showed about $2.12\times$10$^{2}$MPa, $4.40\times$10$^{2}$MPa and $1.32\times$10$^{2}$MPa respectively. The fatigue lives showed about $7\%$ lower FEA result than experiment result showing almost invariable error every analyzed cycle. Stress intensity factor of the FEA result was lower about $3.5~ 10\%$ than that of the experiment result showing that the longer fatigue crack ten添 the hi인or error. It considered that the cause for the difference was the modeled crack tip having always the same shape and condition regardless of the crack growth.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2018.06a
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• pp.107-107
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• 2018

무전해 니켈도금 용액의 성분은 Ni(II)염, 환원제, 적합한 금속 배위 리간드, 안정제 및 특정 특성 요구에 대한 첨가제를 포함한다. 일반적으로 도금 욕에는 미량의 안정제가 함유되어 있다. 만약 적절한 안정화 시스템이 없는 도금 욕에서 도금 공정 시 도금 시작 직후에 많은 양의 니켈 플레이크(Ni flake)가 생성되어 빠르게 도금 용액이 분해되어 더 이상 도금이 어렵게 된다. 그러나 무전해 도금 욕에서 안정제의 역할 및 도금 층에 미치는 영향에 대한 연구는 여전히 부족한 실정이다. 따라서 본 연구에서는 $Pb^{2+}$ 안정제 농도가 도금 층에 미치는 영향과 캐비테이션 침식 실험을 통해 그 내구성을 평가하고자 하였다. 무전해 니켈코팅을 위한 모재는 회주철(FC250)을 $19.5mm{\times}19.5mm{\times}5mm$의 크기로 가공하였다. 회주철의 인장강도는 $330N/mm^2$이며, 그 성분 조성(wt.%)은 3.23 C, 1.64 Si, 0.84 Mn, 0.016 P, 0.013 S 그리고 나머지는 Fe이다. 시험편은 SiC 페이퍼 #1200까지 연마하여 시험편의 표면 거칠기는 $1.6-2.1{\mu}m$ 범위 내로 제작하였다. 무전해 도금 전 시험편은 탈지를 위해 상온의 아세톤 용액에서 3분간 초음파 세척하고, $90^{\circ}C$의 알카리 수용액으로 5분간 세척하였다. 그리고 표면 활성화를 위한 산세척은 5% 황산용액에서 30초 동안 실시하였다. 도금조로 500mL 비커를 사용하였으며, 모든 시험편은 2시간 동안 무전해 니켈도금을 실시하였다. 그리고 니켈도금 층의 내식성과 내구성을 평가하기 위해 전기화학적 분극 실험을 통한 타펠분석과 ASTM G32 규정에 의거한 캐비테이션 침식 실험을 실시하였다. 그 결과 안정제 농도가 도금 속도와 도금 층의 성분 변화에 크게 영향을 미쳤으며, 그에 따라 도금 층의 내식성과 내구성이 크게 변화되었다.

• Korean Journal of Materials Research
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• v.8 no.8
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• pp.766-774
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• 1998

32$0^{\circ}C$, 40%NaOH 용액의 autoclave에서 약 300wppm의 탄소를 함유하고 있는 15Cr-9Fe-balanced Ni 합금 판상시편에 대해 응력부식 저항성을 조사하였다. 부식시편은 $700^{\circ}C$, 100시간 동안의 열처리로 합금내부에 석출될 수 있는 가능한 한 많은 양의 크롬계 탄화물을 석출시킨 후, 다시 재용해에 의해 크롬계 탄화물의 형태를 조절하는 $800^{\circ}C$-$950^{\circ}C$범위의 최종열처리를 시행하고 급냉시킨 다음 U-자형으로 응력을 가하여 준비되었다. 최종열처리 온도가 올라감에 따라 시편들의 입계응력부식균열(IGSCC ) 전파속도는 $900^{\circ}C$까지는 거의 직선적으로 증가하다가 $950^{\circ}C$에서는 $700^{\circ}C$에서 얻은 값보다도 더 낮게 감소하였다. 즉, 크롬계 탄화물이 재용해되어 그 밀도가 감소함에 따라 IGSCC저항성이 감소하다가 완전히 재용해된 $950^{\circ}C$ 열처리 조건에서 오히겨 가장 큰 IGSCC 저항성을 나타내었다. 이와같은 최조열처리 온도에 따른 니켈계 합금 600의 부식거동은 입계에 존재하는 크롬계탄화물의 형태변화 때문이 아니라 입계에서 탄소-크롬계 탄화물-크롬간의 상평형에 의해 이루어지는 탄소의 입계편석량이 크롬계탄화물이 존재할 때에는 열처리 온도에 따라 증가하다가 그것이 완전히 재용해 되었을 때 가장 낮아지기 때문인 것으로 생각된다.

• Journal of Korea Foundry Society
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• v.41 no.6
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• pp.543-549
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• 2021

Aluminum cast iron has excellent oxidation resistance and good resistance to sulfide and corrosion. Compared to Ti and Ni alloys, it is expected to be a substitute material for structural materials and stainless steels because it is relatively inexpensive to use Fe, which is a non-strategic element. This results in a weight reduction effect of about 30% as compared to the use of stainless steel. With regard to aluminum as an alloying material, it is an element that has been widely used for the alloying of cast iron in recent years. Practical use has been delayed owing to the resulting lack of ductility at room temperature and the sharp decrease in the strength above 600℃ of this alloy, however. The cause of the weak room temperature ductility is known to be environmental embrittlement by hydrogen, and the addition of various alloying elements has been attempted in order to mitigate these shortcomings. Although alloying elements such as vanadium, chromium, and manganese are mainly used to increase the hardness and wear resistance of gray cast iron, the price of finished products containing these elements and the problems associated with alloys with this material impose many limitations.

• Journal of the Korean Electrochemical Society
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• v.24 no.2
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• pp.13-27
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• 2021

Stainless steel, which includes Ni and Cr with Fe balance, is most often applied for a wide range of applications such as structure and equipment material. It is not only suitable for use in these applications due to its good corrosion resistance, but also can be applied to catalyst, supporting material, and current collector due to intrinsic properties of Ni and Fe contained in stainless steel. Therefore, in recent years, a lots of surface treatment methods have been studied to activate stainless steel, developing application of water splitting system. In this review paper, the research on the surface treatment technology of stainless steel for water splitting is summarized. It is expected to be able to propose the diverse surface treatment approaches of stainless steel for application to low-cost and highly efficient water splitting electrode.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2004.05a
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• pp.352-355
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• 2004

Inconel 625 is useful in variety of industrial applications because of the resistance to attack in various corrosive media at temperatures from $200^{\circ}C$ to over $1090^{\circ}C$, in combination with good law- and high temperature mechanical strength. Rencently, this material is also used widely in offshore processing piping in order to extend the maintenance tenn and improve the quality of anti-corrosion. In general, high quality weldments for this material are readily produced by commonly used processes. Not all processes are applicable to this material group, Ni-alloys. Metallurgiad characterictics or the unavailability of matching, position or suitable welding processes. Nowadays, the flux cored wire is developed and applied for the better productivity in several welding position including the vertical position. in this study, the weldability and weldment characteristics of inconel 625 are considered in FCAW weld associated with the several shielding gases($80\%Ar\;+\;20\%\;CO_2,\;50\%Ar\;+\;50\%\;CO_2,\;100\%\;CO_2$) in viewpoint of welding productivity.

• Journal of the Korean Society for Precision Engineering
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• v.32 no.8
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• pp.681-685
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• 2015

Hastelloy C-276 composed of Cr, Mo, and Ni is a versatile, corrosion-resistant alloy with numerous industrial applications including its use in nuclear reactors, general chemical plants, and as a superconducting base material. Of especial significance, it can be used as a thin-sheet type whereby lap-joint welding is occasionally necessary. The main welding problems for thin-sheet metals are deformation and burn-through from an excessive heat input. Laser welding can minimize these problems because it has a high energy density and low heat effect on the base material. In this study, the laser-welding characteristics of lap-joint Hastelloy C-276 sheet metal were determined. The criteria of the laser-welding variables were chosen using a heat-conduction analysis, and the optimal welding parameters were selected by experimenting with an Nd:YAG laser.

• Journal of Welding and Joining
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• v.16 no.6
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• pp.59-68
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• 1998

In this study, effects of solidification modes (primary $\delta$-ferrite, primary ${\gamma}$-austenite) on the pit initiation and propagation in the 304L and 316L austenitic stainless steel weld metals were investigated. The solidification mode of weld metal was controlled by the addition of nitrogen to Ar shielding gas. Through the electrochemical experiments (potentiodynamic anodic polarization and potentiostatic time-current transient test) and metallographic examination (microstructure and elemental distribution), the following results were obtained. The more the volume content of nitrogen in the shielding gas were, the lower critical current density for passivity was observed. In comparison with weldments solidified through the primary $\delta$-ferrite solidification mode and the primary ${\gamma}$-solidification mode, the former showed higher critical pitting potential and a longer incubation time for stable pit initiation than the latter. However, in the pit propagation stage the former exhibited a faster dissolution rate than the latter. These results were believed to ee related to the distribution of alloying elements such as Cr, Mo, Ni and S.

• Journal of the Korean Society for Heat Treatment
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• v.19 no.1
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• pp.3-9
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• 2006

The phase changes, nitride precipitation and hardness variations of 14%Cr-6.7Ni-0.65Mo-0.26Nb-0.05V-0.03C super martensitic stainless steel were investigated after nitrogen permeation heat treatment at a temperature range between $1050^{\circ}C$ and $1150^{\circ}C$. The nitrogen-permeated surface layer was transformed into austenite. The rectangular type NbN, NbCrN precipitates and fine round type precipitate were coexisted in the surface austenite layer, while the interior region that was free from nitrogen permeation kept the martensitic phase. The hardness of surface austenite showed 280 Hv, while the interior region of martensite phase represented 340 Hv. When tempering the nitrogen-permeated steel at $450^{\circ}C$, a maximum hardness of 433 Hv was appeared, probably this is attributed to the secondary hardening effect of the precipitates. The nitrogen concentration decreased gradually with increasing depth below the surface after showing a maximum of 0.3% at the outmost surface. The strong affinity between nitrogen and Cr enabled the substitutional element Cr to move from interiors to the surface when nitrogen diffuse form surface to the interior. Corrosion resistance of nitrogen permeated steel was superior to that of solution-anneaed steel in the solution of 1N $H_2SO_4$.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2004.11a
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• pp.234-238
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• 2004

Inconel 625 is useful in variety of industrial applications because of the resistance to attack in various corrosive media at temperatures from $200^{\circ}C$ to aver $1090^{\circ}C$, in combination with good law- and high temperature mechanical strength. Rencently, this material is also used widely in offshore processing piping in order to extend the maintenance term and improve the quality of anti-corrosion. In general, high quality weldments for this material are readily produced by commonly used processes. Not all processes are applicable to this material group, Ni-alloys. Metallurgical characterictics or the unavailability of matching, position or suitable welding processes. Nowadays, the flux cored wire is developed and applied for the better productivity in several welding position including the vertical position. in this study, the weldability and weldment characteristics (mechanical properties) of inconel 625 are considered in FCAW(Flux Core Arc Welding) associated with the several shielding gases($80\%Ar\;+\;20\%CO_2,\;50\%Ar+50CO_2,\;100CO_2$) in viewpoint of welding productivity.